NZ Seismicity in January 2021... & January 1921

Monthly Seismicity Summary #1 - January 2021

This year, as part of my blog posts, I endeavour to produce monthly summaries of New Zealand seismicity. This will primarily be limited to those earthquakes felt in major communities, with note of smaller events too.

In addition to this, a second section of these monthly summary posts will be dedicated to the seismicity which afflicted New Zealand exactly one century ago. Currently only five earthquakes are documented as having struck New Zealand in 1921 - as part of my research I have delved into archival documents (newspapers & Seismological Observatory felt reports) and I shall divulge some of my findings each month. I may also attempt a more detailed blog post for a major quake in June (time shall tell).

January 2020 Seismicity

The first widely felt quake struck on the morning of the 2nd January. This, the second quake in a doublet (the first striking in the early hours of New Years Eve 2020) with epicentres just east of Porirua Harbour, had a local magnitude (ML) of 4.2 and a moment magnitude (Mw) of 4.0. The result of oblique reverse faulting, it was associated with the subduction of the Pacific Plate beneath the Australian Plate. The quake was felt in the Wellington Region, with 7623 felt reports received by GeoNet. Maximum intensities seem to have been MMI V - observations indicative of this intensity include: it was felt by almost all people indoors and some outdoors, alarming a few people, displacing small unstable objects, knocking hanging pictures against walls, and swinging doors.

Small quakes were felt thereafter, with quakes in North Canterbury (ML 3.8) & North Taranaki (ML 3.5) on the 5th January, Central Canterbury (ML 2.5) and South Tasman (ML 3.9) on the 10th, Whanganui (ML 3.4) on the 12th, Gisborne (ML 3.7) on the 13th, North Whanganui (ML 3.4) on the 17th, & a doublet beneath Taupo (ML 2.3, ML 2.0) on the 21st. 

In the early hours of the 22nd January a ML 5.0 quake struck near Secretary Island, Fiordland. At present no moment tensor solution has been produced for this quake by GeoNet, but it seems reasonable on the basis of felt reports - which were few in number, and mainly concentrated at Te Anau, Manapouri and to a lesser degree in Queenstown, with intensities not exceeding MMI IV - that the moment magnitude was in the region of 4.5. Earthquakes which are relatively shallow (<30km depth) in Fiordland, especially those associated with the subduction interface or subducting Australian slab, tend not to be widely felt in Southland and Otago unless they have a local magnitude of 5.5 or higher.

Later on the morning of the 22nd January a ML 4.2 earthquake struck 25km south-west of Whanganui. It was predominantly felt in Whanganui, with scattered felt reports from Palmerston North, Foxton, Levin, Paraparaumu, Porirua & Wellington; the maximum intensity was MMI IV. This offshore area plays host to a plethora of small to moderate magnitude earthquakes, although none have been known to exceed ML 5.0, and as far as I am aware none have been damaging. In a reply on Twitter GeoNet stated that this seismic activity had been ongoing since at least 1940, with activity at its greatest in the 1980s. I do believe this activity predates 1940, with likely candidates throughout the latter half of the 19th, and early decades of the 20th, centuries. 

Starting at 7:47 am on the 25th January, an earthquake sequence began in the volcanic lakes east of Rotorua. A ML 4.5 (Mw 4.6) quake struck at 7:57 near Lake Rotoiti, which was felt widely in the Bay of Plenty region, from Whakatane in the east to Tauranga in the west, with maximum reported intensities of MMI VI at Lake Rotoiti. A quarter of an hour later a ML 4.9 (Mw 5.1) quake struck, which was felt out to Waihi in the west and Opotiki in the east. These two quakes triggered many small landslides within the unstable Haroharo lava fields of the Tarawera Volcano and on the steep flanks of Lake Ōkataina, indicating maximum intensities of MMI VII. A plethora of aftershocks followed, although none exceeded ML 4.0, and the felt extent for these was mostly limited to Kawerau, Lakes Rotomа̄, Rotoehu & Rotoiti, as well as the city of Rotorua. These quakes were the result of normal faulting within the back-arc Taupo Rift, and occurred in an area which has witnessed similar sized quakes in 1998 (Haroharo Sequence) and 2004 (Mw 5.4 Lake Rotoma Quake and aftershocks).

Felt report map of the 25th January 2021 Mw 5.1 Lake Rotoiti Earthquake, produced using a large sample of the felt reports received by GeoNet for the quake.

In the morning of the 27th January a ML 3.6 (Mw 3.6) earthquake struck 5km offshore of New Plymouth. The quake produced maximum intensities of MMI V, and was followed by two small M2.0 & M2.3 aftershocks, the latter of which caused MMI IV shaking.

Less than an hour after the ML 3.6 New Plymouth quake, a ML 4.3 (Mw 3.9) earthquake struck offshore of Foxton Beach, Horowhenua. This quake was felt from Hawera down to Wellington, and produced maximum intensities of MMI IV. It was an aftershock to the May 2020 Mw 5.6 Horowhenua Earthquake, which caused minor damage to coastal Horowhenua communities. This is a good example of how moderate quakes can still produce aftershock months after they struck.

Finally, at just past midday on the 29th January a ML 3.1 earthquake struck south-east of Whanganui, with reported maximum intensities of MMI III-IV.

During this month, 100 years ago...

According to the National Earthquake Information Database there were no earthquakes in New Zealand in January 1921. Evidently this cannot have been the case - for January 2020 ~1700 earthquakes were recorded, and about 2% of those were felt. Therefore, though numbers may have varied, it is statistically likely that there were a minimum of perhaps 10 felt earthquakes in January 1921. However not all of these will be recorded in newspapers (which primarily noted earthquakes felt in the town in which the newspaper was published and occasionally rural settlements in the immediately vicinity) or the Seismological Observatory's felt reports (reports received from observers in towns and cities with reasonable populations across New Zealand).

As a result for January 1921 only 3 earthquakes can be found, the earliest of these occurring quite late in the month on the 21st January 1921. A slight tremor was noted in the Wairarapa Age newspaper (published in Masterton) as having been felt in the town at 7pm. For this period only three newspapers are available on Papers Past, and none of them mention the quake, so whether it was felt in any of the neighbouring Wairarapa towns is unknown.

The second earthquake is of the most interest. At 5:05 am on the 28th January 1921 a strong earthquake was felt in eastern Otago, primarily in and around Dunedin, waking many residents. Available newspaper reports suggest the earthquake was only felt in a limited region - Dunedin, Port Chalmers, Green Island & Waihola. Seismological Observatory reports, however, state that the quake was felt over a far wider area, with reports from Owaka in the south to Oamaru in the north. 

The strongest intensity was MMI V in Dunedin, with many people awakened by the quake, and it is possible the same intensity was observed at Port Chalmers & Green Island. MMI IV intensities almost certainly were observed in Oamaru, Waihola, Taieri, Balclutha & Owaka, with a rather suspicious but plausible felt report found in the Mataura Ensign newspaper stating "a slight earthquake shock, lasting a few seconds, was experienced locally [in Gore] this morning about 5.10."

Felt report map of the 28th January 1921 ML 4.5-5.0 Otago Earthquake. Compare with felt report map of the 1974 Dunedin Earthquake below.

No reports are forthcoming from Central Otago localities (i.e. Naseby, Clyde, Alexandra, Wanaka, Queenstown) which suggests that the quake did not produce sufficiently strong enough shaking to awaken people in these places. This also suggests, unusually for earthquakes felt in Otago and/or Southland, an epicentre which was not located in on the west coast of the South Island in Fiordland but rather on the east coast, not too distant from Dunedin.

The felt extent and intensities are not too dissimilar from those observed during the ML 4.9 April 9th 1974 Dunedin Earthquake; the primary difference is that the 1974 quake caused chimney damage in South Dunedin, indicative of MMI VII intensities, whereas this January 1921 quake produced no damage. A suggestion is that the 1921 quake may have been further offshore, but of a similar magnitude. Irrespective of the lack of damage caused, this is a sizeable moderate earthquake in an unusual location, and is a very helpful find when evaluating the historical seismic record of Otago.

Isoseismal map of the 9th April 1974 ML 4.9 Dunedin Earthquake (Downes & Dowrick, 2014). Note scattered felt reports from rural Central Otago locations which are not likely to be found in 1920s newspapers or Seismological Observatory felt reports.

There is one final remark to make about this quake. In the Evening Star of the 28th January 1921 a paragraph in the small column to do the earthquake notes: "This morning a great quantity of a large type of old seaweed not commonly seen on the beach was washed in at St. Clair, and it is assumed that this is the result of the seismic disturbance." Although the assumption may be quite a jump, it would appear as though the weather at the time was quite fair, with light winds from the north-east. St. Clair is a southerly/south-easterly facing beach, so it is perhaps a bit peculiar that older seaweed may have got washed in such great quantities. Is this suggestive of a minor tsunami, and if so was this event indeed a tectonic earthquake as the 1974 quake was, or perhaps a submarine landslide far offshore on the edge of the continental shelf? Or alternatively could this be a delayed result of a previous storm, observed onshore in the previous week or occurring offshore in Southern Ocean? This is all pure conjecture with only this note as basis, but a tsunamigenic source is a possibility, however small.

The third earthquake struck at 6:44 am on the 29th January 1921, and is undoubtedly a deep earthquake beneath the North Island. Deep quakes are incredibly difficult to discern location or depth for without instrumental recordings, and this quake is a good example. Felt locations roughly form an east-west ellipsoid centred beneath the Rangitikei Valley - the only decent felt report is from Whanganui, with glassware and crockery rattling, indicative of MMI IV, but probable MMI IV felt reports can be found for Taihape, Ohakune and Opunake, with further reports of "slight shocks of earthquake" received from Eltham, Palmerston North, Dannevirke and Waipawa. On this basis the earthquake likely had a local magnitude of 4.5-5.0, with a depth probably in excess of 40km, and likely as much as 100km.

January 1921 is by far the quietest month in terms of seismicity of that year, and besides the ML 4.5-5.0 Offshore East Otago earthquake is not too notable for any significant earthquakes. Succeeding months contain some far more interesting, and damaging, earthquakes...

Acknowledgements

Data on the 2021 earthquakes is sourced from GeoNet, with moment magnitude information courtesy of John Ristau (GNS Science). Data on the 1921 earthquake is sourced from archival newspapers courtesy of the National Library of New Zealand via the Papers Past website, as well as from the GNS Science Archives courtesy of Paul Viskovic.

Reference[s]

Downes, G.L., Dowrick, D.J., (2014). Atlas of isoseismal maps of New Zealand earthquakes - 1843-2003. 2nd rev. edn. Lower Hutt: GNS Science.

Minor edit made to paragraph on seaweed observation, 1st February 2021 (UTC)/2nd February 2021 (NZDT).

Criticism of Seismologists Is Nothing New

Given it has been a little while since I last wrote a blog post, I thought I'd dip my toe in the water again to see if I can get the creative juices flowing with this.

A significant part of the work I am doing currently is transcribing old felt reports and newspaper articles to do with the earthquakes I wish to base my Masters project on (proposal submission is still a long way off in April). For one of the two primary historical earthquakes I wish to focus on this means transcribing ~130 PDF pages, including newspaper articles, letters, telegrams and completed felt report forms. Currently I am ~70% complete in this task, with the other primary earthquake still to do.

One of these newspaper articles is perhaps worthy of comment. The page from which this is taken has three columns cut out of the newspaper issue and stuck onto a specific A4 form page for newspaper extracts. The first column is an opinion piece, the second and third columns report the observations of the quake from different localities. I quote the first in full:

    "Earthquakes, like big totalisator dividends, set a lot of tongues wagging, and it is to be feared that they produce a mushroom crop of prevaricators. There are those walking the streets in perfect assurance to-day who will aver that they were not in the least perturbed by last night's visitor, but the average man must admit that when he was just getting interested over the first tremor, the succeeding jolt made him think that matters were going beyond a joke. However, New Zealanders are not easily frightened by earthquakes, and it is safe to say that a great many persons commented yesterday on the "earthquaky weather" that was hanging about, and is still hanging about, for that matter. And this raises the question as to what New Zealand's official seismologist or vulcanologist was doing not to predict such an unmistakable 'quake as that which shook the city last night. The answer seems to be that he is concerned chiefly with the pie-crust of the North Island volcanic region, and if that is so nobody would be unkind enough to suggest that he should turn his attention elsewhere in the meantime."

        Star (Christchurch, New Zealand), 10th December 1925.

There are several interesting points which can be gleaned from this opinion by, one assumes, the editor of the newspaper. 

The first is how he has compared earthquakes to investments. This article in from the 1920s, when stocks and shares were all the rage in the post-WW1 world and speculative investments were everywhere. Only four years later the Wall Street Crash occurred, the harbinger of the Great Depression of the 1930s.

The second is that, despite this being simply an opinion, it actually describes the experience of the earthquake relatively well. Preliminary tremors were felt - most likely the P-waves and possibly S-waves - followed by stronger tremors (possibly S-waves and definitely surface-waves) just as people in Christchurch probably felt the earthquake was about to die down. The earthquake in question most likely had its epicentre in a region bounded by Nelson, Blenheim and Wellington (250-300km to the north/north-north-east) and fairly large by New Zealand standards (M6.5-6.7). This explains the description of it's felt effects in Christchurch, which is susceptible to strong shaking even from distant earthquakes due to it's location above unusually strong crust and thick alluvial sediments.

The third is that New Zealanders are not easily frightened by earthquakes. In 1925 New Zealand had been experiencing a relatively quiescent period in seismicity - the last deadly earthquake had occurred in 1917 at Tinui in the Wairarapa, and the country hadn't suffered a M7 onshore quake since 1904 (a deep offshore Mw 7.3 struck the Bay of Plenty in November 1914). Although moderate quakes had struck more recently in 1917 (Tinui, Mw 6.7-6.9), 1921 (Hawke's Bay, Mw 6.6-6.8) & 1922 (Motunau, Mw 6.4), none of these had been truly devastating, and much of the country hadn't experienced truly strong shaking for decades. For Christchurch the 1922 Motunau quake had caused minor damage but nothing significant, 1901 Cheviot had yielded similar results, and the last significant damage had occurred in 1888, nearly 40 years earlier. It is little wonder that the editor, and perhaps New Zealanders at large, were not concerned too much by earthquakes as an imminent danger (1929 Buller & 1931 Hawke's Bay would awaken New Zealanders to just how significant a hazard earthquakes posed in their nation).

The fourth point regards the reference to "earthquaky weather". This is a phenomenon which has been alluded to for more than a century and is nothing more than people trying to find something to explain events which they struggle to find an explanation for. Science, and in all honesty simple observations, shows that there is no one type of weather which occurs or precedes earthquakes; therefore it is impossible to say there is such a thing as "earthquake weather", because the weather which occurs at the time of earthquakes is as variable as it is when there are no earthquakes. There is also no evidence of a causal link between atmospheric conditions and earthquakes, and despite many amateur scientists from the mid-19th century (Charles Rous Marten specifically recorded the weather for 1860s earthquakes felt in Otago) to the present day none have provided irrefutable proof of any connection.

This pervading pseudoscientific belief is the basis for the editor's attacks on the scientists at the Hector Observatory in Wellington (who rather ironically preserved the article for posterity - one wonders whether their reaction was one of amusement, muted frustration or indignation). As far as I am aware none of the seismologists at the Hector Observatory (R. C. Hayes, C. E. Adams) ever issued forecasts or predictions (these being far out of the scope of the science at the time - this precedes the invention of a magnitude scale for example). Some people in New Zealand did issue predictions, but as with their successors today these were based on baseless theories utilising astronomical or astrological "methods" and were primarily to attain public fame than for any societally useful purpose.

The accusation that the Hector Observatory's scientists were focussed chiefly on the Taupo Volcanic Zone most likely stems from the most recent significant seismicity in New Zealand. During much of 1922 a significant and damaging earthquake sequence struck north of Lake Taupo, and seismologists were busying themselves investigating the cause and effects of these intriguing and unusual events during the successive years. Besides the 1922 Motunau Earthquake there had been relatively little seismicity outside of the North Island until 1925, and none of these events seem to have produced enough interest from government scientists to go in the field and investigate the source areas of the quakes. The accusation may also have some basis in the regional rivalries that had existed in New Zealand for decades (stemming from the provincial system of the 19th century), although this is more speculation than anything else.

All in all, this is a rather interesting little opinion article, and says a lot about public understanding of earthquakes, and general perception of the incipient scientific study of geohazards. It is important to understand the context of historical earthquakes not just geologically and tectonically, but also in terms of social and cultural history. This is where books such as The Great Quake Debate: The Crusader, the Skeptic, and the Rise of Modern Seismology are important and indeed necessary in the study of historical seismicity: the only way to learn about these early-instrumental and pre-instrumental earthquakes is via historical sources, and thus the data gathering process should be treated as much as a historical study as a geological one. (Whether this makes me any less of a scientist than someone who deals primarily in, for example, seismograms and what they tell us about earth processes is for you to decide).

I wish to extend thanks to GNS Science, and specifically to Paul Viskovic and the GNS Archives team, for providing me with the digitised felt reports from the early 20th century. I also acknowledge Gaye Downes & the late David Dowrick for the fantastic Atlas of Isoseismal Maps of New Zealand earthquakes - 1843-2004 which  provides the magnitudes quoted above.

A final note: it has been fantastic to see from afar the 2020 Geoscience Society of New Zealand Annual Conference in Christchurch via Twitter. I recently became a member of the society and would have dearly loved to have attended. The science being produced is absolutely incredible, and I hope I will be able to join in the research with colleagues in New Zealand in the coming years. Perhaps I'll even be lucky enough to have a poster at a future conference!

Why Study Historical Earthquakes In New Zealand?

I've had to think hard on how to put this together. I really want to share what I do with everyone, but experiences in the past (when I did my fiction writing a decade or so back) make me incredibly hesitant to reveal too much information. Hopefully I can find a balance here; please let me know if there isn't enough information in this in order for you, the reader, to actually understand what I am talking about!

Searching the Archives

In my previous blog post I left off having decided to start looking at historical earthquakes in New Zealand for a prospective, albeit never begun, Masters project at Plymouth University. About a month later (early April 2018) I began to search the internet to see what I could find. The first, and most obvious port of call, was the GeoNet website. On the website you can find all sorts of information about earthquakes: live seismograms, recent earthquakes, slow slip events and much more. The two pages of most use to me however are the historical events, which shows a variety of significant earthquakes which have struck New Zealand between 1848-present, and the earthquake search which enables you to search for earthquakes within the GeoNet database. I'll get back to database later in this blog post.

Both of these pages are very powerful. The historical events summarises all sorts of quakes, from significant 19th century quakes (1848 Marlborough, 1855 Wairarapa, 1868 Cape Farewell & 1888 North Canterbury) to more moderate 20th century events (1966 Seddon, 1976 Waikato & 1990 Lake Tennyson). New Zealand's more recent seismicity has a far greater share of the list's contents, because a) recent seismicity has been the most active to strike New Zealand in 80-90 years and b) most people are going to interested in reading about the more recent significant earthquakes than those in distant living memory or beyond living memory.

Isoseismal Map of the 1868 Cape Farewell Earthquake (Downes & Dowrick, 1995)

And herein lies the problem for New Zealand's historical catalogues. There are three main sources available for historical quakes in Aotaeroa: the GeoNet earthquake database (also known as the National Earthquake Information Database, or NEID for short); the two editions of Gaye Downes & David Dowrick's Atlas of Isoseimal Maps of New Zealand Earthquakes (published 1995 & 2014); & George Eiby's An Annotated List of New Zealand Earthquakes (published 1968). The latter author also has a couple of papers documenting earthquakes which occurred in the early years of New Zealand's historical after the Pākehā arrived in the early 20th century, whilst there are a myriad of papers documenting individual historical earthquakes from different disciplinary perspectives (seismology, tectonics, ground motion, engineering geology, etc.)

This is a fantastic start and in fact the available sources mean that historical earthquakes post-WW2 are fairly well documented, with all earthquakes above magnitude 6.0 known, and many more moderate (magnitude 4.5-6.0) earthquakes also documented. Many of these events are not included in GeoNet's historical events page (e.g. 1948 Mw 6.4 Waiau, 1962 Mw 5.9 Westport, 1974 ML 4.9 Dunedin & 1993 Mw 6.2 Ormond), but can be found in the NEID.

Isoseismal Map of the 1974 Dunedin Earthquake (Downes & Dowrick, 2014)

However, for the region targeted for the prospective Masters project back in 2018, there had been no significant quakes since a Mw 5.9 in 1951 damaged a township familiar with seismicity in the 20th century. This meant that, with a fairly complete catalogue after 1951, to find other earthquakes which may have adjusted the stress in the region I had to look earlier. This meant investigating earthquakes which may not be in the small collection of databases and catalogues available.

Another important discovery from the above sources was found in Downes & Dowrick (2014). In the preface to the Second Edition, the final paragraph notes that "there are many deep and some shallow earthquake with magnitude M≥6 for which there are insufficient data for an isoseismal map", as well as "large earthquakes in remote locations... or locations that were sparsely populated may not have bene recognised yet, as in-depth research on parts of the historical record, principally, 1855-1900, is incomplete." Both the need to look for specific regional quakes prior to 1951, and this paragraph, was encouragement to see just what could be found. Now it was just to find sources of information to do this.

By luck I happened upon a website which may be able to help and I have continued to use ever since. This is Papers Past, a New Zealand government website containing thousands of different digitised newspapers, magazines, journals, letters, diaries and parliamentary papers from the 19th, 20th & early 21st centuries. A treasure trove for researchers into New Zealand's post-colonisation history, it also contains plenty of references to earthquakes. One journal in particular, the Transactions and Proceeding of the Royal Society of New Zealand, contains in every edition from 1869-1902 tables of earthquakes in the respective year felt in the towns & settlements of New Zealand. These are an invaluable source for any wannabe researcher into New Zealand's historical seismicity, and they were one of the first gold mines I happened across - in fact it is the basis of much of my 19th century research.

However my main focus initially was on using the newspapers found in the online archive. Within a week of commencing my research I had got a brief overview of seismicity from the 1900s decade (1901-1910) using issues of the New Zealand Herald newspaper, based in Auckland. Further searching revealed that many quakes were missing, and with my foci changing over the weeks and months that decade still requires a lot of work.

Page 5 of the Monday, September 3 1888 issue of The Press newspaper (Christchurch), showing the some of the articles on the North Canterbury Earthquake two days prior (The Press, 1888)

For the first year and a half I did not have a specific aim besides trying to find as many earthquakes as possible. Due to my increasing understanding of existing knowledge of New Zealand's historical seismicity, I set an arbitrary end date of 1929 (March 1929 Arthur's Pass & June 1929 Buller Earthquakes were the two most significant earthquakes to strike New Zealand in at least a quarter of a century, and in the case of the latter since 1855). A beginning date for the catalogue I was gradually constructing was post-1855 (the Mw 8.1-8.2 Wairarapa Earthquake is the largest quake in post-colonisation New Zealand's history), and currently it is 1862 (the year of earliest quake I have in my catalogue).

By the end of last year I had decided that I must recommence on my education after a year of retail work & gradual recovery from my final year at Plymouth. I began to make enquiries, both in the UK and abroad, with a vague idea of wanting to do a Masters project on historical New Zealand seismicity. At about this time I decided to take a look into seismicity in the latter part of my catalogue's temporal range. In the process of searching I came across a few earthquakes which caught my attention. An email sent to New Zealand scientists in January of this year proved the spark to deciding on two possible Masters projects. Further enquiries ruled out one option - it would require personally looking through archives in New Zealand to find further sources of information on the earthquake, despite the apparent importance of this quake to seismic hazard in New Zealand - and added a lot of possibilities towards the other option.

At present it is still early days in putting together the Master project; for a start it is yet to be proposed, owing to the course not yet commencing. Data collection is still ongoing, although it is making good and steady progress. What feels most exciting to me is being able to share a portion of what I have found*, and perhaps hint at just how much more is yet to revealed. 

At present my All Quakes Database (which ironically excludes those already documented in Downes & Dowrick or the NEID) contains ~1550 individual earthquakes between 1862-1929. Recent data acquisitions (via GNS Science and a personal book acquisition) reveal that in reality, there most likely is a sum four or five times greater than that total to be found and documented. Many of these will most likely only have one felt location with no relevant information from which to derive an estimated intensity, but a sizeable minority will prove helpful in at bare minimum producing isoseismal maps, and at best relatively reliable estimates of location, magnitude and depth.

My desire to complete a Masters project on a small aspect of this research is twofold: to prove to myself that I can produce something worthy of being put out there into the wide scientific world, and to prove to others that what I am attempting to do is worthwhile. A catalogue of moderate and significant earthquakes is all very well and good, but it needs to be applied to things which have relevance to people today. And this is the task I have decided I wish to take on.

Final Note

I shall endeavour to share these holiday finds with you all soon. One of these was indeed an extraordinary find, owing both to it's contents and most certainly it's age. Another was written by the most popular writer on seismicity at the time of it's publication, and contains some beautiful plates. The third is a more recent publication (albeit nearly three decades older than myself) on a really significant New Zealand earthquake and was a nice find to accompany the others.

*This is a fun example of choosing the correct word. Technically speaking I am finding these quakes, but only because they have been forgotten or neglected in terms of research. A term I often unintentionally use is discover which would infer I was the first person to feel the quake, which would be impressive given I would need to be at least 70 years older than I am. Perhaps find and rediscover are the best terms to use, as I am both finding references to them, and in doing so rediscovering earthquakes felt by New Zealanders 90 or more years ago.

Literature References

Downes, G.L., Dowrick, D.J. 1995. Atlas of Isoseismal Maps of New Zealand Earthquakes. Institute of Geological & Nuclear Sciences monograph 11. Lower Hutt: Institute of Geological & Nuclear Sciences.

Downes, G.L., Dowrick, D.J. 2014. Atlas of Isoseismal Maps of New Zealand Earthquakes - 1843-2003. Second edition (revised). Institute of Geological & Nuclear Sciences monograph 25. Lower Hutt: Institute of Geological & Nuclear Sciences.

Eiby, G.A. 1968. An Annotated List of New Zealand Earthquakes, 1460-1965. New Zealand Journal of Geology and Geophysics. 11:3, p. 630-647.

Eiby, G.A. 1968b. A Descriptive Catalogue of New Zealand Earthquakes, Part 1 - Shocks Felt before the end of 1845. New Zealand Journal of Geology and Geophysics. 11:1, p. 16-40.

Eiby, G.A. 1973. A Descriptive Catalogue of New Zealand Earthquakes, Part 2 - Shocks Felt from 1846 to 1854. New Zealand Journal of Geology and Geophysics. 16:4, p. 857-907.

Picture References

Downes, G.L., Dowrick, D.J. 1995. Atlas of Isoseismal Maps of New Zealand Earthquakes. Institute of Geological & Nuclear Sciences monograph 11. p. 29. Lower Hutt: Institute of Geological & Nuclear Sciences.

Downes, G.L., Dowrick, D.J. 2014. Atlas of Isoseismal Maps of New Zealand Earthquakes - 1843-2003. Second edition (revised). Institute of Geological & Nuclear Sciences monograph 25. p. 604. Lower Hutt: Institute of Geological & Nuclear Sciences.

The Press (Christchurch, New Zealand), 1888. "Earthquake", 3 September 1888, p. 5. Accessed via: https://paperspast.natlib.govt.nz/newspapers/press/1888/09/03/5, National Library of New Zealand.

A Little About Me

I'm back from holiday and...

As my second blog post as I rejuvenate this blog, I thought I would write out a post explaining how I have got to point where I have decided to focus on historical seismology. As one prominent current historical seismologist has stated a few times, those who study the subject are a rare breed. How one gets interested in this topic then is perhaps something of interest to you.

A Quick History of My Interest in Seismology

As I stated in my last blog post I visited New Zealand's South Island in February 2010 with my grandfather and parents. I was in my mid-teens, quite impressionable, and I felt I knew what I wanted to do in the future (an author of fantasy fiction, having already gone through dreams of wanting to be an ancient historian & architect - all of these represent key interests of mine today). I had never been on a long-haul flight before (my only previous flight having been in autumn 2009 from Southampton Airport to Dublin, a journey not much more than an hour in length, as opposed to the 9 hour flight to Singapore, the 8 hours to Sydney, and the 2 hour or so to Christchurch). This was an adventure I was unlikely to match in a long time, if ever.

Both my grandfather and my father are avid motorcyclists, and my Granddad had been to New Zealand on two or three occasions in the previous 14 years. During his three visits - all for biker international rallies - he had befriended two different biker couples who lived in Christchurch. In February 2010 there was an International Rally for BSA bikes in Christchurch, and as my Granddad was nearly 80 this was likely to be his last chance to visit them. We arranged to go with him, and despite a very cold January feeling like it was threatening being able to fly away the weather relented by the time we left in early February.

Winter 2009-2010 was a tad bit cold in the UK. Exact date unknown

The first week was spent in Christchurch and involved several runs out to various sights. The first ride out was to Cust, a small village on the road between Woodend and Oxford in the Canterbury Plains. The second involved a ride across the Port Hills, round the north-west side of Lyttelton Harbour, and down to Wigram Airfield & Air Museum. The third day was a free day to do what you wished with, and we went into central Christchurch and saw the Cathedral Square, as well as a small bike museum hidden away in a small brick building, before going through the Lyttelton Tunnel and looking around Lyttelton itself. We saw the Timebell Station from the road (at the time it was having necessary renovations made to it, so sadly we could not take a look around).

Our fourth day took us down to Akaroa Harbour and a nice boat trip around it. The fifth took us to Godley House in Diamond Harbour, on the south side of Lyttelton Harbour. On the return trip I took my favourite picture from the entire holiday - the view of Lyttelton Harbour from the Summit Road looking east, from some point above Governors Bay. This was effectively the final day of the International Rally.

Godley House, 12th February 2010

The remainder of the holiday was spent touring around the southern half of the South Island in a rather cramped and poorly-maintained campervan. Despite the problems, we enjoyed visiting many places in South Canterbury, Otago & Southland - Oamaru, the Moeraki Boulders, Dunedin, the Otago Peninsula (to see the great albatross on a rather cloudy day), Curio Bay, the Bert Munro (The World's Fastest Indian) exhibitions in a department store in Invercargill, the Mirror Lakes and Milford Sound, Queenstown, Cromwell, Wanaka's National Transport and Toy Museum, Mount Cook & the Tasman Glacier (which we landed on), Lake Tekapo and Geraldine.

Unsurprisingly the whole holiday had a profound impact on me. What had struck me wherever we went was how welcoming, compassionate and kind New Zealanders were. That feeling stuck with me as we witnessed from afar - on the television news and in letters from our family friends - the devastating earthquakes which wracked Christchurch and it's surroundings. Places we had seen - the Anglican Cathedral, the Press Building, Lyttelton Timebell, Godley House - were ruined. All of these places, and many more besides, have subsequently been demolished or left to ruin. I witnessed from afar as the soul was ripped from the heart of Christchurch by forces of nature we were powerless to stop or combat.

The Lyttelton Timeball Station, 10th February 2010

At the time they struck I had little knowledge of earthquakes. I had seen news reports of several significant quakes over the years: I believe I can recollect Gujarat in 2001, as well seeing the news reports on Bam in 2003, Sumatra in 2004, Kashmir in 2005, Sichuan in 2008, L'Aquila in 2009 and of course Haiti in January 2010. All of these had been significant moments in my lifetime, but I had never had need to look, nor was taught about, how and why these events occurred. I had no real idea what a fault was, although I had some knowledge about tectonic plates and many previous earthquakes (having an adult Atlas is an invaluable learning tool, even when you are primary school age!).

Therefore I began to actively find out what was causing these earthquakes, using the Internet as my guide. I stumbled across GeoNet at some point, and can recollect what I believe was the original layout of the website (if it was not the original, it was certainly the second iteration). I began to glean information on my own, and by the end of secondary school made the decision to find a local college where I could do a qualification in Geology. I had decided that if I could do anything in the future to help mitigate future earthquakes like Christchurch, I would feel I had done something worthwhile. I had also been encouraged by one of my grandfather's friends in Christchurch to try and do my university degree in New Zealand - something I have not done yet, and perhaps will be able to satisfy her wishes at a later date.

Towards the end of my first year at college a qualification opportunity arose in which one could complete a piece of research on a topic of their choice (it was encouraged that the topic of this Extended Project related to one of the other courses you were studying, though it was not limited to this). I decided I had to do one on New Zealand earthquakes. For several months I had few ideas what exactly to do the project on, until on the morning of the 19th July 2013 a moderate magnitude earthquake struck in the Cook Strait between Wellington and a small Marlborough town called Seddon. The sequence intensified over the next two days, and by the middle of August there had been two mid-M6 strike-slip earthquakes which had caused significant damage in the Seddon and Ward townships and minor damage in Wellington. My topic was found, and by hand-in date in November 2013 I had a document which I was proud of. I got top grades, which greatly pleased me, and I went off to university at Plymouth with high hopes for the years to come.

Fault Map of the Cook Strait Sequence showing the different faults involved (Gurney 2013, unpublished)

Personally, those hopes were somewhat dashed, though through no fault of the department or staff. As many undergraduates find, living away from home is a tough ask, and perhaps my failure to socialise well led to the problems later on in my degree which resulted in my two-year hiatus from education. Whilst my education continued, I kept up with my own personal research. I began my original blog (United Kingdom Earthquake Bulletin) at the start of my Bachelors degree in September 2014, and set up a Twitter account alongside it. Most of you who are reading this will be doing so purely because I started this endeavour 6 years ago. Perhaps this Twitter account has been the most influential part of my development - I have encountered many brilliant and kind people who have helped me learn so much about my passion.

My research into Cook Strait had led me to encounter the concept of stress triggering, particularly relating to the Landers & Big Bear Earthquakes in 1992, and the North Anatolian Fault's unzipping between 1939 and 1999. This seemed to be a way in which future earthquakes could be forecast, if not predicted, and indeed it is still used as a crucial tool in understanding seismic hazard for the near future. Unfortunately I always felt I lacked the skills in mathematics and physics to pursue this (at secondary school I had been fantastic at these subjects; at college I suddenly & inexplicably struggled), and it was never taught during my Bachelors. Nonetheless the idea intrigued me, and I floated the idea of using the method on the November 2016 Kaikoura Earthquake to a couple of lecturers at UK institutions when looking for a Masters in late 2016 and early 2017.

As luck (or perhaps lack of luck) would have it, as I laboured to complete a now deferred dissertation in an unplanned fourth year, a lecturer (Dr Zoë Mildon) began at Plymouth who specialises in stress triggering. By the time she started I had been forced to leave my lodgings in Plymouth (twice, but I digress & do not wish to discuss this subject in a blog), so I managed to have a meeting with her in late February. Despite my misgivings, the fact she had begun at Plymouth made me seriously consider staying on to do a Masters in Geology (which would have been a fifth year at Plymouth). 

During the meeting, we forged an idea inspired by Zoë's work on the historical earthquakes in the Apennines and how stress triggering explained why the earthquakes over the past few centuries had occurred in the order and places they had. Once back home I began to look into how I may be able to find information on historical earthquakes which had occurred in the area of interest in New Zealand, and any which might possibly have been missed. The project never got off the ground - I decided against staying on at Plymouth for personal reasons - but the seed had been sown. Now all that was to be done was to begin the search...

[To Be Continued]

A rather snowy Plymouth & Roland Levinsky Building, 28th February 2018

P.S. I promised to tell you share some exciting things from my recent holiday. I shall do so soon, but I think I'll do this in small doses. Besides I am using my exciting things for research currently and will need to find a good way to photograph them in all their glory!

August 2020 NZ Seismicity Summary & Making A New Start

A Brief Update

It has been a good while since I did a blog post - both of my Earthquake Bulletin blogs have been severely neglected for a year or more, so I thought it might be nice to write a new one as I am beginning on a new stage of my journey.

I graduated with my Bachelors degree in Geology at the University of Plymouth about two years ago (September 2018). I had a terrible last couple of years; I ended up doing four years, one more than normal because I deferred my dissertation, only to get my additional year totally messed up by other things. As a result I have spent the last couple of years doing a job to get some money coming in, recovering my mental health and at the same time developing an idea for a Masters project, and exactly what I would like to do as my career.

I write this blog post at near enough the same time that, 10 years ago, events shook into motion which changed my life. At 4:35 am on the 4th September 2020 a Mw 7.1 earthquake struck about 40km west of Christchurch, causing significant damage to rural areas, and substantial damage to heritage & historical buildings in the Christchurch urban area. I had visited the city only 7 months prior on my "holiday of a lifetime", so this was quite personal to me, especially since family friends lived in the city's western suburbs (Ilam & Riccarton).

When the second Christchurch quake struck in February 2011, I decided that I wanted to know more about why earthquake occurred, and if there was anything that could be done to mitigate them. That began my journey into studying earthquakes, which on a professional level I have done nothing, but on an amateur level I have done plenty. I began my Twitter account about 6 years ago at the beginning of my undergraduate course, and since then have built up a substantial, and frankly a surprisingly kind & considerate, network in the seismological & hazard communities.

At last, with my mental health in a very good state, and with nearly two and a half years of part-time research behind me, I have decided to progress in my career ambitions. In a month's time I will be starting a Masters at the University of Portsmouth in Geological & Environmental Hazards. This should be a really good stepping stone towards where I want to go, and should provide plenty of expertise & assistance in my progress. I also hope it will allow me to do a research project I have been developing since the beginning of this year (more on that in the coming months I hope).

Summary of August Seismicity

Now that I have discussed where things are with me, I thought I would do a short summary of August's New Zealand seismicity. By and large, the years since the November 2016 Mw 7.8 Kaikoura Earthquake have thankfully been relatively quiet for earthquake in New Zealand. That said, there have been some decent earthquakes which have been widely felt across the country, and a couple of damaging quakes too (for example the 25th May Offshore Levin Earthquake, which caused minor chimney & masonry damage in Foxton Beach and other nearby coastal communities).

August has proven to be a relatively quiet month in keeping with post-Kaikoura seismicity, but nonetheless some widely felt quakes have occurred. The first of these, on the 8th August, was a Mw 4.2 earthquake beneath the northern South Island, which was felt from Kaikoura to Paraparaumu, and was also felt on the West Coast (Greymouth, Westport, Karamea), Tasman (Takaka, Motueka, Richmond), Nelson, Marlborough (Picton, Blenheim, Seddon) and the Wellington area. The quake was the result of reverse faulting at a depth of 76km, and was therefore associated with the subduction of the Pacific Plate beneath the northern South Island (Indo-Australian Plate). The quake struck at 8pm in the evening, so would have been more easily noticeable than it would have been during daylight hours.

The second sizeable quake, on the 12th August, was a Mw 4.9 earthquake in northern Fiordland, which was felt from Invercargill up to Greymouth. The quake was strongest in the Lake Country (Glenrochy, Queenstown, Arrowtown, Wanaka, Te Anau), and was a relatively standard Fiordland earthquake. It was the result of oblique reverse faulting at a depth of 17km, and may have been associated with the northernmost part of the Fiordland Subduction Zone. The quake struck at 10pm at night, so was more noticeable in Dunedin than would perhaps have been anticipated owing to many people being in a stationary position (sitting, laying down).

The third significant quake, on the 26th August, was a ML 3.3 earthquake beneath Christchurch. Despite it's very small magnitude, as many ~M3 quakes in Christchurch do, it packed a reasonable punch, with more than 1,000 felt reports received. The quake was felt throughout almost all of the city, and was weakly felt in Lyttelton. No moment tensor or focal mechanism is available for such a small quake, but this was one of the many aftershocks from the 2010-2012 Canterbury Earthquake sequence which is still ongoing, even if the frequency and magnitudes of the earthquakes are significantly reduced from it's peak nearly a decade ago. The quake struck at about midday, so was perhaps not as well observed by people as it would have been had it struck in the evening or during the night.

The final significant quake, on the 29th August, was a Mw 5.5 earthquake beneath the Bay of Plenty, ~120km east of the Coromandel Peninsula. This quake, though a good distance offshore, woke many people up in Tauranga, the Coromandel Peninsula, northern Hauraki & Auckland. It has since been followed by a plethora of aftershocks, at least 10 of which have been Mw 4.0 or larger. The quake was a result of strike-slip faulting at a depth of 5km, but many of the aftershock have been the result of normal faulting. These quakes are the result of extensional processes within the southernmost Havre Trough as it transitions into the Taupo Volcanic Zone through the Bay of Plenty & Rotorua-Taupo areas. This is part of a long extensional back-arc basin within the overlying Indo-Australian Plate, which runs parallel and to the west of the Tonga-Kermadec-Hikurangi Trench. Similar swarms/sequences have afflicted the Bay of Plenty in the past, for example in 1992 (Mw 6.3 north of Matata) and in 1994 (Mw 6.5 northwest of Te Kaha). Though this quake is far smaller than those of the 1990s, the sequence has the potential to rumble along for several weeks, producing mid-M4 aftershocks. The quake struck at about 4am in the morning, which meant most people were at rest at the time of the quake, and therefore even in the relatively aseismic Auckland where people tend to feel quakes every 5 years or so on average, many were awoken by the slow, swaying motion of this distant earthquake.

Overall, about 1600 earthquake were recorded during August 2020, the smallest a ML 0.2 near Ruapehu on the 15th August, the largest the Mw 5.5 Bay of Plenty quake on the 29th August. This is a fairly normal month in New Zealand's seismicity. Since the month ended there has been a Mw ~5.0 earthquake at about 10pm on the 3rd September beneath the northern South Island, in a similar location to the 8th August quake, which garnered more than 25,000 felt reports on the GeoNet website, from Ashburton up to Taumarunui. Shaking most likely didn't exceed MM5 - most years the highest intensity observed from New Zealand earthquakes in MM5-6 (some objects fall from shelves, perhaps some minor plaster damage or dislodged bricks from chimneys).

I will be away for the next week on a much-delayed holiday, but once back I will have a little something to share with my readers from my holiday (however many that may be of course, such a neglected blog will only attract so many readers however much I try to share it), and perhaps some seismicity news to share (hopefully not of too significant a magnitude).

Kia ora!

Jamie

Seismogram of the 3rd September ML 5.7 (Mw ~5.0) St Arnaud Earthquake from Top House broadband seismograph (NZ_THZ) - Source: GNS Science data via ObsPy

Felt reports as received by GeoNet, showing felt reports from Ashburton & Hokitika up to Taumarunui & New Plymouth - Sourced: GeoNet (GNS Science)

NZ Anniversaries: 06/01/2015 Mw 5.6 Wilberforce Quake

The 2015 Wilberforce Earthquake

A simplified felt report map of the January 2015 Mw 5.6 Wilberforce Earthquake on the South Island of New Zealand (Source: GeoNet - Imagery: Google Earth)

Introduction

During 2019 I will be writing blog posts on the anniversaries of the major earthquakes to strike New Zealand since European colonisation began - the first quake chronologically occurred in 1848. Each quake will get a small piece on the cause of the earthquake and it's effects & impacts (both societal and environmental) - for some of these events there will be far more information owing to their recent occurrence, greater impact or greater potential impact if they were to recur in the future.

My aim with this exercise is to hopefully better communicate the seismic hazard in New Zealand to the public - some areas have surprisingly had significant and damaging earthquakes, but these and the risk they pose may not be known to many people besides academics.

Links to sources for the information shared are provided throughout the texts of these posts & the others I produce (including my 2019 >/=Ml 4.0 quakes & 2019 widely felt quakes pages).

Wilberforce

In the early morning of January 6th 2015, at 6:48am, a sharp earthquake rocked much of the South Island. It's origin was approximately 9 kilometres beneath the Southern Alps, in the uppermost streams of the Wilberforce River, Canterbury Province.  The quake most likely took no more than 5 seconds to rupture a small (~5km) fault, but in doing so it produced a moment magnitude 5.6 (local magnitude 6.0) earthquake. 

The earthquake was initially located beneath Lake Coleridge, which led to very confused reactions from people living there who had not experienced the quake to a severe intensity (MM5 or moderate intensity only). This was subsequently revised to it's correct position.

As can be seen in the pictured at the top of the article, the quake was felt from New Plymouth in the north down to Dunedin and Invercargill in the south; it was felt most strongly on the west coast of the South Island (from Fox Glacier up to Westport) and also in the westernmost & southernmost areas of the Canterbury Plain (Timaru, Temuka, Geraldine, Methven, Oxford).

A seismogram of the 2015 Wilberforce Earthquake from the Waitara Valley seismograph station (filtered 1-5 Hz - Source: GeoNet)

The earthquake occurred on a strike-slip fault - that is where two slabs of rock slide past one another laterally, with the fault which slipped being vertical. The below focal mechanism shows the two possible fault planes it could have occurred on - a NNW-SSE sinistral (left-lateral motion) fault or a WSW-ENE dextral (right-lateral motion) fault. GeoNet aftershock locations do not allow an easy analysis of which of these two fault strikes (alignments) the quake occurred on as they follow a NW-SE alignment themselves. The largest aftershocks (Mw 4.5 & Mw 4.6) occurred 6 and 11 hours after the mainshock - both of these were also strike-slip.

The focal mechanism for the 2015 Wilberforce Earthquake shows it was the result of strike-slip faulting (Source: GeoNet)

The primary impact from this quake was the number of landslides which occurred within the Southern Alps around the epicentre. An aerial survey by GNS identified 263 landslides, the second largest of which (25km away from the epicentre) temporarily dammed the Arahura River.

This earthquake has probably been forgotten by most people because it's societal impact was negligible, but this is the most recent example of a sizeable Southern Alps earthquake. The Southern Alps plays host to many different strike-slip and thrust faults which accommodate some of the relative plate motion of the obliquely colliding Australian & Pacific Plates - most of this is accommodated by the Alpine Fault.

Quakes have occurred on faults in this area in the past - the 1929 Arthur's Pass, 1994 Arthur's Pass and 1995 Cass earthquakes are largest to have occurred within the past 170 years and will be discussed further later in the year.

The earthquake was too small really to change the stress regime in the Southern Alps - small segments of the adjacent Alpine Fault were stressed towards failure and destressed away from failure, but the changes exerted were too small to really change the likelihood of an Alpine Fault quake in the short-term or long-term.

This article was written by J H Gurney at 18:00 GMT on the 5th January 2019 (07:00 NZDT on the 6th January 2019).

New Zealand Earthquakes of January 2016

New Zealand Seismic Summary – January 2016

The first month of 2016 has been relatively busy. GeoNet has recorded 1642 earthquakes of magnitude 1.0 or larger, 137 of these being magnitude 3.0 or greater. Of the latter total 33 events were reported felt by members of the public. No earthquake were recorded which had moment magnitudes larger than 5.0; the last such event to strike New Zealand was a moment magnitude 5.3 earthquake, 35 kilometres north-east of Taihape, on the 1^st November 2015.

Poster created on the 29th January 2016 for the West Christchurch Sequence of the previous day.

Mw 4.6, Pongaroa, 4^th January 2016

The first major felt earthquake struck at 1:07pm on the 4^th January, 20 kilometres east of Pongaroa on the North Island. The earthquake, with a local magnitude of 5.0 and moment magnitude of 4.6, struck at a focal depth of 30 kilometres and was the result of extensional normal faulting within the subducting Pacific Plate. This earthquake followed a moment magnitude 5.7 earthquake on the 12^th October 2015, only 5 kilometres to the west and may have been triggered by this prior earthquake due to stress changes within the subducting slab.

The earthquake had 15 aftershocks of local magnitudes 2.1 to 4.1 located in a broadly north-west to south-east area. The largest aftershock (3:10pm on the 4^th January) was given a moment magnitude of 4.2 and was probably a result of reverse faulting at a focal depth of 36 kilometres. This second moment tensor solution had far less confidence due to the seismic waves from the moment magnitude 6.7 earthquake in Manipur, India which struck earlier on the 4^th January.

GeoNet received 1,132 felt reports from the public for the 4^th January earthquake, with a maximum intensity of MM6 (strong shaking) received from Palmerston North. Reports were definitely received from Picton in the south to Ohakune in the north. Other felt reports are displayed on GeoNet’s map but I have less confidence in their validity.

Ml 3.2, Taupo, 4^th January 2016

A moderate earthquake struck at 7:30pm on the 4^th January, 25 kilometres north-west of Taupo. The earthquake, with a local magnitude of 3.2, had a depth of 10 kilometres. The location of this earthquake is fairly normal, with small swarms periodically striking to the north of Taupo at very shallow (less than 15 kilometres) depths.

This earthquake occurred within the Taupo Volcanic Zone, a southern extension of Lau-Havre Basin which runs from west of Fiji down to the Bay of Plenty, parallel to the Kermadec Trench. This back-arc basin is responsible for several submarine volcanoes (seamounts) which erupt periodically. White Island and the other Bay of Plenty islands are within this volcanic belt, as are the North Island volcanoes of Rotorua, Taupo, Tongariro and Ruapehu. It is this particular region which tends to have a large share of the shallow depth earthquakes on the North Island.

GeoNet received 104 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Taupo. Reports were definitely received from Taupo in the south to Atiamuri in the north.

Mw 3.8, Seddon, 5^th January 2016

A moderate earthquake struck at 9:04am on the 5^th January, 15 kilometres north-east of Seddon. The earthquake, with a local magnitude of 3.9 and moment magnitude of 3.8, had a focal depth of 25 kilometres and was a result of strike-slip faulting. This earthquake struck on the north-western fringe of the aftershock sequence to the July and August 2013 Cook Strait Sequence which included two moment magnitude 6.6 earthquakes.

The Cook Strait and Marlborough regions represent a transition from the Pacific Plate subducting beneath the North Island at the Hikurangi Trench to a transpressive regime involving numerous strike-slip and oblique-reverse faults. These have resulted in numerous earthquakes in the past 200 years, including the 1848 Marlborough Earthquake and the 1929 Murchison Earthquake, both of which had estimated magnitudes in excess of 7.5. These faults act a transfer region between the Hikurangi Trench and the 460 kilometres long Alpine Fault.

GeoNet received 195 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Seddon and Blenheim. Reports were definitely received from Richmond in the west to Lower Hutt in the east.

Ml 4.7, Opotiki, 5^th January 2016

The second major earthquake struck at 12:57pm on the 5^th January, 10 kilometres north-east of Opotiki. The earthquake, with a local magnitude of 4.7, had a depth of 71 kilometres and was almost certainly related to the subduction of the Pacific Plate beneath the Indo-Australian under the North Island. Unfortunately no moment tensor has yet been made available, so any further analysis of the source of this earthquake is impossible.

GeoNet received 328 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Whakatane, Opotiki and Opape. Reports were definitely received from Wellington in the south to Te Kaha in the north.

Mw 4.8, Rotorua, 11^th January 2016

The third major earthquake, and largest in January, struck at 1:14pm on the 11^th January, 30 kilometres east of Rotorua. The earthquake, with a local magnitude of 4.3 and a moment magnitude of 4.8, had a focal depth of 150 kilometres and was the result of oblique reverse faulting.

As with the 5^th January Opotiki earthquake this earthquake was the related to the subduction of the Pacific Plate beneath the Indo-Australian under the North Island and was most likely a result of slab tear within the subducting slab of the Pacific Plate. A similar but far larger earthquake struck the Cook Inlet in Southern Alaska on the 24^th January 2016, with a moment magnitude of 7.1 and a focal depth of 122 kilometres (GEOFON, GFZ Potsdam).

GeoNet received 766 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Wellington, Tawa, Hastings and Gisborne. Reports were definitely received from Blenheim in the south to Te Kaha in the north.

Ml 4.1, Whanganui, 12^th January 2016

A moderate earthquake struck at 2:47pm on the 12^th January, 30 kilometres south of Whanganui. The earthquake, with a local magnitude of 4.1, had a depth of 39 kilometres. This struck a very active seismic region beneath the North Taranaki Bight, which is believed to the southernmost tip of the extensional region which extends southwards from the Lau-Havre Basin via the Taupo Volcanic Zone. In this instance, as with the Lau-Havre Basin and the Kermadec Trench, this back-arc basin is parallel to the Hikurangi Trench east of the North Island.

GeoNet received 309 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Marton. Reports were definitely received from Stratford in the west to Wellington in the east.

Ml 3.8, Taumarunui, 13^th January 2016

A moderate earthquake struck at 1:22am on the 13^th January, 30 kilometres of Taumarunui. The earthquake, with a local magnitude of 3.8, had a depth of 15 kilometres. This earthquake, as with the 4^th January Taupo and 12^th January Whanganui earthquakes, was a result of the extensional regime related to the back-arc basin running from the South Taranaki Bight all the way up to Fiji. Earthquakes are common in this particular region, located just west of the volcanic peaks of Tongariro, Ngauruhoe and Ruapehu, running from Taumarunui in the north to Ohakune in the south.

GeoNet received 109 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Ohakune and Taumarunui. Reports were definitely received from Whanganui in the south to Tihoi in the north.

Ml 3.6, Whanganui, 21^st January 2016

A moderate earthquake struck at 11:51am on the 21^st January, 30 kilometres south-west of Whanganui. The earthquake, with a local magnitude of 3.6, had a depth of 13 kilometres. This earthquake, as with the 4^th January Taupo, 12^th January Whanganui and 13^th January Taumarunui earthquakes, was a result of the extensional regime related to the back-arc basin running from the South Taranaki Bight all the way up to Fiji. This earthquake almost certainly occurred on a one of the many wrench and normal faults dominating the North Taranaki Bight, as revealed since the 1970s by seismic tomography conducted by the petroleum industry.

GeoNet received 125 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Aramoho. Reports were definitely received from Wellington in the south to Taupo in the north.

Ml 3.3 & Ml 3.2, Thornton Swarm, Whakatane, 24^th January 2016

A small earthquake swarm struck between 9:17pm and 11:23pm on the 24^th January, 10 kilometres north-west of Whakatane. The 23 earthquakes of this swarm, with local magnitudes between 1.7 and 3.3, had depths of 1 to 11 kilometres. Of these 23 earthquakes, 11 had magnitudes of 2.5 or larger, with the largest two events of local magnitude 3.3 (9:32pm, 7 kilometres depth) and local magnitude 3.2 (9:46pm, 5 kilometres depth).

Such swarms occur fairly often, with a small swarm occurring almost annually. These are again related to the back-arc basin which runs from the South Taranaki Basin in the south to Fiji in the north, running through the Taupo Volcanic Zone and the Bay of Plenty. This particular swarm was not particularly impressive, mainly due to the largest earthquake only being local magnitude 3.3.

GeoNet received over 600 felt reports from the public for these earthquake, with maximum intensities of MM5 (moderate shaking) from Coastlands, Whakatane and Ohope. The magnitude 3.3 earthquake garnered 270 felt reports, whilst the magnitude 3.2 earthquake garnered 101 felt reports. Only three recorded earthquakes in the swarm were not reported as felt.

Mw 3.9, Christchurch, 28^th January 2016

The fourth major earthquake, and largest on the South Island in January, struck at 12:24pm on the 28^th January, 10 kilometres west of Christchurch. The earthquake, with a local magnitude of 4.1 and moment magnitude of 3.9, had a focal depth of 5 kilometres and was the result of right-lateral strike-slip faulting on an east-west striking fault. This earthquake was a major late aftershock to the September 4^th 2010 moment magnitude 7.2 Darfield Earthquake and struck at the far east of the aftershock zone to that earthquake. It was the largest earthquake to hit Christchurch since a moment magnitude 4.0 earthquake on the 26^th December 2014.

The earthquake had 4 aftershocks of local magnitudes 2.4 to 2.9. Three aftershocks had magnitudes of 2.8 to 2.9: a local magnitude 2.9 (12:46pm on the 28^th January) definitely had a maximum intensity of MM5 (moderate shaking) in the southwest of the city and was felt as far away as Duvauchelle in the south and Rangiora in the north (a maximum felt distance of 41 kilometres); a local magnitude 2.9 (9:00pm on the 28^th January) had a maximum intensity of MM5 (moderate shaking) in the southwest of the city and was felt as far away as Sumner in the east (a maximum felt distance of 21 kilometres); and a local magnitude 2.8 (11:47pm on the 29^th January) had a maximum intensity of MM4 (light intensity) and was felt as far away as Northwood in the north (a maximum felt distance of 17 kilometres).

GeoNet received 2,348 felt reports from the public for the 28^th January earthquake, with a maximum intensity of MM6 (strong shaking) received from Hornby, Halswell, Riccarton, Somerfield, Saint Albans and Christchurch (the southwest, west and centre of the city). Reports were definitely received from Akaroa to Rangiora, with possible reports from Greymouth (MM4) and Kaikoura (MM3).

Ml 4.1, Picton, 29^th January 2016

The fifth major earthquake struck at 3:20pm on the 29^th January, 30 kilometres east of Picton. The earthquake, with a local magnitude of 4.1, had a depth of 36 kilometres. Earthquakes in the western part of the Cook Strait are commonplace and if in excess of local magnitude 3.5 are generally well felt both in the Marlborough region of the South Island and the Wellington region of the North Island. This earthquake was no exception and was the strongest earthquake to strike near the capital.

GeoNet received 1,372 felt reports from the public for this earthquake, with maximum intensities of MM5 (moderate shaking) received from Linkwater, Kelburn, Wellington, Lyall Bay, Roseneath, Rongitai, Eastbourne and Days Bay (all around Wellington Harbour, both west and east coasts). Reports were definitely received from Richmond in the west to Masterton in the east.

Written by J H Gurney, 3:25am NZDT, 1st February 2016.

From Devon, United Kingdom.