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).