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