In the last two days, we had two very large earthquakes. In Myanmar, a magnitude 7.7 earthquake caused devastation, not only in Myanmar, but also in the region. The vast majority of earthquakes happen on plate boundaries, and this event was no exception.  At the largest scale, the cause of earthquakes in the region is the collision between the Indian and Eurasian plates,. This is the same collision that is responsible for the Himalaya’s to the Northwest.

Today, a large earthquake happened near Tonga. Here, the Pacific plate subducts under the Australian plate. This earthquake was recorded on all our stations that were up and running at the time. For example, here is a snapshot of one of our newest stations MTMC1 at Mount Maunganui College:

You can see the boundary between the Pacific and the Australian plate in the topography of the ocean floor  (“bathymetry”) on google maps in the form of a darker blue colour, indicating a topographic low:

On this map, you can also observe that this subduction zone extends all the way down to New Zealand, where we have the Hikurangi subduction zone to the East of the North Island.

The big difference between the subduction off the coast of New Zealand’s North Island, Te Ika-a-Māui, and the subduction near Tonga, is the speed at which the plates converge: in Tonga, this is speed is almost twice as fast as near New Zealand, up to 24 cm per year! Unlike in Myanmar, it appears that the damage in Tonga is minimal. And while this type of earthquake can cause a tsunami, the the tsunami warning has been lifted at the time of writing.

Are these earthquakes related?

If you made it this far in reading this post, you must be really interested in earthquakes. People like you may ask if these two earthquakes are related. It does feel that when we do have large earthquakes, they come in bunches.  Your feeling may not be entirely wrong. Recent studies show that the waves from large earthquakes sometimes trigger earthquakes even thousands of kilometres away. For that to happen, the fault on which the triggered earthquake happened had to be “ready to go” at any time, already. Still, it is hard or even impossible to prove that a particular earthquake is triggered by another, but there is statistical evidence it can happen!

In the last two days the Bay of Plenty has been shaken by hundreds of earthquakes, some greater than magnitude 4, and most of them relatively shallow (less than 15km) . This means shaking at the surface has been significant, but we have not read of any people getting hurt, thankfully. Our whānau at Te Kura O Te Pārao are less than 20km from the middle of this swarm, and the evidence on their station is clear:

The region has been at the mercy of Tāwhirimātea *and* Rūaumoko in recent months an our thoughts are with those enduring the shaking while recovering from the recent storms.

This particular swarm appears to happen very near the natural geothermal area of Awakeri, which is part of the surface expressions of the Taupō Volcanic zone (TVZ).  Below is an image from the geothermal website of nz that shows the geothermal areas of the TVZ, including Awakeri:

Yesterday, an eruption on Whakaari led to injuries and loss of lives. Our thoughts are with the whanau of those affected by this tragic event. We have been getting a lot of inquiries about this eruption, so we decided to write up what we know about the seismology associated with this eruption.

Whakaari is a volcano that is part of the Taupo Volcanic Zone, and forms a small island in the Bay of Plenty.

As far as we can tell, none of the Ru stations recorded the signals associated with the eruption, but GEONET operates seismic stations on the island:

The seismic data for station WIZ are plotted below. We annotated the time of the eruption. You can see that in the day(s) leading up to this, there were some small spikes in the data indicative of small local earthquakes. In addition, we can see a few hours of low-amplitude “rumbling” of Whakaari on the 8th of December:

Seismic station WSRZ is closer to the top rim of the volcano, and recorded this:

It looks like the amplitudes of the seismic signals were elevated  on the 7th and 8th of December, but things actually calmed down on the 8th before the eruption…  After the eruption, small impulsive signals may be from brittle failure of the rocks (small earthquakes).

It is difficult — if not impossible — to predict volcanic eruptions, and even after the fact it is hard for us to say whether signals prior to the eruption were out of the ordinary for Whakaari. The experts at GNS have had a elevated warning in place for Whakaari since October, based on seismic signals such as these, and gas sensing. In addition, an M5.9 earthquake at 115 km depth occurred on November 24th with its epicentre about 10km from Whakaari. As with all geological tragedies such as these, we hope we can learn more about the rumblings of Rūaumoku, reducing risk in future calamities.

A recent swarm of earthquakes in Northland prompted an article in the New Zealand Herald that features our Whangarei Girls High School station WGHS1, and station manager Nick Major. What a great article, and congratulations to Nick and the girls in Whangarei to keep an ear out for earthquakes up North!