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:
Last night some of our Rū network members were awoken by an earthquake under Lake Taupō. You can find the estimated epicentre and other information about the earthquake at this page from Geonet. As you can read there, the earthquake’s magnitude was estimated as M4.2. Geonet qualifies this as a “light” event, but if you live in Taupō the shaking probably woke you up! This is because the quake happened only ~10 km from town, where the amplitude of the waves are still relatively large. The same effect can be seen when throwing a rock in the water: ripples are high near where the rock landed, but as the ripples travel away, their amplitude decreases. The distance of this earthquake to the other stations in the Rū network is so large that you probably did not feel its seismic waves, but your TC1 seismometer is sensitive enough to record it. Have a look when you get back to school on Monday!
Lake Taupō is a large caldera volcano. From time to time, Taupō-nui-a-Tia and the surrounding areas see increases in the amount of earthquakes. They often come from a distinct spot. We call this an “earthquake swarm.” Swarms over the years have come from different spots under the lake, and sometimes swarms will stop and start years later in the same location. Exactly why this happens is an area of active research. This particular earthquake is part of a recent swarm from the last weeks. You can read more details about this swarm and seismicity near Taupō here.
Most of you will have seen the news of an underwater volcanic eruption in Tonga, and the resulting tsunami. The eruption has led to unknown amounts of damage in Tonga at the time of writing, but our thoughts are with the people of Tonga, and anyone else in the Pacific region impacted.
To give you an idea of the power of the eruption, please have a look at the attached image. Our school seismometers in Taupo Nui A Tia College and St Mary’s Catholic Primary In Rotorua recorded a cigar-like signal between 6 and 7.30 UTC. This equates to 7pm and 8.30pm local time, and is a recording of the the sound of the eruption, travelling through the air — not through the ground! — from Tonga to Taupo and Rotorua. At ~340 m/s, this sound took almost two hours to travel the ~2000 km from Tonga to Aotearoa.
By the way, you can also see the increase in noise on the station that started at 21.00UTC; this noise in Taupo (!) is caused by ocean waves from Cyclone Cody crashing on Aotearoa’s Eastern shores, and then shaking the ground all the way to Taupo…
The COVID-19 pandemic is paralyzing our ways of life, as we are hunkered down in a Level 4 lock-down. Businesses are closed, and work is moved to home, where possible. Schools are closed, and we live in our “bubbles.” Many of the seismic stations in the Ru network of school seismometers continue to operate, however, and we wanted to share with you some interesting observations. Here is a figure from our station at St. Mary’s Catholic Primary School in Rotorua:
The horizontal scale starts on January 1st of this year, and ends today, April 9th. The vertical scale is the average noise level at the station. There is a lot to see in here. First, the smallest consistent variations are between the days and the nights; Rotorua and St. Mary’s Primary School are much quieter during the nights than during the days. The next scale up, you can see that every 7 days, there is a period of low noise. These are the weekends, which are generally quieter than the weekdays. At an even larger time scale, we see that St. Mary’s station was less noisy during the Summer Holiday than during term 1 of school. Until two weeks ago, when school closed again, and noise levels dropped to its lowest levels.
This shows that seismometers are sensitive to the noise we humans make, even the not fully grown ones. In this case, the exceptionally low noise levels during the lock-down, lower than the Summer Holiday levels, indicate that station SMC2 is (normally) sensitive to human noise beyond the classroom, such as cars driving in the street. If you don’t believe us, please read the GEONET news, where it is reported that the lock-down in Auckland can be sensed all the way in a borehole 300 m under ground!
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!
In recent months we have added many new schools to our network, including Maungatapu Primary School. Under direction of Chris Dixon, this school is very active in the sciences. Through the Ru network, it is able to also connect its Maori language immersion programme, for example; have a look at their “Ru Whenua” posters!
All their hard work has attracted the attention of local media, resulting in this great article about their recordings of a local swarm of earthquakes. Well done, Edward, Jackson, Jamie, and all the others at Maungatapu School!
At the moment, each Ru seismic station is sending a screenshot of its recordings to the University of Auckland. There are several ways to see this screenshot. For example here, and if you click on your station on our network map. If your screenshot is not up-to-date (and remember, seismologists’ clock is in UTC, not local time!), it is worth checking a few things:
Is the computer associated with your station working? If you see signal being recording on the screen, the answer is “yes.”
Is the signal you see on your screen “live?” In other words, is the latest part of the wiggles current? If not, make sure you hit the “now” button in jamaseis. Do you see new wiggles being recorded?
Next, open a browser on the computer of your station. Can you surf the web? If not, make sure you have a working ethernet connection. Because wireless connections are prone to timing out, we recommend wired connections. A reboot may prompt you to (re)make the connection to the internet, or you may need an IT person to help you.
If you have power (1), you are “live” (2) and you have an internet connection (3), but your screenshot is still out of date, please contact the Ru team at the university of Auckland.
The “textbook” method to estimate the epicentre of an earthquake is based on the arrival time difference between the primary and secondary seismic wave. From this time difference, we can estimated the distance from the station to the earthquake; in other words, from a single station, we know the earthquake happened anywhere on a circle centered on the station, where the radius is the estimated epicentral distance. The intersection of at least three station’s circles provides an estimate of the epicentre.
But how do we get the radius for each circle? In the figure below, you can see the seismograms from several of the Ru seismic stations for an earthquake near Rotorua, plotted as a function of their distance to Rotorua. The red and blue curves are predicted arrival times for the primary and secondary wave, based on a spherically symmetric earth. We made this figure for a publication in the European Journal of Physics, but the jupyter notebook that generates these figures is available here.