All posts by kvan637

The sound of the Hunga-Tonga-Hunga-Ha’apai underwater volcano eruption

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…

Finally, as an exclamation point of the powers of Ranginui, Papatūānuku, and Rūaumoko, an Earthquake in Te Araroa (Magnitude 4.7) was also recorded at 3.22 UTC.

The Seismic Sound of Silence in Level 4 Locked-down Aotearoa

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!

 

Seismic data from the Whakaari (White Island) eruption of December 9th, 2019

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.

 

Ru in the news

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!

 

Troubleshooting your seismic station set up

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:

  1. Is the computer associated with your station working? If you see signal being recording on the screen, the answer is “yes.”
  2. 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?
  3. 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.
  4. 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.

A jupyter notebook to illustrate earthquake location

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.

mapwithcircles
circles centered on each seismic station represent the epicentral distance to an earthquake. The intersection of all circles is at 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.

rotoruaSP2
The distinct arrivals of the primary and secondary wave are matched with the arrival time curves predicted for a spherically symmetric earth.

Severe earthquake NNE of Amberley, NZ

Birkenhead
This is a screenshot of the seismic recordings at Birkenhead Primary School, on November 13th, 2016.

Shortly after midnight, last night, a severe earthquake struck the South Island. The full extent of the damage is not clear yet, and of course the members of the Ru network think about those affected by this event.

The seismic networks computed the thrust motion on the fault in a matter of minutes, and in this case the motion on the fault warranted a tsunami warning.

The New Zealand Herald features an article with the first reactions from geonet scientists. The mention of the Hope Fault is interesting. This fault is the southern-most fault of the Marlborough Faults (as far as we know!), which extend from the Alpine Fault. However, both Geonet and the USGS indicate a more southern placement of the epicentre. Besides, the Hope Fault is a strike-slip fault, whereas this event was a thrust fault! We at Ru wouldn’t be surprised if this event was slip (or slips, plural) on a combination of faults. In any case, there will much to learn from this event in the coming time. A discussion about the complexity of the tectonics in this area has already been posted on the USGS website.

Meanwhile, you can expect hundreds of aftershocks to fill your station helicorder screens in the coming days and weeks. If you get this message on Monday November 14th (local New Zealand time), you can see much of the action on our network page, similar to the image at the top of this post from Birkenhead Primary School.

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A report from our Workshop

The Ru workshop took place on the 27th-29th January 2016. It was great to bring a group of people together with such passion and enthusiasm for teaching, to share ideas and contribute towards the  seismometers in schools program.

Day 1 took place at the University of Auckland’s City Campus. The morning revolved around the Auckland Lablet; Physics experimentation on an Android tablet. There were demonstrations to showcase Lablet being used record and analyse several physics experiments and some very useful ideas came out of the discussion.

In the afternoon, the group were able to get their hands on the TC-1 Seismometer and built four from scratch. This showed just how simple it can be to construct the TC-1 and it was a great success when all four completed devices recorded data without a hitch.

Ted and team assembling a TC-1
Ted and team assembling a TC-1

There were several excellent talks over the rest of the afternoon. It was particularly interesting to hear (and see) how Jonathan had utilized an Arduino (a key component of the TC-1), to run a weather station.

Day 2 was spent on Waiheke Island. There were some great presentations by Dan Hikuroa, Caroline Little, Katrina Jacobs, Glenn Vallender, Michelle Salmon and Martin Smith. The day was nicely rounded off with some fantastic refreshments.

Great food on Waiheke
Great food on Waiheke

The highlight of Day 3 was undoubtedly the field trip to Rangitoto Island. The weather was excellent and although it was quite a hike up to the top, it was universally enjoyed. It was great to have Dan along as a guide to share his knowledge of the geology of the island and it was particularly interesting to explore the lava tubes.

Everyone at the top of Rangitoto
Ru on Rangitoto

Overall the workshop had a great turnout. We hope everyone enjoyed the three days and gained some ideas and insights that will be helpful in their schools.

Group photo on Waiheke
Ru Workshop 2016 attendees