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Seismology

Seismology

Lombok and Fiji – or why a M6.9 earthquake can be worse news than a M8.2 event

Two magnitude 6.9 earthquakes in Indonesia in the space of two weeks, 20 km apart. Meanwhile, a magnitude 8.2 event in the Pacific. Did you get any questions about the end of the world being upon us, how come all these quakes happen so close together and why the Fiji event was so harmless?

 

Seismicity in the years 2000-2018 around the Sunda and Banda Arcs, colour coded by depth. The location of Lombok (where the recent series of devastating earthquakes took place) is indicated by the red circle

 

 

Latitude Longitude Origin time depth Magnitude Region
8.2597° S 116.4363° E 2018-08-05 11:46:38 UTC 31.0 km Mww6.9 Sumbawa Region, Indonesia
8.324° S 116.6256° E 2018-08-19 14:56:28 UTC 25.62 km Mww6.9 Sumbawa Region, Indonesia
18.1777° S 178.1109° W 2018-08-19 00:19:37 UTC 563.41 km mww8.2 Fiji Islands Region

 

 

Lombok

The quakes in and around the island of Lombok, Indonesia, fall within a larger series of events. Apart from the two Mw 6.9 quakes on 5 and 19 August, there was a whole set of Mw 5+ and Mw 6+ in the surrounding days, not to mention the hundreds of smaller fore- and aftershocks (see Figure 1).
This huge seismic activity around Lombok is hardly surprising: Indonesia lies on the Pacific Ring of Fire, where the Australian oceanic crust subducts beneath the Eurasian plate along the Sunda Arc with a convergence rate of about 7 cm/yr [Nugroho et al, Tectonophysics 2009], consequently generating a lot of seismic and volcanic activity. To the east, subduction has progressed further, such that the Australian continent is now in collision with the Indonesian islands.
The recent quakes are, however, not direct subduction earthquakes – the main subduction zone lies south of the islands and at the location of these earthquakes (between 20km and 30km), the subducted Australian plate sits much deeper. The location and focal mechanism therefore point to a shallow south-dipping thrust fault that is part of the Flores Back-arc Thrust system [1, 2].

 

Seismicity in the Sunda/Banda arcs region, and the recent earthquakes around Lombok

Left: Map of seismicity in the Sunda / Banda arc region. Events were retrieved for the period 2000 to 21 August 2018 from the USGS NEIC catalog, and are colour-coded by depth. Lombok is indicated by a red circle. Right: The three Lombok events discussed in the post, as well as the fore-and aftershocks as recorded between 1 July and 21 August 2018. Fore- and aftershocks are colour coded by event magnitude

 

The series of earthquakes started with a Mw 6.4 event on 29 July, that killed around 20 people and injured hundreds [3]. About a hundred aftershocks followed, and roughly a week later, on 5 August, an even larger Mw 6.9 event shook the area again, thereby degrading the 29 July event to a foreshock. The Mw 6.9 event was much more destructive with a death toll of more than 450 and counting, not to mention the hundreds of thousands of people who lost their homes, properties and got displaced [4].
A tsunami warning was issued for a while, but was retracted when it became clear no dangerous waves were excited – otherwise the damage might have been even more widespread. Then, two weeks later and just 20 km to the west, another M 6.9 earthquake hit the island on 19 August [5]. Although of the same magnitude as the 5 August event, the 19 August earthquake resulted in far fewer casualties (around ten reported so far [6]) – presumably because of the destruction and evacuation come after the first earthquake.
As anyone living in an earthquake-prone region will tell you, it is common for a whole series of quakes to strike the same area within a short time: the release of stress on one part of a plate contact or system of faults causes another part to become instable, and some argue that this can even happen over very large distances [Stein & Toda, Science 2012]. That’s the reason why, so shortly after this series of destructive events, people on the Lombok island continue to remain on alert, with many preferring to sleep under shelters – even if their homes are still intact.

 

So what about the M 8.2 event near the Fiji islands?
It also happened on 19 August, was more than 30 times as strong, but barely made it to the news.

 

Fiji

The 19 August event is a typical deep-focus earthquake and falls within the Tonga subduction zone. At the Tonga Trench, part of the Pacific Ring of Fire, the Pacific plate subducts steeply beneath the Australian plate. The normal faulting focal mechanism, location and depth indicate deformation within the subducted slab (Figure 3).
19 August 2018 earthquake near Fiji and background seismicity in the Tonga trench region

Figure 3: The recent M8.2 earthquake in the Tonga trench region. Background seismicity is all events of magnitudes M>5 as reported by the USGS NEIC catalog.

Both strong events and deep events occur quite regularly along this trench, but what makes the 19 August earthquake quite unique is the fact that it is both strong and deep. However, the USGS has received very few “Did you feel it” reports*: fortunately for the people living on the Pacific islands, its depth of 560 km meant that any devastating surface waves were hardly excited at the surface and damage seems limited so far [8].
On Lombok, on the contrary, the matter cannot be addressed lightly. Efforts have started to clean the rubble and rebuilding houses, schools and hospitals. But this proves to be hard: most of the infrastructure, especially in the north of the island, has been gravely damaged by the earthquakes themselves, or ensuing landslides. Many people are living in temporary shelters, and it is difficult to supply food, clean water, and medicines to the area.

 

No, the Lombok and Fiji events are not likely to be related, in case someone asks you (although I’m probably preaching to the choir). There are roughly 7000 km between the islands and the only thing they share is a contact with the Australasian plate.

 

 

This post was written by Nienke Blom, with revisions from Marina Corradini

 

 

[*]: The largest ever recorded deep-focus event was the Mw 8.3 Sea of Okhotsk earthquake, which occurred off the Kamchatka Peninsula on May 24th, 2013 at 600 km depth. This earthquake attracted special attention because, among other issues, it was felt all over Asia [Chebrova et al, J Volc. Seismol. 2015] , despite the large epicentral distance.

 

Note: data were downloaded from the IRIS and NEIC catalogs using obspyDMT [Hosseini and Sigloch, Solid Earth, 2017] figures were produced using matplotlib based on obspyDMT scripts.

 

Seismo @ school

Being a seismologist is not just doing research, it is also sharing experience and teaching the next generation.
As early career scientists, we are used to share ‘our science’ during open days and career days at university. Another peculiar moment for Science outreach is the National Science week, where researchers can set up experiments and exhibitions to draw the attention of the general public (both children and adults). During this week, you’d probably get the chance to see the building response to an earthquake, the ground motion recorded by a seismometer, the inner structure of the Earth…
In addition to these actions, a seismologist can also be of help in teaching Earth Sciences and Seismology at school. That’s what we call educational seismology. We provide accessible materials (instruments, lecture notes and practical examples) to guide high school teachers during their Earth Sciences courses.

 

Ever heard of a LEGO seismometer?

Figure 1. BGS Lego seismometer

 

In the UK, the educational seismology is driven by the British Geological Survey which recently proposed to build a LEGO seismometer (figure 1).

 

If you’re skeptical and wondering whether this instrument could actually detect an earthquake, let’s have a look at the seismic trace (in blue) recorded at the bottom!

 

Figure 2. M6.9 earthquake in Japan recorded by the Lego seismometer in a school (http://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/schoolSeismology/seismometers/lego.html)

 

 

But what is educational seismology?

Educational seismology aims at explaining to students of all ages how seismologists interpret a seismogram and locate an earthquake, the concept of earthquake magnitude, the seismic cycle and the seismic risk, and so on.
Another key point of the educational seismology is to provide seismic instruments for teaching purpose [Virieux, 2000; Zollo et al. 2014]. For example, “SISMOS à l’École” in France [Courboulex et al. 2012; Berenguer et al. 2013 and 2014], which celebrated in 2016 its 20th year anniversary, managed to install 75 seismometers in several schools (figure 3). It is a good opportunity for students to see what a seismometer looks like and to observe earthquakes recorded by a seismometer. Moreover, the data recorded by the instruments are useful for the seismic community and available for research purpose on the IRIS database.

 

Figure 3. At the top: location of seismic instruments of SISMOS à l’École (http://www.edusismo.org). At the bottom: distribution of seismic stations in France. Green triangles show instruments currently active. Red triangles show inactive instruments.

 

Useful material for practical purpose:

Figure 4. Simulation of a seismic wave generated by the mass motion on a rough surface and recorded by an accelerometer.

In the beginning of July, Michelle Salmon from the Australian Seismometers in Schools [Balfour et al. 2014] gave a short workshop at the annual conference of the Australian Science Teacher Association (ASTA) and provided materials for high school teachers. At the end of the session, the small group of 15 teachers left with news ideas for their students and some useful materials for practical purpose (figure 4).

 

Another useful tool for teaching seismology is the IRIS earthquake browser. From this interface, you can select a list of earthquakes and plot them on a map showing plate boundaries. In addition, you can produce 3D cross section for a selected region and see the distribution in depth of earthquakes located along subduction zones or mid-oceanic ridges (figure 5).

Figure 5. Location of earthquakes in Central America. Colour scale shows the depth of these events. At the bottom right, the 3D cross section associated with this region with a domain of mid-oceanic ridge in the western part and the subduction zone beneath the central/South America in the eastern part.

 

 

Educational resources for high school teachers are available on different websites (IRIS, AuSiS, Edusismo) and free to access. If you are interested to help the community, or you have some ideas and want to teach the next generation, do not hesitate to make contact with one of the members working in the field.

 

Upcoming events:

 

 

References 
Balfour, N.,Salmon, M, Sambridge, M. (2014). The Australian Seismometers in Schools network: Education outreach, research and monitoring, SRL, 85, 5,1063-1068.
Berenguer, J. L. , Courboulex, F., Balestra, J., Nolet, G., Lognonne, P. (2014). Innovative Resources for seismology@school with the French educational seismological network. AGU Fall meeting abstract.
Berenguer, J. L., Courboulex, F., Tocheport, A., Bouin, M.P. (2013). Tuned in to the Earth from the school Edusismo: The French educational seismological network. Bulletin de la société géologique de France, 184,1-2,183-187.
Courboulex, F., Berenguer, J. L., Tocheport A., Bouin, M. P., Calais, E., Esnault Y., Larroque, C., Nolet, G., Virieux, J., Sismos à l’École (2012): A worldwide network of real-time seismometers in Schools, SRL, 83,5, 870-873.
Virieux, J. (2000). Educational seismological project : EDUSEIS, SRL, 71, 5, 530-535.
Zollo, A., Bobbio, A., Berenguer, J. L., Courboulex, F. Denton, P., Festa, G., Sauron, A., Solarino, S. Haslinger, F., Giardini, D. (2014) The European experience of educational seismology. Geoscience research and outreach, 145-170.

 

 

This post was written by Walid Ben Mansour, with revisions from Marina Corradini

 

Walid Ben Mansour is a post-doctoral research fellow at Macquarie University. He works on multi-observable probabilistic tomography for different targets (mining, seismic hazard). You can reach him at walid.benmansour[at]mq.edu.au

4th TIDES Advanced Training School

The 4th TIDES Advanced Training School was held in Prague, Czech Republic, from the 2nd to the 6th of July 2018. If you missed it, take a look at Michaela and Eric’s short report:

 

Eric Loeberich

It’s the first Sunday of July. It could have been a calm sunny noon in Vienna, but that’s not my plan for today, I’ve to catch a train in 45 minutes! I check the room, take my luggage, ready to start my journey. The next week will be pretty interesting, TIDES (Time Dependent Seismology), an action supported by the COST Association (European funded framework aiming at an enhanced transnational cooperation within the science community), organizes an advanced training school in Prague. What a great opportunity to widen my knowledge and to establish contacts with other early career scientists (ECS) and seismic experts! I’m sure to meet familiar faces… One of them is Michaela. We already attended some workshops together and since this year we are both part of the ECS-representatives team.

 

Michaela Wenner

After a turbulent week of fieldwork, my journey continues to Prague to attend the TIDES training school. This year’s topic is ‘near surface processes’, which perfectly fits my current research project. Near surface processes often include a lot of  fieldwork, as does my PhD, but during this week I hope to sit back and listen to the most experienced people in the community. An airport full of ryan-air tourists makes me saying goodbye to my city quite easy and I’m curious what Prague has to offer. As Eric, I am happy to see some of the people I had the pleasure to meet during my short career but haven’t seen in a while.

 

 

Sunset in Prague – © Eric Loeberich

 

 

The kind of creepy forest – especially by night without lights – leading up to the hotel sets a beautiful scenery for the conference. Many nights of networking on the terrace lie ahead of us – after a dense scientific program, of course. Indeed, the expert talks about earthquakes, volcanoes, glaciers, landslides, reservoirs, cities and other near surface processes gave a good overview on current knowledge and advances in these fields.
Cutting edge science – mostly by ECS – was then presented in the afternoons in short talks and posters. And after work follows the fun – at TIDES 2018 in the form of concerts, drinks and tango lessons. We have the impression that even though we heard many interesting talks and discussed our research, the training school was mostly based on networking. One attempt to include the trainees better into the discussions was conducted via a question hour after lunch. Basically groups of four trainees had to discuss the morning talks over lunch and come up with questions. This was at first met with not too much enthusiasm amongst the trainees, but turned out to be quite interesting with many answered questions.
All in all, we had a great time in Prague and are sad to see that this was the last training school within the TIDES action. We hope that there will be similar programmes soon, that allow ECS to get a better idea of what is going on within the community and to network with potential collaborators.

 

This post was written by Michaela Wenner and Eric Loeberich, with revisions from Marina Corradini

 

Michaela Wenner is a PhD student at ETH Zurich. She works on seismic signals of mass movements, such as rockfalls, debris flows and ice avalanches. You can reach her at wenner[at]vaw.baug.ethz.ch

Eric Loeberich is a PhD student at the University of Vienna. He works on seismic anisotropy in the upper mantle, as produced by lattice-preferred orientation of olivine in lithosphere and asthenosphere. You can reach him at: eric.loeberich[at]univie.ac.at

The new ECS-reps team of the Seismology Division!

At the EGU General Assembly 2018, a new team of Seismology Early Career Scientist representatives was introduced and installed. With more than half of the EGU membership consisting of Early Career Scientists, the team represent an important part of the community and want to be approachable for all. They will be responsible for the Seismology blog, organize the yearly short course “Seismology for non-seismologists” at the General Assembly, and organize outreach and career events. Next to that, they plan to get in touch with industry (where do those seismologists end up who do not continue in academia?), and integrate the short course with similar courses organized by the Geodynamics and Tectonics division. Hopefully they will update you on this here in the next months!

You can reach the team on ecs-sm@egu.eu, and for all those who didn’t make it to the meeting (including some of the new team!) we here give a brief introduction of the new team.

 

Nienke Blom

Nienke is a postdoctoral research associate at the University of Cambridge and works on seismic waveform tomography, with a specific interest in developing methods to image density. She enjoys reading a good book, hiking, cycling, and cooking. As ECS rep, she will mostly be involved with the EGU blog and the EGU short course “Seismology for non-seismologists”, which she’s already helped organise for the past couple of years. Nienke is the EGU point of contact for the ECS rep team. You can reach her at nienke.blom[at]esc.cam.ac.uk.

 

 

 

 

 

Marina Corradini

Marina is an Italian PhD Candidate at the Institut de Physique du Globe de Paris. In her work she investigates the relation between the rupture complexity and the high-frequency seismic radiation through the use of a back-projection technique. When she is not in her office, she works as a scientific divulgator at ‘Cité des Sciences et de l’Industrie’ of Paris. As ECS-reps she would like to promote gender equality in geoscience and explore how the society currently supports postgraduate and postdoctoral female researchers in their career progression. You can reach her at corradini[at]ipgp.fr

 

 

 

 

 

Eric Loeberich

Since 2.5 years Eric is a PhD student at the University of Vienna, working in the field of seismic anisotropy in the upper mantle, as produced by lattice-preferred orientation of olivine in lithosphere and asthenosphere and detectable by shear-wave splitting measurements. During his ECS-reps  time, Eric tries to establish relations between the Seismology Division and the industry together with Michaela, Andrea and Walid and will help Lucile taking care of the Seismology Twitter account (@EGU_Seismo). In his leisure time, Eric plays basketball and football, discovers Vienna or enjoys a coffee (or one more). You can reach him at: eric.loeberich[at]univie.ac.at

 

 

 

 

Michaela Wenner

Michaela just recently started her PhD at ETH Zurich on seismic signals of mass movements, such as rockfalls, debris flows and ice avalanches. Whenever she is not occupied with seismology or fieldwork, she loves to meet up with friends and go to the mountains. Michaela will mostly be involved in the industry connections the ECS team wants to establish. She will also help organizing the EGU short course “seismology for non-seismologists” at the general assembly, where she already participated as a speaker this year. You can reach her at wenner[at]vaw.baug.ethz.ch

 

 

 

Maria Tsekhmistrenko

Maria is currently a PhD student at the University of Oxford. She is focused on the seismic imaging of the western Indian Ocean. More specifically the velocity structures beneath the La Reunion Island from the surface to the core mantle boundary. Maria is also interested in data processing and visualization since she believes it makes research more accessible to a wider range of scientists as well as non-scientists. In her spare time she enjoys a good book, photography, rowing and good food. As an ECS rep she will be engaged in the EGU blog and the EGU short course “Seismology for non-seismologists”. You can reach her at mariat[at]earth.ox.ac.uk

 

 

 

 

Walid Ben Mansour

Walid is a post-doctoral research fellow at Macquarie University and works on multi-observable probabilistic tomography for different targets (mining, seismic hazard) in the Macquarie’s Geophysics and Geodynamic group. In parallel of his work, Walid practices judo, long distance running and organises every year local events for the festival Pint of Science.  As ECS rep, he will work on the bridge between academic and industry field and also on short courses for non-seismologists with the team. You can reach him at walid.benmansour[at]mq.edu.au.

 

 

 

 

 

Lucile Bruhat

Lucile is currently a visiting researcher at the Earthquake Research Institute in Tokyo, Japan. Starting July 2018, she will be a post-doctoral researcher at the Ecole Normale Supérieure (ENS) in Paris, France. Her work aims at improving the description and understanding of the physical processes that underlie the earthquake cycle, through a combination of geodetic/seismological data analysis and numerical modeling of the earthquake rupture. She is originally from Versailles, France, and after an initial training in Paris, went to Stanford University for her PhD. With Eric, she’ll be in charge of the Twitter and Facebook accounts of the Seismology Division. In her spare time, she enjoys reading, cooking, working out at the gym, and discovering new craft beers. You can reach her on Twitter at @seismolucy.

 

 

 

Andrea Berbellini

Andrea is an Italian Post Doc at the University College London. He works on different projects, such as a new method for source characterization from second-order moments and crustal tomography from ellipticity of Rayeligh waves. In his free time he likes to binge-watch tv series, eat too much and go to the beach (if available). You can reach him at: a.berbellini[at]ucl.ac.uk