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Laura Ermert

Laura is a PhD student at ETH Zürich in Switzerland. She is working on ambient noise source inversion with cross-correlation techniques. Her goal on the blog is to showcase PhD students' and young researchers' results, as well as recent seismological highlights. You can reach Laura at lermert att student.ethz.ch.

EGU GA 2016 – You don’t get older, you get better!

EGU GA 2016 – You don’t get older, you get better!

This years general assembly is over and its buzz still ringing in my ears. Here are a few spotlight impressions fresh off my mind. In the coming days and weeks, the complete brand new ECS representative team will present itself on the blog, and we will update you in more detail about highlights and low points of the conference.

SCIENCE!

Do we have to mention it was amazing? Yes, I’ve heard people say “I don’t really like the talks at EGU. The quality is not very high.”  If you really feel this way, think about convening your own session next year! Look out for information on the blog or here. Ideal persons to contact are the newly appointed science officers who represent topical sections of the division (more info soon!).

MEDAL LECTURES!

The Sushi platters of science, medal lectures are full of artfully crafted delicacies and easy to digest. Several can be found online for you to enjoy again, although sadly not the Beno Gutenberg lecture by Roel Snieder. 

WORKSHOPS!

I filled gaps in my science program with useful and entertaining workshops and short courses. You could learn about new software, hear tips on writing grants, watch science communicators in action, or receive hands-on strategies for managing your PhD work from the Gutenberg medalist himself, to name just a few.


LO AND BEHOLD: After EGU is before EGU.

The next general assembly will take place from 23 to 28 April 2017 in Vienna, Austria (surprise, surprise!).

To make the General Assembly even better, please leave feedback. Note that neither the quality of coffee nor the quantity of beer are likely to increase.

Consider uploading your presentation: Colleagues will be glad to revisit your contribution and making a pdf available increases the visibility of you work.

And don’t forget to pass by again soon to hear more about the ECS team’s composition, future activities and EGU2016 highlights!

Listen to the … massive black hole merger song!

Listen to the … massive black hole merger song!

I bet you were every bit as excited as me about the recent announcement of the detection of gravitational waves at two locations of the Laser Interferometer Gravitational-Wave Observatory LIGO*. These waves were sent out to space-time by the merger of two black holes. Call me a nerd, but after reading the news I soon started wondering: What sort of periods do these waves have? In my imagination, something as humongous as black holes combining would cause some sort of vast, incredibly low frequency signal that would stretch over far longer periods than any seismic wave Earth could possibly ring to.

I admit I was ever so slightly disappointed to learn my intuition was completely mistaken: Contrary to many seismic waves (those that matter to me, anyway), the chirp that the physics world got all giddy about is really in the audible range. My disappointment was dispersed immediately by the possibility of actually listening to it here.

Still — the ground of course merrily jiggles along at such frequencies (35 Hz upwards). Moreover, other signals LIGO hopes to observe fall into the typical range of seismic waves. Seismology, therefore, plays a big role in the detection of gravitational waves: As EOS explained, a highly elaborate shielding protects the mirrors at LIGO from ground vibrations. This is achieved both by damping and by actively compensating for ground motion recorded at various sensors – among them, not surprisingly, three broadband seismometers and several geophones.

*Admittedly, this post is not “showcasing young researchers’ result from seismology” — but I couldn’t help it!

Listen to the hum

Listen to the hum

A new global S-wave model has recently been published in Geophysical Journal International. While this sounds exciting enough to tomographers and geodynamicists, this model has been constructed in a rather avant-garde way, too. It is one out of only two global tomographic models ever to be made based on the Earth’s background oscillations, that is long-periodic seismic noise also known as Earth’s hum. This weak signal is thought to be excited mainly by the interaction of the oceans with the solid Earth.

The only previous tomography using the Earth’s hum, published in Science in 2009, had the main aim of demonstrating the feasibility of hum tomography. Thus, the recent model is an actual novelty in the zoo of global S-wave models.

 

Contrary to surface waves radiated by earthquake sources, the waves of the hum don’t even amount to micrometers of displacement amplitude – long-range correlations of such weak signals can be very slow to emerge from the uncorrelated local noise surrounding the stations. Haned and coworkers could extract the very weak, but coherent hum signals using a particular technique. They discarded amplitude information and worked with instantaneous phases of the signal only, both for correlating and the subsequent stack weighting. On the emerging correlation functions, group velocities between station pairs were measured; these formed the basis for regionalization and depth inversion with a probabilistic approach.

 

While the ray coverage of the model is still heavily bundled in the Northern hemisphere, the authors propose that including extra stations in the future, especially OBS stations, will provide a ray coverage that is extending and complementing earthquake-based tomographies. As to the comparison with earthquake-based studies, they note that although correlation coefficients are generally high, discrepancies in detail do exist. An ideal solution would be to invert both types of data, jointly.

Brewing wiggles on Mars

Brewing wiggles on Mars

Creating 1 Hz-seismograms in a 3-D Earth model, and comparing them to observed body waves, is a dream that’s today still too expensive for routine use. But even tackling the problem for a spherically symmetric Earth poses certain challenges. Recently, a new tool entered the stage (you might have followed the Solid Earth discussion): Instaseis allows users to extract seismograms for a spherically symmetric Earth within milliseconds in a convenient Python / Obspy environment.

The idea is simple. Thanks to spherical symmetry and Green’s function reciprocity, a ‘one source fits all’ approach can be taken: A database for a source at the surface and a spherically symmetric model like PREM is computed and stored at each single element of the computational mesh, to a given depth. With this database, a source and a receiver location, any seismogram recorded at Earth’s surface can be synthesized within milliseconds. As long as receiver depth doesn’t change, there is no need to recalculate the database. Interchange ‘source’ and ‘receiver’ for the opposite case.

‘It’s not a 1-D Earth!’, moan the critics. This is true, and it is hard to imagine how a similar tool could ever include 3-D structure. However, there are plenty of questions one can pose and tests one can run, which 3-D models, do to sheer computational effort, preclude. Mars seismograms, testing the plausibility of a multitude of possible Mars interiors, are one very cool future use of instaseis within the InSight mission. Others include shakemovies, ambient noise synthetics, finite source inversions, matched filters, and education…

As one can imagine, the high frequency wave field database quickly reaches unwieldy storage sizes. A promising future development in this regard would be the hosting of such databases on an IRIS server, which is planned but not implemented yet – stay tuned.