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The Sassy Scientist – Earthquake Exoteries Nr. VII

The Sassy Scientist – Earthquake Exoteries Nr. VII

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here or leave a comment below.

In a comment on a post about the key papers in geodynamics, the Curmudgeonly Commenter asked:


Could you please point out some exceptionally important papers in geodynamics and tell us something interesting about the history of the field?


Dear CC,

I feel like a lifetime has passed since I first started answering your question. I hope you are satisfied with the extent of the response. Please don’t ever ask me a question again. Really, I’ve got some important research to do. This is supposed to be some light-hearted banter, not some in-depth material acquisition you can put in your Introductions.

Additional spatio-temporal complexities
Of course, by now you realise that we must know everything about the earthquake cycle. We comprehend exactly what types of deformation takes place at different depths along-dip a fault interface, and no new features are observed anymore. Well, you’re quite mistaken. Not that long ago, a whole new can of worms was opened with the seismological and geodetic discovery of non-volcanic tremor, also dubbed Low-Frequency Earthquakes (LFEs), which corresponds to events known as Episodic Tremor and Slip (ETS) — this is basically the release of a certain seismic moment over a prolonged period of time instead of a single, finite rupture/earthquake (Hirose et al. 1999, Dragert et al. 2001, Obara 2002). The time period over which these occur is variable and segmented along-strike a single, continuous fault interface (Takagi et al. 2019, Rousset et al. 2019a), and are not restricted to subduction interfaces only (Rousset et al. 2019b). Such effect of along-strike locking variability is a common observation (e.g., Métois et al. 2016) and has direct consequences for the accumulation of slip deficit along-strike a subduction interface (e.g., Herman et al. 2018), i.e., an inherent effect of the presence of finite asperities.

Semantics
Lastly, let’s follow Wang and Dixon (2004) closely as they warn about the pitfalls of semantics. Different sub-fields — and I’m not just talking about geodynamics on it own — can understand the same word to mean something different. In this, Wang and Dixon (2004) use “coupling” as a common example of a term where kinematic observations are intertwined with dynamics terms of interaction. One could easily offer up other terms, such as e.g. “afterslip”, that do not have a clear-cut definition but is widely thrown around to mean ‘something that happened after a major earthquake occured’. Let’s not let all of our hard work be in vain by using terminology we either simply misuse, or overly complicate. It never hurts to explicitly explain what you mean with the terminology you use, even though it is crystal clear to you. Just a little tip.

So tell me, Curmudgeonly Commenter, did you learn anything about earthquakes? Was I clear and extensive enough for you? Happy at long last with my response? Let me know in the comments…

Yours truly,

The Sassy Scientist

PS: This series of posts was written with inspiration of the older papers, before the 60’s especially; they’re a lot of fun and meanwhile you’ll obtain some ideas to present in your undergraduate courses. In case you’re working on the earthquake cycle, from a geodynamics or rock mechanics perspective, especially but not exclusively on facets I haven’t mentioned above, I endorse/recommend/applaud mentioning it in the comment section. Or contact us for a proper guest author post! Get your word out to the public and get people interested.

References:
Dragert, H., Wang, K. and James, T.S. (2001), A silent slip event on the deeper Cascadia subduction interface. Science, 292, 1525–1528, doi:10.1126/science.1060152.
Herman, M.W., Furlong, K.P. and Govers, R. (2018), The accumulation of slip deficit in subduction zones in the absence of mechanical coupling: Implications for the behavior of megathrust earthquakes. Journal of Geophysical Research: Solid Earth, 123, 8260–8278. https://doi.org/10.1029/2018JB016336.
Hirose, H., Hirahara, K., Kimata, F., Fujii, N. and Miyazaki, S. (1999), A slow thrust slip event following the two 1996 Hyuganada earthquakes beneath the Bungo Channel, southwest Japan. Geophysical Research Letters, 26( 21), 3237–3240, doi:10.1029/1999GL010999.
Métois, M., Vigny, C. and Socquet, A. (2016), Interseismic coupling, megathrust earthquakes and seismic swarms along the Chilean Subduction Zone (38°–18°S). Pure and Applied Geophysics, 173(5), 1431–1449. https://doi.org/10.1007/s00024-016-1280-5.
Obara, K. (2002), Nonvolcanic deep tremor associated with subduction in Southwest Japan. Science, 296, No. 5573, 1679-1681.
Rousset, B. (2019a), Months-long subduction slow slip events avoid the stress shadows of seismic asperities. Journal of Geophysical Research: Solid Earth, 124. https://doi.org/10.1029/2019JB018037
Rousset, B., Bürgmann, R. and Campillo, M. (2019b), Slow slip events in the roots of the San Andreas fault. Science Advances, doi:10.1126/sciadv.aav3274
Takagi, R., Uchida, N., and Obara, K. (2.019), Along‐strike variation and migration of long‐term slow slip events in the western Nankai subduction zone, Japan. Journal of Geophysical Research: Solid Earth, 124, 3853–3880. https://doi.org/10.1029/2018JB016738.
Wang, K. and Dixon, T.H. (2004), “Coupling” semantics and science in earthquake research. Eos, Transactions of the American Geophysical Union, 108, 180, https://doi.org/10.1029/2004EO180005
Zebker, H.A., Rosen, P.A., Goldstein, R.M., Gabriel, A. and Werner C.L. (1994), On the derivation of coseismic displacement fields using differential radar interferometry—the Landers earthquake. Journal of Geophysical Research, 99:19617–34.

The Sassy Scientist – Earthquake Exoteries Nr. VI

The Sassy Scientist – Earthquake Exoteries Nr. VI

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here or leave a comment below.

In a comment on a post about the key papers in geodynamics, the Curmudgeonly Commenter asked:


Could you please point out some exceptionally important papers in geodynamics and tell us something interesting about the history of the field?


Dear CC,

Over the past few weeks (it’s almost been an entire month! time flies when you’re having fun) we’ve been mainly been discussing early seismological observations and theory, rock mechanics and of course our beloved discipline: geodynamics. But — and I know this is hard to believe — there are also other disciplines looking into earthquakes. Who knew, right? This week, I’ll give them a little limelight.

Other observations: geodesy
Whereas the rock mechanics researchers were focussed on comparing their results to seismological observations, other researchers were more interested in potential surface expressions and uncovering whether any more aspects of the earthquake cycle were missing. Geodesy, can’t live without it. With the advent of GNSS data — GNSS = Global Navigation Satellite System; GPS is the American, GLONASS the Russian, Galileo the European, and (in 2020) BeiDou the Chinese system — came a wondrous richness of information of surface deformation (Ryan and Ma 1998; Bastos et al. 2010). Finally we have a fully 3D vector of surface deformation with (generally) very small error margins. This is nothing compared to the wonder of (Inferometric) Synthetic Aperture Radar — a.k.a (In)SAR — which provides high-resolution, continuous displacement fields of the surface based on the reflection of a beam sent down to the Earth’s surface from a satellite (e.g., Zebker et al. 1994; Bürgmann et al. 2000). One note should be made here; not every environment is particularly suited for every satellite that provides InSAR since the resolution and power of the satellite-sourced beams is variable. Therefore InSAR is most often used to detect the surface deformation of objects such as volcanos and areas that are arid, i.e., unvegetated, even though the newest generation of satellites can actually ‘look through’ vegetation. Since InSAR only measures the reflection of a single beam, one only obtains the deformation in that one (the so-called line-of-sight) direction: not a 3D deformation field as with GNSS. However, combining several images with different lines-of-sight introduces the possibility to resolve a complete 3D deformation field (Fialko et al. 2001; Wright et al. 2004), which can be further enhanced by the inclusion of GNSS vectors (e.g., Wang and Wright 2012). So much more data to invert for! Now we must be exactly sure that we know exactly what happens at and around the fault interface before, during, and after major earthquakes, right? …. Right? Well, we don’t. Easy conclusion, I know.

So why is it still so hard, you ask me? Well, there are some additional complexities. Come back next week if you want to find out what they are.

Yours truly,

The Sassy Scientist

PS: This post was written using only some observations. Some are still missing to foster a full picture.

References:
Bastos, L., Bos, M.S. and Fernandes, R. (2010), Deformation and Tectonics: Contribution of GPS Measurements to Plate Tectonics – Overview and Recent Developments. doi:10.1007/978-3-642-11741-1_5. In: Sciences of Geodesy - I, Chapter: 5, Publisher: Springer Berlin Heidelberg, Editors: Guochang Xu
Bürgmann, R., Rosen, P. and Fielding, E. (2000), Synthetic Aperture Radar interferometry to measure Earth’s surface topography and its deformation. Annual Reviews of Earth and Planetary Science, 28, 169–209.
Fialko, Y., Simons, M. and Agnew, D. (2001), The complete (3-D) surface displacement field in the epicentral area of the 1999 Mw 7.1 Hector Mine earthquake, California, from space geodetic observations. Geophysical Research Letters, 28(16), 3063–3066.
Ryan, J.W. and Ma, C. (1998), NASA-GSFC's geodetic VLBI program: a twenty-year retrospective. Physics and Chemistry of the Earth, 23, 9–10, 1041-1052. 
Wang, H. and Wright, T. (2012). Satellite geodetic imaging reveals internal deformation of western Tibet. Geophysical Research Letters, 39(7).
Wright, T.J., Parsons, B.E. and Lu, Z. (2004). Toward mapping surface deformation in three dimensions using InSAR. Geophysical Research Letters, 31(1).
Zebker, H.A., Rosen, P.A., Goldstein, R.M., Gabriel, A. and Werner C.L. (1994), On the derivation of coseismic displacement fields using differential radar interferometry—the Landers earthquake. Journal of Geophysical Research, 99:19617–34.

The Sassy Scientist – Earthquake Exoteries Nr. V

The Sassy Scientist – Earthquake Exoteries Nr. V

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here or leave a comment below.

In a comment on a post about the key papers in geodynamics, the Curmudgeonly Commenter asked:


Could you please point out some exceptionally important papers in geodynamics and tell us something interesting about the history of the field?


Dear CC,

“Enough with the seismology and rock mechanics”, I hear you scream. You came to this blog for geodynamics! And geodynamics you will get.

There is one key aspect of the earthquake cycle; it should be embedded in a longer time interval than just one earthquake, as fostered recently by e.g., van Dinther et al. (2013a, b). Indeed, how can an earthquake stand alone? Doesn’t the ‘background’ stress field play a major role in loading the faults in a — continuum — lithosphere but especially during the post-seismic ‘relaxation’ period? And what if you’re generating new faults, or changing your continuum material properties over time? Does pore-fluid pressure provide an indication for future earthquakes, or does the presence of water in your fault zone only lead to healing, and eventual long-term locking? Understanding the 3D, geodynamic setting of the region where you expect this earthquake to happen is paramount; the stresses imparted upon a locked fault patch (which can be geometrically complex) due to far-field tectonics may define whether that fault will fail or not.

On the other hand, our understanding of what happens during an earthquake is also … let’s say limited. Yes, we can obtain some slip distribution that shows how much motion was accommodated during an earthquake. This doesn’t provide an inkling of the underlying nucleation process. Dynamic rupture models (e.g., Das 1980; Koller et al. 1992; Tada et al. 2000) can simulate the nucleation of an earthquake on (not just planar) fault planes. This sounds exiting as actual earthquake sources produce seismic waves, and rupture fronts of earthquakes that can even reach super-shear velocities — in a super-shear rupture the rupture front travels faster than the shear wave speed!!! See e.g., Das (2015) for an extensive explanation on this topic. However, and yes there’s always a however, the strength of a fault and the prescribed conditions of failure are limiting factors; are these actually feasible parameters? Besides the rate-and-state of the fault (segments), the stress field may — no — will control the extent, speed and magnitude of the eventual rupture (e.g., Aochi and Fukuyama 2002). So yes, we need geodynamics — sure, a bit of rock mechanics and seismology will do too — but primarily geodynamics to make further steps.

When we consider seismic hazard, should we just focus on the one earthquake lying and waiting in silence, prowling patiently until we have forgotten that it may strike, or be ever so vigilant, obtain every shred of evidence at any scale (both in time and space) of the earthquake cycle and scope every angle to have any hope of cracking the enigma? There’s a lot to be done still. However, not only geodynamicists have pounced on this earthquake problem. Other disciplines smelled societal relevance and funding as well, so that will be the topic of next week.

If you think “but I want to know more about earthquakes and the role of geodynamics in solving this issue” (as well you should), I would like to refer everyone to van Zelst et al. (2019) for an at-length discussion of the inferences of dynamic rupture models that are coupled with geodynamically enforced stress states. What a paper!

Yours truly,

The Sassy Scientist

PS: This post was written under duress by our awe-inspiring Editor-in-Chief to provide some promotion of her recent paper. Some people are just shameless…

References:
Aochi, H. and Fukuyama, E. (2002), Three-dimensional nonplanar simulation of the 1992 Landers earthquake.Journal of Geophysical Research: Solid Earth, 107(B2), ESE–4.
Das, S. (1980), A numerical method for determination of source time functions for general three-dimensional rupture propagation. Geophysical Journal International, 62(3), 591–604.
Das, S. (2015), Supershear Earthquake Ruptures – Theory, Methods, Laboratory Experiments and Fault Superhighways: An Update. In: Perspectives on European Earthquake Engineering and Seismology. Geotechnical, Geological and Earthquake Engineering volume 39, edited by A. Ansal, doi:10.1007/978-3-319-16964-4_1
Koller, M.G., Bonnet, M. and Madariaga, R. (1992), Modeling of dynamical crack propagation using time-domain boundary integral equations, Wave Motion, 16, 339–366
Tada, T., Fukuyama, E. and Madariaga, R. (2000), Non-hypersingular boundary integral equations for 3-D non-planar crack dynamics, Computational Mechanics, 25, 613–626.
van Dinther, Y., Gerya, T.V., Dalguer, L.A., Corbi, F., Funiciello, F. and Mai, P.M. (2013b), The seismic cycle at subduction thrusts: 2. Dynamic implications of geodynamic simulations validated with laboratory models. Journal of Geophysical Research: Solid Earth, 118(4), 1502–1525.
van Dinther, Y., Gerya, T.V., Dalguer, L.A., Mai, P. M., Morra, G. and Giardini, D. (2013a), The seismic cycle at subduction thrusts: Insights from seismo-thermo-mechanical models. Journal of Geophysical Research: Solid Earth, 118(12), 6183–6202.
van Zelst, I., Wollherr, S., Gabriel, A.-A., Madden, E.H. and van Dinther, Y. (2019), A dynamic rupture simulation of a megathrust earthquake constrained by geodynamic and seismic cycle modelling, Journal of Geophysical Research: Solid Earth, accepted manuscript

The Sassy Scientist – Earthquake Exoteries Nr. IV

The Sassy Scientist – Earthquake Exoteries Nr. IV

Every week, The Sassy Scientist answers a question on geodynamics, related topics, academic life, the universe or anything in between with a healthy dose of sarcasm. Do you have a question for The Sassy Scientist? Submit your question here or leave a comment below.

In a comment on a post about the key papers in geodynamics, the Curmudgeonly Commenter asked:


Could you please point out some exceptionally important papers in geodynamics and tell us something interesting about the history of the field?


Dear CC,

After discussing the earthquake cycle last week, I think we can go into friction. Are you still following me? Is this series of blog posts in depth enough? There’s more to come, don’t worry.

Rate-and-state turmoil
During the 60’s and 70’s an additional new theory was fostered in the framework of rock mechanical experiments investigating friction, which still rocks the scientific community to this very day: rate- and state-dependent friction. I cannot get into too much detail about rate-and-state friction here for brevity and clarity — and I won’t — but I’ll leave some references at your disposal for further in-depth reconnaissance. Building on earlier work, Dieterich (1978, 1979) and Ruina (1983) describe that the shear stress on a fault plane is related to the normal stress on said fault plane — as in Byerlee — but additionally the sliding velocity on the fault and a critical slip distance (i.e., a measure of the fault interface smoothness) also come into the equation. The fault interface itself is oftentimes distributed as a fault gouge — i.e., a very fine-grained, usually unconsolidated material in a fault zone between the strong blocks. Faults without such gouge are oftentimes considered immature, and may behave fundamentally different (Marone and Scholz 1988). Putting the complexity of frictional fault slip in layman’s terms: the rate-and-state formalism states that the friction on a fault interface evolves through time, between static ‘locking’ and dynamic ‘sliding’ coefficients. This includes the possibility of healing or weakening of the fault interface (Marone 1998, Scholz 1998, Sleep 2005). Rate-and-state dependent friction provides an explanation for an abundance of observations, e.g., predicting stable, conditionally stable and unstable depth ranges of seismicity, the down-dip slip distribution and loading of a fault plane during the interseismic period, and aftershocks (e.g., Marone 1998, Scholz 1998). One — relatively recent — and exciting observation is that the rate-and-state friction formalism does not only hold for earthquakes generated at interfaces between rocks, but also along the bottom of ice masses (Winberry et al. 2013; Lipovsky and Dynham 2016). That’s just a chilling coincidence.

Just one tiny, eensy-teensy, itty-bitty, wee, slightly negative comment on rate-and-state friction; it’s a kinematic description of the observations, not a mechanical explanation of what’s actually happening at depth along a ‘fault interface’. If you can even call it that.

I think that’s more than enough for one week. Or rather, the Editor-in-Chief thinks that.

Yours truly,

The Sassy Scientist

PS: This post was written in some state at some rate.

References:
Dieterich, J. H. (1978), Time-dependent friction and the mechanics of stick-slip. Pure and Applied Geophysics, 116, 790–805
Dieterich, J. H. (1979), Modeling of rock friction: 1. Experimental results and constitutive equations. Journal of Geophysical Research, 84, 2161–2168
Lipovsky, B. P. and Dunham, E. M. (2016), Tremor during ice-stream stick slip. The Cryosphere, 10, 385–399, doi:10.5194/tc-10-385-2016
Marone, C. J. (1998), Laboratory-derived friction laws and their application to seismic faulting. Annual Reviews of Earth and Planetary Science, 26, 643–696
Marone, C. J. and Scholz, C. H. (1988), The depth of seismic faulting and the upper transition from stable to unstable slip regimes. Geophysical Research Letters, 15, 621–624
Ruina, A. (1983), Slip instability and state variable friction laws. Journal of Geophysical Research, 88, 10359–10370
Scholz, C. H. (1998), Earthquakes and friction laws. Nature, 391, 37–42
Sleep, N. (2005), Physical basis of evolution laws for rate and state friction. Geochemistry, Geophysics, Geosystems, 6, 11, doi:10.1029/2005GC000991
Winberry J. P., S. Anandakrishnan, D. A. Wiens, and Alley, R. B. (2013), Nucleation and seismic tremor associated with the glacial earthquakes of Whillans Ice Stream, Antarctica. Geophysical Research Letters, 40, 312–315, doi:10.1002/grl.50130