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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - F-IMAGE (Seismic Functional Imaging of the Brittle Crust)

Teaser

Summary

***************** Functional imaging of the brittle crust with seismological imaging ***************** By Rosemary Fayjaloun and Michel Campillo

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What is the problem/issue being addressed?
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The physics of the earthquakes are still poorly understood, despite their dramatic impact.
F-image project aims to study the behavior of the rocks around the fault, to understand the mechanical behavior of the faults and the rocks around the faults.

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Why is it important for society?
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The project should lead to new observables that could be used in the framework of earthquake risk assessment: temporal evolution of elastic properties in the vicinity of the faults, changes in the scattering properties indicative of the damage of the rocks, classification of the various components of the continuous motion of the Earth surface (micro-earthquakes, tremors, noise of different origins...).

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What are the overall objectives?
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The physical conditions of the rock vary spatially, from the surface to the deep rock, and from the fault core itself to the host rock. Also, the properties of the rock evolve with time.
F-image will analyse the spatio-temporal changes of elastic properties around fault zones using seismological methods, using 3 approaches for 3 target regions:
- 3D imaging of the damaged zone and its extent (the small scale heterogeneities, the size and density of cracks and voids, the presence of fluids) by studying the effective seismic velocity and the high scattering strength and the attenuation of the rock, via:
. imaging the strong scatterers (like seeing objects in a dense fog)
. imaging the scattering and attenuation strength (like mapping the density of the fog)
- continuous monitoring of the rock properties
. imaging the temporal changes in the rock susceptibility
- detecting the low-amplitude seismicity and studying the relation between the statistical properties of seismicity and the tectonic deformation expected during mechanical changes of the medium (damage, fluid injection).
. using machine learning techniques

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Study regions :
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- Southern California plate boundary, characterised by several major fault segments and by high potential for significant earthquakes. Southern California is very well studied which allows us to better understand of the mechanics of the fault zone.
- Northern Anatolian Fault in the Marmara region, characterised by devastating earthquake and by its seismicity migrating from East to West: In this scenario of migration, the Marmara segment is expected to break in the coming years, which represents a major threat for the city of Istanbul, as one of the largest mega-poles in Europe.
- In the last part of the project, Groningen gas field in Netherlands, the largest gas reservoir in Europe (or another field of applications in better data are available). The temporal changes in the rock properties (velocity and scattering) at this area allows us to study the induced seismicity due to hydrocarbon exploitation and Geothermal fields and therefore the induced earthquake hazards.
- Study of laboratory analogues to highlight the physical processes at work during the observed temporal variations.

Work performed

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Imaging
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-we have studied the scattering properties and their lateral variations around the North Anatolian fault using continuous records (PhD Chantal Van Dinther with Michel Campillo, Ludovic Margerin and Helle Pedersen)
-we have performed a passive body wave imaging of the San Jacinto fault in Southern California (phD Rita Touma with Michel Campillo and Alexandre Aubry)
-we are developping the theory of radiative transfert of elastic waves including surfave to body coupling. We solved a simplified scalar problem treating the coupling of surface to body waves for the first time (PhD Andres Barajas with Ludovic Margerin and Michel Campillo)

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Monitoring
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-we have analyzed the temporal variations of seismic velocities in different areas to evaluate quantitatively the effect of external forcing and tectonic processes (PhD Andres Barajas, PhD Shujuan Mao, postdoc Piero Poli, postdoc Qingyu Wang with Florent Brenguier, Laurent Stehly, Ludovic Margerin, Albanne Lecointre, Gregor Hillers and Michel Campillo)
-we have gathered and pre-processed the data for Southern California and North Anatolian fault (PhD Chantal van Dinther, PhD Shujuan Mao, postdoc Qingyu Wang with Pierre BouÃ© and Michel Campillo

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Classification of signals
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-we are developing a new approach of unsupervised detection and classification based on te Scattering Transform (PhD Eric Beauce, postdoc Leonard Seydoux, postdoc Piero Poli with Maarten de Hoop, Michel Campillo, Glenn Cougoulat and Philippe Roux)
-we detected at global scale a new class of long period seismic signals associated with deep ocean volcanos (postdoc Piero Poli with Michel Campillo)

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Laboratory experiments
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-we have developed a series of experiments with non-cohesive and cohesive granular media that mimic the behavior of the rocks in the fault gouge. The setting includes measurements of acoustic emission associated with damage and temporal evolution of seismic speed in order to eventually compare these results with actual observations on the field (phD Vincent Canel with Xiaoping Jia, postdoc Leonard Seydoux, postdoc Piero Poli and Michel Campillo)
-in parallel, we are developing a numerical code to model the specific behavior of the cohesive granular medium (phD Vincent Canel with Ioan Ionescu and Michel Campillo).

Final results

-we are developing a new passive imaging technique with body waves that is producing images of the structure of the faults
-we are developping the first framekork to handle surface wave to body wave coupling in the multiple scattering regimes. This advance will make possible 4D imaging of the seismic speed in fault systems with unprecedented accuracy.
-we have developed the tools to apply passive monitoring to large arrays and long time series (i.e. huge data sets) in order to discriminate external forcing (meteorology and climate trends) and identify tectonic processes
-we are developping original artificial intelligence tools to identified new signals (unsurvised approach) and to classify the continuous records. We have great expectations for the systematic application of this technique for detection, discovery of new classes of signals and improvement of the correlation approach.
- these new observable will be integrated in a model of fault evolution in the final part of the project.