Feb 08

PhD thesis: Satellite image analysis for measuring rock properties in fault damage zones, a key constraint for earthquake modeling

Context: Decades of research in earthquake sciences have yielded meager prospects for earthquake predictability: we cannot today predict the time, location and magnitude of a forthcoming earthquake. Yet important advances have been made, including the demonstration that certain intrinsic properties of the tectonic faults that produce earthquakes significantly affect their magnitude and hence their devastating potential. It is thus of critical importance to properly document the geometric and other physical properties of the faults. The latest generation of satellite-based imaging sensors (Pleiades, Sentinel, etc.) acquires large volumes of Earth surface’s images with high spatial, spectral and temporal resolution (up to 50cm/pixel, 50 bands, twice per day, covering the entire planet). These data offer the unprecedented opportunity to observe and to measure the faults as they appear at the Earth surface, at a level of detail and precision never achieved before. The proposed work intends to take advantage of these new satellite data to recover a specific fault property–fault rock damage, better understanding of which is needed to improve earthquake modeling.

Subject: As tectonic faults grow over time, they permanently damage the embedding crustal rocks, in the form of dense networks of secondary damage faults and fractures of various scales, spacing, orientations, slip modes, etc., extending off the parent fault. These damage faults/fractures modify the elastic properties of the surrounding crustal volume, actually increasing its compliance. Theoretical models show that the variability of damaged rock compliance around a fault controls the distribution and amplitude of the displacements and the rupture speed of the large earthquakes this fault produces. Rock compliance in fault damage zones thus seems to have a pronounced impact on the seismogenic potential of parent faults (i.e., their ability to produce large earthquakes). Yet, there exist at present very few data that measure the rock compliance in natural fault damage zones. Available earthquake models are thus using at best ad hoc compliance values, but more generally ignore the compliance of the damaged crust. This PhD topic thus intends to provide robust estimates of crust compliance in natural fault damage zones. These measurements will then be used to better constrain earthquake rupture models developed in our group.

There exist sites over the world where the damaged rocks around faults are well exposed to observation. The exposition is at the ground surface, but a few studies that were conducted at local spots in some of those exposed damage zones have shown that the elastic properties recovered from surface data are similar to those inferred by geophysical means in the first 5-6 km of the crust. Measuring damage at the ground surface thus seems relevant to estimate the compliance of the shallow crust. Some of the best exposures of fault damage zones are located in Western USA, in dry and hence well-preserved environments. There, the damage faults are clearly observable, and hence measurable, for they have specific morphological and topographic signatures. The project aims at identifying these signatures so as to measure the sizes, spacing, densities, slip modes, and orientations of the faults/fractures in damage zones all along and across a few target faults. Using theoretical models, we will then convert these measurements into rock compliance estimates.

Work Tasks:

The work includes three principal tasks, pertaining to three scientific domains. The PhD student will thus gain a multi-disciplinary high-level expertise.

1) Satellite data analysis:

  • Building a multi-stereo, high-resolution DEM of 4-5 damage sites in Western USA.
  • Developing a data fusion protocol to combine optical Pléiades and radar TerraSAR-X data.
  • Developing stereoscopic methods to measure 3D damage faults geometries.
  • Developing new machine learning approaches to automate the identification and the measurements of the damage zone faults in the 3D volume.

2) Tectonic analysis:

  • Overall damage fault analysis.
  • High-resolution fault analysis.
  • Building damage fault scaling laws.

3) Mathematical modeling of damage measurements and use in earthquake models:

  • Using a mathematical model to estimate the crustal compliance in fault damage zones.
  • Injecting the compliance estimates into dynamic earthquake rupture models.

More details on this PhD thesis position here