Measuring Co-seismic Deformation from Optical Images. A Proposition for Methodological Improvements.

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Measuring Co-seismic deformation from optical images. A proposition for methodological improvements PI: Jean-Philippe AvouacCollaborators: Sebastien Leprince, Remi MichelThe geometry of fault ruptures and the distribution of coseismic displacements are key data to highlight the mechanics of seismic rupture. It is of interest in earthquake source studies because fault geometric complexities are known to influence rupture dynamics, hence seismic radiation, and because the coseismic slip distribution can be used to test dynamic rupture models. Conversely, such data are necessary to constrain the source kinematics which can be derived from seismological records, because there are various trade-offs between the geometric characteristics of the fault, the distributions of slip and rupture time on the fault. Geometry of fault ruptures and coseismic slip are generally obtained from field investigations and geodetic measurements. It is however not straightforward to assess whether the slip measured at the surface along the fault trace, and it's along strike variability, relates to near-surface complications or really reflect fault slip distribution at depth. Geodetic data are generally too sparse to constrain tightly the slip distribution and differential inSAR generally fails in the near-fault zone where large displacements result in poorly correlated images. We develop an alternative approach based on sub-pixel correlation of satellite optical images acquired before and after an earthquake. This technique suffers from some limitations mostly due to the uncertainties of the satellite attitude or to too approximate account for stereoscopic effects. The proposed methodological research is meant to apply to any satellite optical system but at this stage we focus on the analysis of SPOT images, taking advantage of the technological advances due to SPOT5 that was launched in 2002. These advances should indeed allow correcting the internal (mirror's orientation, thermal deformation of the optical system, misaligned CCD) and external sources of geometric distortion (satellite attitude and position). In addition, the availability of high resolution Digital Elevation Models with global coverage, in particular following the SRTM mission, make it possible to account accurately for stereoscopic effects. We use available data for the Landers and Hector Mine earthquakes, which are both well documented events, making it possible to test the validity and estimate the accuracy of the ground displacements measured from the SPOT images.