Bignami, Christian

A destructive (Mw 7.9) earthquake struck the Sichuan province (China) on May 12, 2008. The seismic event, the largest in China in more than three decades and referred as the Wenchuan earthquake, ruptured approximately 280 km of the Yingxiu-Beichuan fault and about 70 km of the Guanxian-Anxian fault. Surface effects were suffered over a wide epicentral area (about 300 km E-W and 250 km N-S). The huge earthquake took place within the context of long term uplift of the Longmen Shan range in eastern Tibet. The Longmen Shan fault zone is the main tectonic boundary between the Sichuan basin and eastern Tibet and is characterized by a large topographic relief (from 500m a.s.l. to more than 4000m) and large variations in rheological properties. The coseismic deformation is imaged by a set of ALOS-PALSAR L-band SAR interferograms. We use an unprecedented high number of data (25 frames from 6 adjacent tracks) to encompass the entire coseismic area. The resulting mosaic of differential interferograms covers an overall area of about 340 km E-W and 240 km N-S. The complex geophysical context of Longmen Shan and the variations of the fault geometry along its length can be better handled by means of numerical methods. The fault geometry is constrained by inversions of geodetic data and by taking into account the geological features of eastern Tibet and Sichuan basin. As a result, we build a Finite Element (FE) model consisting of two non planar faults embedded in a non-homogeneous medium with real topography of the area. We develop a procedure to perform inversions of DInSAR data based on FE computed Green functions of the surface displacement field. We retrieve a complex slip distribution on the fault segments in a heterogeneous medium with realistic surface topography.

Remote sensing sensors for detecting urban damage and other surface changes due to earthquakes is gaining increasing interest. To this aim optical images can represent useful tools for this application thanks their very high ground geometric resolution, especially when more frequent revisit times will be feasible with the implementation of new missions and future possible constellations of satellites. Sub-meter resolution images at visible frequencies are able to provide information at the scale of a single building. This kind of information is extremely important if provided with sufficient timeliness to rescue teams. In this work, the earthquake that hit the ancient city of Bam, Iran, on December 26th, 2003 has been investigated. The urban area was very close to the epicenter of the seism which caused strong damage to the urban structures. Pre- and post-earthquake Quickbird panchromatic images have been used to show the capability of this data to map damage at building scale by means of segmentation approach based on the application of morphological operators. A validation process has been performed by comparing the map of damage levels at single building scale with a detailed ground-based damage map provided by in situ survey.