A Physical Model for Widespread Near-Surface and Fault Zone Damage Induced by Earthquakes - Seismic observations indicate that material velocities at shallow depths decrease over a large area after large earthquakes. The reductions are widespread, and occur at distances of up to several source dimensions. A persistent low-velocity fault zone has also been documented extensively from seismic and geodetic observations, in which the velocity drops further after large earthquakes. Dynamic stresses carried by seismic waves in the near surface or accompanying rupture at depth in the fault zone, are thought to create these velocity reductions by causing material damage. However, a rigorous physical interpretation as to why modest dynamic stresses can cause widespread near-surface damage, and why fault damage zones form, is lacking. By using a Drucker-Prager yielding criterion to simulate dynamic rupture propagation on a vertical strike-slip fault, I show that the widespread near-surface damage is caused by material yielding induced by seismic waves under the low confining pressure. Because the confining pressure increases with depth, materials yield more easily near the surface. The yielding zone at depth is narrowly confined near the fault, but its thickness broadens dramatically near the surface, forming a ‘flower-like’ damage zone, which is commonly observed in the geologic record. The fault zone damage at depth is induced by the large dynamic stress associated with the rupture front, while is induced by strong seismic waves ahead of the rupture front near the Earth’s surface. These results have important implications for the formation and evolution of fault zones, and possibly for the dynamic triggering of earthquakes as well. - Shuo Ma; Department of Geological Sciences; San Diego State University
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