[J05-3] Crustal dynamics: Multidisciplinary approach to seismogenesis III

[J05-3-01] Spatial heterogeneity of crustal stress

^○Yoshihisa Iio (DPRI, Kyoto University,Uji,Japan)

invited

It has long been believed that crustal stress is highly heterogeneous in space. Direct stress measurements in boreholes show that directions of the maximum compressional stress axis change in location, from a borehole site to another, or even within a same borehole. These measurements imply that crustal stress is fractal. Heterogeneous slip distributions and hypocentral distributions are explained by heterogeneous stress fields. Stress inversion studies of focal mechanisms reveal that estimated stress field changes in space and also in time. Recently, it is reported that directions of the maximum compressional stress axis change before and after large earthquakes. These phenomena are usually explained by small magnitudes of differential stress (maximum minus minimum compressional stress) in focal regions However, the estimated magnitudes are too small compared with those estimated from frictional experiments, thus an alternative explanation is that they reflect heterogeneous stress field. They assume that crustal stress is highly heterogeneous and acts on each fault in a direction to which the fault easily slips. Namely, faults of a different orientation are subjected under different stress field. When slips of large earthquakes generate stress changes, only faults that are favorable for those stress changes can be triggered by the stress changes, and an apparent stress field could be estimated by stress inversions using only those triggered earthquake data.
In this study, we will try to reveal that crustal stress is regarded as uniform in a km scale from stress inversion analyses of focal mechanism, different from the usual idea of a fractal-like stress field. We analyze precise focal mechanisms and do a simple simulation of crustal stress considering heterogeneous structures.