Measuring the heterogeneity of the coseismic stress change following the 1999 Mw7.6 Chi-Chi earthque

Abstract

Seismicity quiescences are expected to occur in places where the stress has been decreased, in particular following large main shocks. However, such quiescences can be delayed by hours to years and be preceded by an initial phase of earthquake triggering. This can explain previous analyses arguing that seismicity shadows are rarely observed, since they can only be seen after this triggering phase is over. Such is the case of the main rapture zone, which experiences the strongest aftershock activity despite having been coseismically unloaded by up to tens of bars. The 1999 Mw7.6 Chi-Chi, Taiwan earthquake is characterized by the existence of several such delayed quiescences, especially off the Chelungpu fault on which the earthquake took place. We here investigate whether these delays can be explained by a model of heterogeneous static-stress transfer coupled with a rate-and-state friction law. We model the distribution of coseismic small-scale stress change τ by a Gaussian law with mean τ̄ and standard deviation στ. The latter measures the level of local heterogeneity of the coseismic change in stress. The model is shown to mimic the earthquake time series very well. Robust inversion of the τ̄ and στ parameters can be achieved at various locations, although on-fault seismicity has not been observed for a sufficiently long time to provide more than lower bounds on those estimates for the Chelungpu fault. Several quiescences have delays that can be well explained by local stress heterogeneity, even at relatively large distances from the Chi-Chi earthquake. Copyright 2007 by the American Geophysical Union.