Ma et al, JGR 2005

Non-technical summary. Advocates of static stress transfer argue that aftershocks and subsequent mainshocks often occur in regions that experienced an increase in Coulomb stress caused by the mainshock, and are less prevalent in regions subject to a Coulomb stress drop. To date, most work has concentrated on earthquakes on strike-slip faults, whose stress change does not vary greatly with depth. For thrust faulting, the stress change becomes strongly depth-dependent, and thus the down-dip geometry and slip of the source fault, and the depth of aftershocks become critical to Coulomb analysis. The 20 September 1999 Mw=7.6 Chi-Chi, Taiwan, earthquake on the Chelungpu fault is probably the world's best recorded continental thrust event, with well determined spatial slip models from seismic, strong motion, and geodetic data. Its background seismicity and aftershock sequence are also recorded in unprecedented detail, making it ideal for investigation.

The correlation between calculated earthquake stress increases and the location of aftershocks has provided the strongest evidence in the scientific literature that stress changes imparted by mainshocks promote subsequent seismicity, a correlation that the 1999 Chi-Chi earthquake well exhibits. Several studies over the past 6 years have deepened the argument by resolving stress changes on aftershock focal mechanisms, which removes the assumption that the fault planes associated with aftershocks have any particular orientation. To assess whether the mainshock has promoted aftershock occurrence, one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand. For Chi-Chi we find a 28% increase for thrust faults and an 18% increase for strike-slip faults, commensurate with increases reported for the 1992 M=7.3 Landers and 1994 M=6.7 Northridge earthquakes.

But perhaps the chief criticism of the static stress triggering hypothesis is the difficulty in observing predicted seismicity rate decreases in the 'stress shadows,' or sites of Coulomb stress decrease. Detection of sustained drops in seismicity rate demands a long record of small (M≥2) earthquakes and a high seismicity rate, conditions that are met at Chi-Chi. We find four lobes with seismicity rate declines of 40-90% for 50 months, and they coincide with the stress shadows. The rate drops are evident when analyzed three ways: in uniform cell calculations, in 100-month-long time series, and by visual inspection of the M≥3 seismicity. Yet another reason why detection of such declines has proven so rare emerges from this study: there is a ubiquitous increase in seismicity rate for the first 3 months after Chi-Chi—and perhaps several other mainshocks—that must be associated with a different mechanism. Understanding this intital seismic response represents, in our judgment, the next frontier of earthquake interaction research.