STRESS TRANSFERRED BY THE 1995 M-W=6.9-OBE, JAPAN, SHOCK - EFFECT ON AFTERSHOCKS AND FUTURE EARTHQUAKE PROBABILITIES

The Kobe earthquake struck at the edge of the densely populated Osaka- Kyoto corridor in southwest Japan. We investigate how the earthquake t ransferred stress to nearby faults, altering their proximity to failur e and thus changing earthquake probabilities. We find that relative to the pre-Kobe seismicity, Kobe aftershocks were concentrated in region s of calculated Coulomb stress increase and less common in regions of stress decrease. We quantify this relationship by forming the spatial correlation between the seismicity rate change and the Coulomb stress change. The correlation is significant for stress changes greater than 0.2-1.0 bars (0.02-0.1 MPa), and the nonlinear dependence of seismici ty rate change on stress change is compatible with a state- and rate-d ependent formulation for earthquake occurrence. We extend this analysi s to future mainshocks by resolving the stress changes on major faults within 100 km of Kobe and calculating the change in probability cause d by these stress changes. Transient effects of the stress changes are incorporated by the state-dependent constitutive relation, which ampl ifies the permanent stress changes during the aftershock period. Earth quake probability framed in this manner is highly time-dependent, much more so than is assumed in current practice. Because the probabilitie s depend on several poorly known parameters of the major faults, we es timate uncertainties of the probabilities by Monte Carlo simulation. T his enables us to include uncertainties on the elapsed time since the last earthquake, the repeat time and its variability, and the period o f aftershock decay. We estimate that a calculated 3-bar (0.3-MPa) stre ss increase on the eastern section of the Arima-Takatsuki Tectonic Lin e (ATTL) near Kyoto causes fivefold increase in the 30-year probabilit y of a subsequent large earthquake near Kyoto; a 2-bar (0.2-MPa) stres s decrease on the western section of the ATTL results in a reduction i n probability by a factor of 140 to 2000. The probability of a M-w = 6 .9 earthquake within 50 km of Osaka during 1997-2007 is estimated to h ave risen from 5-6% before the Kobe earthquake to 7-11% afterward; dur ing 1997-2027, it is estimated to have risen from 14-16% before Kobe t o 16-22%.