HR: 08:45h
AN: S11C-04    [Abstracts]
TI: Estimating Stress Heterogeneity From Aftershock Rate
AU: * Helmstetter, A
EM: agnes@ldeo.columbia.edu
AF: LDEO Columbia University, 61 Rte 9W, Palisades, NY 10964 United States
AU: Shaw, B
EM: shaw@ldeo.columbia.edu
AF: LDEO Columbia University, 61 Rte 9W, Palisades, NY 10964 United States
AB: We estimate the rate of aftershocks triggered by a heterogeneous stress change, using the rate-and-state model of Dieterich [1994]. We show than an exponential stress distribution P(τ) ~ exp (-τ/τ₀) gives an Omori law decay of aftershocks with time ~ 1/t^p, with an exponent p=1-Aσ_n/τ₀, where A is a parameter of the rate-and-state friction law, and σ_n the normal stress. Omori exponent p thus decreases if the stress "heterogeneity" τ₀ decreases. We also invert the stress distribution P(τ) from the seismicity rate R(t), assuming that the stress does not change with time. We apply this method to a synthetic stress map, using the (modified) scale invariant "k²" slip model [Herrero and Bernard, 1994]. We generate synthetic aftershock catalogs from this stress change. The seismicity rate on the rupture area shows a huge increase at short times, even if the stress decreases on average. Aftershocks are clustered in the regions of low slip, but the spatial distribution is more diffuse than for a simple slip dislocation. Because the stress field is very heterogeneous, there are many patches of positive stress changes everywhere on the fault. This stochastic slip model gives a Gaussian stress distribution, but nevertheless produces an aftershock rate which is very close to Omori's law, with an effective p≤ 1, which increases slowly with time. The rate-and-state model cannot explain an Omori law with p>1, unless the stress decreases with time after the mainshock. The inversion of the full stress distribution P(τ) is badly constrained for negative stress values, and for very large positive values, if the time interval of the catalog is limited. However, constraining P(τ) to be a Gaussian distribution allows a good estimation of P(τ) for a limited number of events and catalog duration.
UR: http://www.ldeo.columbia.edu/~agnes/PAP/HS05.pdf
DE: 7215 Earthquake source observations (1240)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005