Because the slip distribution is non-uniform, we use the method of Ripperger and Mai (2004), shown to be consistent with static or dynamic elastic dislocation models, to determine the coseismic stress change (stress drop). This method maps the spatially variable slip on the fault to the spatially variable stress change, or stress drop. Results applied to the SAFOD target event are shown in Figure 2.4B. In regions of high slip, the stress change is positive indicating a stress drop during rupture. The method also determines the degree of stress increase (negative stress change) on the region surrounding the rupture. The model has a peak stress drop of 80 MPa, and averages ranging from 3.7-19.7MPa depending on how the average is calculated.
The very high stress drop we obtain for much of the rupture area of the SAFOD repeaters (Figure 2.4B) is at odds with more traditional spectrally-based estimates (e.g. Imanishi et al., 2004). However, the stress drop averaged from Figure 2.4B over areas with positive stress drop is only 11.6 MPa, which is close to the Imanishi et al. (2004) result. On the other hand, the spatially variable high stress drop we obtain is required to fit the shape of the moment rate functions, and the peak is closer to the estimate obtained using the method of Nadeau and Johnson (1998). Thus, the finite-source results reconcile these disparate estimates of stress drop, illustrating that the two methods are apparently sensitive to different aspects of the rupture.
Assuming an average density of 2000 kg/, hydrostatic pore pressure and a coefficient of friction of 0.4 gives a maximum frictional strength of only 7.8MPa at the depth of the events. On the other hand, it has been proposed that small dimension asperities with strength approaching that of intact rock can concentrate substantial stress levels (Nadeau and Johnson, 1998). High stress drop repeating earthquakes may represent those relatively isolated, small-scale, contact points where large stress concentrations can develop and be released on a fairly regular basis. The much larger fault areas of bigger earthquakes may be frictionally weak, but studded with sparsely distributed high strength asperities producing relatively low average stress-drops during large earthquake rupture.
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