Spatial migration of earthquakes within seismic clusters in Southern California: Evidence for fluid diffusion
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Seismicity within many earthquake swarms is observed to migrate slowly with time, which may reflect event triggering due to slow fault slip or fluid flow. We search for this behavior in Southern California by applying a weighted least squares method to quantify event migration within 69 previously observed seismicity bursts. We obtain bestfitting migration directions and velocities, and compute a statistical migration significancesm for each burst using a bootstrap resampling method. We define 37 bursts with sm0.8 as the migration group, and 32 bursts with sm<0.8 as the nonmigration group. To explore differences between the two groups, for each burst we compute effective stress drop (quasi, the ratio between total moment and radius), the skew of the moment release time series (), the timing of the largest event (tmax), and the distance separation between the first half and second half of the sequence (ds). As expected, the migration group features larger ds and lower quasi, consistent with higher migration significance. It also features lower and higher tmax, similar to observations from swarms in the Salton Trough, while the nonmigration group is more similar to main shockaftershock sequences. To explore possible fluid involvement, we model the migration behavior with the fluid diffusion equation, and identify 18 bursts with diffusion coefficients ranging from 0.01 to 0.8 m2/s, with the majority below 0.16 m2/s. The obtained diffusion coefficients and migration behavior are similar to the Reservoirinduced seismicity beneath the Au reservoir in Brazil. The majority of normal faulting events are associated with these 18 bursts, while the nonmigration group has the most reverse faulting events, indicating a possible link between sequence type and focal mechanism.
