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The large aftershocks of the Northridge earthquake and their relationship to mainshock slip and fault-zone complexity

Seismological Laboratory University of California, Berkeley, Berkeley, California 94720

Abstract

The distribution of fault slip for the Northridge mainshock and two M_W 6 aftershocks is estimated using an approach that inverts the far-field momentrate functions derived by empirical Green's function deconvolution. The slip for these events was found to have a strong negative correlation with the locations of small aftershocks, and there is a notable clustering of aftershocks near the edges of slip, suggesting that these events are triggered by a redistribution of stress following the mainshock rupture. The two largest aftershocks ruptured the updip regions adjacent to the leading edge of the mainshock rupture. One aftershock, 11 hr after the main event, initiated 8 km west of the mainshock slip terminus and ruptured east to the western edge of mainshock slip. This event was found to have ruptured the same south-dipping fault as the mainshock. The two events, however, are separated by a lateral offset in the depth of seismicity that serves as a geometrical barrier to through-going rupture. An M_W 6.2 aftershock approximately 1 min after the main event is inferred to have ruptured an updip region of the mainshock fault that is relatively devoid of small aftershock seismicity. This event was located at the eastern edge of the sequence that overlaps with the north-dipping fault responsible for the 1971 San Fernando earthquake. Although the south-dipping plane is favored for this aftershock, it is not possible to rule out that it occurred on the north-dipping fault structure, or within the hanging block of the mainshock.