Abstract

The 12 September 1994 Mw 5.8 Double Spring Flat, Nevada, earthquake initiated at the intersection of a northeast- and northwest-striking set of conjugate faults within an overlapping zone between the Genoa and Antelope Valley fault zones, of the eastern Sierra Nevadan range frontal fault system. The mainshock ruptured on the northeast-striking fault plane. Eight days after the mainshock, the aftershock activity migrated from the mainshock fault plane to the northwest-striking conjugate fault. Over the next 2 years, aftershocks migrated southward onto another set of conjugate faults and then onto the Antelope Valley fault zone. The focal mechanisms of 17 M > 4 aftershocks were estimated from a time-domain moment tensor inversion using regional broadband data. The T axis (minimum stress direction) is oriented east-west (N80°E to N100°E) for the (M > 4) events as is, commonly observed along the eastern Sierra Nevadan range front in northwestern Nevada. From these results, we make some general points that can be considered in seismic hazard assessment. The maximum magnitude in overlapping normal fault zone is limited to the size of the overlapping zone. This makes small- to moderate-size (M < 6) strike-slip earthquakes more likely than large range-front (M > 7) earthquakes. The seismicity within this overlapping zone may indicate interseismic strain accumulation from east-west extension mainly through strike-slip deformation. The apparent scarcity of modern normal-faulting earthquakes along the Sierran range-front faults suggests a characteristic model, while a Gutenberg and Richter model for the recurrence behavior of earthquakes applies to the overlap zones between the normal faults. The pattern of seismicity and principle stress directions from the aftershock fault-plane solutions suggest a tectonic model of changing fault geometry for the overlapping zone between the Genoa and Antelope Valley fault zones. Two plausible long-term tectonic outcomes may develop with this model: a normal fault growth model where the overlapping segments of the Genoa and Antelope Valley faults eventually become “hard linked” (form a throughgoing fault) or a normal fault growth model where the overlapping segment of the Genoa fault system grows southward while the Antelope Valley fault is isolated in the formation of new basins and ranges.

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