Foreshocks and Mainshock Nucleation of the 1999 M w 7.1 Hector Mine, California, Earthquake

Abstract

Foreshocks provide valuable information on the nucleation process of an upcoming large earthquake. We applied high-resolution similar-waveform techniques for earthquake detection, location, and source parameter estimation to understand the space-time evolution of a foreshock sequence and its relationship to the mainshock hypocenter. The 1999 M w 7.1 Hector Mine, California, earthquake was preceded by 50 foreshocks (−0.4 ≤ M ≤ 3.7) during the 20 hr before the mainshock. Foreshock activity did not accelerate leading up to the mainshock. Their locations moved north with time, rupturing adjacent areas along the fault plane with little overlap, but remained within a compact <2 km 3 volume. The mainshock initiated at a location where previous foreshocks had locally increased the shear stress. These observations are consistent with a triggered cascade of stress transfer, where previous foreshocks load adjacent fault patches to rupture as additional foreshocks, and eventually the mainshock.