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

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 (-0.4M3.7) 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. Plain Language Summary Foreshocks are smaller earthquakes that happen near, but before, a large earthquake. We can use foreshocks as a tool to help understand how large earthquakes begin. One model of earthquake initiation holds that small foreshocks directly trigger other small foreshocks by changing the stress on the fault and, eventually, a large earthquake occurs if the conditions are favorable. Since earthquakes of all sizes would start out the same way, it is impossible to predict ahead of time which earthquakes grow large. An alternative explanation holds that smaller foreshocks do not directly trigger the large earthquake but instead are a symptom of something else happening underground, such as slow fault movement that does not release ground-shaking waves. Several foreshocks occurred in the 20hr before the 1999 magnitude 7.1 Hector Mine, California, earthquake. Analyzing records of the ground shaking from these foreshocks, we found that they are consistent with the former model, where each foreshock triggered the next one to happen like a line of falling dominoes. We discovered that the large earthquake started out the same way as the smaller foreshocks, so we could not tell ahead of time that it would eventually grow large.