The initial subevent of the 1994 Northridge, California, earthquake: Is earthquake size predictable?

We examine the initial subevent (ISE) of the M 6.7, 1994 Northridge, Califo rnia, earthquake in order to discriminate between two end-member rupture in itiation models: the 'preslip' and 'cascade' models. Final earthquake size may be predictable from an ISE's seismic signature in the preslip model but not in the cascade model. In the cascade model ISEs are simply small earth quakes that can be described as purely dynamic ruptures. In this model a la rge earthquake is triggered by smaller earthquakes; there is no size scalin g between triggering and triggered events and a variety of stress transfer mechanisms are possible. Alternatively, in the preslip model, a large earth quake nucleates as an aseismically slipping patch in which the patch dimens ion grows and scales with the earthquake's ultimate size; the byproduct of this loading process is the ISE. In this model, the duration of the ISE sig nal scales with the ultimate size of the earthquake, suggesting that nuclea tion and earthquake size are determined by a more predictable, measurable, and organized process. To distinguish between these two end-member models w e use short period seismograms recorded by the Southern California Seismic Network. We address questions regarding the similarity in hypocenter locati ons and focal mechanisms of the ISE and the mainshock. We also compare the ISE's waveform characteristics to those of small earthquakes and to the beg innings of earthquakes with a range of magnitudes. We find that the focal m echanisms of the ISE and mainshock are indistinguishable, and both events m ay have nucleated on and ruptured the same fault plane. These results satis fy the requirements for both models and thus do not discriminate between th em. However, further tests show the ISE's waveform characteristics are simi lar to those of typical small earthquakes in the vicinity and more importan tly, do not scale with the mainshock magnitude. These results are more cons istent with the cascade model.