SURFACE RUPTURE OBSERVATIONS ASSOCIATED WITH THE M7.1 HECTOR MINE EARTHQUAKE OF OCTOBER 16, 1999

The M_w7.1 Hector Mine earthquake occurred within the Eastern California Shear Zone (ECSZ). The earthquake was accompanied by 48 km of primarily dextral surface rupture along the Lavic Lake and Bullion faults in and adjacent to the northern Bullion Mountains. Lesser rupture occurred across two right-steps to the south, demonstrating, as in the 1992 Landers earthquake, that strain in the ECSZ may be propagating across a series of less-favorably oriented faults. Surface rupture was mapped using post-earthquake aerial photography. Field mapping provided additional detail and over 350 fault rupture observations; approximately 150 slip measurements were used to define the slip distribution. Dextral slip was relatively symmetrical about the epicentral region, averaging nearly 3 m with a maximum of 5.2 m. Vertical slip was secondary and variable with minor west-side down displacements predominating in the Bullion Mountains and east-side down to the north and south. The character of the fault rupture varies with the material faulted and includes linear scarps, dextrally offset stream channels, mole tracks, trenches, shutter ridges, and left-stepping en echelon fractures. Complexity of surface faulting varies from a simple linear scarp to multiple branches within a zone several hundred meters wide. Field and aerial photographic evidence indicates that most of the faults that ruptured had prior Late-Quaternary displacement although only some portions had prior Holocene displacement.

Preliminary results from this earthquake and more complete results from paleoseismic investigation of faults to the west suggest important implications for hazard assessment in this and similar broad shear zones. Although individual faults have long recurrence intervals and low slip-rates, collectively the ECSZ presents a greater regional hazard than would be indicated by the individual faults. The long recurrence interval also implies that faults that last ruptured in latest Pleistocene to early Holocene time may be a greater hazard than faults that show evidence of late Holocene displacement - a reversal of the common understanding of more narrowly defined faults zones elsewhere.

Abstract Authored by The Hector Mine Earthquake Geologic Working Group