Aerial and Field Observations of Surface Rupture; M7.1 Hector Mine Earthquake 16 October, 1999

WALLS, CHRISTIAN, Earth Consultants International, Orange, CA

The 1999 M7.1 Hector Mine earthquake occurred 16 October, 1999, on the NNW striking Lavic Lake and Bullion faults in the eastern California shear zone (ECSZ) of the southern California Mojave desert. The Lavic Lake fault in the rugged Bullion Mountains is demarcated by dramatic color contrasts between ignimbrites, basalts and volcaniclastics. The previously horizontal playa surface of Lavic Lake shows remarkable macro and micro scale structures associated with rupture geometries. The earthquake produced surface rupture approximately 48 km in length, with an average dextral slip of 2.5-2.9 m and a maximum slip of 5.2 m. Local bends and steps produced up to 1.4 m vertical separation. Fault rupture is characterized by simple linear scarps to multiple branches within a zone several hundred meters wide, side-hill benches, offset stream channels, mole tracks, trenches, shutter ridges, offset creosote and left and right-stepping en echelon fractures commonly connected by respective thrust and normal faults. Several splay faults, 0.5-3.0 km long, form bounding structures to intricate conjugate fault systems and exhibit slip gradients. The distribution of horizontal slip along the fault appears to be symmetrical, with a gradual decrease from 50 to 0 cm dextral displacement in the northern 4.1 km of surface rupture. Northeast trending splay faults peeled slip off the main fault strands before the surface rupture terminated as a narrow zone 1.2 km into the Pisgah basalt. The lack of scarps in the Pisgah basalt suggests that this segment of the Lavic Lake fault had not ruptured post flow emplacement.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California