Relationship of the kickapoo fault to the Johnson Valley and Homestead Valley faults, San Bernardino County, California

Abstract

Abstract The 1992 Mw 7.3 Landers earthquake rupture propagated from the Johnson Valley fault to the Homestead Valley fault across Homestead Valley. Rupture was concentrated along the Kickapoo fault (a.k.a, Landers fault), a previously unrecognized north-south-striking, 5-km-long fault in Homestead Valley, and along small secondary fault traces east of the Kickapoo fault. To better understand the role of the Kickapoo fault in accommodating slip between the Johnson Valley fault and the Homestead Valley fault, we performed detailed aerial photograph interpretation and field mapping of alluvial deposits, geomorphology, and co-seismic surface rupture; evaluated existing water-well data; and conducted a gravity survey of the Homestead Valley area. These data show that the Kickapoo fault is a pre-1992 structure best interpreted as the northward continuation of the southern Johnson Valley fault. The Kickapoo fault merges northward with the northwest-striking Homestead Valley fault. Thus, we interpret that the Johnson Valley, Kickapoo, and Homestead Valley faults form an interlinked system. Large ruptures similar to the 1992 Landers rupture may have repeatedly propagated between the Johnson Valley and Homestead Valley faults along the Kickapoo fault. The Kickapoo-Johnson Valley fault intersection defines a 1-km-wide, left-restraining bend. Uplifted alluvial deposits and pressure ridges are common within the bend, indicating the bend is a long-lived contractional feature. Homestead Valley itself is underlain by relatively thin alluvial deposits over crystalline bedrock, showing that no significant subsidence has occurred during the late Quaternary. These observations are consistent with the accommodation of north-south right shear on the southern Johnson Valley and Kickapoo faults in Homestead Valley.