THE TRANSFER OF SLIP BETWEEN 2 EN ECHELON STRIKE-SLIP FAULTS - A CASE-STUDY FROM THE 1992 LANDERS EARTHQUAKE, SOUTHERN CALIFORNIA

Detailed mapping of the fresh surficial ruptures in the right step between the Homestead Valley Fault and the Emerson Fault, two right-lateral en echelon faults that broke in the 1992 Landers earthquake, indicates that the transfer of dextral slip between the faults is accommodated primarily by a series of obliquely striking right-lateral strike-slip cross faults. Right-lateral slip on the cross faults and counterclockwise rotation of the interior blocks are sufficient to transfer virtually 100% of the dextral slip and to accommodate the extension across the jog. Asymmetry of the dextral slip curves along the cross faults indicates that some of them may have been induced to fail by slip on the Homestead Valley Fault, while others were induced to fail by slip on the Emerson Fault. Comparison of the magnitude of slip in 1992 to bedrock and geomorphic offsets suggests that the Homestead Valley Fault and several secondary faults have experienced about 100 nominal Landers events and that the stepover structure originated at the same time as the Homestead Valley Fault, about 1 m.y. ago. Because the Emerson Fault has an order of magnitude greater total offset but only slightly greater surficial slip in 1992, we conclude that it is a significantly older fault. Repetition of 1992-like events should eventually lead to a merging of the Homestead Valley Fault with the Emerson.