STRAIN TRANSFER ACROSS THE TRANSTENSIVE IMPERIAL AND MEXICALI VALLEYS OF SOUTHERN CALIFORNIA AND NORTHERN BAJA CALIFORNIA, MEXICO: TRANSITION FROM EXTENSION TO STRIKE-SLIP DOMAINS AND IMPLICATIONS FOR PATTERNS OF EARTHQUAKE GENERATION

In the Salton Trough region, strain transfer transitions northward from predominantly extensional spreading offshore in the Gulf to primarily conjugate strike-slip faulting and minor block rotation onshore in the Imperial Valley. The Cerro Prieto (CP) fault transfers part of its 40+ mm/yr of slip to the Imperial fault (IF) in the vicinity of the CP geothermal field, with the balance of strike-slip continuing northwest across the border, where slip steps right at the Heber geothermal field to feed slip into the southern San Jacinto (SJ) fault system. Paleoseismic investigations show that the CP extension fault expresses a similar level of activity to the IF, both having sustained two surface ruptures in the past 350-400 years. Similarly, to the west, northward transfer of strike-slip is accomplished within and along the Sierra Mayor and Sierra Cucapa via the series of transtensional basins of Laguna Salada (LS), with sediment thicknesses of up to 4 km. The sierras themselves are superposed uplifts within the northern trough depression that result from transpressive strike-slip faulting within the ranges. North of the border in the Imperial Basin itself, strain is primarily on parallel northwest-striking dextral faults, with northeast-striking sinistral faults, allowing for conjugate block rotations. The NE-striking faults are seismogenic, as seen in moderate earthquakes in 1942 (M6.5), 1980 (M5.9), and 1987 (M6.2). The conjugate fault structure of the northern Imperial Valley may allow for some faults, such as the Clark fault that terminates southeastward in the San Felipe Hills, to transfer strain to other faults via block rotation, in this case to the Superstition Hills fault. Paleoseismic investigations along the major faults of the southern San Andreas (SA) system demonstrate that major earthquake production over the past thousand years has come in clusters of large events separated by up to 2-3 centuries of relative quiescence. The historical seismicity, with M7-7.5 earthquakes on the LS, IF and CP faults during the past century, suggests that the area has entered another period of larger earthquake production, possibly leading to failure of the two largest fault zones, the south-central SJ and southern SA faults, each of which is capable of large earthquakes and neither of which have broken in about 300 years.