Structure of the San Fernando Valley region, California: Implications for seismic hazard and tectonic history

Victoria E. Langenheim, Thomas L. Wright, David A. Okaya, Robert S. Yeats, Gary S. Fuis, Kristina Thygesen, & Hans Thybo

Published March 31, 2011, SCEC Contribution #1447

Industry seismic-reflection data, oil-test well data, interpretation of gravity and magnetic data, and seismic-refraction deep-crustal profiles provide new perspectives on the subsurface geology of San Fernando Valley, home of two of the most recent damaging earthquakes in southern California, from analysis of. Seismic-reflection data provide depths to Quaternary to Miocene horizons; beneath the base of the upper Miocene Modelo Formation are largely non-reflective rocks of the middle Miocene Topanga and older formations. Gravity and seismic-reflection data reveal the North Leadwell fault zone, a set of down-to-the-north faults that do not offset the top of the Modelo Formation, that strikes northwest across the valley, and that may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that influenced deposition of the late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic-refraction data indicate that the basin underlying San Fernando Valley is asymmetric, with the north part of the basin (Sylmar subbasin) reaching depths of 5 to 8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the co-linear Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes about N45°E. That northeast-trending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults.

Langenheim, V. E., Wright, T. L., Okaya, D. A., Yeats, R. S., Fuis, G. S., Thygesen, K., & Thybo, H. (2011). Structure of the San Fernando Valley region, California: Implications for seismic hazard and tectonic history. Geosphere, 7(2), 528-572. doi: 10.1130/GES00597.1.