Active fault hazard in southern California: Ground rupture versus seismic shaking

Abstract

There are exceptions to the common assumption that surface rupture by faulting is always accompanied by a damaging earthquake. Near Ventura, California, faults of the Oak View—Ojai area and Orcutt and Timber Canyons displace late Pleistocene and/or Holocene alluvial materials and soils by several tens of metres. The movement is almost exclusively along bedding planes in response to flexural slip during folding. Therefore these faults constitute a ground-rupture hazard. However, because they do not extend downward to rocks of high shear strength which could store large amounts of elastic strain energy, they do not constitute a seismic-shaking hazard. In contrast, the Oak Ridge fault near the coast does not cut strata shallower than 1,250 to 1,500 m below the surface, although pressure ridges attest to recent activity. Similarly, the Newport-Inglewood fault between Long Beach and Inglewood oil fields in the Los Angeles basin is expressed at the surface as a line of anticlinal hills rather than a throughgoing fault. Historical faulting at Inglewood and possibly at Rosecrans and Dominguez oil fields may be related to oil-field development rather than earthquake-related displacement. Those portions of the Oak Ridge and Newport-Inglewood faults pose seismic-shaking hazard but not ground-rupture potential from earthquakes alone. For planning purposes, it is necessary to distinguish faults with only ground-rupture potential or with only seismic shaking potential from the well-known faults with potential for both.