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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.