Urban areas in Southern California are at risk from major earthquakes, not only quakes generated by long-recognized onshore faults but also ones that occur along poorly understood offshore faults. We summarize recent research findings concerning these lesser known faults. Research by the U.S. Geological Survey during the past five years indicates that these faults from the eastern Santa Barbara Channel south to Dana Point pose a potential earthquake threat. Historical seismicity in this area indicates that, in general, offshore faults can unleash earthquakes having at least moderate (M 5–6) magnitude. Estimating the earthquake hazard in Southern California is complicated by strain partitioning and by inheritance of structures from early tectonic episodes. The three main episodes are Mesozoic through early Miocene subduction, early Miocene crustal extension coeval with rotation of the Western Transverse Ranges, and Pliocene and younger transpression related to plate-boundary motion along the San Andreas Fault. Additional complication in the analysis of earthquake hazards derives from the partitioning of tectonic strain into strike-slip and thrust components along separate but kinematically related faults. The eastern Santa Barbara Basin is deformed by large active reverse and thrust faults, and this area appears to be underlain regionally by the north-dipping Channel Islands thrust fault. These faults could produce moderate to strong earthquakes and destructive tsunamis. On the Malibu coast, earthquakes along offshore faults could have left-lateral-oblique focal mechanisms, and the Santa Monica Mountains thrust fault, which underlies the oblique faults, could give rise to large (M ~7) earthquakes. Offshore faults near Santa Monica Bay and the San Pedro shelf are likely to produce both strike-slip and thrust earthquakes along northwest-striking faults. In all areas, transverse structures, such as lateral ramps and tear faults, which crosscut the main faults, could segment earthquake rupture zones.

ABSTRACT The Northern Channel Islands demonstrate pervasive Quaternary deformation, including faulting, regional warping, and localized folding. Field mapping on Santa Cruz Island, interpretation of seismicreflection profiles north of the island, measurement of fault-zone striations, and study of uplifted and deformed coastal terraces show which structures have been active in the late Quaternary and the pattern and rates of deformation during that time. The Santa Cruz Island fault is the largest and most active ground-rupturing fault on the island. The fault is predominantly left-lateral, although striations and displaced landforms demonstrate that it has a smaller, but significant component of reverse slip. The Poso Beach fault is an active reverse structure that cuts a 125 ka coastal terrace on southwest Santa Cruz Island. Other major faults include the “south branch” of the Santa Cruz Island fault, the Potato Harbor fault, and “Fault C”, but conclusive evidence of late Quaternary slip on these faults is lacking at this time. In addition to brittle deformation, regional warping is recorded by deformation of Pleistocene coastal terraces and progressive submergence and tilting on the island’s northern continental shelf. North of the Santa Cruz Island fault, warping takes the form of subsidence of the shelf and uplift of the island, with uplift reaching a maximum near the fault. On the south half of the island, deformation consists of a broad, low-amplitude south tilt, upon which localized folding associated with the Poso Beach fault and the Christi anticline is superimposed. These secondary folds, as well as southeast-striking right-lateral faults near Valley Anchorage, probably represent local accommodation of right-lateral structures of the California Borderland as they meet the east-west-trending western Transverse Ranges to the north. Overall, the pattern of late Quaternary deformation on Santa Cruz Island is consistent with a model in which the island, and probably the entire Northern Channel Islands chain, is part of a regional anticline, the north limb of which is progressively tilting above a concave-up listric thrust fault.

ABSTRACT A continuous 220 km long anticline underlies the Santa Monica Mountains and northern Channel Islands of southern California. This fold has been explained as the result of slip on a ramp in a blind thrust fault. Here we investigate the part of that structure including northwest Santa Rosa Island and southwest Santa Barbara basin. A U.S. Geological Survey (USGS) multichannel seismic reflection profile, USGS-808 was reprocessed and output in migrated depth and time. This profile, an industry multichannel profile, and numerous USGS high resolution reflection profiles were correlated to petroleum test well logs and to published data from Ocean Drilling Program (ODP) Site 893. A continuous fold limb dips north 10°-15° beneath northern Santa Rosa Island and the adjoining shelf and slope. This fold limb is cut by high-angle strike-slip faults on Santa Rosa Island, and by strands of the steeply S-dipping Santa Cruz Island fault at the shelf-break. The base of the fold limb is deformed by Pliocene slip on a blind S-dipping western continuation of the Oak Ridge-deep Mid-Channel fault. Neogene strata beneath central Santa Barbara Channel are nearly undeformed except that slight tilting and normal separation of Miocene strata is preserved across N-dipping faults. Steeply-dipping Pliocene and older strata associated with the North Channel fault are seen in wells beneath the northern margin of Santa Barbara basin, but are not imaged on USGS-808. The gently-dipping strata indicate that little shortening has been accommodated by folding and faulting across Santa Rosa Island or beneath much of Santa Barbara Channel. The observed wide fold limb can be created by less slip above a concave-up (listric) fault than by the more widely applied ramp-flat fault bend fold models. Progressive limb rotation is interpreted beneath the southern margin of Santa Barbara basin, consistent with a listric thrust model.