HOLOCENE MARINE ABRASION PLATFORMS EAST OF THE SOUTH BRANCH OF THE SANTA YNEZ FAULT NEAR SANTA BARBARA, CA SUGGEST M 7-8 EARTHQUAKES ARE POSSIBLE

Hypothesis: Do small, uplifted Holocene marine abrasion platforms near Santa Barbara, CA suggest a serious earthquake hazard to millions of people in Southern California?

East of Santa Barbara uplift of the coast is about 6 m/ ky. A recent study of several abruptly uplifted Holocene marine platforms identified four uplift events during the past several thousand years. Each event is several m, suggesting that each uplift was produced by an M 7-8 earthquake. Rupture length associated with the large earthquakes should be about 100 km, and evidence of the earthquakes should be present near Santa Barbara.

A marine abrasion platform near Santa Barbara was likely produced by a M7+ earthquake. The platform is about 2 m above the modern abrasion platform, and the coast is uplifting at about 1.6 m/ky. A smaller remnant of the same platform is present about 1.3 km to the NE. At that location, paleo tide pools may be present on the platform. We have discovered a possible uplift event along the Gaviota Coast west of Santa Barbara. Faults that produce ~ 2 m /ky of coastal uplift near Santa Barbara and uplift the adjacent Santa Ynez Range are poorly understood.

The 130km, left –reverse Santa Ynez fault is just north of the segmented Santa Ynez Range. The South Branch of the Santa Ynez fault (SBSYF) crosses Gaviota Canyon at a high angle. At the coast the SBSYF has a 2+m thick zone of crushed rock, suggesting it is a major fault. The canyon is a complex water gap within a left step of the SBSYF, and is a discontinuity between high topography east of the fault and lower topography west of the fault.

Stream steepness (ksn) values, which are sensitive to uplift rate, suggests that the rate of uplift east of the SBSYF is sufficient to produce uplifted Holocene platforms. We have not observed Holocene abrasion platforms west of the SBSYF, suggesting a lesser rate of uplift.