ACTIVE UPLIFT OF THE GAVIOTA COAST, SOUTHERN CALIFORNIA; IMPLICATIONS FOR EARTHQUAKE HAZARD

The Santa Ynez Fault is one of the longest active structures in the Western Transverse Ranges and is capable of generating >M~7.3 earthquakes, threatening millions of people living in Southern California. The Southern Branch of the Santa Ynez Fault (SBSYF) is a major branch of the fault that has poorly understood behavior. Emergent marine terraces appear continuous along the Gaviota Coast, yet this is incompatible with numerical dates from first-emergent marine terraces near the western extent of the Gaviota Coast (marine isotope stage (MIS) 5a) and first-emergent marine terraces on the eastern extent of the Gaviota Coast (MIS 3).

Because the SBSYF is the largest recognized fault in the Gaviota Coast, we test whether the rate up uplift associated with the SBSYF is greater than has been suggested in the published literature. To test this, a geomorphic analysis of the coastline and the SBSYF has been conducted focusing on stream profile analysis and mountain morphology. Normalized stream steepness indices (ksn) of similar rock strength regimes west of the SBSYF (~46) are consistently half of those east of the SBSYF (~77), which would correspond to a ~6-7 fold increase in uplift rates in this coastline. A Kolmogorov-Smirnov test on ksn values from either side of the SBSYF rejects the null hypothesis of no statistical difference with 99.99% confidence. Additionally, the location of the SBSYF corresponds to an abrupt westward decrease (~350m) in mountain crest elevation, and the only water gap that transverses the western Santa Ynez Mountains. We hypothesize that the SBSYF represents a significant tectonic boundary that separates two distinct segments of the Santa Ynez Mountains. Moreover, the fault likely has a greater vertical slip component than previous suggested. This would indicate a significantly greater earthquake hazard than previously assumed.