Abstract

We document evidence for surface‐rupturing earthquakes (events) at two trench sites on the southern Green Valley fault, California (SGVF). The 75–80 km long dextral SGVF creeps ∼1–4  mm/yr. We identify stratigraphic horizons disrupted by upward‐flowering shears and infilled fissures unlikely to have formed from creep alone. The Mason Rd site exhibits four events from ∼1013 CE to the present. The Lopes Ranch site (LR, 12 km to the south) exhibits three events from 18 BCE to present including the most recent event (MRE), 1610±52  yr CE (1σ) and a two‐event interval (18 BCE–238 CE) isolated by a millennium of low deposition. Using OxCal to model the timing of the four‐event earthquake sequence from radiocarbon data and the LR MRE yields a mean recurrence interval (RI or μ) of 199±82  yr (1σ) and ±35  yr (standard error of the mean), the first based on geologic data. The time since the most recent earthquake (open window since MRE) is 402  yr±52  yr, well past μ∼200  yr. The shape of the probability density function (PDF) of the average RI from OxCal resembles a Brownian passage time (BPT) PDF (i.e., rather than normal) that permits rarer longer ruptures potentially involving the Berryessa and Hunting Creek sections of the northernmost GVF. The model coefficient of variation (cv, σ/μ) is 0.41, but a larger value (cv∼0.6) fits better when using BPT. A BPTPDF with μ of 250 and cv of 0.6 yields 30 yr rupture probabilities of 20%–25% versus a Poisson probability of 11%–17%.

Online Material: OxCal model for ages of paleoearthquakes, Lopes site 1998 logs, and description of a model for estimation of rupture frequency.

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