No abstract

A paleoseismic study conducted along the Cholame segment of the San Andreas fault provides evidence for three earthquakes and the amount of lateral offset for the most recent event (1857 Fort Tejon earthquake). Excavations at the Las Yeguas (LY4) site include five fault-perpendicular, two parallel, and several handdug trenches. Abruptly truncated sand and silt layers that are not correlative across the fault zone constrain the timing of the penultimate event (L2) between cal. A.D. 1030-1300 and 1390-1460. Vertical offset, shearing, and fracturing of silty sand and gravel units that overlie L2 and historical artifacts that bracket the timing of the MRE (L1) provide evidence that the most recent ground-rupturing event, L1, occurred between cal. A.D. 1390-1460 and ϳ1865. L1 is likely the 1857 Fort Tejon earthquake. Tectonic silt-filled fractures that dissect historic gray-tan silt and sand suggest a ground shaking or a triggered slip event (L0), which occurred after L1. Threedimensional excavation of an alluvial fan edge (unit 4) indicates that 3.0 ‫ע‬ 0.70 m of near-fault brittle slip occurred during the 1857 earthquake at this site.

Determining the timing of paleoearthquakes is a central goal of paleoseismology and serves as an important input to seismic hazard evaluations. Herein, we develop a Bayesian statistical method for refining the ages of strata and earthquakes and calculating the recurrence of earthquakes based on data from paleoseismic excavations. Our work extends previous paleoseismic Bayesian modeling by simultaneously calculating the joint posterior probability density of recurrence intervals, earthquake ages, and layer ages. To estimate this joint posterior density, we employed the Metropolis-Hastings Markov-Chain Monte Carlo (MCMC) sampling method. We used the Wrightwood paleoseismic excavation site as a test bed for our method by comparing the results derived from our simulation to those of previous studies. We found that for stratigraphic order constraints, our results agreed well with previously used methods; however, when peat growth was used to constrain the minimum time that separates layers from one another, a systematic aging of strata in the lower portion of the stratigraphic column resulted. Inspection of our results indicated that, rather than representing a minimum time that separates layers, peat growth instead determined the strata's ages exactly when a constant peat-growth rate was prescribed. Using this information, we developed a new method of improving layer age, earthquake age, and recurrence interval estimates at sites where peat-bearing layers are present. This method generally produces layer-age estimates with less variance than previous methods and requires that both proximal stratigraphic constraints and trenchwide constraints be obeyed. The benchmarking of the MCMC simulation results shows that simulation provides an effective and efficient way of improving estimates of layer ages, earthquake ages, and recurrence intervals.

No abstract

Determining the timing of paleoearthquakes is a central goal of paleoseismology and serves as an important input to seismic hazard evaluations. Herein, we develop a Bayesian statistical method for refining the ages of strata and earthquakes and calculating the recurrence of earthquakes based on data from paleoseismic excavations. Our work extends previous paleoseismic Bayesian modeling by simultaneously calculating the joint posterior probability density of recurrence intervals, earthquake ages, and layer ages. To estimate this joint posterior density, we employed the Metropolis-Hastings Markov-Chain Monte Carlo (MCMC) sampling method. We used the Wrightwood paleoseismic excavation site as a test bed for our method by comparing the results derived from our simulation to those of previous studies. We found that for stratigraphic order constraints, our results agreed well with previously used methods; however, when peat growth was used to constrain the minimum time that separates layers from one another, a systematic aging of strata in the lower portion of the stratigraphic column resulted. Inspection of our results indicated that, rather than representing a minimum time that separates layers, peat growth instead determined the strata's ages exactly when a constant peat-growth rate was prescribed. Using this information, we developed a new method of improving layer age, earthquake age, and recurrence interval estimates at sites where peat-bearing layers are present. This method generally produces layer-age estimates with less variance than previous methods and requires that both proximal stratigraphic constraints and trenchwide constraints be obeyed. The benchmarking of the MCMC simulation results shows that simulation provides an effective and efficient way of improving estimates of layer ages, earthquake ages, and recurrence intervals.