Stairways to hazard: relating geologic observations to future fault behavior

Abstract:

Geologic slip rates are a time-averaged measurement of earthquake behavior calculated over 10²—10⁶-year time scales, and thus represent only a coarse metric of how fast or slow a fault has released strain over a given time interval. Slip rates are a primary input for probabilistic seismic hazard analyses (PSHA), which forecast expected ground shaking in future earthquakes. Geologic slip rates are typically constructed using a ratio of displacement and age for a geomorphic feature offset by the fault, and can encompass dozens of earthquakes, or as few as one or two events including an open seismic cycle. The wide range in the number of earthquake cycles spanned by geologic slip rates leaves open questions regarding whether geologic rates in different settings (i.e., variations in strain accumulation rate) describe the same processes, and how to assess associated uncertainties in PSHA frameworks. To explore these questions, we develop a numerical methodology to derive synthetic earthquake paths (i.e., displacement—time ‘stairways’) constrained by geologic data. We utilize this new method to model previously determined dated offset data along the Toe Jam Hill and San Andreas faults. We construct earthquake paths that fit these data, and then extrapolate the earthquake paths into the future. Uncertainties derived from modeling of future slip rates using this new methodology are constrained by the total suite of earthquake paths that fit the geologic field observations. We demonstrate that uncertainties in future slip rates, constrained by geologic observations and the total suite of possible earthquake paths, can provide a critical input to hazard calculations.