Modeling Velocity Recordings of the Mw 6.0 South Napa, California, Earthquake: Unilateral Event with Weak High-Frequency Directivity

On 24 August 2014, an Mw 6.0 earthquake struck the Napa area in the north San Francisco Bay region, causing one fatality and damaging many older buildings in the Napa area. First, I employ low‐frequency data (0.05–0.5 Hz) from 10 near‐fault strong‐motion stations to perform slip inversion, revealing (in agreement with other studies) rupture propagating up‐dip and unilaterally along the fault with a dominant shallow asperity. Then I generate broadband synthetics (0.05–5 Hz) using an advanced Ruiz integral kinematic (RIK) source model (Ruiz et al., 2011) and a 1D velocity model. The RIK model is composed of randomly distributed overlapping subsources with a fractal number–size distribution. The particular distribution of the RIK subsources is constrained by the low‐resolution slip model from the inversion. Besides providing stable omega‐squared spectral decay at high frequencies, the RIK model is able to reproduce the frequency‐dependent directivity effect with adjustable strength. Comparison of the synthetic velocity waveforms with the observed data shows that the smallest modeling bias and variance is achieved by a rupture model with complex rupture propagation and thus weak high‐frequency directivity. I link my findings with other studies, including analog experiments by Day et al. (2008), suggesting that this feature is rather common to earthquake sources.

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