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

On 24 August 2014, an M-w 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.