Broadband Ground Motion Simulations for an Earthquake “Doublet” on the Hayward and Calaveras Faults

The occurrence and impact of the February 2023 earthquake doublet in Türkiye, where an Mw 7.8 event was followed roughly 9 hours later by an Mw 7.5 event on an adjacent fault, begs the question: What would be the impact of such a sequence in California? As part of the effort to address this question, this Data Release consists of simulated ground motions for a hypothetical doublet scenario on the Hayward and Calaveras faults in the San Francisco Bay region. The fault locations and orientations are based on the 2023 USGS National Seismic Hazard Model. Both faults have creeping zones, and a simple scheme is employed in the kinematic rupture generator to taper the coseismic slip in the upper 10 km based on the ratio of long-term creep rate to long-term fault slip rate along each section. The resulting magnitudes are Mw 7.15 for the 95-km Hayward rupture and Mw 7.16 for the 100-km Calaveras rupture. Broadband ground motions are computed using a combination of deterministic and stochastic approaches for the lower- and higher-frequency components, respectively. The deterministic calculations use the USGS 3D San Francisco Bay region velocity model (v21.1) with a minimum shear velocity of 400 m/s, and the stochastic simulations use a 1D reference model with Vs30-based site-specific adjustments using period-dependent factors. Three-component waveforms are saved on a 1.2 km by 1.2 km grid of points, and PGA, PGV, and spectral accelerations at periods of 0.3, 1.0, and 3.0 s are extracted to produce scenario ShakeMaps. Comparison of the simulated motions with empirical ground-motion models (GMMs) are generally favorable; however, noticeable differences exist due to rupture directivity and basin response effects.