The 2026 Mw4.6 Boulder Creek earthquake and insights for seismic hazard associated with the southern Santa Cruz Mountains section of the San Andreas Fault

Evan T. Hirakawa, Grace A. Parker, Annemarie S. Baltay, Tara A. Nye, & Tim Clements

Submitted August 30, 2026, SCEC Contribution #15530, 2026 SCEC Annual Meeting Poster #TBD

We investigate the ground motions generated by the April 2, 2026 Mw4.6 Boulder Creek, California, earthquake. This event occurred on the Santa Cruz Mountains section of the San Andreas Fault, and is the largest earthquake to occur there since the 1989 Mw 6.9 Loma Prieta earthquake and its aftershock sequence. Comparison with NGA-West2 empirical ground motion models, including regional adjustments for the San Francisco Bay Area, indicates that the near-source ground motions were relatively large, implying a particularly energetic source and perhaps a high stress drop. We also observe elevated ground motion amplitudes at distances of approximately 80-100km, which we attribute to critical reflections from the Moho (commonly referred to as “Moho bounce”). We compare these observations with similar distance-dependent amplitude patterns observed in the 1989 Loma Prieta earthquake, which may have contributed to the large ground motions observed in San Francisco and Oakland, and find them to be consistent. We perform 3D wave propagation simulations using a finite-difference code and a regional seismic velocity model, with source parameters informed by the empirical ground motion analysis, to investigate 3D wave propagation effects and evaluate the role of a Moho reflection. Preliminary simulations confirm that the near-field ground motions require a source time function that implies an above average stress drop. A primary goal of this work is to address gaps in our understanding of the source, path, and site effects associated with the nearby 1989 Mw 6.9 Loma Prieta earthquake and to develop a framework for interpreting the large ground-motion amplitudes observed around the San Francisco Bay and implied seismic hazard for similar events.

Citation
Hirakawa, E. T., Parker, G. A., Baltay, A. S., Nye, T. A., & Clements, T. (2026, 08). The 2026 Mw4.6 Boulder Creek earthquake and insights for seismic hazard associated with the southern Santa Cruz Mountains section of the San Andreas Fault. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)