Seismogeodesy of the 2014 Mw6.1 Napa Earthquake, California: Rapid Response and Modeling of Fast Rupture on a Dipping Strike-slip Fault

Bill C. Hammond, Richard M. Allen, Yehuda Bock, Jennifer S. Haase, Jianghui Geng, Brendan W. Crowell, & Diego Melgar

Published July 14, 2015, SCEC Contribution #6039

Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 Mw6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

Key Words
Seismogeodesy, Source scaling, Moment tensor, Slip Inversion, Surface rupture

Citation
Hammond, B. C., Allen, R. M., Bock, Y., Haase, J. S., Geng, J., Crowell, B. W., & Melgar, D. (2015). Seismogeodesy of the 2014 Mw6.1 Napa Earthquake, California: Rapid Response and Modeling of Fast Rupture on a Dipping Strike-slip Fault. Journal of Geophysical Research: Solid Earth, 120(7), 5013-5033. doi: 10.1002/2015JB011921.

Related Projects & Working Groups
Tectonic Geodesy