Examining the Causes of Regional Differences in California Based on Ground Motions from Small to Moderate Earthquakes

ABSTRACT Residuals from ground-motion models (GMMs) for California show systematic differences in the long-period event terms for small-to-moderate magnitude earthquakes (Mw < 5) between Northern and Southern California. The spatial pattern of regional differences in event terms followed the boundaries of the seismic networks, suggesting a systematic difference in magnitudes for small-to-moderate earthquakes between the Northern and Southern California networks. Both networks employ the same workflow defined by the Advanced National Seismic System. Therefore, procedural differences in moment tensor inversion (MTI) are unlikely to be the cause of the observed regional differences in event terms. The two networks utilize different seismic-velocity models in the MTI, with lower velocities in coastal Northern California compared to Southern California. A physics-based analysis indicates that the regional amplification differences among the seismic-velocity models used in the MTI are consistent with the observed differences in the event terms between the two regions, providing evidence that the event terms in ergodic GMMs for California also capture regional wave-propagation differences, not just source differences. This affects the development of GMMs: a better methodology is needed to identify these regional wave propagation patterns from event-specific faulting processes, which are currently mapped in the event-terms. In addition, having several earthquakes in the datasets with magnitudes obtained without MTI is a potential source of regional differences in the event terms. Using earthquakes selected from the ongoing Next Generation Attenuation-West3 project, we conduct MTIs on 188 earthquakes, yielding 63 seismic moment estimates and an additional 43 values adopted directly from the Comprehensive Catalog. These 106 moment magnitude estimates can replace those from earthquakes in the dataset that lack a moment magnitude, reducing reliance on conversion models. We conclude this study by providing recommendations to improve the documentation of the MTI maintained by the seismic network operators.