Relationship Between Peak and Cumulative Ground Motions from 49 Mw 3–6 Earthquakes in the San Francisco Bay Area

ABSTRACT In this study, we examine the relationship between peak ground velocity (PGV) and cumulative absolute displacement (CAD) for 49 Mw 3–6 earthquakes in the San Francisco Bay Area (SFBA) and gain insight into the spatiotemporal partitioning of seismic energy in ground-motion records with respect to source, path, and site effects. PGV and CAD are positively correlated, but there can be large deviations from the average trend. For example, ground-motion records with either very long duration resonance or short pulse-like motions have higher or lower CAD, respectively, but could have very similar PGV. We perform principal component analysis (PCA) on PGV–CAD for >7000 records in the SFBA with the goal of investigating what influences positive or negative anomalies in cumulative motions. PCA rotates the PGV–CAD data points into two principal components, for which the one with the larger variance, which we call the “primary intensity” represents mostly the distance dependence of ground-motion amplitudes. The other principal component, which we call the “excess motion,” represents the deviation from cumulative motions that would be typical for a ground-motion record with a given PGV. The excess motion is positive in the case of records with long duration ringing and is negative for short duration pulse-like ground motions. We find that excess motion is generally positive at sites in sedimentary basins and in soft sediments around the San Francisco Bay. Excess motion is generally negative in the very near field, as well as at sites on hard bedrock. We discuss the findings here in terms of implications for seismic hazard applications and other wave propagation phenomena.