Recent studies emphasize the rapid assessment of earthquake source properties, such as moment magnitude, to help alleviate the impact of earthquakes. Depending on local crustal structure, earthquakes occurring at different depths can differ greatly in high‐frequency motions, which emphasizes the importance in constraining focal depth for the predictions of strong motions. For large earthquakes, assessing rupture directivity is also essential in estimating ground‐motion effects throughout the source region. In this article, we perform an in‐depth study on a group of recent earthquakes near the intersection of the San Jacinto and San Andreas fault systems in southern California. We develop a systematic method to accurately estimate moment magnitude and focal mechanism within 3–6 s after the first P arrival. Focal depth can also be constrained within ∼10  s upon the arrival of S waves. To determine the direction of fault rupture, we implement a forward‐modeling method, which takes smaller earthquake recordings as empirical Green’s functions to simulate the rupture direction of the beginning motion generated by larger events. With a small event nearby, we resolve the rupture characteristic of the 2014 Mw 4.4 event using information at stations within 35 km from the epicenters and successfully predict the ground‐motion response at stations at farther distances, where directivity effect is significant. Rupture direction of simulated earthquakes with larger magnitudes can also be accurately resolved using the method proposed, opening a possibility to predict ground motions ahead of time, in particular for hazardous regions.

Online Material: Figures of inversion results, waveform fits, directivity, and ground‐motion prediction analysis; and table of crustal model.

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