Abstract

We use relocation and source parameter analysis, including stress drop and directivity, to investigate fault structure and earthquake interaction of small‐magnitude earthquakes within a sequence in the Walker Lane tectonic region. Two high‐angle, left‐lateral strike‐slip earthquakes (Mw 3.65 foreshock and Mw 3.85 mainshock), occurred 7 min apart, 20  km north of Truckee, California, on 27 June 2017. Both events were felt over a wide area in northeastern California and northwestern Nevada and happened nearby Holocene fault zones including the Polaris, Mohawk Valley, and Dog Valley Quaternary fault zones that pose a significant hazard to the populated regions in the area. We use waveform cross correlation to relocate the earthquakes and empirical Green’s‐function methods to estimate the source parameters of all ML>2 events. Also, we estimate the rupture directivity for the two largest events. We relocate 50 out of 52 earthquakes within the sequence, with an average relative error of <30  m. The events define a single structure between 5 and 6 km depth, trending N45°E and dipping 70°80° to the northwest. The distribution of relocations matches the northeast‐striking plane from both the moment tensor solutions and computed first‐motion focal mechanisms, indicating sinistral strike‐slip motion on a previously unmapped fault. We observe average stress drops of 5  MPa using P and S waves and spatial variation related to the rupture areas of the foreshock and mainshock. We are able to detect components of directivity toward the northeast for the foreshock (Mw 3.65) and directivity toward the southwest for the mainshock (Mw 3.85), both aligning with the fault plane. This analysis illustrates details in source properties and rupture propagation that can be derived with high‐precision event locations within dense regional networks and provides more data and a better understanding as it relates to potential seismic hazard.

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