Earthquake Interaction, Fault Structure, and Source Properties of a Small Sequence in 2017 near Truckee, California

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 (M-W 3.65 foreshock and M-W 3.85 main-shock), occurred 7 min apart, similar to 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 M-L > 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 similar to N45 degrees E and dipping similar to 70 degrees-80 degrees 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 similar to 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 (M-w 3.65) and directivity toward the southwest for the mainshock (M-w 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.