Analysis of Two Magnitude ~4 Earthquakes and Aftershocks Near Truckee, California, 2017

Rachel L. Hatch, Ken D. Smith, & Rachel E. Abercrombie

Published August 15, 2017, SCEC Contribution #7852, 2017 SCEC Annual Meeting Poster #058

On June 27th, 2017, two magnitude ~4 high-angle strike-slip faulting earthquakes occurred seven minutes apart approximately 20 km north of Truckee, CA. Both events were felt over a wide area in northeastern California and northwestern Nevada. The sequence took place within a left-step between the southern extent of the Mohawk Valley and northern terminus of the Polaris fault zones. Both faults pose a significant hazard to the Truckee-Tahoe-Reno areas. Our aim is to apply data from the dense Nevada Seismological Laboratory (NN) network in the region to better understand smaller sequences and advance our knowledge of structures and source properties typical of the area. We develop a set of high-precision locations and estimate the source parameters of all events > M 2. In addition, we estimate the rupture directivity for the two M ~4s. Using both absolute and relative (GrowClust; Trugman and Shearer, 2017) relocations, we successfully relocate 50 of the ~70 events that have occurred in the sequence to date, with an average location error of < 50 m. Relocations show a linear distribution of events confined to a small area at about 5-6 km depth, trending ~N40E, and dipping to the northwest ~65-75°. This trend matches both M ~4 moment tensor solutions (NSL) and several HASH (Hardebeck and Shearer, 2002) focal mechanisms indicating sinistral strike-slip motion. We follow the EGF approach of Abercrombie et al. (2017) to calculate spectral ratios and stress drops for 9 of the events in the sequence. The EGF approach also enables us to calculate source time functions at surrounding stations to observe azimuthal variation in source duration and constrain possible rupture directivity. Stress drop results show an average stress drop of 4 MPa and 15 MPa for P and S waves, respectively; moreover, we see spatial and temporal differences in stress drop within this sequence. First-order observations suggest potential components of directivity towards the NE for the foreshock (M 3.95) at the southwest extent of the sequence area, and directivity towards the SW for the mainshock (M 4.11) at its northeast extent, essentially bracketing the ~ 1 km rupture area. This analysis illustrates details in source properties and rupture propagation estimates that can be derived with high-precision event locations within dense regional networks.

Key Words
earthquake relocation, GrowClust, source properties, stress drop, directivity, Walker Lane

Hatch, R. L., Smith, K. D., & Abercrombie, R. E. (2017, 08). Analysis of Two Magnitude ~4 Earthquakes and Aftershocks Near Truckee, California, 2017. Poster Presentation at 2017 SCEC Annual Meeting.

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