Characteristics of Three Small (Mw < 4.5) Urban Area Sequences in the Walker Lane: Earthquake Interaction, Fault Structure, and Source Properties

Rachel L. Hatch, Rachel E. Abercrombie, Christine J. Ruhl, & Ken D. Smith

Published August 15, 2018, SCEC Contribution #8720, 2018 SCEC Annual Meeting Poster #076

We analyze three well-recorded small earthquake sequences (2015 Thomas Creek; 2017 Truckee; 2014 Virginia City) within urban areas in the Walker Lane region. Using absolute and relative relocation along with source parameter analysis, we detect fault structures, stress drop, directivity, and observe distinct event migration, including fluid and aseismic rates.

In the 2017 Truckee sequence, two events (Mw 3.65 foreshock and Mw 3.85 mainshock), occurred north of Truckee, California, on 27 June 2017. We relocate 50 out of 52 earthquakes within the sequence, which define a single structure between 5 and 6 km depth, trending ∼N45°E and dipping ∼70°–80° to the northwest. The structure matches the northeast-striking plane from two moment tensor solutions and computed first-motion focal mechanisms, indicating sinistral strike-slip motion. We observe stress drops averaging ∼5 MPa 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, and directivity toward the southwest for the mainshock, both aligning with the fault plane.

The Virginia City, Nevada earthquake sequence (ML ≤ 3.2) began in early 2014, with the main part of the sequence occurring over a ~10 day period in late January. We are able to relocate 366 of the 429 catalog events, which reveal three well-defined planar structures spanning ~1 km, ranging from 8-10 km in depth. Observations show a clear migration of seismicity, from east to west, with each structure activating in turn. Stress drops for 12 of the events average ~3.5 MPa. Key observations of event migration and stress drop suggest that fluids and aseismic movement play a role within this sequence.
The 2015 Thomas Creek occurred in south Reno, NV on 22 Dec, 2015 with a period of foreshocks lasting nearly 30 minutes prior to the Mw 4.3 mainshock. We relocate 156 or the 202 detected events in the sequence, which give evidence for a west-dipping structure, interpreted to project to the surface on the west side of the Virginia Range along the eastern boundary of the Reno basin. Average stress drop for 14 events in the sequence is ~8 MPa, and we detect directivity for several events.

This analysis demonstrates the ability to extract details in fault and source properties, and rupture propagation for small sequences. These data can provide a more informed seismic hazard assessment for these urban areas.

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

Citation
Hatch, R. L., Abercrombie, R. E., Ruhl, C. J., & Smith, K. D. (2018, 08). Characteristics of Three Small (Mw < 4.5) Urban Area Sequences in the Walker Lane: Earthquake Interaction, Fault Structure, and Source Properties. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology