Seismic Swarm S20171015.2: Analysis of Activity Northwest of Benton, California

A seismic swarm designated S20171015.2 was recorded 23 km northwest of Benton, California, beginning at 08:06 on 15 October 2017 and concluding at 00:56 on 16 October 2017. In 16 hours and 49 minutes, the sequence produced 26 earthquakes. Magnitudes ranged from 0.4 to 1.6, with the largest event occurring at 12:06 on 15 October at a depth of 5 km. Depths for the full set of events spanned 3–13 km, though the majority clustered between 7 and 9 km. The sequence exhibited typical swarm characteristics: no single dominant mainshock, a rapid onset followed by a gradual decline in rate, and events distributed across a compact source volume.

The swarm unfolded in two main phases. The first 90 minutes after initiation contained six events, including two of magnitude 1.4. Activity continued at a moderate rate through the morning, with an additional magnitude-1.6 event at 12:06. The afternoon and evening produced smaller events at irregular intervals, and the final recorded earthquake (magnitude 1.4 at 8 km depth) marked the swarm’s termination just before 01:00 on 16 October.

The region northwest of Benton lies within the Mina deflection segment of the Walker Lane belt, a northwest-trending zone of distributed right-lateral shear that accommodates a portion of Pacific–North America relative plate motion. Crustal deformation here occurs primarily through normal and strike-slip faulting on Quaternary structures. Seismicity is shallow, generally less than 15 km, consistent with the brittle–ductile transition in this tectonically active corridor. The 2017 swarm depths align with this pattern and with background seismicity documented throughout the central Walker Lane.

Since 1 January 2000, ten swarms have been identified in the same source area. These occurred in 2001 (two swarms), 2004 (two), 2006 (one), 2008 (two), 2009 (one), and 2015 (two). The recurrence of swarm-type sequences rather than isolated mainshock–aftershock sequences reflects the influence of fluid migration and aseismic slip transients along favorably oriented faults.

The October 2017 swarm fits the established pattern of low-magnitude, short-duration clusters that characterize the area’s background tectonic regime. No felt reports or damage were associated with the events, and no escalation into a larger mainshock was observed. Continued monitoring remains warranted given the region’s position within an active transtensional domain.

References
U.S. Geological Survey Earthquake Catalog (ComCat)
California Geological Survey, Quaternary Fault and Fold Database
Nevada Seismological Laboratory regional seismicity reports
Slemmons, D.B., et al., 2001, “Seismotectonics of the Walker Lane,” Geological Society of America Special Paper 353.