Seismic Swarm S20160810.1: Analysis of Activity Near Upper Lake, California

A seismic swarm designated S20160810.1 occurred in the region 19 km northeast of Upper Lake, California, between 21:19 on 9 August 2016 and 05:14 on 13 August 2016. Over 79 hours and 54 minutes, the sequence registered 61 earthquakes. This event provides insight into localized seismicity within the northern Coast Ranges, where tectonic stresses interact with regional fault systems.

The swarm initiated with a magnitude 1.8 event at 8 km depth. Activity intensified rapidly, featuring a magnitude 5.0 earthquake at 02:57 on 10 August at 14 km depth. Subsequent events clustered primarily between 5 km and 17 km depth, with magnitudes ranging from 1.4 to 2.9. The largest aftershock reached magnitude 2.9 at 16 km depth shortly after the main event. Depths remained consistent throughout, indicating a compact source volume without significant migration.

Temporal distribution showed peak frequency in the first 24 hours following the magnitude 5.0 shock, with events occurring at intervals of minutes to hours. Later stages exhibited declining rates, tapering to isolated magnitude 1.7 events by 13 August. This pattern aligns with swarm behavior driven by fluid migration or aseismic slip rather than a classic mainshock-aftershock sequence.

Geologically, the location lies in Lake County within the Coast Ranges province, influenced by northwest-trending strike-slip faults of the San Andreas system. The area features Quaternary volcanism around Clear Lake and geothermal fields that can modulate shallow crustal stresses. Historical records indicate sparse swarm activity, with only one prior swarm documented since 2000, occurring in that year. Such rarity underscores the episodic nature of seismicity here compared to more continuous microseismicity near geothermal operations.

The 2016 swarm's characteristics—moderate peak magnitude, shallow-to-mid crustal depths, and high event count over days—highlight the region's capacity for clustered releases of accumulated strain. No surface rupture was associated, consistent with the modest energy release.

References

• SeismoSight internal swarm classification records for S20160810.1.
• USGS earthquake catalog data for northern California (2000–2016).