Seismic Swarm S20181217.1: Analysis of Activity Near Mono City, California

A seismic swarm designated S20181217.1 was recorded 22 km east of Mono City, California, beginning at 04:26 on 17 December 2018 and concluding at 14:03 on 19 December 2018. Over 57 hours and 37 minutes, the sequence comprised 44 earthquakes. Magnitudes ranged from 0.6 to 2.5, with the largest event occurring on 17 December at 11:26. Focal depths varied between the surface and 18 km, indicating a mix of shallow crustal and mid-crustal sources consistent with regional faulting patterns.

The swarm exhibited typical characteristics of clustered, low-to-moderate magnitude events without a single dominant mainshock. Early activity on 17 December featured several events above magnitude 2.0, including shocks of 2.4 and 2.3, followed by a gradual decline in both frequency and size through 18 and 19 December. Depths showed no strong systematic migration, though many events clustered between 4 km and 12 km, suggesting activation along distributed fractures rather than a single through-going fault plane.

This episode fits within a longer-term pattern of swarm activity in the Mono Basin. Since 2000, nineteen swarms have been documented in the immediate region, occurring in 2001 (two), 2004 (two), 2006 (one), 2008 (two), 2009 (one), 2011 (one), 2015 (two), 2016 (two), 2017 (four), and 2018 (two). Such recurrent clustering reflects ongoing tectonic extension and fluid-related triggering common to the western Great Basin.

Geologically, the Mono Basin lies within the Basin and Range province, where northwest-directed extension accommodates motion between the Pacific and North American plates. The area is bounded by the Sierra Nevada frontal fault system to the west and the active faults of the Walker Lane to the east. Mono Lake itself occupies a structural depression formed by normal faulting and volcanic subsidence. The broader region includes the Long Valley Caldera immediately to the south, which formed during a major rhyolitic eruption approximately 760,000 years ago and continues to exhibit episodic unrest linked to magmatic and hydrothermal processes.

Seismic monitoring in this setting benefits from dense networks operated by the U.S. Geological Survey and partner institutions, providing high-resolution catalogs that capture both tectonic and volcanic-related events. Historical records show that while most swarms remain small, they occasionally coincide with measurable ground deformation or changes in geothermal features, underscoring the value of continued surveillance for hazard assessment.

References

U.S. Geological Survey Earthquake Hazards Program, regional seismicity reports for eastern California.
California Geological Survey, geologic maps and fault activity summaries for Mono County.
Long Valley Observatory, USGS, volcanic and seismic monitoring updates through 2023.