Seismic Swarm S20120215.1: Analysis of Activity Near Swall Meadows, California

Seismic swarm S20120215.1 occurred 19 km ENE of Swall Meadows, California, in the tectonically active eastern Sierra Nevada region. The sequence began at 19:43 on 14 February 2012 and concluded at 19:05 on 16 February 2012, spanning 47 hours and 21 minutes. During this period, 33 earthquakes were recorded, with magnitudes ranging from 0.3 to 2.8 and focal depths predominantly between 6 and 9 km.

The swarm exhibited typical characteristics of clustered seismicity in volcanic and extensional terranes. The largest event, magnitude 2.8 at 8 km depth, initiated the sequence, followed by smaller events that showed no clear mainshock-aftershock decay pattern. Depths clustered around 7–8 km, consistent with brittle failure in the upper crust overlying magmatic or hydrothermal systems. Two magnitude 2.4 events occurred later in the sequence, but overall energy release remained modest.

Geologically, the location lies on the western margin of the Basin and Range province, adjacent to the Long Valley Caldera. This area experiences ongoing extension and is underlain by Quaternary volcanic rocks, including rhyolitic tuffs and basaltic flows. Crustal thinning and faulting along normal faults accommodate regional tectonics, while residual heat from Pleistocene volcanism sustains fluid circulation that can trigger swarm activity through pore-pressure changes.

Historical records indicate three swarms in the region since 1 January 2000. The earlier episodes occurred in 2003 (one swarm) and 2007 (two swarms), suggesting episodic unrest linked to the same structural and magmatic framework. Such recurrent swarms are common in the Long Valley area and are monitored for potential escalation into larger tectonic or volcanic events.

Analysis of the 2012 sequence shows 79 percent of events at depths of 7–9 km, indicating a narrow seismogenic zone. Magnitudes followed a Gutenberg-Richter distribution typical of swarm behavior, with b-values near 1.0. No surface rupture or felt reports of significant intensity were associated with the activity.

This swarm provides insight into the persistent low-level seismicity that characterizes the Swall Meadows vicinity. Continued monitoring remains essential given the proximity to populated areas and infrastructure in Mono County.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for Long Valley Caldera region.
California Geological Survey Quaternary fault and fold database.