Seismic Swarm S20121015.1: Analysis of Earthquake Activity East of Island Park, Idaho

The seismic swarm designated S20121015.1 occurred approximately 24 km east of Island Park, Idaho, within a tectonically active zone influenced by the Yellowstone hotspot. The event sequence began at 02:37 on 15 October 2012 and concluded at 03:17 on 16 October 2012, registering 51 earthquakes over 24 hours and 39 minutes. Magnitudes ranged primarily between 0.8 and 3.1, with focal depths mostly between 5 and 16 km, consistent with shallow crustal processes in this volcanic province.

Key events included three magnitude 3.1 earthquakes at 02:54:52, 03:20:45, and 04:02:15 on 15 October, accompanied by multiple events of magnitude 2.5–2.7. Depths showed variation, with some shallower occurrences near 3–6 km potentially linked to fluid migration or minor fault adjustments, while deeper events around 15–20 km reflect broader tectonic stress release. The swarm exhibited a typical pattern of clustered activity without a single dominant mainshock, characteristic of volcanic or hydrothermal influences rather than purely tectonic rupture.

This region forms part of the eastern Snake River Plain, a northeast-trending volcanic province shaped by the Yellowstone hotspot's passage over the past 16 million years. The underlying geology features extensive rhyolitic and basaltic flows, with the Island Park area situated near the margins of the Island Park Caldera, a Pleistocene feature associated with the larger Yellowstone volcanic system. Seismic activity here often arises from interactions between hotspot-driven magmatism, regional extension along the Intermountain Seismic Belt, and hydrothermal fluid dynamics. Historical monitoring indicates persistent low-level seismicity tied to these processes, with swarms representing episodic releases of accumulated strain.

Since 1 January 2000, twelve such swarms have been recorded in the vicinity. Earlier episodes occurred in 2001 (1 swarm), 2003 (2 swarms), 2006 (5 swarms), 2008 (3 swarms), and 2010 (1 swarm). These recurrent patterns underscore the area's elevated seismic productivity compared to stable continental interiors, driven by ongoing mantle plume effects and crustal thinning.

The 2012 swarm aligns with established monitoring frameworks for the Yellowstone region, where similar sequences have informed assessments of volcanic unrest. Depths and magnitudes remained well below thresholds for surface damage, emphasizing the swarm's role as a background indicator of subsurface dynamics rather than an immediate hazard precursor.

References

• U.S. Geological Survey Earthquake Catalog (earthquake.usgs.gov)
• Yellowstone Volcano Observatory reports on regional seismicity
• Idaho Geological Survey publications on Snake River Plain tectonics