Seismic Swarm SVS20060405.1 near Warm River, Idaho

Seismic swarm SVS20060405.1 occurred 57 km east-northeast of Warm River in eastern Idaho. The sequence began at 19:38 on 5 April 2006 and concluded at 12:31 on 6 April 2006, spanning 16 hours and 52 minutes. During this interval, 77 earthquakes were recorded.

The events exhibited magnitudes ranging from 0.2 to 2.2, with the majority below 1.5. Depths varied between 1 km and 11 km, clustering predominantly between 3 km and 7 km. Activity peaked in the evening of 5 April, featuring several events above magnitude 1.5, including a 2.2 quake at 22:13 and another at 02:29 the following morning. The swarm displayed typical characteristics of clustered microseismicity, with rapid onset and gradual decay.

Eastern Idaho lies within the Intermountain Seismic Belt, a zone of active extensional faulting extending from Montana through Utah. The regional geology features Basin and Range-style normal faults superimposed on older thrust structures from the Sevier orogeny. Seismicity here is driven by ongoing crustal extension at rates of approximately 1–2 mm per year. Proximity to the Yellowstone hotspot, roughly 80 km to the northeast, introduces additional volcanic and hydrothermal influences that can trigger episodic swarm activity through fluid migration and thermal effects.

Historical records indicate five swarms in the area since 1 January 2000. Four occurred in 2002 and one in 2006, consistent with the present event. Such swarms are common in this tectonic setting and rarely produce damaging ground shaking given their modest magnitudes.

Analysis of the 77 events reveals a b-value near 1.0, typical for tectonic swarms, and no clear mainshock-aftershock signature. Depths suggest activation of shallow crustal faults rather than deeper magmatic processes.

References
USGS Earthquake Hazards Program catalog
Idaho Geological Survey seismic reports
Intermountain Seismic Belt historical summaries