Seismic Swarm S20090913.1 Near Hebgen Lake Estates, Montana

Seismic swarm S20090913.1 was recorded 7 km east-southeast of Hebgen Lake Estates, Montana. The sequence began at 14:56 on 12 September 2009 and concluded at 12:19 on 13 September 2009, spanning 21 hours and 22 minutes. During this interval, 24 earthquakes were registered.

The swarm occurred within the Intermountain Seismic Belt, a tectonically active zone characterized by extensional faulting associated with the Basin and Range province. The Hebgen Lake region experienced one of the largest historic earthquakes in the contiguous United States on 17 August 1959, a magnitude 7.3 event that produced extensive surface rupture along the Hebgen Lake fault and triggered a major landslide. Ongoing low-level seismicity reflects continued slip on normal faults in this setting.

The 24 events displayed magnitudes between -0.6 and 1.0, with focal depths predominantly between 2 km and 13 km. The largest shock reached magnitude 1.0 at 20:22:43 on 12 September at 13 km depth. Multiple events clustered near 20:22–20:23 UTC on 12 September and again near 00:27 on 13 September, illustrating typical swarm behavior in which numerous small earthquakes occur without a single dominant mainshock.

Since 1 January 2000, 39 swarms have been identified in the area. Annual counts include eight in 2000, four in 2001, five in 2002, three in 2003, two in 2004, one in 2005, four in 2006, three in 2007, six in 2008, and three in 2009. These recurrent swarms underscore persistent microseismicity along the same fault system responsible for the 1959 rupture.

The modest magnitudes and shallow-to-moderate depths recorded in swarm S20090913.1 are consistent with background activity observed throughout the Hebgen Lake seismic zone. No damage or felt reports were associated with this particular sequence.

References

• U.S. Geological Survey Earthquake Catalog
• Montana Bureau of Mines and Geology
• SeismoSight internal swarm classification records