Seismic Swarm S20090916.1 Near Hebgen Lake Estates, Montana

Seismic swarm S20090916.1 was recorded 5 km south-southeast of Hebgen Lake Estates, Montana. The sequence began at 04:54 on 16 September 2009 and ended at 12:13 on 17 September 2009, lasting 31 hours and 18 minutes. During this interval, 28 earthquakes were detected.

Magnitudes ranged from −0.8 to 1.5, with focal depths between 1 km and 12 km. The largest event reached magnitude 1.5 at 10:40 on 16 September at a depth of 10 km. Multiple events clustered between 08:20 and 10:43 on 16 September, with several registering magnitudes near or above 1.0. Later activity on 17 September included two final shocks at depths of 12 km and 10 km.

The swarm occurred in a region of persistent low-level seismicity within the Intermountain Seismic Belt. This belt extends from the Rocky Mountains into the Basin and Range province and is defined by active normal faulting and crustal extension. The Hebgen Lake area lies near the intersection of the Hebgen Lake fault and related structures west of the Yellowstone volcanic field. Historical deformation in the zone is dominated by east-west extension accommodated along north- to northwest-striking faults.

The 1959 Hebgen Lake earthquake (magnitude 7.3) remains the largest instrumentally recorded event in the immediate vicinity. That quake produced surface rupture along the Hebgen Lake and Red Canyon faults and triggered extensive landsliding. Post-1959 aftershock sequences and subsequent swarms demonstrate that the fault system continues to release strain through both isolated events and episodic clusters.

Since 1 January 2000, 37 swarms have been identified in the same locale. Annual counts vary: six swarms occurred in both 2000 and 2008, five in 2002, four each in 2001 and 2006, three each in 2003, 2007, and 2009, two in 2004, and one in 2005. These episodic clusters typically lack a single dominant mainshock and instead comprise numerous events of similar magnitude distributed over hours to days.

Swarm characteristics in this setting are consistent with fluid migration or aseismic slip along pre-existing faults within an actively extending crust. Depths predominantly between 5 km and 10 km align with the brittle-ductile transition zone inferred for the region from regional velocity models.

References
USGS Earthquake Hazards Program catalog (events 2000–2009)
Montana Bureau of Mines and Geology seismic network reports
Published studies on the 1959 Hebgen Lake earthquake and Intermountain Seismic Belt tectonics