Seismic Swarm S20011202.1: Analysis of Activity Along the Idaho-Montana Border

The Idaho-Montana border region forms part of the northern Intermountain Seismic Belt, where extensional tectonics associated with the Basin and Range Province drive recurrent shallow crustal earthquakes. This tectonic setting produces normal faulting and swarm-type seismicity, with events typically occurring at depths of 5–10 km. Historical records document elevated swarm frequency in the area since systematic monitoring began.

SeismoSight internal classification identifies Swarm S20011202.1 as a distinct sequence that initiated at 02:39 on 2 December 2001 and concluded at 03:28 on 7 December 2001. Within 120 hours and 48 minutes, the swarm comprised 131 earthquakes. Analysis of the first 100 events reveals predominantly microseismic activity, with magnitudes ranging from –0.2 to 2.5 and focal depths clustered between 5 and 10 km. The largest event reached magnitude 2.5 at 13:18 on 4 December 2001. Temporal distribution shows peak rates during the first 48 hours, followed by a gradual decline, consistent with fluid-driven swarm behavior rather than a classic mainshock-aftershock sequence.

Regional context indicates that the Idaho-Montana border has experienced multiple swarms in recent monitoring history. Since 1 January 2000, eleven swarms have been recorded, including nine in 2000 and two in 2001. These episodes underscore the persistent low-level seismic hazard in this tectonically active corridor.

The shallow depths and low magnitudes observed in S20011202.1 align with the broader pattern of induced or naturally triggered swarm activity along Basin and Range normal faults. No significant surface rupture or damage was associated with this sequence.

References

• SeismoSight internal swarm classification database
• USGS Earthquake Hazards Program regional tectonic summaries (Intermountain Seismic Belt)