Seismic Swarm S20000614.1 Near Imperial, California

Seismic swarm S20000614.1 began at 19:00 on 14 June 2000 and concluded at 11:04 on 17 June 2000, centered 8 km northeast of Imperial, California. Over 64 hours and 4 minutes, the sequence produced 145 earthquakes. Analysis of the first 100 events reveals a rapid onset dominated by events of moderate size followed by a prolonged decay of smaller shocks. The initial event reached magnitude 4.1 at 5 km depth. Subsequent activity included multiple events between magnitudes 2.5 and 3.6, with the largest recorded shock of the early sequence reaching magnitude 4.5 at 10 km depth approximately three hours after onset. Depths clustered between 4 km and 14 km, consistent with shallow crustal faulting.

Magnitudes declined steadily after the first day, with most later events falling below magnitude 2.0. Shallow depths predominated, although occasional events extended to 16 km. The temporal pattern shows intense clustering in the first 12 hours, followed by episodic bursts that diminished in frequency and size. This distribution aligns with typical swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence.

The swarm occurred within the Imperial Valley segment of the Salton Trough, a pull-apart basin formed by oblique extension along the Pacific–North American plate boundary. The region hosts the Imperial Fault, Brawley Seismic Zone, and northern extensions of the Cerro Prieto Fault. These structures accommodate right-lateral shear and northwesterly extension at rates of approximately 35–40 mm per year. Historical records document repeated moderate-to-large earthquakes in the valley, including the 1940 El Centro event (M7.1) and the 1979 Imperial Valley earthquake (M6.4), both of which produced surface rupture along the Imperial Fault. The 2000 swarm lies near the northern terminus of the Imperial Fault, where interaction with the Brawley Seismic Zone commonly generates clustered seismicity.

Recent geodetic and seismic monitoring confirms ongoing strain accumulation and microseismicity throughout the Imperial Valley. Updated fault models from the U.S. Geological Survey place the 2000 swarm within a zone of distributed deformation that links the San Andreas and San Jacinto fault systems to the Gulf of California rift. Depths recorded during the swarm correspond to the brittle–ductile transition zone mapped in regional velocity models.

Seismic swarms in this setting frequently precede or accompany aseismic creep episodes and are influenced by high heat flow and fluid circulation associated with the underlying magmatic and geothermal systems of the Salton Trough. The 2000 sequence exhibited no clear mainshock and lacked significant aftershock migration beyond the initial cluster, reinforcing its classification as a swarm.

References

• U.S. Geological Survey Earthquake Hazards Program, Imperial Valley fault database (2023 update)
• Southern California Earthquake Data Center, regional seismicity catalog
• SCEC Community Fault Model version 5.3
• USGS National Seismic Hazard Model, 2023 release