Seismic Swarm S20040612.1 Near Truckee, California

A seismic swarm designated S20040612.1 was recorded 8 km NNW of Truckee, California, beginning at 07:34 on 12 June 2004 and concluding at 03:43 on 14 June 2004. Over 44 hours and 9 minutes, the sequence produced 35 earthquakes. The events clustered tightly in both time and space, consistent with swarm behavior rather than a mainshock-aftershock sequence.

Event magnitudes ranged from -0.4 to 3.6, with the two largest shocks (3.6 and 3.4) occurring within minutes of each other on the afternoon of 12 June. Hypocenters were shallow, mostly between 0 km and 11 km depth, indicating activity within the brittle upper crust. The swarm exhibited typical swarm characteristics: rapid onset, fluctuating rates, and no single dominant event controlling the sequence.

The Truckee region occupies the northern Sierra Nevada, near the transition to the Basin and Range province. Tectonic deformation here is driven by northwest-directed shear and extension along the Walker Lane belt, a zone accommodating a portion of Pacific-North American plate motion. Active faults, including segments of the Tahoe-Truckee system and related normal faults, produce recurrent small-magnitude seismicity. Historical records document moderate earthquakes in the broader Lake Tahoe-Truckee area, underscoring ongoing tectonic strain accumulation.

Since 1 January 2000, four swarms have been identified in the immediate vicinity. Three occurred in 2003 and one in 2004, indicating episodic swarm activity superimposed on background seismicity. Such swarms are commonly attributed to fluid migration or aseismic slip transients that transiently elevate pore pressures and trigger brittle failure on favorably oriented faults.

The 2004 sequence fits this regional pattern. Its shallow depths and low-to-moderate magnitudes align with the structural style of normal and strike-slip faulting prevalent in the northern Sierra Nevada. No damage or felt reports of significance were associated with the largest events, consistent with their modest sizes and rural epicentral location.

Continued monitoring of the Truckee area remains important for understanding how swarm sequences relate to longer-term strain release along the Walker Lane. The 2004 swarm provides a useful case study of clustered microseismicity in this tectonically active transition zone.

References

• U.S. Geological Survey Earthquake Catalog (historical data for Truckee, CA region)
• California Geological Survey, Regional Fault and Seismicity Reports
• SeismoSight internal swarm classification records (S20040612.1 parameters)