Seismic Swarm S20070509.1 Near Carnelian Bay, California

The seismic swarm designated S20070509.1 was recorded 0 km southeast of Carnelian Bay, California, on the north shore of Lake Tahoe. Activity began at 12:49 on 8 May 2007 and concluded at 04:50 on 11 May 2007, encompassing a total of 36 earthquakes over 64 hours. This sequence represents a typical low-magnitude swarm event, with events clustered in both time and space.

Magnitudes ranged from -0.2 to 2.6, with the majority falling below 1.0. Depths varied between 0 km and 19 km, though most events occurred between 7 km and 11 km. The largest shock reached magnitude 2.6 at a depth of 7 km on 8 May at 21:20:20, followed shortly by a magnitude 1.9 event at 8 km depth. Subsequent activity showed a gradual decline in both frequency and size, with several negative-magnitude detections indicating the sensitivity of the monitoring network. Depths remained relatively consistent in the mid-crust until the final hours, when a few events were recorded near 15–19 km.

The Lake Tahoe region lies within the tectonically active transition zone between the Sierra Nevada and the Basin and Range province. Regional deformation is accommodated primarily by the north-northwest-trending normal faults of the Tahoe fault system and associated structures of the Walker Lane belt. These faults accommodate extension and dextral shear, producing both discrete large earthquakes and episodic swarm sequences. Historical records document recurrent seismic swarms in the area, consistent with fluid migration along fault zones or aseismic slip triggering brittle failure at shallow to mid-crustal levels.

Since 1 January 2000, nine swarms have been identified in the immediate vicinity. Earlier episodes occurred in 2003 (three swarms), 2004 (two swarms), and 2005 (four swarms). The 2007 sequence fits the established pattern of short-duration, low-magnitude activity that does not culminate in a mainshock exceeding magnitude 3.0. Such swarms contribute to the long-term strain release along the Tahoe basin margins without producing surface rupture.

Ongoing monitoring by regional seismic networks continues to refine understanding of fault connectivity beneath Lake Tahoe. The 2007 swarm underscores the persistent microseismicity that characterizes this portion of the Sierra Nevada frontal fault system and highlights the value of dense station coverage for resolving fine-scale spatiotemporal patterns.

References

SeismoSight internal swarm classification database
USGS Earthquake Hazards Program regional fault maps and historical seismicity catalogs
Nevada Seismological Laboratory Lake Tahoe basin studies