Seismic Swarm Near Carnelian Bay, California: The April 2014 Event and Regional Context

A seismic swarm designated S20140404.2 was recorded 1 km NNW of Carnelian Bay, California, on the north shore of Lake Tahoe. The sequence began at 21:26 on 3 April 2014 and concluded at 11:06 on 5 April 2014, encompassing 40 earthquakes over 37 hours and 39 minutes. Magnitudes ranged from -0.9 to 2.7, with the largest event occurring at 05:27 on 4 April. Focal depths clustered primarily between 2 km and 11 km, indicating shallow crustal activity typical of the region.

The swarm exhibited classic characteristics of clustered seismicity, with events occurring in rapid succession without a single dominant mainshock-aftershock decay pattern. Early activity included small events near 0.7–1.8 magnitude at depths of 3–7 km, followed by a peak at magnitude 2.7. Subsequent events remained predominantly below magnitude 1.0, with occasional spikes to 1.9 and 0.8. Depths showed modest variation, remaining within the upper 15 km of crust and consistent with fault slip along local normal faults.

Carnelian Bay lies within the Lake Tahoe basin, a region shaped by Cenozoic extension linked to the Basin and Range Province and the northern Walker Lane belt. Active normal faults, including strands of the North Tahoe fault and Tahoe-Sierra frontal fault system, accommodate ongoing east-west extension. These structures have produced the basin's characteristic bathymetry and control the distribution of microseismicity. The Sierra Nevada block to the west experiences uplift relative to the subsiding basin, generating recurrent small-magnitude earthquakes.

Since 2000, fifteen swarms have been documented in the immediate area, occurring in 2003 (3), 2004 (2), 2005 (4), 2007 (1), 2008 (1), 2010 (1), and 2012 (3). This recurrence underscores the persistent low-level tectonic strain release along the same fault network. No damaging events have been associated with these swarms, which typically release energy equivalent to a single moderate earthquake distributed over hours to days.

The April 2014 swarm fits within this established pattern of episodic activity. Such sequences are thought to result from fluid migration or aseismic slip triggering brittle failure on favorably oriented faults. Monitoring by regional seismic networks continues to refine understanding of these processes and their relation to larger regional hazards.

References

USGS Earthquake Hazards Program
California Geological Survey
Nevada Seismological Laboratory historical catalogs