Seismic Swarm S20190706.5 Near Trona, California: July 2019 Earthquake Sequence

The seismic swarm designated S20190706.5 occurred in the Searles Valley region of California, centered approximately 24 km north-northeast of Trona. This sequence began at 08:02 UTC on 6 July 2019 and concluded at 00:24 UTC on 15 July 2019, spanning 208 hours and 22 minutes. During this interval, 302 earthquakes were recorded, providing a detailed record of clustered seismic activity in an area known for episodic swarm behavior.

The first 100 events exhibited magnitudes predominantly between 0.8 and 3.3, with the majority falling in the 1.5–2.5 range. Depths varied from surface levels near 0 km to a maximum of 19 km, though most clustered between 2 km and 10 km. Early activity on 6 July included several events above magnitude 2.5, such as a 3.2 quake at 12:40 and a 3.3 event at 17:09, both at depths under 12 km. Subsequent days showed a gradual decline in peak magnitudes, with events remaining shallow and frequent through 8 July. This pattern reflects typical swarm characteristics, where energy release occurs through numerous moderate events rather than a single dominant shock.

The Trona area lies within the Mojave Desert portion of the Eastern California Shear Zone, a tectonically active region accommodating right-lateral shear between the Pacific and North American plates. Local geology features Precambrian basement rocks overlain by Paleozoic sedimentary sequences and Quaternary alluvial deposits associated with Searles Lake. Faulting in the vicinity includes segments of the Garlock Fault system to the south and distributed northwest-trending structures that facilitate swarm activity through fluid migration and stress transfer.

Historical records indicate 12 swarms in the region since 2000, occurring in 2000, 2002, 2008, 2010, 2012, 2013, 2014, 2015, 2016, and three separate episodes in 2019. These recurrent swarms highlight the area's propensity for clustered seismicity without producing large mainshock-aftershock sequences.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Regional Fault Maps
SeismoSight Internal Swarm Classification Records