Seismic Swarm S20250411.1 Near Johannesburg, California

A seismic swarm designated S20250411.1 was recorded 16 km west of Johannesburg, California, beginning at 16:24 on 10 April 2025 and concluding at 08:43 on 13 April 2025. Over 64 hours and 19 minutes, the swarm produced 43 earthquakes. Magnitudes ranged from 0.7 to 2.4, with the majority of events occurring at focal depths of 5–8 km. The largest event reached magnitude 2.4 at 20:43 on 12 April 2025.

The sequence displayed typical swarm characteristics, featuring multiple events of similar magnitude without a dominant mainshock. Notable magnitude-2.0+ events occurred at 23:05 on 10 April, 02:28 on 11 April, 14:12 on 11 April, 20:43 on 12 April, and 03:39 on 13 April. Depths remained relatively consistent, clustering between 6 km and 8 km for most shocks, indicating activity within the shallow brittle crust.

Johannesburg lies in the Mojave Desert portion of Kern County, within the Eastern California Shear Zone. This region experiences distributed right-lateral shear between the San Andreas Fault to the southwest and the Basin and Range province to the northeast. The local geology is dominated by Precambrian to Mesozoic crystalline basement rocks overlain by Quaternary alluvium and volcanic units. Active faulting is accommodated along northwest-trending strike-slip structures that form part of the broader plate-boundary deformation system.

Since 1 January 2000, four prior swarms have been documented in the immediate area: one in 2009, one in 2016, and two in 2019. The 2025 swarm represents the fifth such episode, underscoring recurrent low-magnitude clustered seismicity in this portion of the shear zone.

References
SeismoSight internal swarm classification S20250411.1
USGS Earthquake Catalog (queried through April 2025)
California Geological Survey fault database