Seismic Swarm S20100517.1 Near Progreso, Baja California: Geological Context and Event Analysis

The seismic swarm designated S20100517.1 occurred approximately 14 km south of Progreso in Baja California, Mexico. It began at 01:00 on 16 May 2010 and concluded at 12:49 on 8 June 2010, spanning 563 hours and 48 minutes. During this period, 365 earthquakes were recorded. This activity aligns with the region's established pattern of episodic seismic swarms driven by tectonic processes at the Pacific-North American plate boundary.

Baja California lies within a tectonically complex zone where the Pacific Plate interacts with the North American Plate through a series of right-lateral strike-slip faults and transtensional basins. The Progreso area sits near the southern extensions of major fault systems, including segments associated with the Agua Blanca and Cerro Prieto faults. These structures accommodate oblique extension linked to the opening of the Gulf of California. Shallow crustal seismicity predominates, with most events occurring at depths of 0–20 km, consistent with brittle failure in the upper crust.

Historical records indicate six swarms in the region since 1 January 2000. These occurred in 2000 (one swarm), 2001 (one swarm), and 2010 (four swarms). Such recurrent swarms reflect periodic fluid migration or stress perturbations along fault networks rather than mainshock-aftershock sequences.

Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from 0.9 to 3.6. The majority clustered between 1.5 and 2.5, with only isolated events exceeding 3.0. Depths remained shallow, averaging 8–10 km and rarely surpassing 19 km. Early events on 16 May showed rapid succession and variable depths from 0 to 10 km. By 17–20 May, magnitudes trended slightly higher, reaching 3.2, while depths stabilized around 7–14 km. From 21–23 May, the sequence exhibited more frequent events above magnitude 2.0, with depths extending to 18 km in several cases. This progression suggests initial diffuse fracturing followed by focused slip on deeper fault segments.

No larger mainshock preceded or followed the swarm, underscoring its character as a swarm-type sequence. The data indicate sustained low-level energy release without escalation to damaging levels.

References

USGS Earthquake Hazards Program regional tectonics summaries for Baja California.
Servicio Sismológico Nacional (Mexico) historical seismicity reports.
SeismoSight internal swarm classification records.