Overview of the tectonic and seismicity in Continental Borderland (offshore) Baja California, México

Abstract

The seismic activity in the Continental Borderland, offshore Baja California, presented a unique opportunity to study a region that is not typically known for high seismic activity, in comparison to the nearby Mexicali Valley. In 2022 and the beginning of 2023, this region experienced unusual activity. Some events caused concern among the population, specifically, two earthquakes occurred near the city of Ensenada on April 15th and July 25th, 2022, with magnitudes of Mw 4.2 and 4.5, respectively. A third event, with Mw6.1, occurred near San Quintin, Baja California, on November 22nd, 2022, and is associated with the San Isidro Fault. This event, surpassing the typical magnitude observed in the region based on historical seismicity, was felt across a significant stretch of the Pacific coast, spanning from Guerrero Negro to Tijuana, México. In June of the same year, a magnitude 4.2 event occurred in the same fault. This fault is a relay for the Santa Catalina Fault in its southeastern sector. The hypocentral locations of the earthquakes varied in depth from 5 to 20 km, mostly occurring between 5 and 20 km. Magnitudes ranged from 1.5 to 6.1. Additionally, continuous offshore seismic activity was observed between Punta Colonet and the town of San Quintin in November–December 2022 and May 2023. The hypocentral locations of these events ranged from depths of 2–18 km, with the majority occurring between 3 and 10 km. Magnitudes ranged from 1.5 to 4.5. This activity appears to be in the form of swarms, as previously reported by other authors. However, the seismic activity observed in May 2023 displays a northeast-southeast trend, with focal mechanisms exhibiting similar strike angles. This leads us to suggest that this seismic activity is associated with the Colonet Fault, which can be observed in the continental region of the coast. Focal mechanisms of major earthquakes indicated strike-slip motion, aligning with the dominant deformation pattern controlled by major faults in the region. A concentration of activity near the coastline becomes apparent upon analyzing seismic activity recorded in various catalogs from agencies such as ISC, RESNOM, USGS, NCEDC, and SCEC. While no reports of moderate inverse mechanism earthquakes have been documented in the Baja California offshore region, they occur in California. Notably, no seismic activity was reported in the area where the Mw6.1 earthquake occurred between 1930 and 2021, suggesting the reactivation of old faults along the continental border, similar to what occurs in the sector of California. One possibility is that the recurrence intervals of these earthquakes are very long, which would explain the lack of historical records. Previous authors have highlighted the potential for large complex ruptures involving combinations of strike-slip and dip-slip faults, particularly exemplified by the 2010 Mw7.1 El Mayor- Cucapah event in northern Baja California. Seismic activity in the region concentrates along the El Descanso, Coronado-Banks, Santa Catalina, Bahía Soledad, and San Isidro faults. This observation is supported by gravimetric and magnetic anomalies found in various structures that have not been previously interpreted, including the newly reported La Chorera fault, based on bathymetry data. Most of the mentioned faults occur in the gravimetric and magnetic anomalies, however, many other structures have not yet been described.