Microseismic activity reveals two stress regimes in southwestern Puerto Rico

Abstract

The seismic activity in the local region of Puerto Rico, lat 17°N–20°N, long 63.5°W–69°W, is the result of the interaction of the North America and Caribbean plates and the response of the Puerto Rico–Virgin Islands microplate. This seismic activity is distributed over a broad region with slightly higher activity concentrated around known fracture zones and prominent topographic features. Within the Island of Puerto Rico, the highest level of onshore shallow seismicity (≤25 km depth) is concentrated to the south of the Great Southern Puerto Rico Fault zone in the southwestern part of the island. We conducted a detailed analysis of hypocenters of 828 shallow quakes (depth ≤50 km) provided by the Puerto Rico Seismic Network (PRSN) and located within the southwestern Puerto Rico seismic zone to determine the stress patterns of this complex area and their relationship with tectonism and local geology. A suite of fi rst motion composite focal mechanisms was computed based on the distribution of seismicity, geology, and topographic features. On average, our solutions resemble the expected stress patterns obtained from regional plate motion and/ or geophysical studies that indicate NW-SE stress fi elds associated with the regime of extension in the Mona Passage and the oblique convergence of the major plates. However, there are local topographic and geologic features like Cordillera Central, Monte Grande, and Sierra Bermeja that are best explained in terms of internal deformations of the southwestern Puerto Rico seismic zone.