Fractal Seismicity and Coulomb Stress Pattern Analysis for 7 January 2020 Mw 6.4 Puerto Rico Earthquake

Abstract

Abstract This study aims to understand the variation of the correlation fractal dimension and static stress changes associated with the 2019 Puerto Rico sequence with special reference to the January 2020 Mw 6.4 mainshock. On examining the seismicity data from 1988 to 2020, we find a shift in the seismicity pattern after the 2019 Puerto Rico sequence. A detailed examination of changes in the correlation fractal dimension (Dc) indicates diffused seismicity throughout the study region until the year 2003 that is demonstrated by relatively high Dc values. Toward the end of 2019, a drastic drop in Dc is observed; it is associated with a tight clustering of events to the south-southwest of Puerto Rico Island (PRI), suggesting the onset of the 2019 sequence. We observed a persistent clustering and significant drop in Dc before the Mw 6.4 earthquake, hinting at the presence of a numerical precursor for this event. Over 30 yr of historical earthquakes have contributed positive Coulomb stress distributions mainly in the northern region of PRI. A deflection in this pattern was reflected after the 2019 Puerto Rico sequence. Based on the static stress analysis, we demonstrated that the Mw 6.4 mainshock was triggered by the 2019 Puerto Rico sequence. The new cluster of events, which contains the Mw 6.4 mainshock, exhibits higher stress levels in the south-southwest region of PRI. The eastern end of Septentrional fault (SF), the zone between SF and North Puerto Rico Slope fault, the middle segment of South Puerto Rico Slope fault, South Lajas fault, Punta Montalva fault, and Central fault are the highly stressed regions at present. Stress perturbation induced by the destructive Mw 6.4 event on the adjacent fault system is a potential threat for PRI, where population density is very high. The combined analysis of Dc and static stress changes indicates the presence of a numerical precursor and highly stressed regions southwest of PRI.