The 2019–2020 Southwest Puerto Rico Earthquake Sequence: Seismicity and Faulting

Abstract

The 2019–2020 Southwest Puerto Rico earthquake sequence ruptured multiple faults with several moderate magnitude earthquakes. Here, we investigate the seismotecton- ics of this fault system using high-precision hypocenter relocation and inversion of the near-field strong motions of the five largest events in the sequence (5:6 ≤ Mw ≤ 6:4) for kinematic rupture models. The Mw 6.4 mainshock occurred on a northeast-striking, southeast-dipping normal fault. The rupture nucleated offshore ∼ 15 km southeast of Indios at the depth of 8.6 km and extended southwest–northeast and up-dip with an average speed of 1.55 km/s, reaching the seafloor and shoreline after about 8 s. The 6 January 2020 (10:32:23) Mw 5.7 and the 7 January 2020 (11:18:46) Mw 5.8 events occurred on two east–southeast-striking, near-vertical, left-lateral strike-slip faults. However, the 7 January 2020 (08:34:05) Mw 5.6 normal-faulting aftershock, which occurred only 10 min after the Mw 6.4 normal-faulting mainshock, ruptured on a fault with almost the same strike as the mainshock but situated ∼ 8 km farther east, forming a set of parallel faults in the fault system. On 11 January 2020, an Mw 6.0 earthquake occurred on a north–northeast-striking, westing-dipping fault, orthogonal to the faults hosting the strike-slip earthquakes. We apply template matching for the detection of missed, small-magnitude earthquakes to study the spatial evolution of the main part of the sequence. Using the template-matching results along with Global Positioning System analysis, we image the temporal evolution of a foreshock sequence (Caja swarm). We propose that the swarm and the main sequence were a response to a tec- tonic transient that most affected the whole Puerto Rico Island.