Seismic Source Models for Southwest Puerto Rico in Light of the 2020 Indios Earthquake Sequence

Session: Early Results from the 2020 M6.4 Indios, Puerto Rico Earthquake Sequence
Type: Oral
Date: 4/30/2020
Time: 02:00 PM
Room: 115
Description: 

A seismic source model was developed for Puerto Rico and adjacent regions (LaForge and McCann, 2005). Although the original model was published 15 years ago, it provides a relevant reference for the recent Indios earthquake sequence in southwest Puerto Rico. The model attempts to account for the following observations: 1) 2.4 mm/yr convergence between fixed Caribbean plate and the Puerto Rico-Virgin Islands microplate in a N79E direction; 2) seismically active, highly oblique subduction of the Caribbean plate beneath southwest PR (Muertos subduction zone), which diminishes to the east and ends as the trench disappears at about 65W longitude, where the Anegada Passage basins become the dominant tectonic features; 3) normal-oblique faulting in SW Puerto Rico as evidenced by the Holocene Lajas fault and identification of the Investigator faults, east-west normal-oblique faults cut into the shelf slope just south of the Indio epicenters; and 4) about 25 degrees of counterclockwise rotation of the PRVI microplate during the past 11 Ma. LM2005 postulate that the Investigator faults mark the transfer of Muertos subduction motion to faults of the Anegada basins, driven by Caribbean-Puerto Rico microplate relative motion, combined with the counterclockwise rotation of the Puerto Rico-Virgin Islands microplate. This results in normal-oblique faulting in a highly complex and seismically active setting and is consistent with the Indios moment tensors. More recent high resolution bathymetry shows faults similar to and north of the Investigator faults, though trending more northeasterly, more consistent with the Indios epicentral patterns. These are consistent with this tectonic model. Rough estimates of slip rates for the Investigator faults are 0.8 mm/yr to the west and 1.5 to east and maximum magnitudes of about 7.0, based on fault length. This implies return periods of 1-4 thousand years for these magnitudes and less for smaller magnitudes. There are no earthquakes of this size in southern Puerto Rico in the 500 year historic record.

Presenting Author: Roland LaForge

Authors