Preliminary Results From a Multichannel Seismic Reflection Survey to Identify the Fault Source of the Indios Earthquake Sequence, Puerto Rico

Session: Early Results from the 2020 M6.4 Indios, Puerto Rico Earthquake Sequence
Type: Oral
Date: 4/30/2020
Time: 02:15 PM
Room: 115
Description: 

The Indios earthquake sequence can be divided into 3 phases. The first, lasting between 12.28.2019 – 01.06.2020, was focused offshore Guanica and consisted of ESE-WNW strike-slip focal mechanism. The second, between 01.06-13.2020, was focused immediately to the NE of the first phase and consisted mostly of normal fault mechanisms striking SW-NE. The third phase surrounded the area of the second phase and extended south and west. It consisted mostly of E-W strike-slip focal mechanisms. The second phase produced the largest magnitude earthquakes and may represent rupture of normal fault(s) dipping to the northwest compatible with InSar vertical displacement onshore. The strike and rake are sub-parallel to the Caja de Muertos (CdM) fault on the shelf NE of the earthquake area. The CdM fault bounds a several 100s-m-deep half-graben, the Ponce Basin, and is perhaps responsible for the uplifted CdM island. The shelf edge in the earthquake area is the only part of southern Puerto Rico which had eroded close to the shoreline to form an amphitheater at the headwall of the only significant submarine canyon along the southern coast. The amphitheater is asymmetric with the long axis sub-parallel to the presumed strike of the normal fault(s). The bathymetry within the amphitheater is convex with its apex ~10 km SE of the shelf edge. This unique bathymetry and the presence of CdM fault and Ponce Basin, suggest repetitive normal displacement over geologic time on SW-NE striking fault(s). GPS-measured displacements suggest that SW Puerto Rico west of CdM island moves to the SW, relative to stable Caribbean, together with Mona Passage and the Dominican Republic whereas the rest of Puerto Rico is moving to the NW. We report on the results of a marine high-resolution multichannel seismic survey to identify the location and geometry of the normal fault(s), to investigate where additional strike-slip faults on the shelf and upper slope lie, and to identify the presumed block boundary.

Presenting Author: Uri ten Brink

Authors