Rapid-response seismic reflection survey to identify the fault sources of the southwestern Puerto Rico seismic sequence

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Intense seismic activity started in southwestern Puerto Rico on December 28, 2019 and is continuing to the present time.

This article is part of the October-November 2020 issue of the Sound Waves newsletter.

The activity to date includes 12 Magnitude 5 and larger earthquakes (black and white beachballs on map below) and thousands of smaller earthquakes. The largest earthquake, a M6.4 on January 7, 2020, caused ≤20 cm of shoreline subsidence near Guayanilla and numerous landslides and rock falls, including the collapse of a famous rock arch along the shore. The seismic activity was mostly located under water within 10 km of the shoreline in an area of industrial facilities such as an electric power station, liquefied natural gas storage tanks, and an inactive oil refinery. The earthquakes caused considerable damage to buildings in the area and frayed the nerves of the local population. Seismic activity in this area came as a total surprise, and concern was rightly raised whether this is the beginning of an even more intense seismic activity.

Map

Map of seismic activity in Southwestern Puerto Rico. Twelve Magnitude 5 and larger earthquakes (black and white beach balls on map); more than 250 line-kilometers of high-resolution multichannel sparker seismic reflection data collected during March 7-13, 2020 (black lines on map); identification of numerous faults (red marks on map); unusual seafloor lineaments (marked in yellow on map) are interpreted to have been formed over many millennia.  

Old black and white photo of ocean and scenery

ocean and beach

The largest earthquake, a M6.4 on January 7, 2020, caused ≤20 cm of shoreline subsidence near Guayanilla and numerous landslides and rock falls, including the collapse of a famous rock arch along the shore. Collapsed arch (right) and the famous arch in 1970 (left).

 

Less than two months after the largest earthquake in the seismic sequence, the USGS Woods Hole Coastal and Marine Science Center organized a rapid response seismic survey. The goal of the survey was to map the faults which ruptured during the seismic activity and understand the regional context and potential for other earthquake activity in the area. More than 250 line-kilometers of high-resolution multichannel sparker seismic reflection data (black lines on map) were collected during March 7-13, 2020 with our seismic equipment, which was shipped to Puerto Rico. The data was collected using a Sparker sound source and received by an array of 32 hydrophone groups embedded within a cable. Both source and receiver were towed behind the vessel R/V Sultana, which travelled at a speed of about 4 nautical miles per hour. The ship, the staging area at the University of Puerto Rico Isla Magueyes marine science laboratory in La Parguera, and the ship’s crew, were generously provided at no cost by the University of Puerto Rico. The lab director, Professor Ernesto Otero; the lab communication specialist, Aldo Acosta; the ship’s captain, Orlando Espinoza; and their staff and crew went out of their way to provide support and advice. The USGS field crew, consisting of Wayne Baldwin, Jason Chaytor, Eric Moore, Alex Nichols, and Uri ten Brink experienced many events of seismic shaking during their stay, but none caused damage. They were able to complete their mission and obtain good data despite rough seas and technical problems. The USGS field crew returned home as lockdown due to the pandemic was declared both in Puerto Rico and in Massachusetts.

Group of people posing for photo

boat at dock

(Left) From left to right – Uri ten Brink, Eric Moore, Wayne Baldwin, Alex Nichols, Jason Chaytor, and Captain Orlando Espinoza. (Right) R/V Sultana.

 

Signal processing of the seismic data post cruise by Dave Foster and Wayne Baldwin allowed the identification of numerous faults (red marks on map) offsetting the shallow sediments (less than 0.5 km below the sea floor) were identified along an ENE trend ~5 km seaward of the shelf edge and parallel to it and may represent the near-surface rupture of the largest earthquake, the January 7, 2020, M6.4. Other faults were identified ~15 km from the shelf edge and their relationship to specific fault rupture is unclear. Near-surface fault offsets were also identified on the shelf in Guayanilla Bay, at the entrance to the bay, at the seaward extension of the Punta Montalva fault, and south of La Parguera. Some of these fault offsets can be correlated with known faults or known moderate earthquakes, but others could not.

People working on boat in body of water

Alex Nichols, Wayne Baldwin, and Eric Moore deploying a streamer in Puerto Rico.

The activity appears to have been recurring during recent geological time as evidenced by the bathymetry and by analysis of faults on land. The area of seismic activity is the only part of southern Puerto Rico where the shelf is indented, and the shelf edge becomes as narrow as 1 km. It is also the source area of the only large submarine canyon along southern Puerto Rico, the Guayanilla Canyon, despite not having large feeding terrestrial rivers. Unusual seafloor lineaments (marked in yellow on map above) are interpreted to have been formed over many millennia. The average extension axis of all the recent earthquakes with magnitudes larger than 5 is NW-SE, similar to the extension directions of Pleistocene-age faults on land.

 

Ocean and beach

Iguana

(Left) Permanently flooded shore in Guayanilla, Puerto Rico.

 

The seismic activity appears to have activated a zone of diffuse deformation, where ruptures may be transitioning from one fault to another (see regional map below). This activity may reflect the establishment of a new block boundary between Puerto Rico and Hispaniola, where Hispaniola is moving west and southwest relative to Puerto Rico. We hope to study this block boundary in greater detail to help assess its hazard to the people of Puerto Rico and their infrastructure.

 

Map

Regional map of Puerto Rico.