Earthquake Swarms in the Puerto Rico Trench Monitored by Ocean-Bottom Seismometers


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Earthquake Swarms in the Puerto Rico Trench Monitored by Ocean-Bottom Seismometers

By Uri ten Brink and Alberto Lopez
December 2007
[revised February 2008]

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Inspection of ocean-bottom seismometers after recovery

Above: Inspection of OBSes aboard the U.S. Coast Guard cutter Dauntless after recovery. Standing: WHOI technicians Alan Gardner (left) and Dave DuBois. Kneeling: USGS scientist Uri ten Brink (left) and USCG Chief Electrician's Mate Tim Glasgow. Photograph by Alberto Lopez, USGS. [larger version]

Scientists monitored sea-floor earthquake activity northeast of Puerto Rico during a 6-month period this year, collecting data that could lead to a better understanding of the danger from large subduction-zone earthquakes and tsunamis in the Caribbean region.

Five ocean-bottom seismometers (OBSes) were deployed on March 8, 2007, by researchers from the U.S. Geological Survey (USGS) and the Woods Hole Oceanographic Institution (WHOI). Each OBS is a self-contained data-acquisition system that free-falls to the ocean floor, where it converts motions of the sea floor into electrical signals that are recorded digitally (see URL http://woodshole.er.usgs.gov/operations/obs/). The scientists deployed short-period OBSes that record frequencies from about 2 to 80 Hz. The instruments were picked up on September 3, 2007, after spending 6 months collecting data at water depths of more than 5,000 m.

The area northeast of Puerto Rico is characterized by frequent swarms of seismic tremors lasting a week or two at a time. One such swarm in 2001 included three earthquakes with moment magnitudes of 5.5 to 6. Although the cause of these recurring swarms is unknown, a study of earthquake hypocenters (points where earthquake ruptures begin), using data from OBSes deployed by the USGS in 2005, suggests that the tremors originate along the subduction zone between the North American and Caribbean tectonic plates. (See article in Sound Waves, June 2005, URL http://soundwaves.usgs.gov/2005/06/fieldwork.html.) Because of the geography of the northeastern Caribbean, earthquakes in this region cannot be accurately located by using land stations alone—hence the need to deploy ocean-bottom seismometers.

Above: Caribbean region, showing study area. See detailed map below. [larger version]

Above: Preliminary locations of epicenters of earthquakes that took place during 2007 OBS deployment, determined by the Puerto Rico Seismic Network (PRSN; URL http://redsismica.uprm.edu/english/). Also shown are locations of OBSes (inverted blue triangles), a temporary broadband seismometer in Anegada (blue triangle), and GPS stations installed in Anegada and St. Maarten (purple stars) as part of the project, as well as broadband seismometers (yellow triangles) operated by the PRSN, the Royal Netherlands Meteorological Institute (KNMI), and the Meteorological Service of the Netherlands Antilles and Aruba (MDNA&A). [larger version]

If confirmed by analysis of the recently acquired OBS data, the locations of the tremors at the plate interface may have profound implications about the capability of the Puerto Rico Trench to generate large earthquakes. Additionally, the tectonic setting of the Puerto Rico Trench is sometimes compared to that of the Sumatra subduction zone, the site of the earthquake that triggered the devastating Indian Ocean tsunami of December 2004. This similarity has caused great interest in the assessment of potential tsunami hazard to the United States east coast and the northeastern Caribbean from a large subduction-zone earthquake along the Puerto Rico Trench.

Above: GPS dome antenna (background) and seismic vault housing temporary broadband seismometer (foreground) on the island of Anegada. Photograph by Alberto Lopez, USGS. [larger version]

All five OBSes recorded data during the entire 6-month deployment. As it happened, the region was unusually active, with three swarms and at least 518 earthquakes. The OBS data are currently being merged with seismic data recorded on land in Puerto Rico and the Virgin Islands by the Puerto Rico Seismic Network, seismic data recorded on land in the Netherlands Antilles by the Royal Netherlands Meteorological Institute and the Meteorological Service of the Netherlands Antilles and Aruba, and data recorded by a temporary broadband seismometer on the island of Anegada, British Virgin Islands. A global-positioning-system (GPS) station on the island of Anegada, the island closest to the swarm, will help determine whether the swarm is at the edge of an aseismic segment of the subduction zone—a phenomenon observed elsewhere in the world, where swarms of minor earthquakes occur at the edge of subduction-zone segments that are not producing earthquakes, either because they are locked or because they are moving smoothly, without the "stick slip" motion that triggers earthquakes. An integrated analysis of the various data sets will likely provide accurate locations of the earthquakes, knowledge about their focal mechanisms (earthquake-slip directions and orientations of the earthquake fault), and an understanding of the potential earthquake and tsunami hazards from the Puerto Rico Trench.

The five OBSes were deployed by Alberto Lopez, a Mendenhall Postdoctoral Research Fellow at the USGS Woods Hole Science Center in Woods Hole, Massachusetts, and David DuBois of WHOI, aboard the National Oceanic and Atmospheric Administration (NOAA) ship Nancy Foster (URL http://www.moc.noaa.gov/nf/) on transit to Puerto Rico from her home port in Charleston, South Carolina. The instruments were picked up by Lopez, Uri ten Brink (USGS Woods Hole Science Center), DuBois, and Alan Gardner (WHOI) aboard the U.S. Coast Guard cutter Dauntless (URL http://www.uscg.mil/lantarea/cutter/dauntless/dauntless.htm), a 210-ft Reliance class cutter out of Galveston, Texas. The cutter's crew, under the command of Commander Dwight Mather, helped stage the recovery operation. Assisting with logistics on land was Chief Warrant Officer Michael Mullen of the U.S. Coast Guard station in San Juan, Puerto Rico. While on the way to pick up the OBSes, the Dauntless was diverted to pick up 31 Dominican migrants drifting on a small boat in the high seas (see URL https://www.piersystem.com/go/doc/586/170631/), an interesting and moving experience for the scientific team on board.

Additional partners in this study are Victor Huérfano Moreno and Christa von Hillebrandt-Andrade from the Puerto Rico Seismic Network and Jay Pulliam from the University of Texas' Institute for Geophysics.

Joint Spanish-United States Cruise Investigates Tsunami and Earthquake Hazards in the Northeastern Caribbean
June 2005