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Fieldwork

Puerto Rico OBS Study


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Deploying an OBS
Above: Deployment of an Ocean Bottom Seismometer off Puerto Rico, showing external sensor arm in the cocked position. The arm will release upon contact with the sea floor.
Puerto Rico and the Virgin Islands are located at an active tectonic-plate boundary between the North American plate and the northeast corner of the Caribbean plate. Large-magnitude earthquakes and devastating tsunamis have occurred in historical times in this densely populated (3.7 million American citizens) part of the United States. Particularly vulnerable are the San Juan urban corridor and the city of Mayaguez. Lack of geologic understanding of the tectonic movements in this part of the Caribbean has hampered our ability to assess the seismic and tsunami hazards. Puerto Rico and the Virgin Islands are unique among the seismically active regions of the U.S. in that the islands are mostly covered by water, which presents both opportunities and challenges for geological and geophysical studies.

A temporary array of land and ocean-bottom seismometers (OBSs) was deployed in early May in and around western Puerto Rico for a period of 45 days to record local and regional earthquakes. This deployment is one of a series of studies planned to better assess the hazard from earthquakes, submarine slides, and tsunamis to Puerto Rico and the Virgin Islands. The data collected by the temporary array will be augmented by data collected by the permanent seismic stations of the Puerto Rico Seismic Network. The combined data will be used to locate earthquakes more accurately than is presently possible, because the accuracy of an earthquake location depends on the distance from the earthquake to the seismometers and on the geographic distribution of the seismometers. Many of the marine and land seismometers were placed at intervals of 15-25 km along two perpendicular arrays. One array was located along a 150-km-long north-south line crossing the Puerto Rico platform on the western side of the island. The other was located along a 250-km-long east-west line on the south part of the island with continuation to Mona Island. The data from these arrays will be used to construct seismic-velocity maps of the Earth's crust beneath the arrays. Seismic velocity is a good proxy for the types of rocks at depth. We also hope to learn about the attenuation of seismic energy from the earthquake at particular seismic stations.

Preparing the OBSs
Above: Preparing thirteen Ocean Bottom Seismometers at the Port Authority in Mayaguez, Puerto Rico. The internal components of each instrument are tested extensively before being loaded onto the ship.
This study is being carried out jointly by the USGS Coastal and Marine Program (CMGP) in Woods Hole and by the Puerto Rico Seismic Network at the University of Puerto Rico. The study is co-funded by the CMGP, the Puerto Rico Department of Natural and Environmental Resources, the Sea Grant Office at the University of Puerto Rico in Mayaguez, and by the President's Office of the University of Puerto Rico. The offshore deployment was carried out by Uri ten Brink, Greg Miller, Erich Roth, Philipp Molzer (WHFC), Robert Iuliucci (contractor), and Rafael Abreu (Puerto Rico Network) aboard the University of Puerto Rico's R/V Isla Magueyez. Instrument recovery is planned for June 13th through 15th aboard the University of Puerto Rico ship R/V Chapman. The marine component represents the largest number of OBSs deployed to date in a single emplacement, the first time that all newly designed USGS OBSs with external geophones were deployed, and the first time that USGS OBSs were deployed in a regional network to record earthquakes. The marine component is the culmination of several years of hard work by Greg Miller and Ray Davis (WHFC) to modify and upgrade the OBS to address their new mission of acquiring better earthquake records. Three land seismometers were also temporarily deployed on Puerto Rico by Russell Sell (Earthquake Hazards Program, EHP, Menlo Park) and Erich Roth (WHFC) to augment the permanent network.

Several press representatives visited the R/V Isla Magueyez prior to leaving port and were briefed by Christa von Hillebrandt-Andrade, Director of the Puerto Rico Seismic Network, and Uri ten Brink about the goals of the cooperative experiment and the instruments. Their reports and photos appeared on the evening news of the Telemundo TV network, in the El Nuevo Dia, the largest Puerto Rico daily newspaper, and in the weekly newspaper Vision. The generous help of Joe Fletcher and Russell Sell (EHP, Menlo Park), the Puerto Rico Port Authority of Mayaguez, the President's office of the University of Puerto Rico, and the staff of the Puerto Rico Seismic Network in various aspects of the experiment are central to the success of the experiment.


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