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Gravity Coring Offshore Puerto Rico and the U.S. Virgin Islands to Investigate the Timing of Submarine Landslides and Large Earthquakes
Recent analysis of the destructive 1918 tsunami that devastated western Puerto Rico has shown that the tsunami was likely caused by a large (10-km3 volume) earthquake-induced submarine landslide in the Mona Passage west of Puerto Rico. Many landslides have also been mapped along the edge of the submerged carbonate platform about 40 km seaward of the north shore of Puerto Rico. Volume analysis and hydrodynamic modeling indicate that at least eight of these submarine landslides could have caused damaging tsunamis; however, because the age of these landslides is unknown, their recurrence interval could not be estimated.
The goal of a recent USGS research cruise was to core and date submarine-landslide-related sediment north of Puerto Rico in order to determine the age of these landslides. Dating of these landslides is needed in order to calculate tsunami probability for the north coast of Puerto Rico. Another coring target of the cruise was the Virgin Islands Basin, the epicenter of the devastating 1867 earthquake and tsunami. There we focused on dating disturbances in the sedimentary layers caused by ground shaking, in hopes of extending the area's earthquake record backward in time. A long earthquake record is needed to adequately estimate the probability of additional earthquakes in the area in the near future.
Coring was carried out aboard the Harbor Branch Oceanographic Institution's research vessel (R/V) Seward Johnson during a 9-day cruise from March 16 to 24, 2008, leaving from and returning to San Juan, Puerto Rico. Cores were collected with the Woods Hole Oceanographic Institution (WHOI)'s Giant Gravity Corer, a 4-in.-diameter pipe, suspended from a cable, that penetrates the sea floor under its own weight and can collect cores as long as 6.5 m. In this cruise, many of the coring sites were expected to have a relatively hard sandy bottom, which is difficult to penetrate, and so we rigged the corer for a maximum core length of only 3 m.
A total of 40 coring sites were occupied during the cruisewithin Mona Canyon, on the south wall and floor of the Puerto Rico Trench, and in the Virgin Island, Vieques, and Whiting Basins south of the U.S. Virgin Islandswith site depths ranging from 1,350 m down to 8,000 m. Coring sites were chosen on the basis of multibeam bathymetry and seismic-reflection data collected by the USGS on several recent cruises in the northeastern Caribbean (for example, see "New Bathymetric Map of Mona Passage, Northeastern Caribbean, Aids in Earthquake and Tsunami-Hazard Mitigation," Sound Waves, May 2007), as well as GLORIA sidescan-sonar data (URL http://pubs.usgs.gov/dds/dds15/).
Just 2 days after we left port, the worst storm of Puerto Rico's winter arrived, generating 16-ft swells along the north side of the island, closing San Juan harbor for 3 days, and forcing the ship to seek shelter along the south side of the island. In spite of these conditions, the dedication and efforts of the science team and Captain George Gunther and the crew of the R/V Seward Johnson resulted in the recovery of more than 24 m of sediment from 21 of the 40 coring sites. The cores are now undergoing nondestructive testing, including multisensor core logging (MSCL), X-ray fluorescence (XRF), and lithologic logging, after which the sediment above and below submarine-landslide-debris deposits will be sampled for 14C radiometric dating.
Participants in the cruise included Matt Arsenault, Sandy Baldwin, Brian Buczkowski, Michael Casso, Claudia Flores, Emily Himmelstoss, Kate McMullen, and Uri ten Brink from the USGS Woods Hole Science Center, Jason Chaytor (chief scientist, WHOI-USGS Postdoctoral Scholar), and Jim Broda (coring specialist, WHOI).
For additional information about submarine landslides and tsunamis in the Caribbean region, see:
in this issue:
Submarine Landslides and Large Earthquakes
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