Earthquake, Landslide, and Tsunami Hazards in the Northeastern Caribbean—Insights from a 2013 E/V Nautilus Expedition
[Based on an article from the journal, “Oceanography”]
The first systematic deep-sea exploration of the northeastern Caribbean has changed scientists’ understanding of geologic hazards around Puerto Rico and the Virgin Islands, and it has provided a glimpse into the region’s hitherto unknown deep-sea ecology.
The multidisciplinary expedition was conducted by the Ocean Exploration Trust from October 4–18, 2013, aboard the exploration vessel (E/V) Nautilus. Expedition members used the remotely operated vehicles (ROVs) Hercules and Argus to view features that U.S. Geological Survey (USGS) chief scientist Uri ten Brink and his colleagues had studied on their seafloor maps and subseafloor records but had never before had the opportunity to see. The primary goal was to investigate faults and landslides that might have triggered earthquakes and tsunamis in the region. The impact of earthquake and tsunami hazards on Puerto Rico and the Virgin Islands is potentially very significant, because more than 4 million people inhabit the islands, numerous tourists visit them each year, and many civilian and military installations are spread along their coastlines.
Earthquakes and submarine landslides are common around Puerto Rico and the Virgin Islands, which lie near the boundary between two tectonic plates—the North American plate and the Caribbean plate. These vast blocks of the Earth’s outermost rigid layer are grinding past each other and, in the process, causing earthquakes and lifting or pulling down the Earth’s surface. Among the peculiarities of this plate boundary is the Puerto Rico Trench, an unusually deep trench north of Puerto Rico (the deepest place in the Atlantic Ocean) where the North American plate is sliding beneath the Caribbean plate at a highly oblique angle (see map, below).
Some of the features explored during the expedition are submarine fault systems thought to have generated damaging earthquakes in Hispaniola (1842), the Virgin Islands (1867), and western Puerto Rico (1918). The researchers also examined landslide scarps (scars in slopes where material has broken away) left by the Mona landslide—a submarine slide thought to have triggered a devastating tsunami during the 1918 earthquake. Additionally, they viewed the Noroît Seamount east of the Virgin Islands, a feature suspected to be a submarine volcano. What they saw changed some of their earlier interpretations and confirmed others.
ROV transects of the Mona landslide scarps, for example, showed the walls to be covered by slow-growing iron-manganese deposits called “varnish,” an indicator that the slide occurred long before 1918. A steep vertical cliff several hundred meters high and approximately 20 kilometers (12 miles) north of the Mona landslide appeared to be a fresh scarp, probably caused by a large landslide or a series of smaller incremental slides. Further analysis will shed light on whether the younger landslide might have triggered the 1918 tsunami.
The expedition also explored the Septentrional fault, a major strike-slip fault that is similar to California’s San Andreas fault and stretches from Cuba to the Mona Rift (see map). Exploration dives revealed no surficial evidence for the fault’s eastward continuation beyond the Mona Rift. This finding suggests that the fault is shorter and thus less of an earthquake hazard to Puerto Rico than previously thought.
Likewise, the researchers found no evidence of a fresh fault surface running from Desecheo Island into western Puerto Rico. On the other hand, the previously proposed fault trace of the 1867 Virgin Islands earthquake was found to be associated with a 30-meter (100 foot)-high linear and somewhat fresh rock wall—a good candidate for a fault. An additional possible fault, cut in places by landslide scars, was mapped by the ship’s multibeam echosounder closer to St. Thomas and could be associated with past earthquakes in the area.
The Noroît Seamount was mapped in detail and verified as a fairly young but inactive volcano. If this seamount is part of the Lesser Antilles volcanic chain, then the Lesser Antilles chain extends almost to Barracuda Bank in the middle of Anegada Passage. Barracuda Bank is a sliver of seafloor a few hundred meters wide that lies just below the sea surface and is surrounded by steep walls that descend almost 2,000 meters (6,500 feet) deep. Some of the walls are subvertical, probably formed by a steep landslide or a fault trace.
The exploration dives were carried out with the ROV Hercules, equipped with high-definition video cameras, a manipulating arm for collecting rock and biological samples, push cores for collecting sediment samples, and equipment for sampling water. Throughout the dives, Hercules was illuminated by its companion ROV, Argus, hovering a few hundred feet above Hercules. The ROVs spent a total of 4.5 days of sea-bottom time at seven dive sites in Mona and Anegada Passages and reached a maximum depth of 3,000 meters (nearly 10,000 feet). The only previous diving expeditions in the Greater Antilles were several dives by the manned submersible Alvin and a series of dives by the Navy submarine NR-1 expedition in Mona Passage in the 1970s, all of which covered a much shallower depth range.
In addition to new geologic information, the 2013 ROV dives yielded new insights into the region’s deep seafloor (benthic) organisms, including corals, sponges, other invertebrates, and fishes. For example, deep-sea corals (living deeper than 200 meters [650 feet]) appeared to be much less abundant in the Greater Antilles region than in canyons along the northeast U.S. Atlantic margin and the Bahamas—although their patchy distribution in the areas viewed by the ROVs may not be representative of their abundance in the broader region. No clear distinction was evident between the benthic communities northwest of Puerto Rico (Atlantic Ocean) and the Virgin Islands (transition to the Caribbean Sea).
Noroît Seamount stood out in its complex and diverse communities of benthic organisms. Large gastropods (sea snails) and the largest primnoid octocorals (corals) were encountered only here. The numbers of fish observed during the dives were generally low except near Noroît Seamount, where the ROVs recorded species not seen during the other dives. Also at Noroît Seamount, the ROV encountered a Bluntnose Sixgill Shark (Hexanchus griseus) longer than the 11-foot-long ROV, lurking at a depth of 1,800 meters (6,000 feet). (See the shark on a video clip.)
Seafloor mapping by the ship's Kongsberg EM302 multibeam echosounder was used to plan the dive transects and to improve the resolution and coverage of current USGS bathymetry of the region. A total of 2,028 line-kilometers of multibeam bathymetry were collected and processed onboard.
The entire expedition was broadcast live to numerous schools, youth clubs, and science museums around the United States. Public interest in the expedition surpassed that in all previous Nautilus expeditions, with 195,276 website visits, 91,733 unique visitors, and 790,649 page views. Interest was particularly intense in Puerto Rico and the Virgin Islands, due in part to a focused publicity effort with local Puerto Rico news outlets. The expedition was also filmed by National Geographic for a film titled “Caribbean’s Deadly Underworld”.
Expedition information and imagery are archived on the Nautilus Live website. View an interview with chief scientist Uri ten Brink. Explore dive photographs and video footage at http://www.nautiluslive.org/albums and http://www.nautiluslive.org/videos (cataloged by dive date).
To learn more about results of the expedition, see “2013 Field Season Summary—Earthquake, Landslide, and Tsunami Hazards and Benthic Biology in the Greater Antilles” in Oceanography, v. 27, no. 1, supplement.
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