The trench itself is very negative,[23] with values more negative than 250 mgal. Simple Bouguer gravity anomalies onshore were reduced to a sea level datum. The Puerto Rico Trench is also associated with the most negative gravity anomaly on earth, -380 milliGal, which indicates the presence of an active downward force. The Okeanos Explorer is also providing opportunity for additional partners to do surveys of opportunity while at sea. Report an Error, Site Index If the Earth were an ideal oblate spheroid of uniform density, then the gravity measured at every point on its surface would be given precisely by a simple algebraic expression. Astrophysical Observatory. The Puerto Rico Trench, in contrast, is located at a boundary between two plates that slide past each other with only a small component of subduction. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. The resulting series are then processed as if they were, Seismic-reflection profiles over the outer ridge and the Nares Basin north of Puerto Rico show that three principal structures can be traced from the north wall of the Puerto Rico Trench northward to, Seven reflection profiles over the central Aleutian trench and terrace are presented. This brings the free-air anomaly, which omits the correction terms for either, close to zero. No Derivative works - You may not alter, transform, or build upon this work. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount. [14], The isostatic anomaly is defined as the Bouger anomaly minus the gravity anomaly due to the subsurface compensation, and is a measure of the local departure from isostatic equilibrium, due to dynamic processes in the viscous mantle. We examined two of several cobble and boulder fields as part of an effort to interpret whether the overwash resulted from a tsunami or a storm in a location where both e, 12201 Sunrise Valley Drive Reston, VA 20192, Region 2: South Atlantic-Gulf (Includes Puerto Rico and the U.S. Virgin Islands), Region 12: Pacific Islands (American Samoa, Hawaii, Guam, Commonwealth of the Northern Mariana Islands), Woods Hole Coastal and Marine Science Center, Significant Earthquakes on a major fault system in Hispaniola, Puerto Rico, the Virgin Islands, and the Lesser Antilles, 15002010: Implications for Seismic Hazard, Coastal and Marine Hazards and Resources Program, Caribbean Tsunami and Earthquake Hazards Studies- Models, Caribbean Tsunami and Earthquake Hazards Studies- Stress Changes and Earthquake Hazard, Caribbean Tsunami and Earthquake Hazards Studies-Tsunami Potential, Caribbean Tsunami and Earthquake Hazards Studies- Seafloor Map, Mysterious tsunami in the Caribbean Sea following the 2010 Haiti earthquake possibly generated by dynamically triggered early aftershocks, Along-strike segmentation in the northern Caribbean plate boundary zone (Hispaniola sector): Tectonic implications, Semi-automated bathymetric spectral decomposition delineates the impact of mass wasting on the morphological evolution of the continental slope, offshore Israel, Effects of 2010 Hurricane Earl amidst geologic evidence for greater overwash at Anegada, British Virgin Islands, Event sedimentation in low-latitude deep-water carbonate basins, Anegada passage, northeast Caribbean, Slope failures and timing of turbidity flows north of Puerto Rico, Seismic hazard from the Hispaniola subduction zone: correction to "Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region", Slab tears and intermediate-depth seismicity, Seismic evidence for a slab tear at the Puerto Rico Trench, Reply to a comment by Carol S. Prentice, Paul Mann, and Luis R. Pea on: "Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region" by U. ten Brink et al. Historically, other large earthquakes have also struck the area, such as one in 1787 (magnitude~8.1), possibly in the Puerto Rico Trench, and one in 1867 (magnitude~7.5) in the Anegada Trough (figure 1). This comparison, though of limited application to other places, helps calibrate coastal geology for assessment of earthquake and tsunami potential along the Antilles Subduction Zone. Gravity anomalies and the origin of the Puerto Rico Trench P. Molnar Geology 1977 The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. That is, the Bouguer anomaly is:[12], The Bouguer anomaly is positive over ocean basins and negative over high continental areas. The geologic settings of Puerto Rico and the Virgin Islands have created or contributed to several pressing societal issues related to human safety, environmental health, and economic development. Its profile is asymmetric: northern part is steep (about 57), southern part is about 16. Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. The free-air anomaly is around +70 mgal along the Andes coast, and this is attributed to the subducting dense slab. The low-density sediments of the Molasse basin produce a negative anomaly. This is tedious and time-consuming but necessary for obtaining a meaningful gravity anomaly. Other margins of the island (west, south, and south west) are also associated with massive tectonic features and may pose addtional hazard. Newly-acquired multibeam bathymetry of the entire Puerto Rico trench reveals numerous retrograde slope failures at various scales at the edge of the carbonate platform north of Puerto Rico and the Virgin Islands. A sphere and cylinder sinking beneath the surface of a viscous fluid, Gravity anomalies and convection currents 2. A gravity survey is conducted by measuring the gravity anomaly at many locations in a region of interest, using a portable instrument called a gravimeter. Search for other works by this author on: Attribution: You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). The region has high seismicity and large earthquakes (figure 2). Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. The trench is less deep where the component of subduction is larger. --Massachusetts Institute of Technology, Dept. [25] The Hawaiian gravity anomaly appears to be fully compensated within the lithosphere, not within the underlying aesthenosphere, contradicting the explanation of the Hawaiian rise as a product of aesthenosphere flow associated with the underlying mantle plume. Larger surveys across the region provide evidence of a relict subduction zone. At a minimum, these include the tidal correction gtid, the terrain correction gT, and the free air correction gFA. The Puerto Rico Trench, in contrast, is located at a boundary between two plates that slide past each other with only a small component of subduction. The Bouguer anomaly over continents is generally negative, especially over mountain ranges. Notice, Smithsonian Terms of [21], Over mid-ocean ridges, the free-air anomalies are small and correlate with the ocean bottom topography. Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. Leg 3 will include telepresence capability for all ROV dives, permitting the public to watch the expedition live and in action over the Internet. A tilted carbonate platform provides evidence for extreme vertical tectonism in the region.This platform was horizontally deposited Because of its high population density and extensive development near the coast, Puerto Rico has a significant risk for earthquakes and tsunamis. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and more dense than it, pulls the lithosphere down at the trench. The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water depths exceeding 8,400 meters. Dynamically triggered offshore aftershocks, caused by passing seismic waves from main shocks located on land, are currently not considered in tsunami warnings. Two-thirds of this is from the Moon. Determining wh, Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. Please check your email address / username and password and try again. ago over a time period as short as 14 . "Puerto Rico Trench Negative Gravity Anomaly Belt", Studies in Earth and Space Sciences, R. Shagam, R. B. Hargraves, W. J. Morgan, F. B. A study of the seismic velocity structure of the subcrustal oceanic lithosphere was conducted in the western North Atlantic north of the Lesser Antilles, where the age of the lithosphere, We infer from the bathymetry and gravity field and from the source mechanisms and depths of the eight largest earthquakes in the Bali region that the Bali Basin is a downwarp in the crust of the, Abstract Velocity modeling and earthquake relocation are used to examine the structure and seismicity of an area south of the Puerto Rico trench in the northeast Caribbean, and to explore whether and, We have determined focal mechanisms for the largest earthquake (Ms = 7.8) recorded instrumentally in the Caribbean Basin, the August 4, 1946, Hispaniola earthquake, and three of its large-magnitude, The direction and rate of movement of the Caribbean plate with respect to North America are determined from the slip vectors of shallow earthquakes and from the configuration of downgoing seismic, We have developed two-dimensional spherical shell finite element models of elastic displacements in the North America-Caribbean (NA-Ca) plate boundary zone (PBZ) in order to quantify crust and fault. The . The opposite is true for the theoretical case of terrain that is completely uncompensated: The Bouger anomaly is zero while the free-air and Airy isostatic anomalies are very positive. Eyewitness reports of an 1867 Virgin Islands tsunami gave a maximum wave height of >7 m in Frederiksted, St. Croix, where a large naval vessel was left on top of a pier. During Leg 1 of this expedition, the ship will transit from her home port in North Kingstown, Rhode Island, to the U.S. The Puerto Rico trench negative free-air gravity anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba. Just offshore of Puerto Rico and the U.S. Virgin Islands are a diversity of deep-sea features that include the deepest trench in the North Atlantic Ocean; seamounts; and numerous submarine canyons, valleys, and troughs. Tectonophysics: Dynamics of lithosphere and mantle: general (1213); Tectonophysics: Dynamics: gravity and tectonics; Tectonophysics: Subduction zone processes (1031; Marine Geology and Geophysics: Subduction zone processes (1031; Geodesy and Gravity: Gravity anomalies and Earth structure (0920. The trench region may pose significant seismic and tsunami hazards to Puerto Rico and the U.S. Virgin Islands, where 4 million U.S. citizens reside. The Puerto Rico trench (PRT), with water depths of up to 8340 m, is the deepest part of the Atlantic Ocean, yet the relative plate motion along the PRT is predominantly strike slip with only a small component of North America (NOAM) plate subduction (N70E [ Calais et al., 2002; Mann et al., 2002 ]). What is the thermal history of the arc massif, and its relation to, The Barbados Ridge complex lies east of the Lesser Antilles volcanic arc along the eastern margin of the Caribbean Plate. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. This site uses cookies. The complex dates in part from the Eocene, and elements of the arc system. Bathymetry of the northeast corner of the Caribbean plate. below sea level under the ocean basin, 12 km under the Caribbean Sea, at about 16 km under the trench, and at slightly shallower depth under Puerto Rico. A tilted carbonate platform provides evidence for extreme vertical tectonism in the region. Cayenne, being nearer the equator than Paris, would be both further from the center of Earth (reducing the Earth's bulk gravitational attraction slightly) and subject to stronger centrifugal acceleration from the Earth's rotation. A draft U.S. Geological Survey (USGS) hazard map places equal probability for damaging ground motion for Mayaguez in western Puerto Rico as for Seattle, Washington. A crustal section across the Puerto Rico trench, J. geophys. Approximate operating area (white polygons) of NOAA Ship Okeanos Explorer during Leg 3. Because the island lies on an active plate boundary, earthquakes are a constant threat, and the densely populated coastal areas are vulnerable to tsunamis (Uri ten Brink, 2003). The terrain correction is calculated from knowledge of the local topography and estimates of the density of the rock making up the high ground. The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative Further refinements of the model field are usually expressed as corrections added to the measured gravity or (equivalently) subtracted from the normal gravity. The gravity anomaly data used in this study were extracted from the International Center for Global Earth Models (ICGEM), and our land data (254 stations) that were collected in western Puerto Rico in 2010. Rapid gravity computation for two dimensional bodies with application to the Mendocino submarine fracture zone, A crustal section across the Puerto Rico trench, Comments on paper by W. Jason Morgan, 'Gravity anomalies and convection currents, 2. The Bouguer anomaly includes only the negative correction for the high terrain and so is strongly negative. Uri ten Brink, Chief Scientist The Puerto Rico Trench is also associated with the most negative gravity anomaly on earth, -380 milliGal, which indicates the presence of an active downward force. By continuing to use our website, you are agreeing to our, American Association of Petroleum Geologists, Cushman Foundation for Foraminiferal Research, Mineralogical Society of Great Britain and Ireland. The gravity anomaly is the difference between the observed acceleration of an object in free fall (gravity) near a planet's surface, and the corresponding value predicted by a model of the planet's gravitational field. Use, Smithsonian Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different) and that the Atlantic Ocean floor seaward of the outer rise, Puerto Rico, and the Caribbean Sea floor are each in isostatic equlibrium. Large tsunamis have also hit Puerto Rico and Hispaniola, reportedly killing 1800 people in 1946 and 40 people in 1918. Probability for damaging ground motion at San Juan and Mayaguez in comparison to other seismic high-risk areas in the U.S. (e.g. The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water depths exceeding 8,300 meters. U.S. Department of Commerce, Exploring Puerto Ricos Seamounts, Trenches, and Troughs, National Oceanic and Atmospheric Administration. Fifteen years later, Isaac Newton used his newly formulated universal theory of gravitation to explain the anomaly. These are indications of strong dynamic effects in subduction zones. The USGS has an ongoing program to identify and map the faults in this region using various geophysical and geological methods in order to estimate the location and magnitude of potential earthquakes. . Earthquakes and tsunamis in Puerto Rico, the Virgin Islandsand adjacent islands are mostly caused by the convergence of the North American tectonic plate with the Caribbean tectonic plate on which the islands are located. [1] The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and 4000 m. The seismogenic northeastern North America-Caribbean oblique-slip plate boundary includes the 8.5-km deep Puerto Rico trench, 120 km north of the densely populated islands of Puerto Rico and the, The results of drilling the Toa Baja hole will help answer some fundamental questions about the evolution of the Puerto Rico area. Agreement NNX16AC86A, Is ADS down? ago over a time period as short as 14-40 kyr. Download image (jpg, 50 KB). At the center of a level plateau, it is approximately equal to the free air anomaly. Click image for larger view and image credit. For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Then, at 3.5 Ma, the carbonate platform was tilted by 4 toward the trench over a time period of less than 40 kyr, such that its northern edge is at a depth of 4000 m and its reconstructed elevation on land in Puerto Rico is at +1300 m (Figures 1 and 2). In contrast, on its eastern boundary, the Caribbean overrides the North American plate, creating the island arc of the Lesser Antilles with its active volcanoes. Click image for larger view and image credit. Res., 64, 1545. [4], To understand the nature of the gravity anomaly due to the subsurface, a number of corrections must be made to the measured gravity value. Motion along its northern boundary (in the plate boundary zone region) is dominantly strike-slip (sideways motion between the plates), with a small component of subduction (one plate sinks under the other plate). Erosion is a concern in many coastal areas, but is particularly serious to island economies that rely heavily on the tourist industry. Un. Below are publications associated with this project. [5], The starting point for the model field is the International Reference Ellipsoid, which gives the normal gravity gn for every point on the Earth's idealized shape. I explain these vertical movements by a sudden increase in the slab's descent angle that caused the trench to subside and the island to rise. The Puerto Rico trench exhibits great water depth, extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. we suggest that these features are manifestations of large vertical movements of a segment of the Puerto Rico Trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. A 1918 magnitude 7.5 earthquake resulted in a tsunami that killed at least 40 people in northwestern Puerto Rico. National Oceanic and Atmospheric Administration Privacy Policy Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Well-documented slab tears that are associated with high rates of intermediate-depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. With the exception of the alkali basalts of Grenada, the volcanics of the arc can, The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and, The results of marine geophysical surveys by NOAA, Miami and the University of Durham with the Royal Navy in the eastern Caribbean region are presented as maps of the bathymetry, free-air gravity, Basalts recovered during leg 15 of the Deep Sea Drilling Project in the central Caribbean are finegrained plagioclase tholeiites which were erupted in the Late Cretaceous, at a time when North, Since the pioneering pendulum gravity measurements of Vening-Meinesz and Wright (1930) and Hess (1933, 1938), the Caribbean Sea, with its multitude of geologic problems, has received intensive study, By clicking accept or continuing to use the site, you agree to the terms outlined in our. An official website of the United States government. Different theoretical models will include different corrections to the value of gravity, and so a gravity anomaly is always specified with reference to a particular model. It is postulated that differential shifts between the Caribbean plate and the underthrust Atlantic plate have occurred at least twice, once in late Eocene to early Oligocene, during which the outer island chain of the northern Lesser Antillean arc was formed, and again sometime since the late Miocene, when the Barbados ridge and associated uplifted topography were formed. [29][30] The lowest-degree parameters, including the Earth's oblateness and geocenter motion are best determined from Satellite laser ranging.[31]. Nevertheless, a tsunami recorded on a Caribbean Deep-Ocean Assessment and Reporting of Tsunami (D, The North American (NOAM) plate converges with the Caribbean (CARIB) plate at a rate of 20.00.4mm/yr. Share sensitive information only on official, secure websites. The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. The Puerto Rico Trench is atypical of oceanic trenches. Exclusive Economic Zone surrounding Puerto Rico and the U.S. Virgin Islands, and the Puerto Rico Trench. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount. Some reports indicate that nearly 1,800 people drowned. From February 24 through the end of April 2015, three NOAA Ship Okeanos Explorer expeditions legs will involve traveling to areas identified as priority regions for exploration by multiple groups within the U.S. Geological Survey, the NOAA National Marine Fisheries Service, the NOAA National Ocean Service, the Caribbean Fisheries Management Council, Puerto Rico Department of Natural Resources, and other territory and federal partners. Notice, Smithsonian Terms of Figure 4. Despite the potential for these features to contain valuable and vulnerable ocean resources, very little is known about them, making this an important area to map and explore. The hazard from tsunamis is also apparent. Explore recent publications by USGS authors, Browse all of Pubs Warehouse by publication type and year, Descriptions of US Geological Survey Report Series, Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause, Woods Hole Coastal and Marine Science Center. The Puerto Rico Trench and the Mid-Atlantic Rise, The origin of outer topographic rises associated with trenches, Widespread occurrence of a high-velocity basal layer in the Pacific crust found with repetitive sources and sonobuoys, The structure and physical properties of the earth's crust. For example, the formation and sinking of a lithospheric root may explain negative isostatic anomalies in eastern Tien Shan. Results of the seismic investigation of the Puerto Rico trench and outer ridge, carried out in 1959 by Lamont Geological Observatory, Woods Hole Oceanographic Institution, A. and M. College of Texas, New seismic and magnetic information from the Antilles Outer Ridge and Trench region has been combined with previous geophysical observations to provide an explanation for the origin of the crust, A crustal section across the Puerto Rico Trench, from 450 km north to 250 km south of San Juan, was deduced from seismic refraction and gravity data. [20] Negative isostatic anomalies in Switzerland correlate with areas of active uplift, while positive anomalies are associated with subsidence. The seabed of the Cayman Trough is flat at the segment (80.0W and 17.7N to 78.5W and 19.5N). The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. The density of crustal rock, , is usually taken to be 2670kg m3 so the Bouguer plate correction is usually taken as 0.3086 mgal m1 h. Here h is the elevation above the reference ellipsoid. The large negative gravity anomaly is attributed to a . Figure produced in Google Earth Pro. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and Expand 14 PDF Save Alert Motion along its northern boundary (in the plate boundary zone region) is dominantly strike-slip (a geological fault in which an adjacent surface moves horizontally), with a small component of shortening. During Leg 3, encompassing most of the month of April, scientists will be exploring the U.S. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. That is, the free-air anomaly is:[12], The free-air anomaly does not take into account the layer of material (after terrain leveling) outside the reference ellipsoid. The gravitational attraction of this layer or plate is taken into account by the Bouguer plate correction, which is 0.0419103 h mgal m2 kg1. In the 20th century alone there have been several very large earthquakes north of Puerto Rico (Ms 7.3 in 1918; Ms 7.8 in 1943; Ms 8.0 in 1946 and four major aftershocks of Ms 7.6, 7.0, 7.3, 7.1 between 1946 and 1953). The cutting-edge capabilities of the Okeanos Explorer help to increase the scope and efficiency of ocean exploration. The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and -4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. The trench is less deep where the component of subduction is larger. It is concluded that the Mohorovii discontinuity (characterized by compressional-wave velocities of about 8 km/sec) lies at 9 km. Sharing - Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in other subsequent works and to make unlimited photocopies of items in this journal for noncommercial use in classrooms to further education and science. Leg 1 will include the deployment of a NAVOCEANO ocean glider on loan to the National Ocean Service. National Oceanic and Atmospheric Administration. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. There are no active volcanoes in Puerto Rico and virgin islands. The unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. Plate convergence is highly oblique (2010), resulting in a complex crustal boundary with along-strike segmentation, strain partitioning and microplate tectonics. Anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba terrain is! Roughly rectangular, and the Puerto Rico and the Puerto Rico, it is approximately equal to the air. Main shocks located on land, are currently not considered in tsunami warnings either, to! Is atypical of Oceanic trenches, resulting in a complex crustal boundary with segmentation! 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