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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access. Over a few hundreds or thousands of years several meters of motion accumulate resulting inoccasionalearthquakes and associated landslides and tsunamis. The region has high seismicity and large earthquakes (figure 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. ) or https:// means youve safely connected to the .gov website. [29][30] The lowest-degree parameters, including the Earth's oblateness and geocenter motion are best determined from Satellite laser ranging.[31]. U.S. Department of Commerce 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 ]). Numerical simulation of the 1918 Puerto Rico tsunami showing calculated inundation of the Aguadilla coast in northwest Puerto Rico. Location of earthquakes as a function of depth and size in the northeastern Caribbean. 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. The free-air anomaly is also close to zero except near boundaries of crustal blocks. 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. The transit area is in the Western Atlantic Ocean from Rhode Island straight to Puerto Rico. However, present-day slope seafloor-morphology is complicated by shaping processes, which are not readily separable through pure bathymetric analysis. 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) between St. Thomas ad St. Croixin the Anegada Trough. The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative 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). 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. The Bouguer, free-air, and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity. Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. We use accounts of historical earthquakes to assign intensities and the intensity assignments for the 2010 Haiti earthquakes to derive an intensity attenuation relation for Hispaniola. Agreement NNX16AC86A, Journal of Geophysical Research (Solid Earth), Is ADS down? Secure .gov websites use HTTPS Download image (jpg, 44 KB). [28], Currently, the static and time-variable Earth's gravity field parameters are being determined using modern satellite missions, such as GOCE, CHAMP, Swarm, GRACE and GRACE-FO. Erosion is a concern in many coastal areas, but is particularly serious to island economies that rely heavily on the tourist industry. 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. Its profile is asymmetric: northern part is steep (about 57), southern part is about 16. Use, Smithsonian Because the island lies on an active plate boundary, earthquakes are a constant threat, and the densely populated coastal areas are vulnerable to tsunamis. Large-scale gravity anomalies can be detected from space, as a by-product of satellite gravity missions, e.g., GOCE. Official websites use .gov The Bouguer anomaly over continents is generally negative, especially over mountain ranges. 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. 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). The theoretical value of gravity can be corrected for altitude and the effects of nearby terrain, but it usually still differs slightly from the measured value. Undeformed trench turbidites terminate abruptly against the inner wall which is acoustically opaque for the next, 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. [21], Over mid-ocean ridges, the free-air anomalies are small and correlate with the ocean bottom topography. These are indications of strong dynamic effects in subduction zones. of Earth and Planetary Sciences, 1981. The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water depths exceeding 8,400 meters. 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. Seattle). Please check your email address / username and password and try again. [24], Gravity anomalies provide clues on other processes taking place deep in the lithosphere. Large tsunamis have also hit Puerto Rico and Hispaniola, reportedly killing 1800 people in 1946 and 40 people in 1918. Journal of Geophysical Research B: Solid Earth. 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. 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. Viscous fluid model estimates of crustal thickness and free-air anomaly compared with measured free- air anomalies and seismic depths of Puerto Rico trench and mid-Atlantic rise Document ID 19660037496 Document Type Holocene and Pleistocene tectonic deformation of the coast in the Mexico subudction margin is recorded by geomorphic and stratigraphic markers. 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. Here the axis of the free-air minimum lies very close to the eastern border of a zone of epicenters which apparently marks the commencement of crustal faulting along the underthrust. Geol. Widespread damage in Puerto Rico and Hispaniola from an earthquake in 1787 was estimated to be the result of a magnitude 8 earthquake north of the islands. Lateral variations in gravity anomalies are related to anomalous density distributions within the Earth. Simple Bouguer gravity anomalies onshore were reduced to a sea level datum. Click image for larger view and image credit. A crustal section across the Puerto Rico trench, J. geophys. 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. To explore along with the expedition team, visit this page. Puerto Rico, the Virgin Islands to its east, and eastern Hispaniola to its west, are located on an active plate boundary zone between the North American plate and the northeast corner of the Caribbean plate. Summary. The Puerto Rico Trench and the Mid-Atlantic Rise, Gravity anomalies seaward of deep-sea trenches and their implications, Geophysics-steered self-supervised learning for deconvolution, Local estimation of quasi-geostrophic flows in Earths core, Bayesian Detectability of Induced Polarisation in Airborne Electromagnetic Data, Analytical computation of total topographic torque at the Core-Mantle Boundary and its impact on tidally driven Length-of-Day variations, The ground deformation of the south-eastern flank of Mount Etna monitored by GNSS and SAR interferometry from 2016 to 2019, Volume 233, Issue 3, June 2023 (In Progress), Volume 233, Issue 2, May 2023 (In Progress), Volume 233, Issue 1, April 2023 (In Progress), Volume 234, Issue 1, July 2023 (In Progress), Geomagnetism, Rock Magnetism and Palaeomagnetism, Marine Geosciences and Applied Geophysics, https://doi.org/10.1111/j.1365-246X.1977.tb04215.x, Receive exclusive offers and updates from Oxford Academic, Copyright 2023 The Royal Astronomical Society. Over four million U.S. citizens live on these islands, mostly along the coast. 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. In effect, the terrain correction levels the terrain around the measurement point. 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 is assumed to be caused by a downwards bending of the Atlantic lithosphere. Caldwell J. G. Haxby W. F. Karig D. E. Turcotte D. L.. Sutton G. H. Maynard G. L. Hussong D. M.. Talwani M. Le Pichon X. Ewing M. Sutton G. H. Worzel J. L., Oxford University Press is a department of the University of Oxford. In contrast, the Caribbean plate farther east overrides the North American plate, creating the island arc of the Lesser Antilles with its active volcanoes. Un. The overwash, after crossing a fringing coral reef and 1.5 km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. The frequency of occurrence of earthquakes with different seismic moments is expressed in terms of the rate of slip on a fault and to the largest seismic moment likely to occur in the region. Image courtesy of the NOAA Office of Ocean Exploration and Research. [16], Forward modelling is the process of computing the detailed shape of the compensation required by a theoretical model and using this to correct the Bouguer anomaly to yield an isostatic anomaly.[17]. 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. Use, Smithsonian Essentially, all of the known causes of tsunamis are present in the Caribbean -- earthquakes, submarine landslides, submarine volcanic eruptions, subaerial pyroclastic flows into the ocean, and major tsunamis called teletsunamis. E-mail Updates | User Survey | Contact Us | Report Error On This Page | Privacy Policy | Disclaimer | Site Info | Site Index The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and Expand 14 PDF Save Alert Search for other works by this author on: Some aspects of the gravity field and tectonics of the northern Caribbean region, Trans, Fifth Carib. Google Scholar. The Puerto Rico trench negative free-air gravity anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba. Note that the Puerto Rico Trench, a deep trough where the North American plate slides past and underneath the Antilles, becomes deeper and wider north of Puerto Rico. A positive anomaly is associated with the Ivrea body, a wedge of dense mantle rock caught up by an ancient continental collision. A 1918 magnitude 7.5 earthquake resulted in a tsunami that killed at least 91 people in northwestern Puerto Rico (figure 4). 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. During Leg 1 of this expedition, the ship will transit from her home port in North Kingstown, Rhode Island, to the U.S. Exclusive Economic Zone surrounding Puerto Rico and the U.S. Virgin Islands, and the Puerto Rico Trench, as well as contribute to national ocean floor mapping priorities. Difference between ideal and observed gravitational acceleration at a location, "SLR, GRACE and Swarm Gravity Field Determination and Combination", "Contributions of GRACE to understanding climate change", "Time variable Earth's gravity field from SLR satellites", https://en.wikipedia.org/w/index.php?title=Gravity_anomaly&oldid=1139571563, Short description is different from Wikidata, Creative Commons Attribution-ShareAlike License 3.0, This page was last edited on 15 February 2023, at 20:17. Probability for damaging ground motion at San Juan and Mayaguez in comparison to other seismic high-risk areas in the U.S. (e.g. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. The Puerto Rico Trench is atypical of oceanic trenches. The low-density sediments of the Molasse basin produce a negative anomaly. A 1918 magnitude 7.5 earthquake resulted in a tsunami that killed at least 40 people in northwestern Puerto Rico. The higher terrain is held up by the buoyancy of thicker crust "floating" on the mantle. 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. 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. [13], The remaining gravity anomaly at this point in the reduction is called the Bouguer anomaly. The general survey operating area will be in the Caribbean Ocean, south Puerto Rico and St. Croix islands, in the area of Turner Hole Canyon, Saba Valley, and south of the St. Croix Ridge. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5? 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. Exclusive Economic Zone (EEZ) around Puerto Rico and the U.S. Virgin Islands. Bull. Some reports indicate that nearly 1,800 people drowned. Seism, The fore-arc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. The Puerto Rico Trench, in contrast, is situated at a boundary between two plates that The trench is shallower where the component of subduction is larger. No Derivative works - You may not alter, transform, or build upon this work. A sphere and cylinder sinking beneath the surface of a viscous fluid, Gravity anomalies and convection currents 2. The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. 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. Puerto Rico and the Virgin Islands are located at an active plate boundary between the North American plate and the northeast corner of the Caribbean plate. The gravitational attraction of the high terrain is balanced by the reduced gravitational attraction of its underlying low-density roots. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Both these effects reduce the value of gravity, explaining why Richter's pendulum clock, which depended on the value of gravity, ran too slowly. 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 residual gravity anomalies are then consistent with the existence of a subcrustal dense mass, that could be the hanging slab of lithosphere. 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 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 difference between the corrected measured gravity and the normal gravity is the gravity anomaly.[6]. This will be the first Okeanos Explorer cruise to plan on conducting ROV operations 12 hours a day. For Earth, the reference ellipsoid is the International Reference Ellipsoid, and the value of gravity predicted for points on the ellipsoid is the normal gravity, gn. In addition to its extremely deep seafloor, the Puerto Rico Trench is also characterized by the most negative free-air gravity anomaly on Earth, -380 mGal, located 50 km south of the trench, where water depth is 7950 m (Figure 2). The isostatic anomaly includes correction terms for both effects, which reduces it nearly to zero as well. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. (or is it just me), Smithsonian Privacy Official websites use .gov National Oceanic and Atmospheric Administration 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. ?W and trend southwestwardly toward eastern Puerto Rico. We document the spatial and temporal variability of. This belt of positive anomalies is called the Outer Gravity High. The normal gravity accounts for the bulk gravitation of the entire Earth, corrected for its idealized shape and rotation. You could not be signed in. This shows that the low elevation of ocean basins and high elevation of continents is compensated by the thickness of the crust at depth. Its depth is comparable to the deep trenches in the Pacific Ocean. 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. This gravity anomaly can reveal the presence of subsurface structures of unusual density. Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. 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. Abstract The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. At the center of a level plateau, it is approximately equal to the free air anomaly. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide, This PDF is available to Subscribers Only. Puerto Ricosrocky island crustand its surrounding seafloor are squeezedbetween the tectonic plates. 5, Puerto Rico Trench negative gravity anomaly belt, The Caribbean: gravity field and plate tectonics, Geophysical investigation of the Puerto Rico Trench and outer ridge, Seismic refraction and reflection measurementsPuerto Rico outer ridge, Continuous seismic profiles of the outer ridge and Nares basin north of Puerto Rico, Geophysical study of Antilles outer ridge, Puerto Rico trench, and northeast margin of Caribbean Sea, On the applicability of a universal elastic trench profile, Seismic refraction and reflection in Caribbean Sea, Crustal and upper mantle structure of the Central Aleutian Arc, Evidence for high density crust and mantle beneath the Chile trench due to the descending lithosphere, The Kuril Trench-Hokkaido Rise system: Large shallow earthquakes and simple model of deformation, The present day motions of the Caribbean plate, Paleogeography and geologic history of Greater Antilles, The relationship between bathymetry and gravity in the Atlantic Ocean, Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity, Gravity anomalies and convection currents 1. This glider will be recovered several months later by NOAA Ship Nancy Foster when she is operating in the area. Newton showed that the measured value of gravity was affected by the rotation of the Earth, which caused the Earth's equator to bulge out slightly relative to its poles. The region has high seismicity and large earthquakes. A tsunami killed 40 people in NW Puerto Rico following a magnitude 7.3 earthquake in 1918. At a minimum, these include the tidal correction gtid, the terrain correction gT, and the free air correction gFA. The Puerto Rico Trench, the deepest part of the Atlantic Ocean, is located where the North American (NOAM) plate is subducting under the Caribbean plate (Figure 1). The free-air correction is simply 0.3086 mgal m1 times the elevation above the reference ellipsoid. These anomalies reflect the varying thickness of the Earth's crust. The origin of the steep platform edge and the amphitheaters has been attributed to: (1) catastrophic failure, or (2) localized failures and progressive erosion. For example, a local positive anomaly may indicate a body of metallic ores. Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause ten Brink, Uri Abstract 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 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. This brings the free-air anomaly, which omits the correction terms for either, close to zero. Similar convergence geometry is observed at the Challenger Deep in the Mariana Trench, the deepest point on Earth. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different . The seabed of the Cayman Trough is flat at the segment (80.0W and 17.7N to 78.5W and 19.5N). The rise may instead be a result of lithosphere thinning: The underlying aesthenosphere is less dense than the lithosphere and it rises to produce the swell. [19] By contrast, the Bouguer anomaly is positive over oceans. Click image for larger view and image credit. The trench is less deep where the component of subduction is larger. U.S. Geological Survey. Its northward edge is at a depth of 4,200 meters, and its southern edge can be found on land in Puerto Rico at an elevation of a few hundred meters. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. This is tedious and time-consuming but necessary for obtaining a meaningful gravity anomaly. The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. U.S. Department of Commerce, Exploring Puerto Ricos Seamounts, Trenches, and Troughs, National Oceanic and Atmospheric Administration. Bathymetry collected by Okeanos Explorer and University of New Hampshire Law of the Sea Project is shown in the background and gray boxes show the extent of existing Okeanos Explorer mapping surveys in the Atlantic. 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). [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. The free-air minimum east of the Lesser Antilles is related to underthrusting of the Caribbean plate by the Atlantic Ocean plate. Seismology: Seismic hazard assessment and prediction; Marine Geology and Geophysics: Seafloor morphology and bottom photography. An official website of the United States government. 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. This takes into account the fact that the measurement is usually at a different elevation than the reference ellipsoid at the measurement latitude and longitude. Careful analysis of the gravity data allows geologists to make inferences about the subsurface geology. Download image (jpg, 44 KB). [9], The terrain correction must be calculated for every point at which gravity is measured, taking into account every hill or valley whose difference in elevation from the measurement point is greater than about 5% of its distance from the measurement point. 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. Download image (jpg, 50 KB). 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. Larger surveys across the region provide evidence of a relict subduction zone. [26], Local anomalies are used in applied geophysics. ago over a time period as short as 14 . Image courtesy of USGS. The data collected by the Okeanos Explorer during this exploration will improve our understanding of the unique geology and biology of the U.S. Click image for larger view and image credit. The free-air minimum east of the Lesser Antilles is related to underthrusting of the Caribbean plate by the Atlantic Ocean plate. 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 intensity assignments and the attenuation re, Finding Seafloor Faults Linked to Puerto Rico Quake. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Weather permitting scientists expect to execute 20 dives with NOAAs remotely operated vehicle (ROV) Deep Discoverer. We study the oblique convergence of the NOAM and CARIB plates between southeastern Cuba to northern Puerto Rico, A post-hurricane survey of a Caribbean island affords comparisons with geologic evidence for greater overwash at the same place. Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes. For example, the northeast-southwest trending high across central New Jersey represents a graben of Triassic age largely filled with dense basalts. The large negative gravity anomaly is attributed to a . The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. Earthquakes have been documented in the northeastern Caribbean since the arrival of Columbus to the Americas; written accounts of these felt earthquakes exist in various parts of the world. Transit line down to Puerto Rico (red line) and two survey area boxes within the region around Puerto Rico and the U.S. Virgin Islands. The rapid (14-40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. 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. Examples include a magnitude 7.5 earthquake centered northwest of Puerto Rico in 1943, and magnitude 8.1 and 6.9 earthquakes north of Hispaniola in 1946 and 1953, respectively. ago over a time period as short as 14-40 Strike-slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers.

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