The free-air minimum east of the Lesser Antilles is related to underthrusting of the Caribbean plate by the Atlantic Ocean plate. 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. [22], There are intense isostatic and free-air anomalies along island arcs. The Bouguer anomaly over continents is generally negative, especially over mountain ranges. Joint analyses of the ocean bottom seismographs and land-based seismic data reveal that the swarms are locat, Historical records indicate frequent seismic activity along the north-east Caribbean plate boundary over the past 500 years, particularly on the island of Hispaniola. 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 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. The profiles of the Puerto Rico Trench are asymmetric due to the tectonic factors. The Bouger anomaly is very negative over elevated terrain. 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. 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. Because of its high population density and extensive development near the coast, Puerto Rico has a significant risk for earthquakes and tsunamis. 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. 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). For a measurement point above the reference ellipsoid, this means that the gravitational attraction of the bulk mass of the earth is slightly reduced. However, the Earth has a rugged surface and non-uniform composition, which distorts its gravitational field. geophys. Dynamically triggered offshore aftershocks, caused by passing seismic waves from main shocks located on land, are currently not considered in tsunami warnings. The normal gravity accounts for the bulk gravitation of the entire Earth, corrected for its idealized shape and rotation. Subsequent cooling thickens the lithosphere again and subsidence takes place. The hazard from tsunamis is also apparent. 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. For example, the formation and sinking of a lithospheric root may explain negative isostatic anomalies in eastern Tien Shan. Large landslide escarpments have been mapped on the seafloor north of Puerto Rico, although their ages are unknown. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. 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. 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. Legs 1 and 2 will focus on 24-hour mapping using the ships three scientific sonars as the ship travels to Puerto Rico and the U.S. Virgin Islands. 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. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and Expand 14 PDF Save Alert Plate movements have caused large magnitude earthquakes and devastating tsunamis. 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. The unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. 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. [18], More generally, the Airy isostatic anomaly is zero over regions where there is complete isostatic compensation. 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. Notice, Smithsonian Terms of This brings the free-air anomaly, which omits the correction terms for either, close to zero. Figure produced in Google Earth Pro. 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 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. We explore the interaction between thrust events on the subduction interface and strike-slip faults within the forearc region using 3-D models of static Coulomb stress change. The free-air anomaly is around +70 mgal along the Andes coast, and this is attributed to the subducting dense slab. [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]. Image courtesy of USGS. For example, a local positive anomaly may indicate a body of metallic ores. Larger surveys across the region provide evidence of a relict subduction zone. For example, a mass of dense ore below the surface will give a positive anomaly due to the increased gravitational attraction of the ore. Different theoretical models will predict different values of gravity, and so a gravity anomaly is always specified with reference to a particular model. Astrophysical Observatory. Gravity anomalies and convection currents. A 1918 magnitude 7.5 earthquake resulted in a tsunami that killed at least 91 people in northwestern Puerto Rico (figure 4). For example, the northeast-southwest trending high across central New Jersey represents a graben of Triassic age largely filled with dense basalts. A tilted carbonate platform provides evidence for extreme vertical tectonism in the region.This platform was horizontally deposited 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. 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. 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. 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. 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. U.S. Department of Commerce The Bouguer, free-air, and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity. Figure 1. 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 ]). 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. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different . The Puerto Rico trench negative free-air gravity anomaly belt extends from south of Barbados, around the Antillean arc, to eastern Cuba. Thesis (Sc.D.) 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. 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. [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. Uri ten Brink, Chief Scientist The residual gravity anomalies are then consistent with the existence of a subcrustal dense mass, that could be the hanging slab of lithosphere. The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water depths exceeding 8,400 meters. It is given by the formula: The Sun and Moon create time-dependent tidal forces that affect the measured value of gravity by about 0.3 mgal. Click image for larger view and image credit. 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. 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. 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. There is a large Bouger positive, of over 350 mgal, beyond 1,000 kilometers (620mi) from the ridge axis, which drops to 200 over the axis. Exclusive Economic Zone (EEZ) around Puerto Rico and the U.S. Virgin Islands. 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. [10], The next correction is the free-air correction. The rocks are naturally full of fractures and faults and given enough push, some of these faults may move abruptly to relieve the stress;causing earthquakes. Google Scholar. Cooling of continental lithosphere by thermal conduction to the surface following a heating event imposes a load on the lithosphere and causes subsidence as the basement rocks contract. Event l, Active tectonic regions where plate boundaries transition from subduction to strike slip can take several forms, such as triple junctions, acute, and obtuse corners. We document the spatial and temporal variability of. [8], The local topography of the land surface affects the gravity measurement. 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. The . To explore along with the expedition team, visit this page. Local measurements of the gravity of Earth help us to understand the planet's internal structure. This effect is very well understood and can be calculated precisely for a given time and location using astrophysical data and formulas, to yield the tidal correction gtid. 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 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 (Fig ure 2). The Bouguer anomaly includes only the negative correction for the high terrain and so is strongly negative. 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. 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. This is taken into account by the terrain correction gT. 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 unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. The slumped material comprises carbonate blocks, which are cohesive and the edge of the carbonate platform is steeper than most continental slopes, resulting in a higher potential runup To help understand the origin of the unusual bathymetry, gravity, and vertical tectonics of the plate boundary and to provide constraints for hazard assessment, the morphology of the entire 770-km-long trench from the Dominican Republic in the west to Anguilla in the east was mapped with multibeam echosounder during three cruises in 2002 and 2003. 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. (or is it just me), Smithsonian Privacy [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. 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. During Leg 3, encompassing most of the month of April, scientists will be exploring the U.S. A 1918 magnitude 7.5 earthquake resulted in a tsunami that killed at least 40 people in northwestern Puerto Rico. 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. towards 2541. Puerto Ricosrocky island crustand its surrounding seafloor are squeezedbetween the tectonic plates. [11], The remaining gravity anomaly at this point in the reduction is called the free-air anomaly. A lock () or https:// means youve safely connected to the .gov website. 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. 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. 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. Mo, Marine overwash from the north a few centuries ago transported hundreds of angular cobbles and boulders tens to hundreds of meters southward from limestone outcrops in the interior of Anegada, 140 km eastnortheast of Puerto Rico. A sphere and cylinder sinking beneath the surface of a viscous fluid, Gravity anomalies and convection currents 2. Contact Us (or is it just me), Smithsonian Privacy Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none have gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. 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. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. II - The Puerto Rico trench and the mid- Atlantic rise. The Caribbean plate is roughly rectangular, and it slides eastward at about 2 cm/yr relative to the North American plate. Share sensitive information only on official, secure websites. 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. The result, a refinement of previous work, was, Summary . Download image (jpg, 44 KB). An official website of the United States government. Weather permitting scientists expect to execute 20 dives with NOAAs remotely operated vehicle (ROV) Deep Discoverer. 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. The trench is less deep where the component of subduction is larger. Figure 3. A tsunami killed 40 people in NW Puerto Rico following a magnitude 7.3 earthquake in 1918. Use, Smithsonian 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 value of this excess mass depends upon more arbitrary assumptions for the crustal mass in the Puerto Rico Trench and its landward wall, but if the other assumptions above are realistic, the dense mass is required and is adequate to bend the surface down at the trench. [24], Gravity anomalies provide clues on other processes taking place deep in the lithosphere. Carl Bowin, 1972. Other Puerto Rican cities also have substantial risk.The hazard from tsunamis is also apparent. The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative Approximate operating area (white polygons) of NOAA Ship Okeanos Explorer during Leg 3. 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 difference between the corrected measured gravity and the normal gravity is the gravity anomaly.[6]. Plate convergence is highly oblique (2010), resulting in a complex crustal boundary with along-strike segmentation, strain partitioning and microplate tectonics. ago over a time period as short as 14-40 kyr. Careful analysis of the gravity data allows geologists to make inferences about the subsurface geology. The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. 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. 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. 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. Agreement NNX16AC86A, Journal of Geophysical Research (Solid Earth), Is ADS down? 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). On this Wikipedia the language links are at the top of the page across from the article title. A lock ( Search for other works by this author on: Some aspects of the gravity field and tectonics of the northern Caribbean region, Trans, Fifth Carib. 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. 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 Bathymetry of the northeast corner of the Caribbean plate. 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 active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. Lateral variations in gravity anomalies are related to anomalous density distributions within the Earth. Over 4 million U.S. citizens live on these islands, mostly along the coast. A positive anomaly is associated with the Ivrea body, a wedge of dense mantle rock caught up by an ancient continental collision. Numerical simulation of the 1918 Puerto Rico tsunami showing calculated inundation of the Aguadilla coast in northwest Puerto Rico. Semantic Scholar is a free, AI-powered research tool for scientific literature, based at the Allen Institute for AI. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. 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. Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none has gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study. Leg 1 will include the deployment of a NAVOCEANO ocean glider on loan to the National Ocean Service. These satellite missions aim at the recovery of a detailed gravity field model of the Earth, typically presented in the form of a spherical-harmonic expansion of the Earth's gravitational potential, but alternative presentations, such as maps of geoid undulations or gravity anomalies, are also produced. 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.