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. The free-air minimum east of the Lesser Antilles is related to underthrusting of the Caribbean plate by the Atlantic Ocean plate. This shows that the low elevation of ocean basins and high elevation of continents is compensated by the thickness of the crust at depth. 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 arises from the low-density ocean water and sediments filling the trench. [29][30] The lowest-degree parameters, including the Earth's oblateness and geocenter motion are best determined from Satellite laser ranging.[31]. Some reports indicate that nearly 1,800 people drowned. 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 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 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. Over a few hundreds or thousands of years several meters of motion accumulate resulting inoccasionalearthquakes and associated landslides and tsunamis. 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). 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. Image courtesy of USGS. The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. Its northward edge is at a depth of 4,200 m, and its southern edge can be found on land in Puerto Rico at an elevation of a few hundred meters. 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. 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. 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. 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. Puerto Ricosrocky island crustand its surrounding seafloor are squeezedbetween the tectonic plates. Careful analysis of the gravity data allows geologists to make inferences about the subsurface geology. Un. 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 trench itself is very negative,[23] with values more negative than 250 mgal. Astrophysical Observatory. The unusually deep sea floor is not limited to the trench and extends farther south toward Puerto Rico. Further refinements of the model field are usually expressed as corrections added to the measured gravity or (equivalently) subtracted from the normal gravity. These anomalies reflect the varying thickness of the Earth's crust. Other Puerto Rican cities also have substantial risk. 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 ]). Plate movements have caused large magnitude earthquakes and devastating tsunamis. The profiles of the Puerto Rico Trench are asymmetric due to the tectonic factors. The surveyed island, Anegada, is 120 km south of the Puerto Rico Trench and is near the, The Virgin Islands and Whiting basins in the Northeast Caribbean are deep, structurally controlled depocentres partially bound by shallow-water carbonate platforms. 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. 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 (figure 1). Conf., Geol. 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. For example, the formation and sinking of a lithospheric root may explain negative isostatic anomalies in eastern Tien Shan. [24], Gravity anomalies provide clues on other processes taking place deep in the lithosphere. In both of these cases the driving mass has an optimum depth of about 100 km. Erosion is a concern in many coastal areas, but is particularly serious to island economies that rely heavily on the tourist industry. However, present-day slope seafloor-morphology is complicated by shaping processes, which are not readily separable through pure bathymetric analysis. This is taken into account by the terrain correction gT. A tilted carbonate platform provides evidence for extreme vertical tectonism in the region. The region has high seismicity and large earthquakes. Leg 1 will include the deployment of a NAVOCEANO ocean glider on loan to the National Ocean Service. This gravity anomaly can reveal the presence of subsurface structures of unusual density. The transit area is in the Western Atlantic Ocean from Rhode Island straight to Puerto Rico. For example, a mass of dense ore below the surface will give a positive anomaly due to the increased gravitational attraction of the ore. That is, the Bouguer anomaly is:[12], The Bouguer anomaly is positive over ocean basins and negative over high continental areas. 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. 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. The active island arc of Lesser Antilles marks the junction between the Atlantic and Carribbean lithospheric plates. Google Scholar. Image courtesy of the NOAA Office of Ocean Exploration and Research. Below are publications associated with this project. 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 kyr. This site uses cookies. Exclusive Economic Zone surrounding Puerto Rico and the U.S. Virgin Islands, and the Puerto Rico Trench. The hazard from tsunamis is also apparent. [8], The local topography of the land surface affects the gravity measurement. --Massachusetts Institute of Technology, Dept. Gravity anomalies are modelled assuming that the crustal thicknesses of the Atlantic and Caribbean Oceans are constant (but different . The gravitational attraction of the high terrain is balanced by the reduced gravitational attraction of its underlying low-density roots. Figure 2. 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. This is tedious and time-consuming but necessary for obtaining a meaningful gravity anomaly. 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. 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. However, the Earth has a rugged surface and non-uniform composition, which distorts its gravitational field. Dynamically triggered offshore aftershocks, caused by passing seismic waves from main shocks located on land, are currently not considered in tsunami warnings. 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. 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. Notice, Smithsonian Terms of 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. Weather permitting scientists expect to execute 20 dives with NOAAs remotely operated vehicle (ROV) Deep Discoverer. 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 ridge and its flanks appear to be fully isostatically compensated. Download Citation | Gravity anomalies and the origin of the Puerto Rico Trench | The Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. Fifteen years later, Isaac Newton used his newly formulated universal theory of gravitation to explain the anomaly. 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. [13], The remaining gravity anomaly at this point in the reduction is called the Bouguer anomaly. 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. 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 Puerto Rico Trench is assumed to be caused by a downwards bending of the Atlantic lithosphere. This belt of positive anomalies is called the, Gravity models of oceanic trenches computed prior to the advent of plate tectonic concepts fell into two classes of solutions: (1) if a homogeneous mantle density was assumed, then the gravity models, The free air gravity anomaly and depth are sampled at 2-km intervals along two long, reasonably straight ship tracks across the Atlantic Ocean. At the center of a level plateau, it is approximately equal to the free air anomaly. For example, a local positive anomaly may indicate a body of metallic ores. The gravitational body force that acts on a flap of lithosphere, hanging in the aesthenosphere and Expand 14 PDF Save Alert NOTICE: This page is being retained for reference purposes only and is no longer being updated or maintained. Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. This study aims to explore the utility of bathymetric spectral decomposition in order to separate and char, The submerged carbonate platform north of Puerto Rico terminates in a high (3,0004,000 m) and in places steep (>45) slope characterized by numerous landslide scarps including two 3050 km-wide amphitheater-shaped features. For more information on past exploration of this region by the NOAA Office of Ocean Exploraiton and Research, visit this page. 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 ]). Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Large tsunamis have also hit Puerto Rico and Hispaniola, reportedly killing 1800 people in 1946 and 40 people in 1918. The origin of the steep platform edge and the amphitheaters has been attributed to: (1) catastrophic failure, or (2) localized failures and progressive erosion. 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. 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. 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 proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5? Click image for larger view and image credit. Thesis (Sc.D.) 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. A positive anomaly is associated with the Ivrea body, a wedge of dense mantle rock caught up by an ancient continental collision. Erosion is a concern in many coastal areas, but is particularly serious to island economies that rely heavily on the tourist industry. Other corrections are added for various gravitational models. Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes. Please check your email address / username and password and try again. The free-air anomaly is around +70 mgal along the Andes coast, and this is attributed to the subducting dense slab. Correcting for these effects removed most of this anomaly. A 1918 magnitude 7.5 earthquake resulted in a tsunami that killed at least 40 people in northwestern Puerto Rico. Immediately after the 1946 earthquake, a tsunami struck northeastern Hispaniola and moved inland for several kilometers. The terrain correction is calculated from knowledge of the local topography and estimates of the density of the rock making up the high ground. 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. Secure .gov websites use HTTPS 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 seabed of the Cayman Trough is flat at the segment (80.0W and 17.7N to 78.5W and 19.5N). 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. These are indications of strong dynamic effects in subduction zones. Studies of free-air gravity anomaly profiles across island arcs show an important belt of positive anomalies seaward of deep-sea trenches. 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 risk to life and economic infrastructure is significant due to the fact that over 3 million U.S. citizens live along the coastlines of Puerto Rico and the Virgin Islands. Official websites use .gov 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. Click image for larger view and image credit. [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. In both of these cases the driving mass has an optimum depth of about 100 km. Van Houten, C. A. Burk, H. D. Holland, L. C. Hollister. The Bouguer, free-air, and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity. 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. Similar convergence geometry is observed at the Challenger Deep in the Mariana Trench, the deepest point on Earth. Because the island lies on an active plate boundary, earthquakes are a constant threat, and the densely populated coastal areas are vulnerable to tsunamis. This latter deformation caused the disappearance of a trench opposite the southern Lesser Antillean arc and a displacement of the Puerto Rico trench axis oceanward away from the axis of the negative free-air anomaly belt near the northeast corner of the Caribbean plate. Images of the slope north of Puerto Rico disclose massive slope failure scars, as much as 50 km across, that probably generated tsunamis along the north shore of the island. Contact Us Numerical simulation of the 1918 Puerto Rico tsunami showing calculated inundation of the Aguadilla coast in northwest Puerto Rico. 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. E-mail Updates | User Survey | Contact Us | Report Error On This Page | Privacy Policy | Disclaimer | Site Info | Site Index Secure .gov websites use HTTPS 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 unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. The line of trend of historic volcanoes of the Lesser Antillean arc is equidistant (160 km) from the axis of minimum free-air anomaly. subducts, or slides, under another one. 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. 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. Search for other works by this author on: Some aspects of the gravity field and tectonics of the northern Caribbean region, Trans, Fifth Carib. Disclaimer A lock ( Large-scale gravity anomalies can be detected from space, as a by-product of satellite gravity missions, e.g., GOCE. Uri ten Brink, Chief Scientist Subsequent cooling thickens the lithosphere again and subsidence takes place. This brings the free-air anomaly, which omits the correction terms for either, close to zero. 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. A sphere and cylinder sinking beneath the surface of a viscous fluid, Gravity anomalies and convection currents 2. A lock () or https:// means youve safely connected to the .gov website. 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. U.S. Department of Commerce 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. Plate convergence is highly oblique (2010), resulting in a complex crustal boundary with along-strike segmentation, strain partitioning and microplate tectonics. 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. [26], Local anomalies are used in applied geophysics. 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. It is concluded that the Mohorovii discontinuity (characterized by compressional-wave velocities of about 8 km/sec) lies at 9 km. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide, This PDF is available to Subscribers Only. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. United States. Its depth is comparable to the deep trenches in the Pacific Ocean. (or is it just me), Smithsonian Privacy [15] The isostatic correction is dependent on the isostatic model used to calculate isostatic balance, and so is slightly different for the Airy-Heiskanen model (which assumes that the crust and mantle are uniform in density and isostatic balance is provided by changes in crust thickness), the Pratt-Hayford model (which assumes that the bottom of the crust is at the same depth everywhere and isostatic balance is provided by lateral changes in crust density), and the Vening Meinesz elastic plate model (which assumes the crust acts like an elastic sheet). Larger surveys across the region provide evidence of a relict subduction zone. of Earth and Planetary Sciences, 1981.