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Relocation of Earthquakes Western Puerto Rico Region Using 17th Caribbean Geological Conference 2005, San Juan PR Relocation Of Earthquakes Western Puerto Rico Region Using Waveform Cross Correlation And Double Difference Techniques Abreu Rafael, Eugenio Asencio and Jay Pulliam Department of Geology, University of Puerto Rico at Mayagüez; Stephanie Ross, Andrew Michael and Shirley Baher USGS Menlo Park, California The Puerto Rico Region has a significant seismic hazard due to large local, although relatively infrequent, earthquakes. To properly quantify this hazard we need to improve our understanding of this seismicity by identifying and understanding specific fault structures in the region. It is important to determine the location of such faults because the hypocentral distance affects the amount of on-land shaking, which is a factor used for earthquake planning by government agencies. Determining fault structure is important because it helps define fault segmentation, which controls the sizes and expected repeat times of earthquakes. In this study, we seek to improve earthquake locations recorded by the Puerto Rico Seismic Network (PRSN) by using waveform cross-correlation and double- difference relocation techniques. This project is a collaborative effort between the U.S. Geological Survey, in Menlo Park, the Department of Geology of the University of Puerto Rico at Mayagüez and the Puerto Rico Seismic Network. Standard PRSN locations for the region show a diffuse cloud of seismicity distributed over a broad region, preventing proper correlation of events to specific active seismogenic sources. The methods mentioned above have the advantage of simultaneous analysis of large seismic datasets, significantly improving the accuracy of earthquake locations and allowing the identification of clusters of earthquakes belonging to specific faults. Our preliminary results show improved definition of active seismogenic sources in the western part of the island of Puerto Rico. Receiver Functions And SKS Splitting Measurements At Three Tectonically Distinct Locations Around The Caribbean Support The Continental Undertow Hypothesis Baez Sanchez Gisela, Rafael Abreu París, Antonio Cameron Gonzales, Eugenio Asencio, and Jay Pulliam Red Sísmica de Puerto Rico, Dept. de Geología, Universidad de Puerto Rico, Mayagüez, PR The degree to which plate tectonic processes are controlled and/or affected by the Earth’s mantle is unclear, yet tectonic processes that deform the Earth’s crust and upper mantle apparently leave seismically-observable signatures in the form of large-scale anisotropy and some structural features of the crust appear to have analogs in the upper mantle. To examine remanent deformation we can study shear waves that penetrate the Earth’s core. These waves (called “SKS phases”) are converted to compressional waves for their path through the outer core and, when they re-enter the mantle and are converted back into shear waves, their energy is polarized in the SV plane. If, when recorded at broadband seismographic stations at the Earth’s surface, rotated into the proper, great- 1 17th Caribbean Geological Conference 2005, San Juan PR circle frame of reference, and examined, these SKS phases have energy polarized in the SH direction, it means that the waves have encountered an anisotropic material somewhere along their path from the core to the surface. We observed such “bi-refringence” or “SKS splitting” on the three broadband stations around the Caribbean that have been operating the longest: SDV in Venezuela, FDF in Martinique, and SJG in Puerto Rico. We measured the polarization direction and time lag indicated by the bi-refringence. SDV completes a transition between station BOCO, to the southwest, and stations MNVV,HSPV, and ECPV, to the northeast that suggests bending of the upper mantle deformation around the continental root of South America. This is similar to the pattern shown—more densely in their case—for SKS splitting parameters around the continental root of North America by Fouch et al. (2000). The two oceanic islands have very different signatures. Results for SJG indicate a generally east- west polarization and are consistent with the results of Russ et al. (1996). The polarization found for FDF differs dramatically, however—it is approximately north- south in orientation. Potential interpretations—including possible sources of error—will be discussed for this result. “Receiver functions”, formed by deconvolving the vertical seismic trace from the radial trace to isolate the shear wave reverberations in the crust and upper mantle, allow us to build an image of sharp boundaries (“layers”) in the Earth. We applied this technique to stations SDV, FDF, and SJG in order to study the deep structure beneath these distinct Caribbean locations. A synthesis of the receiver function and SKS interpretations will be presented in the context of Caribbean seismicity, plate motions, and other geophysical observables. Late Cretaceous and early Tertiary Radiolaria from the Pacific coast of Costa Rica Bandini, A.N. & Baumgartner, Institute of Geology and Palaeontology, BFSH2, University of Lausanne, 1015 Lausanne, Switzerland. [email protected], [email protected] We are in the process of a systematic study of late Cretaceous and early Tertiary radiolarian bearing siliceous mudstones and cherts from the Nicoya and Osa peninsulas. In these areas a variety of oceanic and arc derived terranes include mafic volcanics and associated pelagic and hemipelagic rocks rich in biogenic silica which has favoured the preservation of common moderately to well preserved radiolarians. Late Cretaceous to early Tertiary palaeoenvironments along the pacific margin were characterised by high terrestrial nutrient input derived from volcanic sources and active up-welling. High radiolarian productivity is mostly caused by high nutrients concentration in surface waters (Yamashita et al., 2002). Radiolarian preservation in the sediment is often enhanced by the presence of silica-saturated volcanic tuffs and debris. Currently, we have dated the Berrugate Formation and rocks mapped as Sabana Grande Formation in the Nicoya Peninsula. The Berrugate Formation is a thick hemipelagic- turbiditic sequence containing arc-derived volcanoclastic deposits (Flores et al. 2003). On the bases of published zonations we can state a Coniancian radiolarian age for the Berrugate Formation. We have determined the co-occurrence of Dictyomitra koslovae, 2 17th Caribbean Geological Conference 2005, San Juan PR Pseudoaulophacus florensis, Pseudoaulophacus lenticulatus and Theocampe salillum (species that have not been cited earlier than Coniacian) of with Stichomitra communis (a species that has not been cited later than Coniacian). The Sabana Grande formation consists of pelagic, hemipelagic and turbiditic siliceous and calcareous shales and mudstones (Flores et al. 2003). This formation is currently assumed to be of Cenomanian-Turonian age on the base of its stratigraphic position below the Nambi and Piedra Blancas formations that only indicate a pre-Campanian age. However, the formation itself has not been directly dated. In addition, many isolated outcrops of Sabana Grande lithologies are not stratigraphically constrained and could therefore have a different age. Such an outcrop, sampled east of Nicoya, has yielded an early Eocene age by the occurrence of Phormocyrtis striata striata. More data will be presented during the conference, since our laboratory work is currently in progress. Figure 1 SEM-illustrations of late Cretaceous and early Tertiary radiolarians from the Nicoya Peninsula, Costa Rica : 1. Dictyomitra koslovae Foreman, 2. Theocampe salillum Campbell & Clark, 3. Stichomitra communis Squinabol, 4a-b. Pseudoaulophacus lenticulatus White, 5a-b. Pseudoaulophacus floresensis Pessagno, 6. Phormocyrtis striata striata Brandt References Flores. K., Denyer, P. & Aguilar, T. (2003): Nueva propuesta estratigráfica: Geología de las hojas Matambú y Talolinga, Guanacaste, Costa Rica.- Rev. Geol. América Central, 28, 131-138. Yamashita, H., Takahashi, K. & Fujitani, N. (2002): Zonal and vertical distribution of radiolarians in the western and central Equatorial Pacific in January 1999. Deep- Sea Research, II, 49, 2823–2862. 3 17th Caribbean Geological Conference 2005, San Juan PR Water Resources Development In Barbados--- A Geological Perspective. Barker Leslie H. 2005 Barbados is a small island developing statewhich is the most easterly of the Lessser Antilles Island Arc. Stratigraphically it is composed of three main rock groups: The Basals or Scotland formation , a group of deep water turbidites of mid. to upper Eocene age ; The Oceanic formation, a group of deep water argillaceous deposits of contemporaneous age with the Scotlands; and a thin cover of Pleistocene to Recent reefal limestones which cover some six- sevenths of the island. This stratigraphic sequence together with the Pleistocene to Recent tectonic history of the island have formed the basis for the mode of occurrence of ground water on the island. The general lack of surface water is due to the widespread occurrence of permeable limestone on the island. The ground water occurs in a coastal phreatic aquifer which stretches almost entirely around the island except in the east where the Scotland formation outcrops. With the increasing demand for potable water one of the major problems facing us todayis the need for optimizing the exploitation of groundwater in the face of increasing seawater intrusion. Although engineering science has played a major
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