Abstract: The main purpose of this study is to develop ground motion prediction equations (GMPEs) for the Gulf Coast region of the United States using a hybrid empirical method (HEM). This project contributes toward development of a new set of GMPEs for the Gulf Coast region, which will be consistent with the available recordings. This research supplement recent GMPEs developed by the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation (NGA-East) GMPEs for the Central and Eastern North America (CENA) regions. Recently, a number of GMPEs for CENA are developed as part of NGA-East project conducted by the PEER. However, in majority of them, ground motions recorded in the Gulf Coast region were excluded due to considerably different attenuation attributes in this region (EPRI, 1993). The Gulf Coast region exhibits significantly different ground-motion attenuation because of the thick sediments in the region (Dreiling et al. 2014). The purpose of this study is to develop specific GMPEs for use in the Gulf Coast region using the HEM. Because the strong motion data set is sparse in the Gulf Coast region, the hybrid empirical method represents an appropriate and robust approach which has been generally accepted to develop GMPEs.
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Abstract: SPiRaL version 1.0 is a global-scale joint image of shear and compressional wave speeds derived from millions of travel time arrivals and global surface wave dispersion estimates for Rayleigh and Love waves. We incorporate several modeling elements used to construct the previous LLNL-G3D series of models (Simmons et al. 2011; 2012; 2015) including multiple-event relocation procedures, multi-resolution imaging with spherical tessellation hierarchies, joint inversion with mineral physics constraints and 3-D ray tracing for P- and S-wave phases at regional and teleseismic distances. The new model also incorporates surface wave dispersion estimates for Rayleigh and Love group and phase velocities from recent surface wave maps spanning periods from 25 to 200 seconds (Ma et al. 2014; Ma and Masters 2014). The SPiRaL model consists of over 1.7 million nodes with 5 modeled values at each node including Vp, Vs, and 3 parameters needed to fully account for transverse isotropy for P-, Sh-, and Sv-waves at any arbitrary direction of travel. The bulk (average) strength of anisotropy in the upper mantle is consistent with past waveform- and surface-wave based studies, however there is an indication that the radial anisotropy in the transition zone might be stronger than previously realized. The SPiRaL model represents a significant step towards a global-scale model, with regional-scale details, that can predict multiple seismic observables significant to seismic monitoring including accurate travel times at all distances as well as waveform features important to source characterization. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
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Abstract: The Nevada Bureau of Mines and Geology and the Nevada Seismological Laboratory convened the Working Group on Nevada Seismic Hazards in February 2018. The purpose of the workshop was to review ongoing earthquake hazard research in Nevada, provide insight and advice regarding technical issues related to earthquake hazards, and to identify priorities for future research that will benefit the National Seismic Hazard Map (NSHM). The workshop will occur after the abstract deadline for this meeting, so outcomes were not known when this abstract was prepared. Our presentation will summarize the outcomes of the workshop as they pertain to future updates of the NSHM. Specifically, we will present research priorities for paleoseismic, seismologic, geophysical, and geodetic studies determined to be important for improving the NSHM in the Reno/Carson/Lake Tahoe and Las Vegas urban areas. Building on the outcomes of the workshop, it is anticipated that future collaborative research between the USGS, State government, and academia will contribute valuable information to the NSHM.
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Abstract: Northwestern Himalaya is one of the most tectonically active domains of the Himalaya. This complex collisional tectonic setup is able to produce destructive earthquakes, most recent being the 8 October 2005 Kashmir earthquake (Mw 7.6). In this study, we present the probabilistic seismic hazard assessment of the Kashmir basin of northwestern Himalaya. The motivation for this study is the active tectonic setup surrounding a major population centre in the NW Himalaya, home to about 7 million people. Especially, when historical archives and instrumental earthquake records prove that the basin has suffered damage from historical as well as the recent earthquakes. The seismic hazard is assessed using point (zone free seismicity) and areal (seismic source zones) source models, employing appropriate ground motion prediction equations to predict the ground motions. The seismic hazard maps are expressed in terms of g, with seismic hazard curves and design response spectra at 5% damping for four major towns of the basin at the engineering bedrock. The hypocentral depth-wise hazard maps are shown in ranges of 0-25 Km, 25-70 Km and >70 Km with 10% probability of exceedance in 50 years. This computation is based on smoothly-gridded seismicity for each respective depth zone with a return period of 475 years. While as based on seismic source zones the seismic hazard maps show predicted peak ground acceleration (PGA) and Pseudo Spectral acceleration (PSA) with 2% and 10% probability of exceedance within 50 years for engineering bedrock sites. The PSA maps are expressed in g at 0.2 sec and 1 sec. From this study it is evident that, overall Kashmir basin shows a high seismic hazard, with southeastern part showing a higher hazard as compared to northwestern part. Among the major towns all show high predicted PGA, Anantnag shows the highest (0.65 g) with 10% probability of exceedance in 50 years. The present study thus advocates a significantly higher seismic hazard as compared to the BIS (2002) and consequently recommends updating of the building codes in the region.
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Abstract: In October 2014, a 2-week long slow slip event (SSE) occurred near Gisborne at the northern Hikurangi Margin, New Zealand. It was recorded by offshore instruments, deployed by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. This study uses data compiled from May 2014 to July 2015, recorded on 15 HOBITSS ocean bottom seismometers as well as 12 Geonet stations around the time of this uniquely recorded SSE. We use the S-wave splitting technique to detect stress and fluid changes associated with slow slip. The shear wave splitting fast polarization direction is often inferred to represent the maximum horizontal stress directions (SHmax) in crustal studies and has been shown to vary temporally at volcanoes and in association with large earthquakes. Using Multiple Filter Automatic Splitting Technique (MFAST) we analyse more than 3000 local earthquakes to look for temporal changes in S-wave fast polarization directions and delay times during the Gisborne 2014 SSE. Because S-wave splitting results are sensitive to variations in earthquake locations, we also analyse results from individual spatial earthquake clusters to test the robustness of temporal changes and to better indicate where the measured anisotropy originates. The mean fast directions in the area show a NE-SW trending fast polarization direction similar to local SHmax directions mapped by previous studies and to local fault trends. Preliminary SWS results at LOBS stations show approximately 5-10 degrees of change in fast polarization direction from before to after the SSE. We also observe increased delay times (~ 0.5 seconds) occurring around the time of the SSE with a slow decrease in delay time in the following months. We hypothesize an initial movement of fluids in cracks due to changes in stress during the SSE, followed by a relaxation period where cracks slowly return to their previous state.
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Abstract: The Dominican Republic (DR) comprises the eastern two-thirds of the island of Hispaniola, which is located on the Caribbean-North American plate boundary. Geological studies have led to a description of the shallow structure and stratigraphy of the island but little is known about the lower crust and mantle due to a dearth of broadband seismic data. Since August 2013 a seismic network consisting of 20 broadband, three-component seismic stations has been deployed in the DR. Here we report on a receiver function study of crustal thickness and bulk composition using teleseismic P-wave data. We compute teleseismic receiver functions using an iterative time domain deconvolution technique and determine crustal properties via H-κ stacking. Three distinct, fault-bounded crustal domains, defined by characteristic Moho depths and bulk crustal Vp/Vs, are found in the DR. In the northern domain, the Moho has an average depth of ~32 km with Vp/Vs ratio 1.84 ± 0.1. Moho depths in the southern domain average ~ 33 km and Vp/Vs averages 1.68 ± 0.1. Thicker crust is found beneath the Cordillera Central, in the island’s center, where Moho depths average ~ 39 km and Vp/Vs is ~1.72 ± 0.1, while the crust at coastal areas of Dominican Republic has Moho depths of ~ 29 km and Vp/Vs of 1.77 ± 0.1. An E-W transect across the island’s center and four N-S transects were produced to show variations of crustal thickness and bulk Vp/Vs ratios. We interpret the southern domain to be a terrane accreted from the Caribbean Large Igneous Province. The thicker crust and average Vp/Vs ratio in the island’s center likely indicates an igneous root of the Cordillera Central, which includes Pico Duarte, the highest peak in the Caribbean.
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