Abstract: Seismic gaps along the subduction zones are locations where large earthquakes have not occurred in a long time. These areas are considered locked and are accumulating large amounts of strain energy that will ultimately be released in major earthquake. The Nicoya Peninsula in northwestern Costa Rica was considered a zone with this type of seismic gap. The previous major earthquakes in Nicoya occurred on 1853, 1900 and 1950, which indicates about a 50-year recurrence interval for the characteristic earthquake cycle. With the goals to: 1) record and locate strong subduction zone mainshocks [and foreshocks, “early aftershocks”, and preshocks] in Nicoya Peninsula, at the entrance of the Nicoya Gulf, and in the Papagayo Gulf regions of Costa Rica, and 2) record and locate any moderate to strong upper plate earthquakes triggered by a large subduction zone earthquake in the above regions, a seismic strong motion array (SSMAP project) was installed in the Nicoya Peninsula, array composed of 10 sites with Geotech A900 accelerographs. Also, the OVSICORI-UNA network was upgraded with ES-T episensors that could record the large event. On September 5, 2012, a Mw=7.6 earthquake occurred in the seismic gap and appears to be the expected event based on the 50 years recurrence interval, but was instead 62 years later. The main shock focal mechanism was thrust faulting of the Cocos plate in the Middle America trench with strike N54W and dip 20 degrees NE. The mainshock location was 9.671 N and 85.878 W. The maximum accelerations from two A900 stations perpendicular to the trench, Fortuna (distance 112 km) and Pedernal (distance 128 km) were: 13.8% and 8.9 % g; although the main acceleration was recorded at Dulce Nombre de Nicoya 122% g. The October 10 (MW 5.3) and 24 (Mw 6.6) aftershocks recorded at Tamarindo were accelerations of 2.4% and 8.2% g. We also relocated 50 events from 2006 to 2016 for moderate magnitudes (4 < Mw < 6.5), mainly located in Nicoya Peninsula region.
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Abstract: Vertical ground motions have been traditionally neglected in seismic-hazard analysis, because they were believed to have minor effects on civil structures. Specifically, the vertical site-response and the physical parameters which influence it are still poorly understood. In recently-published vertical GMPEs, the site-response component is still represented only by Vs30. However, several studies have shown that in some frequencies, the P-wave contribution to the upright direction could be as large as that of the SV-wave or even more. In this study, we hypothesize that the site-response component in vertical GMPEs can be better predicted by combining P- and S- related site parameters. Because the cost of probabilistic-based seismic hazard evaluation is directly related to the uncertainty estimate, such a knowledge gap motivates a proper study of P/SV relation. In order to examine the relative contribution of P- and SV- waves on the vertical ground motion component, we conduct a series of 3D earthquake scenario simulations. The simulations are performed using the 4th order seismic wave propagation platform (SW4), which is an open-source code utilizing a finite difference approach in a 3D heterogeneous geological model. The P- and SV- wave components are identified using spatial derivatives of the motion, and their ratio is studied in the frequency range of 0.1 to 10 Hz. The dependence of the P/SV ratio on source and path features, such as the inclination angle and the number of soft layers is studied. This study is expected to facilitate a general understanding that will lead to a better implementation of site-response component in future vertical GMPEs.
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Abstract: We investigate the effects of crustal structure on high-frequency (f>1 Hz) ground motions from induced earthquakes in Oklahoma using recorded waveforms and 1- and 3-D waveform modeling. This work was motivated by previous comparisons of observed response spectral accelerations with the values from ground motion prediction equations (GMPEs) suggesting that the induced ground motions from Oklahoma exhibit a faster-than-expected attenuation with distance. We compiled records from well-recorded earthquakes and computed 5-percent-damped single-degree-of-freedom response from well-recorded earthquakes for various oscillator periods. The acceleration response waveforms comprise strong multiples that develop with increasing distance and exhibit the feature that the relative amplitudes of these multiples evolve with distance. Waveform modeling reproduces these multiples and indicates strong sensitivity of the ground motion attenuation to focal depth, to the thickness of the Paleozoic sediments, and to seismic impedance between the basement rocks and the Paleozoic sediments. We employ 3-D earthquake simulations to investigate the effect of the thinning of the Paleozoic sediments in Oklahoma to the northeast on ground motions. Simulations indicate strong azimuthal- and frequency-dependent variations in the response waveforms. These results indicate that the unique wave-propagation characteristics from induced earthquakes in Oklahoma should be considered in regional ground motion prediction and seismic hazard analyses.
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Abstract: Seismic surface waves are valuable for investigating subsurface structures. However, in many other applications such as in seismic reflection imaging, it is desirable to separate the surface waves from the data. We propose a data-driven approach to predict and separate surface waves from the data based on the nonlinear dispersion measurement. In addition, we can also separate the surface waves into fundamental mode and overtones. The procedure has two steps. We first estimate high-resolution surface wave phase velocities from the recorded data using our nonlinear signal comparison (NLSC) approach. This enables us to predict the surface waves at each receiver location. We then subtract the predicted surface waves from the input seismic data. We applied our approach on two synthetic datasets and one field active-source seismic gather. From these examples, we can see that our new approach could effectively predict and separate surface waves with high fidelity.
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Abstract: Ceboruco volcano in Nayarit, Mexico, is one of the only historically active volcanoes at the western end of the Mexican volcanic belt. The need to characterize the magma chamber and the hydrothermal system motivated this first seismic tomography of the volcano, with a focus on the upper 15 km of the crust. Seismic interferometry applied to ambient seismic noise is increasingly used to retrieve the Green’s function between pairs of stations. This technique allows producing high-resolution images of the upper crust with the advantage of using continuously available, non-destructive data. We use the cross-correlations of the ambient seismic wavefield recorded by a dense network of 25 temporary short-period stations deployed to image shallow crustal structure of Ceboruco. We present the preliminary shear-wave velocity model based on this analysis.
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Abstract: We present the results of a surface wave group velocity tomography study for the Rio de la Plata craton (RPC). This craton represents the oldest Precambrian region of the end of southwest Gondwana in South America. The results were then inverted to estimate crustal and lithospheric thicknesses. Previous studies carried out in South America did not map some areas of the continent such as the RPC, clearly because of the insufficient number of crossing paths. To improve the resolution of the previously obtained crustal and upper mantle images, the number of group velocity measurements for the craton area was increased, achieving a better coverage of paths and a more uniform azimuthal distribution which enhances the tomographic images. Surface wave dispersion curves were obtained by a multiple filter technique with a phase-matched filter to better isolate the fundamental mode. A 2D tomographic inversion of the group velocity was applied using a conjugate gradient method. Our results include both Love- and Rayleigh-wave inversions for periods from 10 to 100 s. The obtained group velocity maps correspond well with the main tectonic structures along the studied area. Inversions of the group velocities were carried out to obtain the S-wave velocity distribution in the crustal region.
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