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: 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: The Parkfield segment of the San Andreas Fault (SAF) is one of the best instrumented seismic regions, due to the famous Parkfield Earthquake Prediction Experiment (Bakun et al., 2005). As a result, the 2004 Mw6.0 Parkfield earthquake generated a wealth of data to study seismic activity before and after the mainshock. Recently, Shelly (2017) released a 15-yr catalog of more than 1 million low-frequency tremor events along the Parkfield-Cholame section of the SAF, based on waveform matching with 88 tremor families. However, except a few previous studies that focused on microseismicity in a short time period (Peng and Zhao, 2009; Meng et al., 2013) or repeating earthquakes only (Lengline and Marsan, 2009), there is no systematic long-term detection of microseismicity in this region. Building upon previous studies of microearthquake detection in the region (e.g. Peng and Zhao, 2009), we perform a systematic detection of microearthquakes with a template matched filter detection technique within one year around the 2004 Mw6.0 Parkfield earthquake mainshock. We take advantage of additional seismic datasets that have not been fully used before and improve the detection of early aftershocks in the first tens seconds after the mainshock. With the analysis of a larger time span, we intend to further investigate and understand the migration patterns of the aftershock sequence and determine if any seismic signals are detected in the time period before the mainshock that could indicate a foreshock sequence. We also plan to relocate those newly detected microearthquakes and search for repeating events that occur at virtually the same location. Updated results will be presented at the meeting.
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Abstract: Backprojection (BP) of teleseismic P waves is a widely-used method to image in time and space the evolution of earthquake radiation. Theoretical studies indicate that the high-frequency bursts can be related to abrupt changes in rupture velocity. Furthermore, recent studies propose that the BP results, which exhibit difference in the frequency content, can reflect the stress status, frictional and/or geometrical heterogeneity on the fault surface. However, the relation between earthquake physics and the spatiotemporal evolution from BP imaging remains unknown. Fukahata et al. (2014) suggest that the BP images are equivalent to either slip or slip rate on the fault, provided that the Green’s functions from the sources to the receivers are incoherent. In this study, we relate the BP results and earthquake source process by testing both kinematic (pseudo-dynamic) and dynamic source models. With these source models, we perform a synthetic backprojection exercise in a simple purely elastic homogeneous whole space, allowing us to solve analytically for the far-field body waves. A 2D correlation coefficient is calculated between BP images and the spatial distribution of various source characteristic field (rupture velocity, slip, peak slip rate, and their gradient) for quantitative comparisons. Because kinematic models are computationally efficient, we run a suite of scenario sources to draw a statistical comparison between the source kinematics and the BP images. From dynamic models, we are able to explore the source physical space by looking at variations in frictional properties and draw comparison with the synthetic BP results.
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Abstract: At the Lesser Antilles arc, the American plate subducts under the Carribean plate with a low velocity (2 cm/a). The seismic potential of the northern segment (between Barbuda and Virgin Islands), where no major historic earthquake is recorded, raises many questions. Recent studies show that the area is weakly coupled, suggesting a seismic cycle that could last thousands of years. Here, the obliquity of the convergence (up to 55°) is partially accommodated by diffuse normal faults in the Anegada Passage and by the Bunce strike-slip fault which decouples the accretionary prism from the margin. The seismic behavior of the megathrust is thus likely different from the adjacent segments. The CASEIS cruise (DOI:10.17600/16001800) collected 42 sediment cores and geophysical data (backscatter, bathymetry, sub-bottom profiles) along the forearc and accretionary prism in order to constrain the seismic potential of the Lesser Antilles through a paleoseismological investigation based on earthquake-triggered turbidites. In the northern segment, turbidity currents flow through canyons incising the slope and megastructures like the Tintamarre spur. They deposit in pull-apart basins (Sombrero, Malliwana) or in 2-6 km-large sinks near the Bunce fault. Decimetric turbidites or metric turbidite-homogenite complexes (HmTu) with intervening hemipelagites are retrieved in seven cores from the northern segment. We propose a preliminary correlation based on 14C dates, comparable sedimentary facies and sequences. The uppermost 20 to 70 cm-thick turbidites are dated at 8 ka BP over 100 km, supporting a regional earthquake trigger. Four deeper HmTu complexes possibly related to mega-earthquakes and/or tsunamis are older than 25 ka BP. The most proximal core contains more turbidites, but likely records local tectonic activity. At this stage, potential seismic events appear diachronous compared to the adjacent Guadeloupe segment, but further analyses are required to improve the time series.
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