Abstract: The 24 August 2014 Mw 6.0 South Napa earthquake caused $400 million in damage to private and commercial properties. Previous site response studies indicate areas that experienced extensive structural damage are underlain by deposits with low Vs30 values. The goal of our study was to evaluate shear-wave velocity to 30 m depth (Vs30) using body waves (see Catchings et al.) and surface waves at six broadband seismometer sites that registered PGA values between 0.3 to 0.6g during the South Napa earthquake in 2014. We used active seismic sources, and up to 120 channels of 4.5-Hz sensors to record seismic waves near broadband seismometers in the City and County of Napa and in the City of Vallejo. To increase the resolution of our data in the upper 30 m, we used 3-m geophone and shot spacing along each profile, which ranged in length from 85 to 260 m. We used a 226-kg accelerated weight-drop and seisgun to generate P- and Rayleigh-wave sources and a 3.5-kg sledgehammer and block to generate S- and Love wave sources. We found that Vs determined from Love Waves were generally lower than Vs determined from Rayleigh Waves at sites with Vs less than 500 m/s, and Vs from refraction tomography show similar results to those determined from multichannel analysis of surface waves (MASW) using Love waves. Our results suggest S-wave velocities vary up to ~30% between refraction tomography and MASW methods, with the latter method generally producing lower velocities. The three methods are complimentary and can be highly useful in accurately evaluating site response.
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Abstract: The Tangshan fault belt is deemed as the seismogenic fault for the 1976 Ms7.8 Tangshan earthquake, which is one of the most devastating earthquakes in the last 100 years in the world and caused more than 240,000 deaths. The dense array ambient vibration surveys with ~ 4 km inter-station distance 146 stations were applied in the spring of 2017. The sedimentary resonance frequencies were measured by the Horizontal-to-Vertical Spectral Ratio (HVSR) method to gain sedimentary thickness distribution and basement morphology. Furthermore, a ~ 1 km inter-station distance seismic profile was deployed along a line perpendicular to the strike of the Tangshan fault belt to investigate the detailed shallow sedimentary structures. Extensive tests are conducted to evaluate and verify the reliability of the HVSR curves. With these HVSR curves, the two-dimensional Quaternary sedimentary structures cross the fault belt are imaged by frequency-to-depth conversion. Two-dimensional HVSR profile clearly reveals two seismic impedance interfaces at ~ 100 m and 300 ~ 800 m depth. The thickness of the unconsolidated and semi-consolidated sedimentary is ~ 100 m with very small variations, which is consistent with the seismic reflection interface from shallow seismic reflection exploration. While the buried depth of the Quaternary sedimentary basement increases from 300 to 800 m from the west to the east along the profile, which is also consistent with the Quaternary sediment depth derived from previous studies. It is worth noting that the depth of the Quaternary sedimentary basement just beneath the Tangshan fault belt varies rapidly with ~ 200 meters, which well agrees with the spatial characteristics of the Tangshan fault revealed by deep seismic reflection profiling. It may suggest that the Tangshan fault belt has been significantly ruptured and modified by strong earthquake activities since Quaternary.
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Abstract: Recent studies showed that microearthquakes recorded by a dense array of receivers at the surface have the same raypath geometry as Reverse Vertical Seismic Profiling (RVSP) surveys, which in principle, allows the use of microearthquake sources combined with RVSP processing to produce high-resolution 3D reflection images of structure beneath the receiver array. Earlier examples of the method produced 2D and 3D reflection profiles from a single microearthquake source, with a later example using multiple events to generate a 3D reflection volume underneath the Piedmont Province in Central Virginia. Although the true power of using microearthquakes for reflection imaging lies in the prospect of using redundancy generated from multiple sources, the complexities that result from double couple sources present several challenges. Some of these challenges have been addressed extensively in previous studies, such as hypocenter uncertainty, and sparse illumination from an aftershock distribution, but other issues, such as spurious phases (SD, PzS, SzP, SzS) in P-wave reflection images have not been properly addressed, mostly because previous studies were restricted by using vertical component seismographs. In this study we take advantage of 3C nodal dense arrays that recorded local seismicity to tackle the issue of these spurious phases. Given that the phases PzP, PzS, SzP, and SzS are all illuminating the structure beneath the receiver array we use wavefield separation and RVSP processing of microearthquakes to generate different structural images from reflected and converted phases.
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Abstract: The Caribbean and Central America region (CCAR) undergoes the entire spectrum of earthquake types due to its complex tectonic setting comprised of transform zones, young oceanic spreading ridges, and subductions along its eastern and western boundaries. CCAR is, therefore, an ideal setting in which to study the impacts of long-term tectonic deformation on the distribution of present-day seismic activity. In this work, we develop a continuous tectonic strain rate model based on inter-seismic geodetic data and compare it with known active faults and earthquake focal mechanism data. We first create a 0.25o x 0.25o finite element mesh that is comprised of block geometries defined in previously studies. Second, we isolate and remove transient signals from the latest open access community velocity solution from UNAVCO, which includes 339 velocities from COCONet and TLALOCNet GNSS data for the Caribbean and Central America, respectively. In a third step we define zones of deformation and rigidity by creating a buffer around the boundary of each block that varies depending on the size of the block and the expected deformation zone based on locations of GNSS data that are consistent with rigid block motion. We then assign each node within the buffer a 0 for the deforming areas and a plate index outside the buffer for the rigid. Finally, we calculate a tectonic strain rate model for CCAR using the Haines and Holt finite element approach to fit bi-cubic Bessel splines to the the GNSS/GPS data assuming block rotation for zones of rigidity. Our model of the CCAR is consistent with compression along subduction zones, extension across the mid-Pacific Rise, and a combination of compression and extension across the North America – Caribbean plate boundary. The majority of CCAR strain rate magnitudes range from -60 to 60 nanostrains/yr. Modeling results are then used to calculate expected faulting behaviors that we compare with mapped geologic faults and seismic activity.
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Abstract: Accurate earthquake locations are critical to identifying faults, understanding tectonics, and estimating a region’s seismic hazard. The Dominican Republic and neighboring Haiti have been the site of numerous devastating earthquakes. A complex pattern of tectonic stresses is created by (a) the oblique subduction of the southward-dipping North American plate in the north, (b) the east-west-trending Septentrional Northern transform fault system, (c) Enriquillo Plantain Garden fault zone in the south, and (d) convergent motion generating great topographical variations, including the Cordillera Central in the center of the island. However, the seismic hazard associated with the island’s faults and the precise locations and characteristics of the faults are poorly known. To improve understanding of seismic hazard on the island of Hispaniola, we relocated events recorded by a temporarily densified broadband network from 2013 to 2017. Fifteen hundred events of magnitude greater than 2.5 were recorded in the span of four years, 150 of which have magnitudes greater than 4. Each event has an average of 15 P wave arrival picks and 8 S wave arrival picks. Both single-event and double-difference techniques are used, in conjunction with a new 1D model for the Dominican Republic, to improve the accuracy of earthquake locations and the geometry of associated fault zones. We will compare the “traditional” (single-event) relocations with those generated by a double difference technique. The catalog relocated in the new 1D produces several nearly linear features that may be previously undiscovered faults. We intend to relocate events in new 3D models, as well, and thereby investigate the distribution of faults at crustal depths, as well.
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Abstract: We present attenuation (t*) and source parameters (Δσ y Mw), obtained through spectral analysis, for earthquakes with hypocenter within the basin of Mexico. These events were recorded by the Red Sísmica del Valle de México (RSVM, Valley of Mexico Seismological Network), composed of 31 broadband stations distributed in Mexico City and surroundings. We analyze events from 2013 to 2016, and selected five with the objective to compare t*, Δσ y Mw and look for spatial variations within the basin. Based on a Brune’s model spectrum, we analyze all the easily-distinguishable S wave recorded on the horizontal components (N-S and E-W). Large part of the basin is in an old dry lake. This fact makes it complicated and heterogenous. By studying t* and Δσ, we can characterize the basin.
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