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: Since the 1985 Michoacan Earthquake, the soft, high water content clays filling the lakebed of Lake Texcoco are known to exhibit resonance at a frequency of about 0.5 Hz and cause significant site response in Mexico City. The data from the Puebla Earthquake on 19 September 2017 also displays this behavior in the ground motion amplification. Relative to nearby sites, the high ground acceleration in the lakebed sediments are a contributing factor of damage to some of the Mexico City’s buildings, a similar outcome to that of the 1985 Michoacan Earthquake. Since 1985, there have been significant improvements to the seismic network, allowing for a more detailed evaluation of ground motions and site response across Mexico City. In this study, we used recorded ground motion data from 61 CIRES stations located in Mexico City during the Puebla earthquake. To get values of soil resonance and their associated amplifications at sites across Mexico City, we used the H/V spectral ratio method and the spectral ratio of the station on the lakebed and a station on firm ground. We mapped the amplification in terms of resonance frequency and amplification across Mexico City. In addition, we compare the results with building damage data from the Puebla and Michoacan earthquake. The spatial distribution of building damage during the Peublo earthquake differs from the Michoacan earthquake, but is consistent with site response.
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Abstract: The iMUSH project (imaging Magma Under St. Helens) included a 3D active source seismic experiment consisting of 23 large shots recorded at ~6000 seismograph stations. The experiment included 3 dense linear profiles: Two profiles of 8 shots each were recorded by > 1000 receivers (~150m apart) and strike NW-SE and NE-SW. A third profile of 4 shots and > 300 receivers strikes E-W. We have made CMP stacked sections and Kirchhoff depth migrations of the profiles using travel-times from 2D and 3D tomography velocity models derived from the dataset. We have also made autocorrelation estimates of the reflectivity beneath the receivers using shot and noise data, and depth converted them using the tomography velocity models. Bright reflections in the CMP sections, depth migrations, and autocorrelations coincide with abrupt velocity changes in the 2D velocity models in the mid to lower crust and at the Moho: Reflections appear at 20-25 km depth at the tops of two lower crustal high velocity (Vp > 7.5 km/s) bodies. One of these is directly beneath the MSH edifice, the other is ~40 km to the SE under the 9ka Indian Heaven basaltic volcanic field. We interpret the high Vp bodies as cumulates from Quaternary or Tertiary volcanism. Separating the two high Vp bodies is a lower velocity column (Vp ≤ 6.5 km/s) dipping SE from the midcrust to the Moho. The Moho reflection is bright under the region of low velocity and dims beneath both of the high velocity lower crustal bodies. The 1980 eruption seismicity extended from the MSH summit to ~20 km depth, stopping above the bright reflection at the MSH high Vp body. Deep long period events under MSH, often associated with motion of magmatic fluids, cluster at 20-30 km depth along the SE edge of this reflection. We suggest that lower crustal magmas migrate from the southeast at the edge of the high velocity body, and then laterally across its top before ascending vertically to the magma storage zone under the summit.
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Abstract: The surge of using geophoned autonomous nodes for scientific projects is advancing passive imaging and monitoring techniques for scientific research, oil and gas projects, hydrology, civil engineering and new applications for short term dense seismic monitoring. The concept of a small, minimal configuration, low power, simple deployments without worry of environmental conditions can be applied to temporary broadband sensor system deployments using the next generation of direct bury broadband sensors. We explore some preliminary scientific deployment scenarios that can be used for broadband studies or as an example of a much smaller logistics next generation Earthscope array type station with little if any difference in noise performance and a potentially order of magnitude less cost. The concept of research grade latency can be applied for telemetry to recover quasi realtime data or very low cost system state of health.
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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: 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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