Abstract: With the exception of isolated and largely near-shore deployments of ocean-bottom seismometers (OBSs), most seismic instrumentation is located on land, although two-thirds of Earth’s surface is covered with oceans. Large earthquakes are generally confined to subduction zones or other plate boundaries, leading to an uneven distribution of seismic sources. This heterogeneity, coupled with the land-based limitations for most for the Earth’s interior, leads to significant unsampled parts of the Earth. Our work is motivated by the planning of a Joint Task Force to develop concepts and applications for Science Monitoring and Reliable Telecommunication (SMART) cables. Over a million kilometers of submarine telecommunication cables currently exist, which are unavailable to the scientific community for acquisition of geophysical data. If these cables are gradually replaced by SMART cables with oceanographic and seismic sensors at roughly 75 km intervals, one significant benefit to our science will be the near-ubiquitous extent of seismic receivers across the oceans, affording an unprecedented opportunity for both monitoring and modeling. In previous work we presented ray tracing through a 1D reference model to predict improvements to ray coverage afforded by sensors on SMART cables, compared to existing land-based seismic network coverage. Here we extend that modeling, tracing P and S rays through the SALSA3D global tomographic model. We compare results of this exercise to those for the iasp91 model with, and without, the SMART cable sensors.
Poster:
SSA_2018_poster_Rowe_final
Abstract: Two-dimensional (2D) spatial attenuation maps are produced for the crust of western Tibet using local earthquakes which are recorded by an array of 30 broadband stations operated from July 2007 to May 2011. Initially estimation of coda waves attenuation (Qc-1 ) is done using single isotropic model and subsequently relative contribution of scattering (Qsc-1 ) and intrinsic (Qi-1) attenuation have been calculated using Multiple Lapse Time Window Analysis (MLTWA) under the assumption of uniform distribution of multiple isotropic scattering effects for the frequency bands of 1-2, 2-4, 4-8, 8-16 and 12-24 Hz, respectively. All the events are selected on the basis of high signal to noise ratio having hypocentral distance within 200 km from the respective stations. The obtained Q values show a strong frequency dependent nature which can be correlated to the degree of tectonic complexity and the heterogeneities present in the medium. As a whole, the intrinsic absorption is found to be the dominant mechanism at all the frequencies which is quite consistent with the geo-tectonic complexity in the region. The obtained values of Q are in good agreement with other segments of Himalaya and Tibet as well as different tectonic regions in the world. Keywords: MLTWA, Intrinsic, scattering, coda wave, western Tibet
Poster:
Rahul Biswas
Abstract: The NGA-Subduction project is currently under development, focusing on analyzing the collected data for different regions affected by subduction-zone earthquakes around the world. As part of this effort, an earthquake-source database has been developed for events with ground motion recordings obtained in Chilean territory. Data from the South American subduction zone, and particularly from Chilean earthquakes, are critical for the success of this project, due to the availability of data from many large (M > 7.5) interface subduction events and the importance of significant regional path effects previously observed in Chile but not evident elsewhere. The source database contains information on seismic moment, moment magnitude, hypocenter location, nodal planes, and fine-fault geometric parameters. More than 40 different finite-fault models have been collected and evaluated, introducing a uniform and consistent procedure for fault trimming, where only the trimmed portion of the fault plane with significant slip is used for source-to-site distance calculations. For events that lack published finite-fault models, uniform protocols are applied to assign the source parameters and a simulation-based representation of the finite-fault parameters is used for distance calculation. To support the use of these simulations for subduction-zone earthquakes, relationships for M-rupture area, M-aspect ratio, and hypocenter locations in the down-dip and along-strike directions are developed.
Poster:
POSTER_SSA_20180517_NGA-Sub_Source_parameters_Chilean_earthquakes
Abstract: More than 300 earthquakes have been reported by the Colombian Seismic Network (RSNC) in a seismic zone in southwestern Colombia since 2012 ranging from Ml 0.8 up to Ml 5.3, with at least 3 M4.5+ earthquakes in a span of 1 month in 2016. Some of these events have been widely felt in major cities in Colombia and are well recorded by the local network. We study the temporal and spatial evolution of these earthquakes, and in particular focus in the short seismic sequence in 2016. Earthquakes show migration towards the SW, along an apparent fault trace and show clear Omori-type aftershock behavior. Using an empirical Green’s function approach, we study the changes in source parameters of the seismic sequence and try to identify if ruptures show any preferential directivity. Temporal or spatial variations of these source parameters may be indicative of changes in the medium before and after the mainshock.
Poster:
poster_AA
Abstract: Mexico City’s building code considers rules to include the bidirectional building response in the design process. The so-called α combination rules account for the impact of 100% and α times the ground motion acting in orthogonal directions. Thus, it allows estimating the forces a structure will resist. Here, based on fully 3D ground motion simulations including the soil-structure interaction, we present a preliminary analysis of the uncertainty in the α parameter in soft soil deposits in Mexico City. The study is performed for a typical structure located in the Lake zone coupled with a realistic velocity model of the basin and a detailed model of the building. The seismic wave propagation and the building response is performed using the Finite Element Method for eight scenarios, mostly subduction earthquakes. We observe that differences in the optimal α parameter estimated per simulation are not adequate to establish a preferred value.
Poster:
Poster_59_JoaquinSanchez
Abstract: Seismic source discrimination at local distances (< 200 km) is becoming increasingly important within the nuclear monitoring community. A study conducted by Koper et al. (2016) found that the difference between local magnitude (ML) and coda/duration magnitude (MC) could distinguish between mining-induced seismicity and natural seismicity in Utah. They found that the shallower mining-induced earthquakes (depths < 2–3 km) had more negative ML-MC values than the generally deeper (> 5 km) tectonic events. Similar results showing that ML-MC decreases as source depth approaches the surface have recently been found in Yellowstone, Oklahoma, and Italy. Here we investigate how well direct measurements of peak amplitude (A) and duration (t) made at individual stations can be used as proxies for the network averaged ML–MC values. In particular, we investigate how log10(A/t) varies as a function of distance, how quickly the variance decreases as more stations are averaged together, and whether individual station corrections are warranted. We also examine how changes to the procedures used to measure A and t affect the performance of log10(A/t) as a depth discriminant. We aim to replicate and explain our ML-MC observations using a three-dimensional, fourth-order, finite-difference code (SW4) to synthesize high-frequency waveforms in realistic Earth models that contain topographic and volumetric scattering. The ultimate goal of this study is to introduce a new depth discriminant to nuclear monitoring practices that can be applied to all networks and help differentiate mid- and lower- crustal earthquakes from potential explosions.
Poster:
Scales_SSA2018
