Abstract: Correlations of the ambient seismic field have been used successfully for tomographic imaging of the Earth on a wide range of scales. This is based on the theoretical and experimental observations that correlation functions computed between the signals recorded by two stations contain an approximation of the impulse response (Green’s function) between these stations. The waves that comprise the ambient field are subject to scattering due to the heterogeneous earth which can generate supplementary arrivals on the correlations functions. It is possible to use these effects of scattering that do not correspond to the propagation between the two stations considered, to locate potential external sources of signal. For this analysis, we use correlation functions computed from continuous signals recorded between 2001 and 2017 by seismic stations in Central California. We identify supplementary arrivals in the correlation functions and use array analysis to map the source of scattering effects linked to strong structural variations. We are particularly interested in imaging scatters that result in coupling between the P-SV and SH systems because they are a consequence of lateral heterogeneities in Earth structure. For that reason, we particularly focus on components of the correlation tensor, different from the Vertical-Vertical component, and we expect the Radial/Vertical to Transverse components to be particularly helpful.
Slidecast:
https://vimeo.com/278010585
Abstract: The NET-VISA software produces an automatic combined seismic, hydroacoustic and infrasound bulletin resulting from the key step of assembling detections from multiple stations within the processing chain of the International Data Centre (IDC) of the Comprehensive Nuclear-Test-Ban Treaty (CTBTO). The IDC waveform analysts are systematically evaluating the results of using it as a complement to the current operational software Global Association (GA), which is nearing its 19th anniversary in continuous operation at the IDC. Events that otherwise have been missed by the standard processing are presented to the analysts from a processing pipeline running in parallel with the GA software. After just seven days of evaluation, the number of events added to the Reviewed Event Bulletin (REB) that originate with NET-VISA represent on average 12.3% of the total of the events in the REB. Out of this total, the number of events with a valid body wave (mb) or local magnitude (ML) larger than or equal to 4 is 3.5%, indicating that most added events fall below this threshold. This paper will present a more complete analysis based on multiple weeks of operational use.
Slidecast:
https://vimeo.com/277699087
Abstract: The Las Vegas Valley fault system (LVVFS) is a complex set of north- to northeast- trending, intra-basin Quaternary fault scarps up to 30 m high that displace alluvial fan, fine-grained basin fill, and paleo-spring deposits in the densely populated Las Vegas metropolitan area. Characterizing the seismic hazard of the LVVFS is currently the focus of a multi-year collaborative study involving researchers from the Nevada Bureau of Mines and Geology, University of Nevada, Las Vegas, and the U.S. Geological Survey. The Eglington fault is the only LVVFS fault currently included on the National Seismic Hazard Map (NSHM), and is a priority focus in the early stages of the investigation. Substantial uncertainty remains regarding the seismogenic potential of the LVVFS. Two endmember hypotheses have been proposed regarding the mechanisms responsible for producing the scarps associated with the LVVFS, including the Eglington fault: 1) tectonic (e.g., coseismic surface rupture) and 2) non-tectonic (e.g., prehistoric differential sediment compaction). In this presentation, we will summarize existing geologic, geodetic, geophysical, and geochronologic data that provide insight into the mechanism(s) responsible for scarp formation within the LVVFS, and present unresolved problems with both endmember tectonic and non-tectonic scenarios. We will also discuss in-progress efforts to characterize the seismogenic potential of the Eglington fault including: planned paleoseismic trenching, geologic mapping using lidar and predevelopment topography derived from historical aerial photographs, Optically Stimulated Luminescence (OSL) dating of the Las Vegas basin stratigraphy, and evaluation of the potential for differential sediment compaction across the fault scarps. In addition, we will present the recommendations from the 2018 Working Group on Nevada Seismic Hazards, including the details of a logic tree framework to address uncertainty in the LVVFS hazard assessment.
Slidecast:
https://vimeo.com/277703108
Abstract: The 8 September 2017 earthquake took place in the Tehuantepec gap, in southeastern Mexico, where no large quakes have taken place since 1902. The 8 September earthquake, however, did not rupture the megathrust plate contact between the Cocos and the North American plates. The 2017 earthquake occurred within the subducting slab at a depth of 57 km, immediately downdip of the interplate locked zone. The source mechanism shows normal faulting on a nearly vertical fault plane. The rupture initiated at the bottom of the subducted lithosphere and propagated towards the surface, breaking through the entire subducted lithosphere. An unusually long, complex, and copious aftershock sequence followed the earthquake. The distribution of the aftershock sequence shows a complex distribution. The main rupture is clearly delineated by a 160-km-long fault, sub-parallel to the oceanic trench, immediately beneath the interplate contact. The aftershocks also concentrate in intraslab faults ~50 km down-dip from the main rupture. These aftershock clusters have the same dip as the main rupture and also suggest rupturing through the entire lithosphere. The tensional state of stress is reflected at a large scale within the subducted plate. This deformation pattern suggests the detachment of the slab induced by its own gravitational weight, pulling it away from the strongly coupled interplate contact. Recent earthquakes in the region, in 1931 (Mw7.8) and 1999 (Mw7.5), also show the large-scale detachment of the downgoing Cocos plate, suggesting that the megathrust plate interface is locked and possibly primed for a great earthquake. A very large event in 1787 (Mw 8.6) occurred in an adjacent segment of the Mexican subduction zone, indicating that this region has been the site of great events in the past.
Slidecast:
https://vimeo.com/277698323
Abstract: The IRIS Data Management Center (DMC) has operated a public repository of seismological data for 3 decades supporting thousands of researchers. Since its founding, the DMC has operated its own infrastructure to support the computational and storage resources needed to support its mission. In the GeoSciCloud project, supported by the National Science Foundation’s (NSF) EarthCube program, the DMC is deploying a subset of its archive and key software components into two cloud environments. This project will allow the DMC to evaluate the realities of operating in the cloud and explore the potential advantages and disadvantages. The two cloud environments selected for this project are Amazon’s AWS and XSEDE’s Jetstream and Wrangler systems. The XSEDE resources are operated on behalf of NSF by Indiana University jointly with the Texas Advanced Computing Center. The DMC has deployed a ~40 terabyte test data set and a subset of its web service-based data access architecture to both environments. The DMC is conducting an extensive evaluation of the capabilities of these deployments. To ensure these systems support and, ideally, improve upon real-world research use cases, the DMC is collaborating with scientists who will perform their own tests designed to meet their research needs. A promising, expected gain from cloud-like environments over DMC-operated systems is the ability to scale-out in order to handle more simultaneous users, both with respect to storage I/O and processor intensive tasks. Another potential advantage is providing data within, or very near to, a powerful computing environment that researchers may also use. Also, evaluating the relative costs of the cloud environments against the DMC’s own infrastructure will be critical. We will report on the status of this work and lessons learned so far.
Slidecast:
https://vimeo.com/278017567
Abstract: In recent years, various techniques exploiting the waveform similarity of clustered earthquakes have been used to increase the detection sensitivity of earthquakes, typically by more than an order of magnitude. These approaches are powerful because they use exact copies of seismograms to match against, but for this same reason, are of limited utility in detecting events that have never been seen before. To overcome this challenge, we develop and implement a framework for generalized seismic phase detection using deep learning. The networks are trained and validated on millions of hand-labeled records and are shown to detect seismic phases with high reliability. We demonstrate the full applicability of the method on a large aftershock sequence and compare the results to that of template matching and STA/LTA detectors. The developed approach is shown to be a promising alternative for earthquake detection, and has potential applications to earthquake early warning.
Slidecast:
https://vimeo.com/277700855
