Abstract: PSHA models typically use instrumental seismic data, but seismic networks are too young to record many large (and thus infrequent) earthquakes. Nevertheless, PSHA source models should be consistent with the instrumental record. Comparing instrumentally observed earthquakes with fault models can be complicated because large earthquakes may not occur precisely on prescribed faults and they may involve multiple faults. Some tests amenable to probabilistic evaluation include participation (occurrence of rupture on specified fault sections) and breaching (rupture crossing specified boundaries along faults). The relevant sections and boundaries may but need not correspond to “segments” as assumed in characteristic earthquake models. The Working Group on California Earthquake Probabilities has published increasingly sophisticated earthquake rupture models since 1988, each specifying fault sections with rates of participation, breaching, or both. Here I compare some of the earlier models with instrumentally observed earthquakes both before and after the model start date. I only count earthquakes after 1933, when seismic networks became adequate. Some discrepancies occur even in retrospective comparisons, because the models were not constrained to fit only those seismic data. The discrepancies don’t invalidate any of the models, but they deserve consideration because the models all share fault geometry and assumptions relating slip rate to earthquake rate.
Slidecast:
https://vimeo.com/276982635
Abstract: Near-surface structure is crucial in understanding earthquake hazards, since seismic speed is directly related to the level of ground shaking. A novel technique to constrain seismic wave speed immediately beneath a seismic instrument is introduced based upon the polarization measurements of teleseismic body-wave arrivals. We demonstrate a counter-intuitive relationship that the P-wave polarization direction is only sensitive to subsurface shear wave speed but not to compressional wave speed, while the S-wave polarization direction is sensitive to both wave speeds. This approach is applied to the High-Sensitivity Seismograph Network in Japan, where the results are benchmarked against the borehole data available at most stations. There is a good agreement between polarization-based estimates and the well measurements at as shallow as 100 m, confirming the efficacy of the new method in resolving the shallow structure. The lateral variation of wave speeds shows that sedimentary basins and mountainous regions are characterized by low and high wave speeds, respectively. Moreover, the analysis is extended into three dimensions by examining the frequency dependence, and preliminary results using US broadband stations show zones of high seismic risk. This versatile technique requires minimal computational resources and can be applied to any single three-component seismograph. It can also provide means of monitoring changes that occur within the very upper crust such as from volcanic or hydrological phenomena. It opens a new path to a reliable, non-invasive, and inexpensive hazard assessment even for locations where a drilling or a field experiment using vibro-trucks or explosives is not a practical option, and reduces uncertainties and ambiguities in the measurements of the near-surface seismic wave speeds.
Slidecast:
https://vimeo.com/276979407
Abstract: Earthquake ground motion records reveal period-dependent correlations, which has implication for seismic risk (Bayless and Abrahamson, 2017). The empirical inter-period correlations of epsilon (the residual between simulations and the mean of the simulations in Fourier Amplitude Spectra (FAS) space) using the Effective Amplitude Spectrum (EAS) computed from the PEER NGA-West2 database resemble a two-sided exponential function. We attempt to incorporate such correlation into the current San Diego State University (SDSU) Broadband (BB) ground motion generator module, which combines deterministic (low-frequency) and stochastic (high-frequency) components. Here, we assume that the Fourier amplitude at frequency f0 is correlated with the Fourier amplitude at f with correlation coefficient exp(-|f-f0|/a) and define a one-sided decaying exponential filter function g = H(f)exp(-f/a), where a is a constant. To improve the EAS correlation in the current SDSU module, we first generate uncorrelated uniformly distributed Fourier spectral amplitudes with unit mean for different realizations, and convolve them with g, which are then multiplied with the Fourier amplitude of the high-frequency ground motion synthetics calculated using Zeng et al. (1991)’s scattering theory. Using our improved method, the BB results for 7 western U.S. events and 2 Japan events with Mw5.0-7.2 show that the empirical inter-period correlations of EAS are well predicted in the SDSU module for a large number of realizations from a single event with unbiased goodness-of-fit of the spectral accelerations in the presence of correlated synthetics.
Slidecast:
https://vimeo.com/276979821
Abstract: Specifying the location and extent of rifted crystalline crust (Precambrian basement) associated with the opening of the Iapetus Ocean in the eastern United States is important for seismic hazard evaluation. As currently mapped by the Central and Eastern U.S. Seismic Source Characterization for Nuclear Facilities (CEUS-SSC) Project, the western limit of major extensional faults and thinned extended crust produced by Iapetan rifting is within the Appalachian Plateau, to the west of the Valley and Ridge province. This limit was defined more than 20 years ago when little information was available about the crustal structure beneath the Appalachians and was based primarily on the distribution of earthquakes in two intraplate seismic zones, as well as some known basement faults. New estimates, using EarthScope, USArray and other data, show that the crustal thickness below the Blue Ridge, Valley and Ridge, and Appalachian Plateau provinces generally exceeds 45 km and is as much as 60 km in places. Such thick crust (equal to that typical of full-thickness continental crust) does not indicate significant extension as in rifted crust. We suggest that the western limit of major extensional faulting along the Iapetan rifted margin is located in the central Piedmont east of the Blue Ridge beneath the Blue Ridge–Piedmont megathrust sheet, and is closely associated with a prominent Bouguer gravity gradient (Appalachian gravity gradient). Only small Iapetan grabens and half grabens have been imaged with crustal seismic reflection data in the footwall beneath the Blue Ridge sheet. Our suggested location for the western limit of Iapetan rifting agrees with palinspastic reconstructions of the Iapetan rifted margin. Intracratonic grabens, inboard from the rifted margin, show only minor crustal extension and thinning. This seismotectonic model is a more credible representation of the basement below the southern and central Appalachians than the current model involving rifted crust.
Slidecast:
https://vimeo.com/276981987
Abstract: Earthquake detection remains one of the fundamental operations in observational seismology. Despite the explosion in quality and quantity of seismic data our ability to build dense and complete seismicity catalogs remains limited. Recent approaches exploit the self-similarity of earthquakes and rely on using the waveforms of a few well known events as templates. Terabytes of continuous seismic signals at multiple stations are cross-correlated against these templates and thousands of events with high waveform similarity may be easily identified. This procedure, known as template matching, is computationally expensive and fails for events whose waveforms are significantly different from the templates. Also, the magnitudes of completeness of intermediate-depth earthquake catalogs are typically large. A dense catalog with a small magnitude of completeness not only allows for a detailed study of seismicity patterns, but also enables the mapping of small-scale b-value anomalies. We propose a Convolutional Neural Network (CNN) architecture for the simultaneous detection of intermediate-depth earthquakes, the precise picking of p- and s- wave arrival times and the estimation of their local magnitude. We test our implementation using a synthetically generated dataset and propose a training scheme that leverages both real and synthetic data for optimal results. As an example we apply our technique to a cluster of intermediate-depth intraplate earthquakes in northern Chile. We are able to detect eight times more events than in the initial catalog and use the picked arrival times to relocate them. We clearly resolve a double-planed structure at depth. Furthermore, we build a detailed b-value map and find that high b-value anomalies correlate well with regions were dehydration is expected.
Slidecast:
https://vimeo.com/276978538
Abstract: This presentation provides an overview of the NGA-Sub, a large multidisciplinary community-based research initiative to develop a comprehensive ground-motion database and multiple ground-motion models (GMMs) for subduction events. In this community-based project, we developed a database of ground motions recorded in worldwide subduction events. The database includes the processed recordings and supporting source, path, and site metadata from Japan, Taiwan, the US Pacific Northwest, Alaska, Latin America (including Mexico, Peru and Chile), and New Zealand. The NGA-Sub database includes 1,570 events with moment magnitudes ranging from 4 to 9.1. The subduction events are classified as interface, intraslab, or outer-rise events. The NGA-Sub ground-motion database has over 180,000 component records. This is by far the largest ground motion database that we have ever developed in any NGA project. Pseudo-spectral acceleration as well as Fourier amplitude spectra for frequencies from 0.1 to 100 Hz have been included in the database. In NGA-Sub, using the empirical ground-motion database and the supporting ground-motion simulations, multiple GMMs are developed. Following the tradition of previous NGA projects, the GMM modeling teams as well as database developers have had continuous technical interactions which resulted in much higher quality of the final products than each researcher could achieved individually. An overview of the NGA-Sub project is presented in this presentation.
Slidecast:
https://vimeo.com/276981569
