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  • 1
    Publication Date: 2015-01-30
    Description: Offshore seismicity at the Cascadia margin is poorly constrained because nearly all previous recordings of earthquakes were made using land-based networks. We conducted earthquake monitoring off Vancouver Island in northern Cascadia using ocean-bottom seismographs. Our results show that most of the offshore seismicity is concentrated along the Nootka fault zone. Otherwise seismicity is extremely low, with no earthquakes located along the shallow, seismogenic part of the megathrust. The lack of interplate seismicity may indicate complete healing and locking of the megathrust over three centuries after the great earthquake of 1700 and a somewhat lower degree of structure heterogeneity, such as subducting seamounts. Events along the Nootka fault zone occur over a 10–15 km depth range. This wide distribution and the previously reported overall moment release rate suggest that a significant part of deformation of this fault zone is aseismic. Several earthquakes beneath the continental shelf may be related to faults dividing tectonic terrains within the overriding plate.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2013-11-22
    Description: For performance-based design, nonlinear dynamic structural analysis using various types of input ground motions is required. Stochastic (simulated) ground motions are sometimes useful as input motions, because their properties can be varied systematically to study the impact of ground-motion properties on structural response, and producing large numbers of ground motions is simple. This paper describes an approach by which the wavelet packet transform can be used to characterize complex time-varying earthquake ground motions, and it illustrates the potential benefits of such an approach in a variety of earthquake engineering applications. A model is developed that requires 13 parameters to describe a given ground motion. These 13 parameters are then related to seismological variables such as earthquake magnitude, distance, and site condition, through regression analysis that captures trends in mean values, standard deviations, and correlations of these parameters observed in recorded strong ground motions from 25 past earthquakes. The resulting regression equations can then be used to predict ground motions for a future earthquake scenario. This model is analogous to widely used empirical ground-motion prediction equations (formerly called attenuation models) except that this model predicts entire time series rather than only response spectra. The ground motions produced using this predictive model are explored in detail, and have elastic response spectra, inelastic response spectra, durations, mean periods, and so forth, that are consistent in both mean and variability with existing published predictive models for those properties.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 3
    Publication Date: 2017-02-03
    Description: Alignments of fractures and cracks in marine sediments may be controlled by various mechanisms such as horizontal compaction and extension and basement faulting. The orientation of these alignments can be estimated through analyses of S -wave splitting. If sensors in ocean-bottom observations are deployed through free fall, sensor orientation needs to be determined in order for the recorded data to be used for such analyses. Here, we estimate the sensor orientation from the linear particle motions of P -to- s ( Ps ) phases converted at the sediment–basement interface and also from T waves that are excited by earthquakes and propagate in the seawater. We examine waveforms of local earthquakes recorded by 32 ocean-bottom seismometers (OBSs) that were deployed through free fall for three months in 2010 off Vancouver Island where the strike-slip Nootka fault zone (NFZ) intersects the deformation front of the Cascadia subduction zone. Because the particle motion of the Ps wave was corrected by estimating splitting parameters, the fast polarization direction, which reflects S -wave anisotropic structure within the sediment, can also be evaluated. Consequently, we could estimate the fast polarization direction at OBSs deployed near the NFZ and west of the deformation front. The obtained fast directions appeared to correspond to alignments of shear fractures in the marine sediments associated with the left-lateral motion of the fault in the basement along the NFZ, margin-normal cracks due to horizontal compression west of and slightly away from the deformation front, and frontal thrust faults within the accretionary prism near the deformation front.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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