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Graehl N.A.,Lettis Consultants International Inc. | Kelsey H.M.,Humboldt State University | Witter R.C.,U.S. Geological Survey | Hemphill-Haley E.,Humboldt State University | Engelhart S.E.,University of Rhode Island
Bulletin of the Geological Society of America | Year: 2015

The Sallys Bend swamp and marsh area on the central Oregon coast onshore of the Cascadia subduction zone contains a sequence of buried coastal wetland soils that extends back ~4500 yr B.P. The upper 10 of the 12 soils are represented in multiple cores. Each soil is abruptly overlain by a sandy deposit and then, in most cases, by greater than 10 cm of mud. For eight of the 10 buried soils, times of soil burial are constrained through radiocarbon ages on fi ne, delicate detritus from the top of the buried soil; for two of the buried soils, diatom and foraminifera data constrain paleoenvironment at the time of soil burial. We infer that each buried soil represents a Cascadia subduction zone earthquake because the soils are laterally extensive and abruptly overlain by sandy deposits and mud. Preservation of coseismically buried soils occurred from 4500 yr ago until ~500- 600 yr ago, after which preservation was compromised by cessation of gradual relative sea-level rise, which in turn precluded drowning of marsh soils during instances of coseismic subsidence. Based on grain-size and microfossil data, sandy deposits overlying buried soils accumulated immediately after a subduction zone earthquake, during tsunami incursion into Sallys Bend. The possibility that the sandy deposits were sourced directly from landslides triggered upstream in the Yaquina River basin by seismic shaking was discounted based on sedimentologic, microfossil, and depositional site characteristics of the sandy deposits, which were inconsistent with a fl uvial origin. Biostratigraphic analyses of sediment above two buried soils-in the case of two earthquakes, one occurring shortly after 1541-1708 cal. yr B.P. and the other occurring shortly after 3227-3444 cal. yr B.P.-provide estimates that coseismic subsidence was a minimum of 0.4 m. The average recurrence interval of subduction zone earthquakes is 420-580 yr, based on an ~3750-4050-yr-long record and seven to nine interearthquake intervals. The comparison of the Yaquina Bay earthquake record to similar records at other Cascadia coastal sites helps to defi ne potential patterns of rupture for different earthquakes, although inherent uncertainty in dating precludes defi nitive statements about rupture length during earthquakes. We infer that in the fi rst half of the last millennia, the northern Oregon part of the subduction zone had a different rupture history than the southern Oregon part of the subduction zone, and we also infer that at ca. 1.6 ka, two earthquakes closely spaced in time together ruptured a length of the megathrust that extends at least from southwestern Washington to southern Oregon. © 2014 Geological Society of America. Source


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

This study presents a novel approach for evaluating ground motion selection and modification (GMSM) procedures in the context of probabilistic seismic demand analysis. In essence, synthetic ground motions are employed to derive the benchmark seismic demand hazard curve (SDHC), for any structure and response quantity of interest, and to establish the causal relationship between a GMSM procedure and the bias in its resulting estimate of the SDHC. An example is presented to illustrate how GMSM procedures may be evaluated using synthetic motions. To demonstrate the robustness of the proposed approach, two significantly different stochastic models for simulating ground motions are considered. By quantifying the bias in any estimate of the SDHC, the proposed approach enables the analyst to rank GMSM procedures in their ability to accurately estimate the SDHC, examine the sufficiency of intensity measures employed in ground motion selection, and assess the significance of the conditioning intensity measure in probabilistic seismic demand analysis. © 2015 John Wiley & Sons, Ltd. Source


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

In this short communication, we respond to the comments made by Dr Brendon A. Bradley and provide additional context to our paper under discussion. © 2015 John Wiley & Sons, Ltd. Source


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

This study develops a framework to evaluate ground motion selection and modification (GMSM) procedures. The context is probabilistic seismic demand analysis, where response history analyses of a given structure, using ground motions determined by a GMSM procedure, are performed in order to estimate the seismic demand hazard curve (SDHC) for the structure at a given site. Currently, a GMSM procedure is evaluated in this context by comparing several resulting estimates of the SDHC, each derived from a different definition of the conditioning intensity measure (IM). Using a simple case study, we demonstrate that conclusions from such an approach are not always definitive; therefore, an alternative approach is desirable. In the alternative proposed herein, all estimates of the SDHC from GMSM procedures are compared against a benchmark SDHC, under a common set of ground motion information. This benchmark SDHC is determined by incorporating a prediction model for the seismic demand into the probabilistic seismic hazard analysis calculations. To develop an understanding of why one GMSM procedure may provide more accurate estimates of the SDHC than another procedure, we identify the role of 'IM sufficiency' in the relationship between (i) bias in the SDHC estimate and (ii) 'hazard consistency' of the corresponding ground motions obtained from a GMSM procedure. Finally, we provide examples of how misleading conclusions may potentially be obtained from erroneous implementations of the proposed framework. © 2014 John Wiley & Sons, Ltd. Source


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

This paper develops a procedure to select unscaled ground motions for estimating seismic demand hazard curves (SDHCs) in performance-based earthquake engineering. Currently, SDHCs are estimated from a probabilistic seismic demand analysis, where several ensembles of ground motions are selected and scaled to a user-specified scalar conditioning intensity measure (IM). In contrast, the procedure developed herein provides a way to select a single ensemble of unscaled ground motions for estimating the SDHC. In the context of unscaled motions, the proposed procedure requires three inputs: (i) database of unscaled ground motions, (ii) IM, the vector of IMs for selecting ground motions, and (iii) sample size, n; in the context of scaled motions, two additional inputs are needed: (i) a maximum acceptable scale factor, SFmax, and (ii) a target fraction of scaled ground motions, γ. Using a recently developed approach for evaluating ground motion selection and modification procedures, the proposed procedure is evaluated for a variety of inputs and is demonstrated to provide accurate estimates of the SDHC when the vector of IMs chosen to select ground motions is sufficient for the response quantity of interest. © 2015John Wiley & Sons, Ltd. Source

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