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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Article
Earthquake Engineering (2007)

In engineering practice, it is common for the ground motion intensity measures used in design to be estimated using a combination of probabilistic and deterministic procedures. Formal probabilistic seismic hazard analyses are performed to estimate intensity measures (IMs) for rock site conditions. This is followed by a deterministic modification of the rock IMs to account for site effects, which is typically done using prescribed site factors available in the literature or in seismic code provisions. In this article we investigate the extent to which ground motions estimated using this semi-probabilistic approach approximate ground motions evaluated in a fully probabilistic context in which the nonlinear site response is integrated into the hazard calculations. Using two existing California sites as examples, we demonstrate that the deterministic application of nonlinear site factors underestimates the ground motions evaluated using a formal probabilistic approach. This misfit arises from multiple sources including different standard deviation terms for rock and soil sites and different controlling earthquakes. In particular, sites having a significant nonlinear site response tend to attract larger contributions from distant earthquakes than do rock sites. Fortunately, a new set of ground motion prediction equations developed through the Next Generation Attenuation project directly incorporate nonlinear site response effects, and hence as those models are integrated into hazard codes, there will be no need to continue in practice the semi-probabilistic approach for ground motion estimation.

    Cover page: Probabilistic versus Deterministic Implementation of Nonlinear Site Factors in Seismic Hazard
    • Article
    Earthquake Engineering (2006)

    Results of 1D ground response analyses are typically not incorporated into probabilistic seismic hazard analyses (PSHA) in a statistically robust way. Often ground response is incorporated into PSHA using deterministic amplification factors. This simplistic method generates results that are intrinsically arbitrary and often unconservative. The main problem in probabilistically linking PSHA and ground response lies in quantifying the dispersion that is appropriate for use with ground response analysis results. We review two alternative procedures for quantifying this uncertainty and illustrate their differences with respect to dispersion values of spectral acceleration at the surface for various site conditions.

      Cover page: Implementation of 1D Ground Response Analysis in Probabilistic Assessments of Ground Shaking Potential
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2024)
      Simulated ground motions can advance seismic hazard assessments and structural response analyses, particularly for conditions with limited recorded ground motions such as large magnitude earthquakes at short source-to-site distances. Rigorous validation of simulated ground motions is required before hazard analysts, practicing engineers, or regulatory bodies can be confident in their use. A decade ago, validation exercises were mainly limited to comparisons of simulated-to-observed waveforms and median values of spectral accelerations for selected earthquakes. The Southern California Earthquake Center (SCEC) Ground Motion Simulation Validation (GMSV) group was formed to increase coordination between simulation modelers and research engineers with the aim of devising and applying more effective methods for simulation validation. In this presentation, we summarize what has been learned in over a decade of GMSV activities. We categorize different validation methods according to their approach and the metrics considered. Two general validation approaches are to compare validation metrics from simulations to those from historical records or to those from semi-empirical models. Validation metrics consist of ground motion characteristics and structural responses. We discuss example validation studies that have been impactful in the past decade and suggest future research directions. Key lessons learned are that validation is application-specific, our outreach and dissemination need improvement, and much validation-related research remains unexplored. This presentation is a summary of our recent paper, Rezaeian et al. (2024), referenced below.
        Cover page: Findings from a decade of ground motion simulation validation research and a path forward
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2018)
        We present a model for the site-to-site component of aleatory variability for use with NGA-East site factors presented in Stewart et al. (2019). This model provides a higher degree of variability at short periods than is present in similar models for the western US.
        • 1 supplemental PDF
        Cover page: Site-to-Site Standard Deviation Model for Central and Eastern North America
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2022)
        Simulated ground motions can advance seismic hazard and structural response analyses, particularly for conditions with limited recorded ground motions, such as large magnitude earthquakes at short source-to-site distances. Rigorous validation of simulated ground motions is required before regulatory bodies, practicing engineers, or hazard analysts can be confident in their use. A decade ago, validation exercises were mainly limited to comparisons of simulated to observed waveforms and median values of spectral accelerations. The Southern California Earthquake Center (SCEC) Ground Motion Simulation Validation (GMSV) group was formed to increase coordination among simulation modelers and research engineers with the aim of devising and applying effective methods for simulation validation. Here, we categorize alternate validation methods according to their approach and the metrics considered. Two general validation approaches are to compare various metrics from simulations to their counterparts from historical records or to their estimated values from existing empirical models. Validation metrics consist of ground motion characteristics and structural responses. We describe this categorization, provide examples that have been valuable in the past decade, and provide potential research directions. Key lessons learned by our GMSV group are that validation is application specific, our outreach and communication warrants improvement, and much research remains unexplored.
          Cover page: Lessons Learned from a Decade of Ground Motion Simulation Validation (GMSV) Exercises and a Path Forward
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2020)
          The 2019 Ridgecrest earthquake sequence produced a 4 July M 6.5 foreshock and a 5 July M 7.1 mainshock, along with 23 events with magnitudes greater than 4.5 in the 24 hr period following the mainshock. The epicenters of the two principal events were located in the Indian Wells Valley, northwest of Searles Valley near the towns of Ridgecrest, Trona, and Argus. We describe observed liquefaction manifestations including sand boils, fissures, and lateral spreading features, as well as proximate non-ground failure zones that resulted from the sequence. Expanding upon results initially presented in a report of the Geotechnical Extreme Events Reconnaissance Association, we synthesize results of field mapping, aerial imagery, and inferences of ground deformations from Synthetic Aperture Radar-based damage proxy maps (DPMs). We document incidents of liquefaction, settlement, and lateral spreading in the Naval Air Weapons Station China Lake US military base and compare locations of these observations to pre-and postevent mapping of liquefaction hazards. We describe liquefaction and ground-failure features in Trona and Argus, which produced lateral deformations and impacts on several single-story masonry and wood frame buildings. Detailed maps showing zones with and without ground failure are provided for these towns, along with mapped ground deformations along transects. Finally, we describe incidents of massive liquefaction with related ground failures and proximate areas of similar geologic origin without ground failure in the Searles Lakebed. Observations in this region are consistent with surface change predicted by the DPM. In the same region, geospatial liquefaction hazard maps are effective at identifying broad percentages of land with liquefaction-related damage. We anticipate that data presented in this article will be useful for future liquefaction susceptibility, triggering, and consequence studies being undertaken as part of the Next Generation Liquefaction project.
            Cover page: Liquefaction and Related Ground Failure from July 2019 Ridgecrest Earthquake Sequence
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