Directivity-Based Probabilistic Seismic Hazard Analysis in California
Directivity-Based

Probabilistic Seismic Hazard Analysis in California

Tools

Directivity-Based Intensity-Measure Interactive Maps
Silvia Mazzoni, Linda Al Atik, Nick Gregor, Yousef Bozorgnia
Natural Hazards Risk and Resiliency Research Center (NHR3), UCLA
April 2023, DOI:10.34948/N3101P

Directivity-Based PSHA Interactive Tool
Silvia Mazzoni, Linda Al Atik, Nick Gregor, Yousef Bozorgnia
Natural Hazards Risk and Resiliency Research Center (NHR3), UCLA
December 2022, DOI:10.34948/N34S39

Directivity-Based PSHA Data Tables
Silvia Mazzoni, Linda Al Atik, Nick Gregor, Yousef Bozorgnia
Natural Hazards Risk and Resiliency Research Center (NHR3), UCLA
September 2023, DOI:xxx

Figure 1. Sample of Geographic distribution of intensity measure (With-Directivity PSA @ T=4sec, Return-Period=2475yr, Vs30=760m/s)

These datasets and web tools are the third in a series of three products on documentation of the directivity-based state-wide PSHA study for California:
  • The first report, "Directivity-Based Probabilistic Seismic Hazard Analysis For The State Of California: Report 1, No-Directivity Baseline Case", by Linda Al Atik, Nick Gregor, Silvia Mazzoni, and Yousef Bozorgnia, described the input seismic source and ground motion models (click here for report)
  • The second report, "Directivity-Based Probabilistic Seismic Hazard Analysis For The State Of California: Report 2, Directivity Implementation", by Linda Al Atik, Nick Gregor, Yousef Bozorgnia, and Silvia Mazzoni, described the implementation of near-field directivity effects on the ground motion for the state-wide hazard study (click here for report)
  • The third report, "Directivity-Based Probabilistic Seismic Hazard Analysis for the State of California: Report 3, Datasets & Tools", by Silvia Mazzoni, Linda Al Atik, Nick Gregor, and Yousef Bozorgnia, describes the tools and dataset presented in this page (click here for report)


The tools provided in this portal can conveniently be used by researchers and practicing professional and represent the most advanced state-of-the-knowledge implementation of directivity-based ground-motion and seismic-source models.

You may access Directivity-based PSHA results for California via three different means:
  • The Directivity-Based Intensity-Measure Interactive Maps provide a quantitative understanding of the distribution of hazard with and without directivity, as well as the directivity adjustment factor, across the entire state, with an overlay of the fault sources and geographic features, at a user-specified oscillator period, Vs30, and return period.
  • The Directivity-Based PSHA Interactive Tool interpolates the state-wide PSHA results to provide uniform-hazard spectra for PSA with and without directivity, as well as for the directivity adjustment factor, at a user-specified location, Vs30, and return period.
  • The Directivity-Based PSHA Data Tables provides access to the state-wide PSHA results via downloadable tables.
Deaggregation plots for magnitude-distance bins, source type, and individual fault sources for each study site are also provided in the first two portals.

NOTE:The mapped directivity adjustment factors can be used on their own to modify the results of no-directivity hazard to account for expected directivity effects at any location in California. This allows users to incorporate directivity effects with the UCERF3 source model without having to implement the models in a hazard program.


Introduction

An independent state-wide seismic hazard analysis was conducted for California using standard probabilistic seismic hazard analysis (PSHA) methodology. This PSHA study and the resulting mean hazard curves, ground motions, and deaggregation results are the first step of, and input for, a larger seismic risk analysis and resiliency study conducted for the natural gas infrastructure system including transmission lines and storage facilities throughout the state of California (…more). These results have also been processed, recombined, and made available here so that the public may have access to DIRECTIVITY-BASED seismic-hazard intensity measures (IMs) in California, a sample of which is shown in Figure 1.
A total of 19,316 sites based on a grid spacing of 0.05 by 0.05 degrees longitude and latitude were used in the seismic hazard analysis (Figure 2). At each site location, PSHA was conducted for multiple VS30 values (ranging from 180m/s to 1100m/s), including the site-specific VS30 value estimated from three-dimensional velocity structure maps in California, shown in Figure 3. Hazard results are also provided for site-specific estimates of basin depth parameters, Z1.0 and Z2.5, where these estimates are deemed reliable, shown in Figure 4 and Figure 5, respectively. Click here to access the NHR3 Project's main project on Natural Gas Infrastructure Safety And Integrity, Seismic Risk Assessment And Enhanced Training.

NO-Directivity PSHA

The seismic source characterization (SSC) model consists of the current UCERF3 (Field et al., 2014) seismic source model for crustal sources and the USGS (2014) seismic source model for the Cascadia interface and intraslab sources in Northern California. For the ground motion characterization (GMC) model, the Next Generation Attenuation – West2 (NGA-West2, Bozorgnia et al., 2014) ground motion models were used including the recommended epistemic uncertainty model of Al Atik and Youngs (2014). For the Cascadia subduction sources, a sensitivity study was conducted for the recently developed NGA-Subduction program (Bozorgnia et al., 2021) ground motion models for the evaluation of these models compared to the original BC Hydro model (Abrahamson et al., 2016). The GMC model for subduction sources is based on the regionalized Cascadia version of the NGA-Subduction ground motion models along with the BC Hydro model.
The hazard analysis was performed using a modified version of HAZ45 (HAZ45-CEC) that was developed to accommodate the UCERF3 crustal sources, shown in Figure 6. Implementation choices were made in HAZ45-CEC for the UCERF3 sources that are different than those made in the development of the 2018 National Seismic Hazard Models.
Click here for to view Report GIRS 2022-12 "Directivity-Based Probabilistic Seismic Hazard Analysis For The State Of California: Report 1, No-Directivity Baseline Case" on the statewide probabilistic seismic hazard study performed for the state of California. Input models, methods, and no-directivity hazard results are provided in addition to a detailed discussion of the implementation approach and the obtained results

WITH-Directivity PSHA

The complexities in both UCERF3 fault ruptures and modern directivity models and the incompatibilities between the two have made their implementation together in probabilistic hazard analysis difficult. As a result, hazard studies involving the UCERF3 source model do not typically consider directivity effects. The study whose results are made available in this portal presents the first implementation of directivity models with UCERF3 fault sources in a state-wide probabilistic hazard study for California. This implementation required simplifications to the complex statewide UCERF3 fault ruptures to render them compatible with the directivity models. Selected directivity models were implemented in a modified version of HAZ45 (HAZ45-CEC) along with the simplifications of the UCERF3 fault ruptures for directivity calculations.
These hazard directivity adjustment maps can be used to modify the results of no-directivity hazard to account for expected directivity effects at any location in California. This allows users to incorporate directivity effects with the UCERF3 source model without having to implement the models in a hazard program. Click here for to view Report GIRS 2023-05 "Directivity-Based Probabilistic Seismic Hazard Analysis For The State Of California: Report 2, Directivity Implementation" on the statewide probabilistic seismic hazard study performed for the state of California.

Directivity Models

Three directivity models were used in this study:
  • CS13: Chiou and Spudich (2013) — This model is based on the direct point parameter (DPP)
  • BS13: Bayless and Somerville (2013) — This model is an update to the Somerville et al. (1997) model, having the simplest formulation with few predictive parameters.
  • BSS20: Bayless et al. (2020) — This model is an update to their BS13 directivity model to include narrowband characteristics and better accommodate complex and multi-segment ruptures. The updated Bayless et al. (2020) model generally retains some of the computational simplicity of the BS13 model and uses both empirical ground-motion data and finite-fault simulations in the model development. Note that this update to model does not include an update for the fault normal and fault parallel components and is only an update to the average horizontal component including directivity.
Based on an evaluation presented in the study report, in computing the weighted with-directivity PSA in the PSHA, the three directivity models, CS13, BS13, and BSS20 were assigned weights of 0.5, 0.25, and 0.25, respectively, for the characterization of directivity effects from active crustal fault ruptures. Directivity was not applied to gridded seismicity or to subduction sources included in the seismic source characterization model for Northern California sites.

Fault-Normal & Fault-Parallel Components

In addition to the average directivity effects, hazard results were also computed for the fault normal (FN) and fault parallel (FP) effects as defined in the Bayless and Somerville (2013) model. The other directivity models do not provide FN/FP predictions and any application of the FN/FP results from only the Bayless and Somerville (2013) model should be evaluated prior to full adoption given the differences in the newer models of their supporting data and characteristics.

Intensity Measures

For each site, Vs30 (180-1100m/s), Return Period (52-5000yr), and Oscillator Period (0.01-10sec + PGV), there are three categories of Intensity Measures (IMs) that are made available:
  • RotD50-PSA NO directivity. Computed at each site for all oscillator periods, and cases.
  • RotD50-PSA WITH directivity. Computed at each site for oscillator periods >= 0.5sec. PSA values are available for the weighted-directivity model, as well as for each individual directivity model.
  • Fault-Normal and Fault-Parallel components of PSA based on the BS13 directivity model. Computed for oscillator periods >=0.5sec.
  • Directivity Adjustment Factor. This factor represents the ratio of WITH-Directivity RotD50-PSA to NO-Directivity RotD50-PSA. Computed at each site for oscillator periods >= 0.5sec. PSA values are available for the weighted-directivity model as well as for each individual directivity model. This adjustment factor can be used to a no-directivity UHS to estimate the effects of directivity. Adjustment-factor values are available for the weighted-directivity model, as well as for each individual directivity model.
  • Fault-Normal and Fault-Parallel-components Directivity Adjustment Factor. For ease of use, this factor represents the ratio of WITH-Directivity Fault-Normal and/Fault-Parallel WITH-Directivity PSA to NO-Directivity RotD50-PSA.
Deaggregation plots for magnitude-distance bins, source type, and individual fault sources are also provided in both portals.

How to Cite this Work

  • Silvia Mazzoni, Linda Al Atik, Nick Gregor, Yousef Bozorgnia (2023): Directivity-Based Intensity-Measure Interactive Maps. The B. John Garrick Institute for the Risk Sciences. Dataset. https://doi.org/10.34948/N3101P (https://doi.org/10.34948/N3101P)
  • Silvia Mazzoni, Linda Al Atik, Nick Gregor, Yousef Bozorgnia (2023): Directivity-Based PSHA Interactive Tool. The B. John Garrick Institute for the Risk Sciences. Dataset. https://doi.org/10.34948/N34S39 (https://doi.org/10.34948/N34S39)
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    Figures

    Figure 2. Location & ID of All Analysis Sites

    Figure 3. Estimated Vs30 at Each Site Location (click here for an interactive version of the map)

    Figure 4. Estimated Site-Specific Z1.0, where available (click here for an interactive version of the map)

    Figure 5. Estimated Site-Specific Z2.5, where available (click here for an interactive version of the map)

    Figure 6. UCERF-3=Based Fault Sources used in the analyses (click here for an interactive version of the map)

    Sponsors


    This web site was developed by Silvia Mazzoni, 2022. (smazzoni at ucla dot edu)