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  • 1
    Online Resource
    Online Resource
    Exploring the Effect of Minimum Magnitude on California Seismic Hazard Maps
    McGill University Library and Archives ; 2023
    In:  Seismica Vol. 2, No. 1 ( 2023-06-22)
    Details
    In: Seismica, McGill University Library and Archives, Vol. 2, No. 1 ( 2023-06-22)
    Abstract: A recent topic of interest is the performance of probabilistic seismic hazard maps relative to historical shaking datasets. Maps developed for different countries appear to overpredict shaking relative to historical shaking datasets. To explore whether this discrepancy arises because of incompleteness in historical datasets, we consider maps and historical data from California. Current probabilistic seismic hazard maps for California appear to predict stronger short period shaking than historical maxima captured by the California Historical Intensity Mapping Project (CHIMP) dataset between 1857 and 2019. We estimate that CHIMP has a magnitude completeness between M 6-6.6, whereas California hazard maps assume a minimum magnitude (MMin) of 5. Disaggregating the maps shows that earthquakes smaller than M 6 and 6.6 respectfully contribute about 25% and 45% of the hazard across California. Increasing the hazard map's MMin to 6 and 6.6 respectively reduces the discrepancy between predicted and observed shaking by approximately 10-20% and 30-35%. These reductions are not enough to bring the maps and data in alignment. Thus, MMin inconsistencies contribute to, but are not the sole cause of, the discrepancy between predicted and historically observed shaking. These results may be generally applicable to maps elsewhere, although MMin will vary for different historical datasets.
    Type of Medium: Online Resource
    ISSN: 2816-9387
    URL: Article
    DOI: 10.26443/seismica.v2i1.212
    Language: Unknown
    Publisher: McGill University Library and Archives
    Publication Date: 2023
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  • 2
    Online Resource
    Online Resource
    Revisiting California’s Past Great Earthquakes and Long-Term Earthquake Rate
    Seismological Society of America (SSA) ; 2021
    In:  Bulletin of the Seismological Society of America Vol. 111, No. 1 ( 2021-02-01), p. 356-370
    Details
    In: Bulletin of the Seismological Society of America, Seismological Society of America (SSA), Vol. 111, No. 1 ( 2021-02-01), p. 356-370
    Abstract: In this study, we revisit the three largest historical earthquakes in California—the 1857 Fort Tejon, 1872 Owens Valley, and 1906 San Francisco earthquakes—to review their published moment magnitudes, and compare their estimated shaking distributions with predictions using modern ground-motion models (GMMs) and ground-motion intensity conversion equations. Currently accepted moment magnitude estimates for the three earthquakes are 7.9, 7.6, and 7.8, respectively. We first consider the extent to which the intensity distributions of all three earthquakes are consistent with a moment magnitude toward the upper end of the estimated range. We then apply a GMM-based method to estimate the magnitudes of large historical earthquakes. The intensity distribution of the 1857 earthquake is too sparse to provide a strong constraint on magnitude. For the 1872 earthquake, consideration of all available constraints suggests that it was a high stress-drop event, with a magnitude on the higher end of the range implied by scaling relationships, that is, higher than moment magnitude 7.6. For the 1906 earthquake, based on our analysis of regional intensities and the detailed intensity distribution in San Francisco, along with other available constraints, we estimate a preferred moment magnitude of 7.9, consistent with the published estimate based on geodetic and instrumental seismic data. These results suggest that, although there can be a tendency for historical earthquake magnitudes to be overestimated, the accepted catalog magnitudes of California’s largest historical earthquakes could be too low. Given the uncertainties of the magnitude estimates, the seismic moment release rate between 1850 and 2019 could have been either higher or lower than the average over millennial time scales. It is further not possible to reject the hypothesis that California seismicity is described by an untruncated Gutenberg–Richter distribution with a b-value of 1.0 for moment magnitudes up to 8.0.
    Type of Medium: Online Resource
    ISSN: 0037-1106 , 1943-3573
    URL: Article
    DOI: 10.1785/0120200253
    Language: English
    Publisher: Seismological Society of America (SSA)
    Publication Date: 2021
    detail.hit.zdb_id: 2065447-9
    SSG: 16,13
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  • 3
    Online Resource
    Online Resource
    California Historical Intensity Mapping Project (CHIMP): A Consistently Reinterpreted Dataset of Seismic Intensities for the Past 162 Yr and Implications for Seismic Hazard Maps
    Seismological Society of America (SSA) ; 2020
    In:  Seismological Research Letters Vol. 91, No. 5 ( 2020-09-01), p. 2631-2650
    Details
    In: Seismological Research Letters, Seismological Society of America (SSA), Vol. 91, No. 5 ( 2020-09-01), p. 2631-2650
    Abstract: Historical seismic intensity data are useful for myriad reasons, including assessment of the performance of probabilistic seismic hazard assessment (PSHA) models and corresponding hazard maps by comparing their predictions to a dataset of historically observed intensities in the region. To assess PSHA models for California, a long and consistently interpreted intensity record is needed. For this purpose, the California Historical Intensity Mapping Project (CHIMP) has compiled a dataset that combines and reinterprets intensity information that has been stored in disparate and sometimes hard-to-access locations. The CHIMP dataset also includes new observations of intensity from archival research and oral history collection. Version 1 of the dataset includes 46,502 intensity observations for 62 earthquakes with estimated magnitudes ranging from 4.7 to 7.9. The 162 yr of shaking data show observed shaking lower than expected from seismic hazard models. This discrepancy is reduced, but persists, if historical intensity data for the largest earthquakes are smoothed to reduce the effects of spatial undersampling. Possible reasons for this discrepancy include other limitations of the CHIMP dataset, the hazard models, and the possibility that California seismicity throughout the historical period has been lower than the long-term average. Some of these issues may also explain similar discrepancies observed for Italy and Japan.
    Type of Medium: Online Resource
    ISSN: 0895-0695 , 1938-2057
    URL: Article
    DOI: 10.1785/0220200065
    Language: English
    Publisher: Seismological Society of America (SSA)
    Publication Date: 2020
    detail.hit.zdb_id: 2403376-5
    SSG: 16,13
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  • 4
    Online Resource
    Online Resource
    A study on the effect of site response on California seismic hazard map assessment
    Frontiers Media SA ; 2022
    In:  Frontiers in Earth Science Vol. 10 ( 2022-9-26)
    Details
    In: Frontiers in Earth Science, Frontiers Media SA, Vol. 10 ( 2022-9-26)
    Abstract: Prior studies have repeatedly shown that probabilistic seismic hazard maps from several different countries predict higher shaking than that observed. Previous map assessments have not, however, considered the influence of site response on hazard. Seismologists have long acknowledged the influence of near-surface geology, in particular low-impedance sediment layers, on earthquake ground-motion at frequencies of engineering concern. Although the overall effects of site response are complex, modern ground-motion models (GMMs) account for site effects using terms based on V S30 , the time-averaged shear-wave velocity in the upper 30 m of the Earth’s surface. In this study, we consider general implications of incorporating site terms from modern GMMs using site-specific V S30 as a proxy in probabilistic seismic hazard maps for California. At the long periods (1–5 s) that affect tall buildings, site terms amplify the mapped hazard by factors of 1–3 at many sites relative to maps calculated for the standard reference soft-rock site condition, V S30 = 760 m/s. However, at the short periods of ground-motion that are the main contributors to peak ground acceleration (PGA) and thus affect smaller structures, only negligible effects occur due to nonlinear deamplification of strong ground-motion at high frequencies. Nonlinear deamplification increases as the shaking level increases. For very strong shaking, deamplification can overcome the linear amplification, yielding net deamplification. We explore the implications of these results for the evaluation of hazard maps. Because site effects do not change the maps appreciably at short periods, we can exclude site response as an explanation for why the maps overpredict historically observed shaking as captured by the California Historical Intensity Mapping Project (CHIMP) dataset. The results are expected to be generalizable to regions that are comparable to California in terms of structure and seismicity rates. In low-to-moderate-seismicity regions where the hazard reflects weaker shaking, nonlinear site response is expected to be less important for the hazard.
    Type of Medium: Online Resource
    ISSN: 2296-6463
    URL: Article
    DOI: 10.3389/feart.2022.931340
    Language: Unknown
    Publisher: Frontiers Media SA
    Publication Date: 2022
    detail.hit.zdb_id: 2741235-0
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  • 5
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    Online Resource
    New Methodology for Unbiased Ground-Motion Intensity Conversion Equations
    Seismological Society of America (SSA) ; 2023
    In:  Bulletin of the Seismological Society of America Vol. 113, No. 3 ( 2023-06-01), p. 1133-1151
    Details
    In: Bulletin of the Seismological Society of America, Seismological Society of America (SSA), Vol. 113, No. 3 ( 2023-06-01), p. 1133-1151
    Abstract: Ground-motion intensity conversion equations (GMICEs) allow for conversions between ground-motion amplitude and shaking intensity. The current methods used to develop GMICEs model the dependence of the intensity on the peak ground-motion (PGM) parameter. For some models, there is a second step that models the magnitude and distance dependence of the residuals from the initial regression. We show that this approach for developing GMICEs works well for estimating the intensity from median ground motions, but for ground-motion values away from the medians, the intensities estimated by the GMICE can have large bias, with overprediction for positive PGM residuals and underprediction for negative PGM residuals. The bias is due to an implicit assumption in the current approach that there is a direct causal relation between intensity and the ground-motion parameter and that the residuals of the intensity and ground-motion parameter are fully correlated. We present two alternative methodologies for developing GMICEs that do not suffer from this bias. The first method includes the magnitude and distance scaling of the GMICE in the initial regression for the scaling with the PGM. The second method excludes the magnitude and distance terms but includes the number of standard deviations of the PGM (ϵ) as an additional parameter in the GMICE. Using a synthetic data set of intensity and peak ground acceleration values, we show that the GMICE developed using the proposed method leads to more accurate estimates of the intensity than current methods. We also discuss implications of using GMICEs based on the proposed methods for the evaluation of probabilistic hazard maps and as input to ShakeAlert estimates.
    Type of Medium: Online Resource
    ISSN: 0037-1106 , 1943-3573
    URL: Article
    DOI: 10.1785/0120220224
    Language: English
    Publisher: Seismological Society of America (SSA)
    Publication Date: 2023
    detail.hit.zdb_id: 2065447-9
    SSG: 16,13
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  • 6
    Online Resource
    Online Resource
    Uncertainties in Intensity-Based Earthquake Magnitude Estimates
    Seismological Society of America (SSA) ; 2023
    In:  Seismological Research Letters Vol. 94, No. 5 ( 2023-09-01), p. 2202-2214
    Details
    In: Seismological Research Letters, Seismological Society of America (SSA), Vol. 94, No. 5 ( 2023-09-01), p. 2202-2214
    Abstract: Estimating the magnitude of historical earthquakes is crucial for assessing seismic hazard. Magnitudes of early-instrumental earthquakes can be inferred using a combination of instrumental records, field observations, and the observed distribution of shaking intensity determined from macroseismic observations. For earthquakes before 1900, shaking intensity distributions often provide the only information to constrain earthquake magnitude. Considerable effort has been made to develop methods to estimate the magnitude of moderate-to-large historical earthquakes using shaking intensities derived from macroseismic data. In this study, we consider earthquakes in California with known instrumental magnitudes to explore uncertainties in estimating the magnitude of historical earthquakes from intensity information alone. We use three California-specific intensity prediction equations (IPEs) and an IPE based on a global ground-motion model (GMM) to determine optimum intensity-based magnitudes for 33 moderate-to-large California earthquakes between 1979 and 2021. Intensity-based magnitudes are close to instrumental magnitudes on average. However, intensity-based magnitudes for individual events differ by as much as 2.2 magnitude units from instrumental magnitudes. This result reflects the weak dependence of ground motions and shaking intensities on moment magnitude and their strong dependence on stress drop. Considering the intensity distributions of the 1906 San Francisco and 1989 Loma Prieta earthquakes, we show that information that could constrain rupture length is discarded when considering only the 2D decay of intensity with distance. We also show that ground-motion intensity conversion equations used in a GMM-based approach may cause a systematic overestimation of large historical earthquake magnitudes. This study underscores both the reducible and potentially irreducible uncertainties associated with using intensity data to estimate magnitudes of historical earthquakes using IPEs and highlights the value of using additional information to constrain rupture dimensions. Using intensity observations alone, moment magnitude uncertainties are typically on the order of a full unit.
    Type of Medium: Online Resource
    ISSN: 0895-0695 , 1938-2057
    URL: Article
    DOI: 10.1785/0220230030
    Language: English
    Publisher: Seismological Society of America (SSA)
    Publication Date: 2023
    detail.hit.zdb_id: 2403376-5
    SSG: 16,13
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  • 7
    Online Resource
    Online Resource
    A compilation of igneous rock volumes at volcanic passive continental margins from interpreted seismic profiles
    Elsevier BV ; 2020
    In:  Marine and Petroleum Geology Vol. 122 ( 2020-12), p. 104635-
    Details
    In: Marine and Petroleum Geology, Elsevier BV, Vol. 122 ( 2020-12), p. 104635-
    Type of Medium: Online Resource
    ISSN: 0264-8172
    URL: Article
    DOI: 10.1016/j.marpetgeo.2020.104635
    Language: English
    Publisher: Elsevier BV
    Publication Date: 2020
    detail.hit.zdb_id: 1494910-6
    detail.hit.zdb_id: 48427-1
    SSG: 13
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