Uncertainty of Southern California Basin Depth Parameters

Stewart, Jonathan P.; Choi, Yoojoong; Shaw, John H.

doi:10.1785/0120050016

Cited by 7 publications

(9 citation statements)

References 10 publications

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“…In addition, in general, it was observed that the SCEC model underestimates the velocities near the center of the basin and overestimates them near the border. These results are consistent with those of Stewart et al (2005), who found that the SCEC velocities are too high in the shallow parts of the basin and too low in the deeper parts. According to Süss and Shaw (2003), the observed differences are due to the fact that the SCEC model uses a 1D function to represent the velocities within a given stratigraphic interval, which ignores lateral velocity gradients due to facies variations across the basin.…”

Section: The Scec Velocity Modelsupporting

confidence: 93%

High-Resolution 3D P-Wave Velocity Model for the East Ventura-San Fernando Basin, California, and Relocation of Events in the Northridge and San Fernando Aftershock Sequences

Pujol¹,

Mueller²,

Shen³

et al. 2006

Bulletin of the Seismological Society of America

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We determined a high-resolution 3D P-wave velocity model for an 80 km ‫ן‬ 80 km area around the San Fernando Valley extending 22 km below sea level. We used events recorded from 1981 to 2000 as well as 799 aftershocks of the 1971 San Fernando earthquake recorded by a portable network. The total number of events and stations used were 13,455 and 101, respectively. Most of the events are aftershocks of the 1994 Northridge earthquake. The inversion software includes a raytracing subroutine that can handle the sharp lateral velocity variations that exist in the area. The resulting model shows an overall good agreement with the Southern California Earthquake Center (SCEC) 3D velocity model, but images the deeper structure of the San Fernando valley in more detail. In addition, our results are supported by the inversion of synthetic data and by the remarkable agreement with a density model derived from gravity data along a 55-km-long profile. An approximately 10km-wide and sharply defined zone of Northridge aftershocks occurred in basement rocks between about 8 and 20 km depth and identifies the fault that slipped during the mainshock. Horizontal slices at deep (8-18 km) levels show that the Northridge earthquake occurred within a high-velocity basement block that may have controlled the size of the earthquake. The fault plane inferred from the seismicity is in good agreement with the geodetic fault plane determined by Hudnut et al. (1996) assuming uniform slip. Most of the aftershocks of the Northridge earthquake occurred within the sedimentary rocks of the San Fernando basin, with a large concentration of events located to the east of the eastern boundary of the mainshock fault plane. The latter events form a zone that dips about 45Њ to the southwest at depths less than about 14 km. These may represent a lateral ramp. This zone terminates at the steeply inclined eastern edge of the San Fernando basin. These events do not lie on the mainshock fault plane, and because the San Fernando aftershocks also lie east of the Northridge fault plane, the San Fernando and the Northridge mainshocks must have occurred on conjugate en echelon faults (i.e., they do not abut each other). In addition, an 8-km-wide cross section along the density model referred to previously suggests the Northridge aftershocks east of the mainshock and most of the aftershocks of the San Fernando earthquake illuminate two thrust faults that have a common highvelocity basement block in their footwalls. The velocity model produced by our analysis suggests that thrust faults formed by modern contraction in this region are strongly influenced by prior basin and basement block geometry.Online material: 3D P-wave velocity models without seismicity. Name /mea_ssa966_605411/966_05237/Mp_2

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Section: The Scec Velocity Modelsupporting

confidence: 93%

High-Resolution 3D P-Wave Velocity Model for the East Ventura-San Fernando Basin, California, and Relocation of Events in the Northridge and San Fernando Aftershock Sequences

Pujol¹,

Mueller²,

Shen³

et al. 2006

Bulletin of the Seismological Society of America

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“…After comparing the J-SHIS depths with their corresponding depth measurements derived from selected KiK-net velocity profiles, we find that the J-SHIS model underestimates site depths at shallow sites and overestimates depths at deep sites. Stewart et al (2005) reported similar results with the southern California basin model (Magistrale et al, 2000). The difference between J-SHIS estimates and measurements (PS logging) can result in a significant overestimation of ground motions, especially at soft sites, using BSSA14.…”

Section: Discussionsupporting

confidence: 59%

“…The color version of this figure is available only in the electronic edition. (Magistrale et al, 2000) by Stewart et al (2005). For the southern California basin model, the mean and standard deviation of residuals between inferred and measured Z 1:5 are 107 and 408 m, respectively, but they are 8.88 and 78.71 m for the J-SHIS model.…”

Section: Site-specific Depth Measurementsmentioning

confidence: 97%

“…Quality of site data is of great significance for empirical studies on site effects. Stewart et al (2005) investigated the bias of southern California basin depth parameters (Magistrale et al, 2000), which were found to have an underestimation bias near the basin margin and overestimation bias near the middle of the basin. The J-SHIS velocity model was queried to establish depth database for sites without depth measurements to 1.0 and 2:5 km=s in Japan during the NGA-West2 project.…”

Section: Introductionmentioning

confidence: 99%

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Testing the Depths to 1.0 and 2.5 km/s Velocity Isosurfaces in a Velocity Model for Japan and Implications for Ground‐Motion Modeling

Zhu

Cotton

Pilz

2019

Bulletin of the Seismological Society of America

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In the Next Generation Attenuation West2 (NGA‐West2) project, a 3D subsurface structure model (Japan Seismic Hazard Information Station [J‐SHIS]) was queried to establish depths to 1.0 and 2.5 km/s velocity isosurfaces for sites without depth measurement in Japan. In this article, we evaluate the depth parameters in the J‐SHIS velocity model by comparing them with their corresponding site‐specific depth measurements derived from selected KiK‐net velocity profiles. The comparison indicates that the J‐SHIS model underestimates site depths at shallow sites and overestimates depths at deep sites. Similar issues were also identified in the southern California basin model. Our results also show that these underestimations and overestimations have a potentially significant impact on ground‐motion prediction using NGA‐West2 ground‐motion models (GMMs). Site resonant period may be considered as an alternative to depth parameter in the site term of a GMM.

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“…The model is largely based on geological structure models. While uncertainties in basin depth parameters were not compiled, they have been examined for an earlier version of CVM-S4 by Stewart et al (2005). For the shallower sediments, velocities are taken from geotechnical borehole measurements.…”

Section: Basin Models In Southern Californiamentioning

confidence: 99%

NGA-West2 Site Database

Seyhan

Stewart

Ancheta³

et al. 2014

Earthquake Spectra

100

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The NGA-West2 site database (SDB) contains information on site condition and instrument housing for 4,147 strong-motion stations with recordings in the project flatfile. The stations are from active tectonic regions, mainly in California, Japan, Taiwan, China, and the Mediterranean area. The principal site parameter is the time-averaged shear wave velocity in the upper 30 m (V S30 ), which we characterize using measurements where available (2,013 stations) and proxy-based relationships otherwise. We also provide basin depths from published models for 2,761 sites mostly in California and Japan. We improved the documentation and consistency of site descriptors used as proxies for V S30 estimation (surface geology, ground slope, and geotechnical or geomorphic categories) and analyzed proxy performance relative to V S30 values from measurements. We present protocols for V S30 estimation from proxies that emphasize methods minimizing bias and dispersion relative to data. For each site, we provide the preferred V S30 and its dispersion.

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Uncertainty of Southern California Basin Depth Parameters

Cited by 7 publications

References 10 publications

High-Resolution 3D P-Wave Velocity Model for the East Ventura-San Fernando Basin, California, and Relocation of Events in the Northridge and San Fernando Aftershock Sequences

High-Resolution 3D P-Wave Velocity Model for the East Ventura-San Fernando Basin, California, and Relocation of Events in the Northridge and San Fernando Aftershock Sequences

Testing the Depths to 1.0 and 2.5 km/s Velocity Isosurfaces in a Velocity Model for Japan and Implications for Ground‐Motion Modeling

NGA-West2 Site Database

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