Project Title/ID Number NGA: Site Condition Metadata from Geology—1L05
Start/End Dates 4/1/03 – 6/30/04
Project Leader C. Wills (CDC-CGS/O)
Team Members K. Clahan (CDC-CGS/O)

F=faculty; GS=graduate student; US=undergraduate student; PD=post-doc; I=industrial collaborator; O=other

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1. Project Goals/Objectives:

Provide geology-based estimates of shear-wave velocity at all strong-motion recording sites in the PEER-NGA database.

2. Role of this project in supporting PEER’s mission (vision):

The PEER Next Generation Attenuation Equation project and developers of attenuation codes need to consider site conditions in seismic hazard analysis. To accommodate this need, CGS is contributing to PEER’s database of site conditions information for all strong motion stations that have recorded earthquake ground motion.

3. Methodology Employed:

For this project CGS compares the locations of measurements of shear-wave velocity with geologic maps to develop correlations between Vs and geologic units; the summary statistics can then be applied to earthquake strong-motion recording stations on those or similar geologic units.

4. Brief Description of past year’s accomplishments (Year 6) & more detail on expected Year 7 accomplishments:

To expand the database of strong ground motion data and metadata, CGS has added preliminary estimates of the average shear-wave velocity to 30 m each of approximately 1200 sites based on existing GIS data. These estimates have then been refined from the preliminary statewide site conditions map based on existing data from measured shear-wave velocity profiles and newly measured profiles, principally in areas where existing profile data is sparse or lacking. Based on the available site conditions information, CGS has estimated of the average shear wave velocity to 30 m for each site, including the basis for the estimate and uncertainty.

5. Other Similar Work Being Conducted Within and Outside PEER and How This Project Differs:

This work is based on generalizing the available measured shear-wave velocity data by geologic unit, then finding the mean and range of shear-wave velocity values for a particular unit. This approach allows for the calculation of summary statistics of shear wave velocity properties that can be applied to any location underlain by a particular unit.

fig 1a fig 1b

In this project we have examined weather units that have similar geologic conditions have similar shear-wave velocity characteristics. As shown in the charts above, alluvium in deep basins is not always similar in velocity. The alluvium in the Imperial Valley is both lower in velocity and less variable than in other basins, such as the Los Angeles basin. Assembling these statistics enables us to provide estimates of velocity and uncertainty for stations throughout California.

6. Plans for Year 8 if project is expected to be continued:

Future work will focus on providing velocity estimates for specific sites where the generalized approach does not provide a sufficiently accurate estimate, mainly sites where the geologic conditions change in the upper 30 m.

7. Describe any actual instances where you are aware your results have been used in industry:

Velocity estimates based on this technique are the basis of the “Preliminary site conditions map of California” which is used in estimating earthquake ground shaking for emergency response through “ShakeMap”s and is used in estimating future potential ground shaking used in setting earthquake insurance rates in California though the California Earthquake Authority.

8. Expected Milestones & Deliverables:

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