Memphis, Shelby County Seismic Hazard Mapping Meeting
June 10, 2003
All participants introduced themselves.
Buddy provided a very brief overview of project - what we did and why.
Current Products
Probabilistic maps
Chris Cramer described the draft probabilistic ground motion maps for the six quads covering most of Memphis. These use the identical input and computations as the USGS National Seismic Hazard maps except that the Memphis maps also include the effects of the local soils and sediments. These new maps include nine maps; 3 probability levels (2%, 5%, and 10% in 50 year probabilities of exceedance) for 3 measures of ground motions (peak ground acceleration or PGA, 0.2 second period and 1.0 second period). He showed and described the 2% in 50 year maps, noting that for higher frequency ground motions (PGA and 0.2 s period) the Memphis ground motions are deamplified relative to National maps. At 1 sec and longer periods, motions are amplified.
Discussion followed about how the maps might be used by geotechnical engineers. It was emphasized (repeatedly!) that Memphis maps are not intended to replace site-specific studies or to become part of building codes.
Geologic maps
Roy VanArsdale described the geologic maps of the six quads he, Randy Cox, and Dave Moore have completed and showed an example. The maps show surficial materials and include cross-sections. There was some discussion of liquefaction features found along some of the rivers, and their possible different liquefaction susceptibility. Dave Moore emphasized that in his quads along Nonconnah Creek and at Presidents Island exposures are very poor due to the engineered structure, so that although not observed one can’t rule out possibility of liquefaction features.
The maps will be available digitally, in GIS format.
Subsurface database
Brian Waldron described the evolution of the database, its table structure, and use of the Oracle database and its query facilities. The database includes information about hundreds of well and boring logs from Memphis and a large surrounding area, as well as other information, and is well documented. An important feature of the database is that it is publicly accessible using GIS ARCInfo applications and also via Internet using web-based map-server software. The Ground Water Institute maintains it for a variety of applications, beyond their use in the seismic hazard maps.
Liquefaction susceptibility maps
Glenn Rix summarized their (his and Sal Romero’s) strategy, progress, and plans for completing probabilistic susceptibility maps. They have focused on determining the liquefaction capacity from the existing data – geologic maps, CPT, and SPT. The capacity gets combined with the ‘demand’ (ground motion) to determine the liquefaction potential. Only a few very high quality CPT measurements exist and SPT measurements are more abundant but are of poorer or unknown quality. From CPT or SPT, assuming some demand, and using standard analyses they derive ‘factor of safety’ profiles. From these the ‘liquefaction potential index’ (LPI) is derived, which is a measure of potential to liquefy (from none to major liquefaction, 0 to 15). For each surficial material they combine LPIs to determine probability density functions, which provide measures of the probability of liquefaction for that unit. These can be combined with the geologic maps to produce probabilistic liquefaction susceptibility maps. An issue still to confront is how to differentiate between engineered and un-engineered fill. The SPT analyses should be done by end of July, with final maps done by the end of the year. Buddy emphasizes that the maps should not be used in place of site-specific studies; rather they could be used as guidelines for engineers.
Shear wave velocities and 3-D geologic model
Joan Gomberg summarized how the well and boring log stratigraphic interpretations were combined with measured shear wave velocity profiles to determine a relationship between the two. The abundant log information was used to estimate laterally varying boundaries between stratigraphic units, throughout the study area. These and the relationship between unit and shear wave velocity provide a means of estimating velocity profiles anywhere in the study area. These profiles are essential inputs to the ground motion calculations.
Discussion of derivative products
Gary Patterson showed an example outreach map from Seattle, which shows lifelines overlaid on a surficial geologic map. He described his project with the Memphis Chamber of Commerce to make GIS overlays for the Memphis maps. The Chamber wants ‘code-relevant’ maps. A long-term goal is to make 3-D automated computer visualizations.
Examples of scenario and predictive maps from other communities were shown. These included relative or amplification maps, composite hazard maps, and a variety of earthquake scenario maps showing either ground motion directly or intensities. The documentation and some derivative products available from the National Seismic Hazard map website were also shown.
Dave Rogers suggested that some maps showing the range of hazard would be useful products, including scenarios from the most probable, smallest significant earthquake (e.g. the 1843 earthquake, the last event that might have significantly affected Memphis) and from the largest, less probable event.
Sammy Crews from the Memphis EMA says any map format would be useful and they prefer scenario maps to probabilistic maps. His office plans from an all-hazards perspective. Earthquakes are perceived as the event that would affect the largest area most severely.
Straightforward guidelines on how to use the maps will be needed. SCEC has produced some for southern California, which might serve as useful examples.
There definitely is a need to educate the public, particularly politicians. The newspaper circular distributed in California some years ago was very well received.
Insurance companies are concerned with state-of-the-art, not building codes. Thus, they should be most interested in the Memphis maps.
The need for data being downloadable to other GIS systems was emphasized repeatedly.
The maps could be used to show how HAZUS could be improved with new data.