A Plan for Urban Seismic Hazard Mapping in the Evansville, Indiana Area

(Last  update,  September  2003)

One type of seismic hazard map depicts the expected level of ground shaking caused by seismic waves and how that level varies spatially. Urban seismic hazard maps differ from the USGS national seismic hazard maps in that they are higher resolution and they account for the effects of the shallow rocks, sediments, and topography on earthquake ground shaking (i.e. site effects). These maps typically show the motion at a single frequency or parameter and for a particular probability of being exceeded during a particular time span. Such maps are probabilistic, and explicitly consider the likelihood and potential contribution of all possible earthquakes, a range of possible seismic wave propagation characteristics and site effects, and account for observational uncertainties. Deterministic or scenario maps, showing the ground motion for a single set of input parameters (e.g. a specific earthquake at a particular location, etc.), can be derived from a subset of the information used to make probabilistic maps. In addition to ground motion, another type of seismic hazard maps may show the spatially variable likelihood of ground failure (such as landslide or soil liquefaction), by combining the potential of materials to lose strength or fail when shaken and the level of shaking expected.

Generating useful high resolution seismic hazard maps is both a technical and non-technical process. The maps and derivative products should be useful in meeting the needs of a broad constituency and thus will be widely effectively advertised, disseminated and updated. These primarily non-technical aspects of mapping must be accomplished in close coordination with the technical work.
     This plan serves as a 'blue print' for the Evansville area hazard mapping project.  We divide the plan below into three parts; the first concerns organizational issues, the second product generation and dissemination, and the third scientific work.  Short-term  action items may be found on a linked 'Action Items' web page.  This plan is a 'living' document, which will be updated and modified as we progress.  

Organization

I. Defining users and their needs.

Useful products satisfy user needs. Thus, we have identified who the most likely users are and what each needs.

• Building code officials and engineers having to conform to building codes are likely to use urban seismic hazard maps as guides; e.g. to identify locations where site-specific work might be needed.  At present, Indiana has adopted the International Building Code (IBC) and the International Residential Code (IRC) with a cap on the maximum PGA of 0.5 g, Kentucky has adopted the IRC and is working o amendments to the IBC (considering a 5% map) before adoption, and Illinois has no statewide building code.
• INDoT, KYDoT, LEPCs, levy authorities, surveyors all are likely users.
  
• Economic development offices would find the maps useful in guiding development.  The Southwest Indiana Regional Development Office and Kentucky Economic Area Development District offices  exist for the counties being mapped in this project.
• Risk managers from various businesses are likely users.  Most of these belong to the SWIDRCC and to the area’s Disaster Resistance Business Alliance.

II. Identifying resources, organization, scheduling.

The project will be coordinated by a coordinating committee.  This committee includes the following people, and the aspects of project work they're most responsible for:

Dave Williams (KGS) - state geologic surveys
Christine Martin (SWIDRCC) - administration, education and outreach
Jennifer Haase (Purdue) – seismological activities
Joan Gomberg (USGS)  - USGS liaison
Norm  Hester (CUSEC) -  CUSEC  liaison

Project resources may come from a variety of sources.  Among these are:

• The USGS may provide support through a variety of methods.  The external grants portion of the USGS National Earthquake Hazards Reduction Program (NEHRP) emphasizes urban hazard mapping in the Central and Eastern US (CEUS). Approximately one million dollars has been available for the NEHRP in the CEUS grants, which are applied for through a competitive process, although some of these funds may be reserved for targeted projects.  The internal portion of the NEHRP in the CEUS supports Buddy Schweig, Joan Gomberg, Chris Cramer, and Tony Crone, who all have some of their time allotted for this project.  Tony and Buddy will be doing paleoseismologic field work in the Wabash Valley during FY04.  The National  Cooperative Geologic Mapping Program will be supporting Rich Harrison and Dave Moore to assist in geologic mapping during FY04.
• The Indiana Dept. of Transportation (INDoT) is most likely to provide support if it is matched by other agencies.  Jennifer Haase has been leading a statewide assessment of seismic hazard for INDoT, with a focus on determining how much the hazard would be affected if site effects were accounted for.  Their current work shows methodology and how, if data could be collected, the work would be done and the potential results.  They are also preparing a proposal to continue this an additional two years, for submittal on Oct. 1, 2003 to INDoT's Seismic Advisory Committee.
• The 21st Century Fund are Indiana state funds that match federal funds for economic development.  The DRC, with Sandia labs, is responding to the 21st Century Fund RFP with a proposal to develop a comprehensive disaster management system.  Purdue may join their efforts. 
• The State Geological Surveys of Kentucky, Indiana, and Illinois all will provide personnel and use of their facilities as noted throughout this plan.  Participation by the SWIDRCC and scientists from Purdue University also represent commitments of resources.   

III.  Advisory Board.

An Advisory Board will facilitate communications between users and those involved in map production, and review project plans and products.  Finalizing a tentative participant list (below) is an action item.

Jerry Thompson - INDoT
Paul Doss – Univ. of S. Indiana geologist
??????  - Insurance sector representative
Jim Morley (Morley and Assoc.), John Donen – consulting engineers
Glen Given - KYDoT, CUSEC transportation task force.
Vince Drenovich, Antonio Babette - Purdue geotechnical engineers
Julio Ramirez, Mete Sozen - Purdue structural engineers
Joe Rachel, Richard Roth - FEMA reps from regions 5, 12
John Steele - Indiana EMA
Kent Arhenholtz - BLA transportation consulting
Barbara Smith - Vectren
???????? - DARBA rep
Paul Wagner – Bristol Meyers Squib
Jeff Schaffer – Louisville engineer
Don Yule – Army Corp, Vicksburg
Randy Heidorn – Evansville School Safety
Sherman Greer – County Emergency Manager
???????? - Congressional Staffer from Hosstedtler’s office
Henry Morgan – KY Geological Survey's Oil and Gas division (retired)
Ed Woolery - University of Kentucky
Al Zeiny - Univ. of Evansville structural engineer

Product Generation, Dissemination

I. Products

Hazard maps and derivative products will be produced for seven 7.5' quadrangles; Evansville North, Evansville South, Daylight, West Franklin, Wilson, Henderson, and Newburgh.  (Note that five quadrangles span Indiana and Kentucky, with only the Evansville North and Daylight quadrangles in Indiana alone.) Two types of hazard maps and derivative products will definitely be generated; probabilistic earthquake ground motion and liquefaction susceptibility maps.  The ground motion maps must be consistent with the national seismic hazard maps and to the extent possible, with other USGS urban seismic hazard map. An approach for mapping liquefaction susceptibility and assessment of the need for landslide susceptibility maps are current action items.  

All supportive data, maps, and derivative products will reside in GIS's and queriable databases, and be accessible via standard Internet Web browser interfaces. Project databases and products will build on existing GIS facilities at each state's geological survey.  Ultimately an interface can be built that links all three together, thus eliminating any duplication or reformatting of data.   The city of Evansville, Vanderburgh county, and the DRC also all have strong GIS capabilities and the DRC is a likely place ultimately to maintain the server that hosts project products.  Since a number of users in the region do not have GIS or Internet  access, paper maps, etc. also will be produced.

II. Education and Outreach

The mapping process provides an ongoing opportunity to educate the public about earthquake hazards. From the start the public can be engaged through workshops (e.g. topics might cover 'what are hazard maps and how might they help the community'), engaging undergraduate or high school students in some of the data compilation and collection, participation in complementary community activities, inviting media coverage of field work, etc. Once complete, strategies to market the map products and insuring that they are correctly understood and used need to be implemented.

Education and outreach activities in the region have been coordinated among the SWIDRCC, the Red Cross, regional universities, EMA offices, and local schools (e.g., PEPP public seismic stations now operate in New Harmony and Harrison High School in Evansville).  State Farm Insurance also has sponsored activities.   The SWIDRCC will take the lead in coordinating project education and outreach.  Christine Martin suggests holding a DARBA workshop, which in addition to marketing the project, could reveal new sources of data.  The USGS will participate in a short program during the 2003 Evansville Earthquake Week (November), coordinated by Christine Martin and Dave Williams.

Scientific Work

I. Introduction

The basic input parameters to high-resolution seismic hazard map calculations include information about 1) earthquake sources, 2) ground motion attenuation, and 3) near surface materials. For the first two of these it is critical to use the same input as used in the USGS national seismic hazard maps. The difference between urban hazard maps and the national hazard maps is in the third input, which this project will provide. The near surface geological materials, and the 3-dimensional variation in their thicknesses and physical properties, either amplifies or de-amplifies the level of earthquake ground shaking and lengthens or shortens its duration. They also affect the potential for ground failures to occur. A goal of seismic hazard mapping is to forecast these effects. The Memphis, Seattle, and other urban hazard mapping projects have developed the methodology for mapping, or forecasting ground motions and liquefaction susceptibility. However, as noted below, we anticipate new discovery and development.

In this context 'near surface' refers to different depths, depending on the type of map (ground motion, liquefaction or landslide susceptibility). The key controlling characteristics of the rocks and sediments also depend on the map type, but some are common to all. We discuss these common inputs below, and then those specific to each type of map in separate sections.

II. Surficial and 3-D geologic maps

The general state-of-the-art is to assume that relevant characteristics of near surface materials (or soil in engineering terms) vary with their geologic classification or lithology. If such an assumption is true, it is useful because surficial geologic maps and 3-dimensional pictures of the lithology can be made at higher resolution than one can map most other material properties. Thus, 3-D geologic maps can be used as proxies, or interpolation tools, for mapping the physical properties. The appropriateness and accuracy of using such proxies determines the accuracy of the maps, and undoubtedly varies regionally and locally. In addition to the geologic mapping itself, a major effort must be dedicated to establishing the degree to which the lithology correlates with the relevant material properties.

Surficial geologic maps showing the distribution of geologic units, particularly the unconsolidated materials, will be generated using standard mapping techniques. Because the surficial geologic mapping work will involve several geologists, efforts must be made to insure uniformity in mapping style, format, etc. The Indiana Geological Survey has no special funding to do surficial mapping but are committed to completing it in Indiana, with the assistance of USGS geologists Rich Harrison and Dave Moore.   (Some work was started under the StateMap program in four quads.) The Kentucky Geologic Survey will be making surficial maps for the Kentucky portions of the quads with a completion target of April 2004.  Bedrock maps already exist.  The Kentucky surficial mapping only is being supported by the State Map program, but efforts to collection of all subsurface information are underway using Survey support.  All maps, data etc. will be entered into the GIS of each Survey.  

A geologic mapping sub-committee will consist of Rich Harrison, Dave Moore, John Hill, Ron Counts, and Drew Andrew. All field mappers will meet on  the field trip in late October/early November 2003, primarily for the purpose of deciding on how to do mapping uniformly.

Data constraints on the stratigraphy come primarily from surface geologic mapping and from logs of various types, from which the depths to lithologic boundaries are estimated. The Indiana Geological Survey has an ARC-GIS water-well database (ILITH) that will soon be expanded to include hydro-carbon wells.  Engineering logs have been lost but they're committed to collecting them again and entering them into the database.   If needed, additional data might be acquired using Indiana Geologic Survey personnel, auguring and gamma logging, sedimentological testing, and SPT measurement equipment.  The  Kentucky Geologic Survey (Ron Counts) plans to acquire engineering and other log data as part of the surficial mapping effort.

The basic analysis for deriving stratigraphic boundaries requires fitting discontinuous surfaces to point measurements of the boundary depths. A number of possible fitting procedures may be applied, as well as structural interpretations that are compatible with the surficial geology and basic geologic principles.  Once we have determined what data exist, have collected and interpreted them (including measures of their accuracy), we will determine if and where additional information is needed. Once answered, plans for additional data collection and analyses can be made and executed.

III. Ground motion maps

To estimate probabilistic ground motions that include the effects of shallow rocks and sediments we will follow the analysis procedure described in Cramer (2003). The key input information includes the shear-wave velocity (Vs), compressional-wave velocity (Vp), initial damping (Qs, Qp), and density of the sub-surface materials, how these vary in 3-dimensions, and measures of the accuracy of these characteristics. For more complex (non-linear) soil response calculations, additional soil properties will need to be known (geotechnical properties such as soil class (sand versus clay), plasticity, porosity, water saturation, and dynamic soil properties). 'Shallow' in this context refers to depths extending to the base of the unconsolidated sediments. Minimally certain characteristics of the rock just beneath the sediments also should be known (Vs, Vp, density, and damping).

Currently the Indiana Geological Survey has ~35 Vs profiles (to bedrock), all in Indiana. The Kentucky Geological Survey may have some shear velocity profiles but no details about these are known.  The CUSEC State Geologists have a new NEHRP project to assess what Vs data exist (for Evansville and elsewhere), and to put them in a database that will reside at the Illinois Geological  Survey. Bob Bauer and Norm Hester are Project  Chiefs.    No information on density exists thus far.

Because measurements of the above characteristics are relatively costly to make, we will begin by asking whether they correlate with lithology.  Once the above data gathering is complete, and using the surficial and 3-D lithologic maps also being generated, we will assess if and where additional shear-wave velocity, compressional-wave velocity, and density measurements should be made to be able to assess their correlation with lithology, and devise a strategy for collecting them.  Should more profiles be needed, the Illinois and Kentucky Surveys have downhole Vs logging equipment that might be used. Whether assessing the utility of existing data or designing new data collection tasks, questions regarding the resolution and accuracy required must be considered along with those pertaining to resources, scheduling, data processing and archiving, etc.

State-of-the-art ground motion estimation considers the non-linear response of sediments to ground shaking. Non-linear effects may amplify or reduce ground motions relative to those motions estimated assuming that the output motion is simply proportional to the motion input to the base of the sediments. For sufficiently large input ground motions, the non-linear sediment response is thought to limit or cap the motion at the surface. While this implies a lower ground motion hazard, severe non-linearity ultimately may result in ground failure. Without question, predictive models of the non-linear response are highly uncertain, both in terms of the underlying theory and the input parameters that must be measured in the field. The crudest approach to include non-linear affects employ 'site-amplification factors' (e.g. those recommended in the 2000 NEHRP seismic provisions), which are standard multiplicative factors that depend on the gross sediment characteristics and input ground motion amplitude and frequency. More complete approaches account for the specific properties of the sediments and their underlying physical behaviors. In addition to requiring shear-wave and compressional-wave velocities and densities, such approaches also require specification of the 'modulus reduction' and 'damping' properties of the sediments, which describe how the sediments lose strength and dissipate energy with shaking, respectively. These parameters vary with depth, sediment type, etc. and are extremely difficult to measure. However, some modern geotechnical tools exist for doing so. Use of these should be considered as well as compilation of whatever relevant information might exist to constrain these properties.

IV. Liquefaction susceptibility maps

Susceptibility here refers to the inverse of the 'capacity' of the sediments to maintain their strength when shaken (i.e. to not liquefy). The liquefaction potential depends both on the capacity and on the 'demand', or the shaking levels the sediments are likely to experience. Thus, liquefaction potential maps combine ground motion and liquefaction susceptibility maps. Our strategy to mapping the susceptibility makes direct use of both geologic and geotechnical information, rather than only one as is often done in more traditional approaches. As for the ground motion parameters, this work focuses on establishing the relationship between the lithology and the properties that control liquefaction capacity.

Susceptibility is best estimated from cone penetrometer test (CPT) measurements, and also may be derived from standard penetration test (SPT) measurements that generally are more abundant but are of poorer or unknown quality. For a specified ground motion amplitude and duration, standard analyses are applied with the CPT or SPT measurements to derive a 'factor of safety' profile and then a 'liquefaction potential index' (LPI). LPI is a measure of potential to liquefy (from none to major liquefaction, 0 to 15). All the LPIs estimated for each surficial geologic unit are combined to determine probability density functions. These thus provide measures of the correlation of LPI and geologic unit, or equivalently the probability of liquefaction for each unit. These correlations or probability density functions then allow the geologic maps to be transformed into probabilistic liquefaction susceptibility maps.

In addition to considering compilation of existing and collection of new CPT and SPT measurements, information about grain size and other geotechnical properties, and the ground water table are needed. In contrast to the ground motion mapping, 'shallow' in this context refers to the top few tens of meters and geologic units must be distinguished with greater resolution (e.g., different types of fill may have very different susceptibilities while ground motion estimates are completely insensitive to these).

All geotechnical data once held by the Indiana Geological Survey have been lost.  Tom Holzer will collect some CPT and other geotech data in the Evansville area during the spring of 2004. Jennifer Haase notes that INDoT has geotechnical data from hundreds of sites (scanned CPT data on CD) and she will work with Christine Martin on a request to INDoT.  Jennifer also will consult with colleagues at Purdue who have worked with INDoT on these data. Some relevant data are being collected for the geologic mapping (see above).   John Kiefer says that  KYDoT has geotechnical data but locating them is  difficult.  Assessing what data exist is an action item, required to be able to decide if/more are needed. The potential to use INDoT's  newly acquired CPT and seismic velocity profiling truck is another action item. 

References

Cramer, C.H. (2003). Site-specific seismic hazard analysis that is completely probabilistic, Bull. Seismo. Soc. Am. Vol. 93, No. 4 (August), in press.