Geologic and Seismic Hazards (GASH) 

The Geologic and Seismic Hazards Technical Working Group (GASH TWG) was formed in 2020 and focuses on disseminating information to AEG membership regarding regulatory and technical guidance, existing and emerging methods and technologies, and case history examples related to assessment of geologic and seismic hazards for a variety of facility types and structures.  GASH expands the scope of the previous Seismic Hazard TWG by covering a wider range of geologic hazards.  Information disseminated by the GASH TWG includes, but is not limited to, that which addresses the following topics: 

  1. Characterization of seismic sources (i.e., faults) as input into probabilistic seismic hazard analysis (PSHA).
  2. The Senior Seismic Hazard Analysis Committee (SSHAC) process for performing a PSHA.
  3. Characterization of hazards due to tectonic and non-tectonic (e.g., subsidence due to dissolution) ground deformation using field and remote sensing techniques.
  4. Probabilistic Fault Displacement Hazard Analysis (PFDHA).
  5. Paleoseismology in the evaluation of fault displacement history and seismic hazard.
  6. Characterization of volcanic hazards for location of proposed nuclear power facilities in regions containing volcanic centers and volcanic deposits of Quaternary age (2.58 million years to present).
  7. Probabilistic Volcanic Hazards Analysis (PVHA).
  8. Integration of geologic, geotechnical, hydrologic, and geospatial data to ensure robust characterization of hazards and foster cooperation with other AEG TWGs.
  9. Evaluation of faulting and earthquake ground motion for hydroelectric power projects.
  10. Communicating with members of the public, particularly non-scientists and key decision-makers, about issues related to geologic and seismic hazards.

The GASH TWG acts to enhance public awareness of the role geoscientists play for ensuring public health and safety and protecting the environment. As practitioners who apply existing and emerging methods and technologies for assessment of geologic and seismic hazards, the GASH TWG discusses the range of information used for such assessments at technical seminars, symposia, conferences, workshops, forums, and field trips that are open to the public and address current practices showcasing the state of the art. Certain of these events will be regularly organized and convened by the GASH TWG for AEG annual meetings and as specialty presentations separate from the annual meeting.                                                                         


NOTE: It is intended that the leadership roles for the GASH TWG will be rotated on a regular interval of about three years to avoid imposing an excessive continuous time commitment upon the co-chairs and to promote new ideas within the TWG.

Gerry L. Stirewalt, PhD, PG, CEG - Senior Geologist, U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Engineering and External Hazards, [email protected]

Gerry is a structural geologist who has polished his passion for geoscience through more than 53 years of experience that includes teaching geology classes at the University of British Columbia (Vancouver, Canada), the University of North Carolina at Chapel Hill, and Furman University (Greenville, South Carolina); geologic characterization of nuclear and non-nuclear power plant sites at locations in the U.S. and abroad; tracking the geologic characterization effort for the proposed high-level radioactive waste (HLW) disposal site at Yucca Mountain (YM) in Nevada; 3D geospatial modeling of faults and subsurface rock units at the proposed HLW disposal site at YM; and geologic characterization of a proposed Canadian HLW disposal site in southern Manitoba. He has served at the U. S. Nuclear Regulatory Commission (NRC) since 2005 with the primary goal of protecting public health and safety and the environment. This goal is attained by ensuring that applicants for proposed new nuclear power facilities characterize the geology of their respective potential sites as required by NRC regulations.

Courtney Johnson, MS, PG, CEG - Principal Geologist, Slate Geotechnical Consultants, [email protected]

Courtney is an engineering geologist and one of the founders of Slate Geotechnical Consultants in California. With 15 years of consulting under her belt, her experience has been quite varied. She has provided geologic input for geologic and seismic hazard assessments, geotechnical analyses, and subsurface investigation in multiple projects. She has conducted projects for structures including hospitals, nuclear facilities, pipelines, offshore platforms, dams, levees, high-rise buildings, and federal buildings located across the United States and abroad.

Previous Activities:

September 2021 at the AEG 64th Annual Meeting in San Antonio, TX – The GASH TWG convened a 1.5-day symposium titled “Evaluating Geologic and Seismic Hazards and the Potential Need for Hazard Mitigation”.

Planned Activities: 

15-17 March 2022 – The GASH TWG will convene a virtual conference titled “Methods and Case Histories for Characterization of Potential Geologic and Seismic Hazards”.

September 2022 – AEG 65th Annual Meeting in Las Vegas, NV: The GASH TWG will review submitted abstracts and convene a technical session related to focus of the GASH TWG as defined above in the “Purpose Statement”.

September 2023 – AEG 66th Annual Meeting in Portland, OR: The GASH TWG plans to convene a technical symposium, and possibly organize a field trip. Ideas for the symposium are currently being discussed among TWG members, with an initial thought to focus on use of lidar in assessment of geologic and seismic hazards.

Coordinating with other TWGs in a timely manner as necessary to define efficient ways to cooperate for planning sessions, symposia, conferences, and field trips where there is an overlap between purpose of the GASH TWG and that of other TWGs.

Future Goals:

September 2024 – AEG 67th Annual Meeting: The GASH TWG  will consider convening a technical symposium or a technical session for this Annual Meeting.

Longer term goals of the GASH TWG will continue to support the stated purpose of the TWG. These goals include the following actions as opportunities arise to implement them:

  • Disseminating information to AEG membership on important issues pertinent to the purpose of the TWG.
  • Continuing meetings of the GASH TWG, or email exchanges, as necessary to pursue and plan upcoming activities. (See “RESPONSIBILITES” below.)
  • Providing an article or note for AEG NEWS related to a specific issue related to the purpose of the GASH TWG annually as noteworthy issues arise.
  • Organizing a topical symposium for the annual meeting once every two to three years.
  • Conducting a technical specialty symposium, conference, or professional forum once every four years outside the annual meeting, if feasible.
  • Promoting papers to be published in the AEG Journal by GASH TWG members or others knowledgeable about efforts related to purpose of the TWG.
  • Reviewing a pertinent textbook or recognizing an important article related to purpose of the GASH TWG in the Insider. 


Based on the Operating Policies of AEG, the GASH TWG is generally responsible for the following actions:

  • Submitting event information related to purpose of the GASH TWG to the AEG News Editors and AEG Headquarters to post on the AEG website as such information develops.
  • Developing a short course, technical session, symposium, and/or field trip for an annual meeting at least once every three years.
  • Providing subject-specific materials for the AEG website annually, or as new materials are developed. The format may be a white paper, a case study, a fact sheet, or another format as deemed acceptable.
  • Developing and maintaining a list of references related to information about geologic and seismic hazards for the AEG website. Efforts will be made to update the list annually.
  • Providing input to interested parties on local, state, or federal regulations related to geologic and seismic hazards that potentially impact critical and non-critical infrastructure as opportunities arise.
  • Outreach to high schools and universities regarding careers in engineering geology focused on identification and mitigation of geologic and seismic hazards as opportunities arise. NOTE: This effort should be coordinated with the AEG Committee on Diversity, Equity, and Inclusion (DEIC) and be part of the overall effort of the DEIC to encourage broader diversity, equity, and inclusion in AEG membership.
  • Submitting an annual written report outlining the GASH TWG Action Plan to the Board of Directors prior to each Annual Meeting, with 01 August as the requested submission date.
  • Including the current Past President on GASH TWG calls and all relevant communications because that individual is the TWG Coordinator who functions as liaison between TWGs and the AEG Board of Directors.

Select Website Links Related to Geologic and Seismic Hazards:

Hazards Caucus Alliance

Earthquake Engineering Research Institute (EERI)

U.S. Geological Survey (USGS) Quaternary Faults and Folds Database

USGS Earthquake Hazards Program

USGS National Earthquake Hazards Reduction Program (NEHRP)

USGS Geologic Hazards Science Center (GHSC)

USGS Volcanic Hazards

USGS Landslide Hazards | U.S. Geological Survey (

Seismological Society of America

Select References Related to Geologic and Seismic Hazards:  

Abrahamson, N. A., and J. J. Bommer, 2005, Probability and uncertainty in seismic hazard analysis: Earthquake Spectra, v. 21, no. 2, p. 603–607.

Abrahamson, N.A., Silva, W.J., and Kamai, R., 2014, Summary of the ASK14 Ground Motion Relation for Active Crustal Regions: Earthquake Spectra: v. 30, no. 3, pp. 1025-1055.

Abrahamson, N.A., Kuehn, N., Gulerce, Z., Gregor, N., Bozorgnia, Y., Parker, G., Stewart, J., Chiou, B., Idriss, I.M., Campbell K., and R. Youngs, 2018, Update of the BC Hydro Subduction Ground-Motion Model using the NGA-Subduction Dataset: Pacific Earthquake Engineering Research (PEER) Center, University of California, Berkeley, CA, PEER Report 2018/02.

Allen, C.R., 1975, Geologic criteria for evaluating seismicity: Geological Society of America Bulletin, v. 86, p. 1041-1056.

Atkinson, G. M., J. J. Bommer, and N. A. Abrahamson, 2014, Alternative approaches to modeling epistemic uncertainty in ground motions in probabilistic seismic hazard analysis: Seismological Research Letters 85, 1141– 1144.

Bear-Crozier, A.N., V. Miller, V. Newey, N. Horspool, and R. Weber, 2016, Probabilistic Volcanic Ash Hazard Analysis (PVAHA) I - Development of the VAPAH tool for emulating multi-scale volcanic ash fall analysis: Journal of Applied Volcanology, v. 5, no. 3, 20 p.

Biasi, G. P., and S. G.Wesnousky, 2016, Steps and gaps in ground ruptures - Empirical bounds on rupture propagation: Bulletin of the Seismological Society of America, v.96, no. 3, p. 1110–1124.

Biasi, G. P., and S. G. Wesnousky, 2017, Bends and ends of surface ruptures: Bulletin of the Seismological Society of America, v. 107, no. 6, p. 2543–2560.

Bonilla, M.G., and Lienkaemper, J.J., 1991, Factors affecting the recognition of faults in exploratory trenches: U.S. Geological Survey Bulletin 1947, 54 p.

Boore, D.M., Stewart, J.P., Seyhan, E., and Atkinson, G.M., 2014. NGA-West2 Equations for Predicting PGA, PGV, and 5% Damped PSA for Shallow Crustal Earthquakes: Earthquake Spectra, Vol. 30, No. 3, pp. 1057-1085.

Borchardt, G., 2010, Determining Relative Age of Faulting Using Soil Stratigraphy - Problems and Misconceptions: Environmental & Engineering Geoscience, Vol XVI, No.1, p. 31-39.

Campbell, K.W. and Y. Bozorgnia, 2014. NGA-West2 Ground Motion Model for the Average Horizontal Components of PGA, PGV, and 5% Damped Linear Acceleration Response Spectra: Earthquake Spectra: August 2014, Vol. 30, No. 3, pp. 1087-1115.

Chiou, B.S-J., and R.R. Youngs, 2014, Update of the Chiou and Youngs NGA Model for the Average Horizontal Component of Peak Ground Motion and Response Spectra: Earthquake Spectra: v. 30, no. 3, p. 1117-1153.

Electric Power Research Institute (EPRI), 2012, Technical Report: Central and Eastern United States Seismic Source Characterization for Nuclear Facilities: EPRI, U.S. DOE, and U.S. NRC.

Electric Power Research Institute (EPRI), 2015, Technical Report on Central and Eastern United States Seismic Source Characterization for Nuclear Facilities - Maximum Magnitude Distribution Evaluation: EPRI.

Federal Energy Regulatory Commission (FERC), 2018, Engineering Guidelines for the Evaluation of Hydropower Projects, Ch 13, Evaluation of Earthquake Ground Motions: Division of Dam Safety and Inspections.

Field, E.H. et al, 2017, A Synoptic View of the Third Uniform California Earthquake Rupture Forecast (UCERF3): Seismological Research Letters v. 88, no. 5, p. 1259-1267.

Frankel, A., R. Chen, M. Petersen, M. Moschetti and B. Sherrod, 2015, 2014 Update of the Pacific Northwest Portion of the U.S. National Seismic Hazard Maps: Earthquake Spectra, v. 31, no. S1, pages S131–S148

Goulet C.A., Y. Bozorgnia, N. Abrahamson, et al., 2018, Central and eastern North America ground motion characterization - NGA-East Final Report: Report no. 2018/08. Berkeley, CA: PEER, 817 pp.

Goulet C.A., Y. Bozorgnia, N. Kuehn, et al., 2017, NGA-East ground-motion models for the US Geological Survey National Seismic Hazard Maps. Report no. 2017/03. Berkeley, CA: PEER, 207 pp., and 12 pp. addendum.

Haller, K.M., M.P. Moschetti, C.S. Mueller et al., 2015, Seismic hazard in the Intermountain West: Earthquake Spectra, v. 31, no. S1, p. S149–S176.

Hashash,Y.M.A., O. Ilhan, J.A. Harmon, et al., 2020, Nonlinear site amplification model for ergodic seismic hazard analysis in central and eastern North America: Earthquake Spectra, v. 36, no. 1, p. 69–86.

International Atomic Energy Agency (IAEA), 2012, Volcanic Hazards in Site Evaluation for Nuclear Installations: IAEA Specific Safety Guide SSG-21, Vienna, Austria.

International Atomic Energy Agency (IAEA), 2021, An introduction to probabilistic fault displacement hazard analysis in site evaluation for existing nuclear installations: IAEA TECDOC-1987, 144 p., Vienna, Austria.

International Atomic Energy Agency (IAEA), 2022, Seismic hazards in site evaluation for nuclear installations, Seismic Safety Guide SSG-9 (Rev. 1): IAEA STI/PUB/1950, 77 p., Vienna, Austria

Lienkaemper, J.J, and C. Bronk Ramsey, 2009, OxCal - Versatile Tool for Developing Paleo Earthquake Chronologies - A Primer: Seismological Research Letters, v.80, n.3. p. 431-434.

McCalpin, J.P., 2009, editor, Paleoseismology (second edition): Burlington, Massachusetts, Academic Press (Elsevier), 613 p.

Moschetti, M.P., P.M. Powers, M.D. Petersen et al., 2015, Seismic source characterization for the 2014 update of the U.S. National Seismic Hazard Model: Earthquake Spectra, v 31, no. S1, p.S31-S57.

Moss, R. and Z. Ross, 2011, Probabilistic Fault Displacement Hazard Analysis for Reverse Faults: Bulletin of the Seismological Society of America, 101(4). 

Noller, J. S., J.M. Sowers. and W.R. Lettis, 2000, Quaternary Geochronology - Methods and Applications: American Geophysical Union, Reference Shelf 4, 582 p.

Pierson, T.C., N.J. Wood, and C.L. Driedger, 2014, Reducing risk from lahar hazards - Concepts, case studies, and roles for scientists: Journal of Applied Volcanology, v.3, no.16, 25 p.

Petersen, M.D., T.E. Dawson, R. Chen, T. Cao, C.J. Wills, D.P. Schwartz, and A.D. Frankel, 2011, Fault Displacement Hazard for Strike-Slip Faults: Bulletin of the Seismological Society of America, v. 101, no. 2, p. 805–825.

Petersen et al., 2020, The 2018 update of the US National Seismic Hazard Model: Overview of model and implications, Earthquake Spectra, v. 36, no. 1, p. 5–41

Reiter, L., 1990, Earthquake Hazard Analysis – Issues and Insights, Columbia University Press, 254 p.

Stirling, M., T. Goded, K. Berryman, and N. Litchfield, 2013, Selection of Earthquake Scaling Relationships for Seismic-Hazard Analysis: Bulletin of the Seismological Society of America, v. 103, no. 6, p. 1–19.

U.S. Nuclear Regulatory Commission, 1997, Senior Seismic Hazard Analysis Committee (SSHAC)  recommendations for probabilistic seismic hazard analysis - Guidance on uncertainty and use of experts: Washington, D.C., U.S. Nuclear Regulatory Commission Report, NUREG/CR-6372.

U.S. Nuclear Regulatory Commission, 2012, Practical Implementation Guidelines for SSHAC Level 3 and 4: Washington, D.C., U.S. Nuclear Regulatory Commission Report, NUREG–2117.

U.S. Nuclear Regulatory Commission, 2018, Updated Implementation Guidelines for SSHAC Hazard Studies: Washington, D.C., U.S. Nuclear Regulatory Commission Report, NUREG–2213.

U.S. Nuclear Regulatory Commission, 2021, Volcanic Hazards Assessment for Proposed Nuclear Power Reactor Sites: Regulatory Guide 4.26, Rev 0, 28p. NRC Agencywide Documents Access and Management System (ADAMS) Accession Number ML20272A168.

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Wells, D.L., and K.J. Coppersmith, 1994, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement: Bulletin of the Seismological Society of America, v. 84, no. 4, p. 974-1002.

Wesnousky, S. G., 2008, Displacement and geometrical characteristics of earthquake surface ruptures: Issues and implications for seismic hazard analysis and the process of earthquake rupture, Bull. Seismol. Soc. Am. 98, 1609–1632.

Wilson, T.M., C. Stewart, V. Sword-Daniels, G.S. Leonard, D.M. Johnston, J.W. Cole, J. Wardman, G. Wilson, and S.T. Barnard, 2012,  Volcanic Ash Impacts on Critical Infrastructure, Physics and Chemistry of the Earth, 45: p5–23.

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Youngs, R.R. et al., 2003, A Methodology for Probabilistic Fault Displacement Hazard Analysis (PFDHA),Earthquake Spectra, v.19, no. 1, p. 191–219.