National Seismic Hazard Maps

National Seismic Hazard Maps
Name	National Seismic Hazard Maps
Country	United States
Agency	United States Geological Survey; Federal Emergency Management Agency (endorsement)
First	1976
Latest	2023

National Seismic Hazard Maps are probabilistic representations of earthquake ground shaking levels across the United States of America intended to inform building code development, risk assessment, and mitigation planning. They synthesize observations from historical events like the 1906 San Francisco earthquake, instrumental catalogs including data from the Global Seismographic Network, and geologic studies of faults such as the San Andreas Fault. Produced primarily by the United States Geological Survey in collaboration with agencies including the Federal Emergency Management Agency, the maps translate seismicity and fault behavior into engineering parameters used by entities such as the American Society of Civil Engineers and the International Code Council.

Overview

National hazard maps quantify the likelihood of exceeding specified ground motions over prescribed time intervals, expressed as metrics that include peak ground acceleration and spectral acceleration for periods used in structural engineering design. They integrate inputs from seismicity catalogs compiled by institutions like the International Seismological Centre and paleoseismic investigations on features such as the New Madrid Seismic Zone and the Wasatch Fault. Outputs guide regulatory frameworks created by bodies such as the National Institute of Standards and Technology and inform risk frameworks employed by insurers like Munich Re and Swiss Reinsurance Company.

Development and Methodology

Map development applies probabilistic seismic hazard analysis methods standardized by panels convened under agencies like the National Science Foundation and advisory committees including the Building Seismic Safety Council. Teams combine earthquake recurrence models calibrated against catalog data from the Advanced National Seismic System with geologic slip-rate estimates from studies of faults such as Hayward Fault and Cascadia Subduction Zone. Ground motion prediction equations from researchers associated with universities such as the California Institute of Technology, Massachusetts Institute of Technology, and University of California, Berkeley are used to translate event magnitude and distance into shaking intensity. Peer review processes have engaged organizations like the Seismological Society of America.

Data Sources and Models

Primary data sources comprise instrumental seismic catalogs maintained by the National Earthquake Information Center, paleoseismic trenching records from university research groups, and geodetic measurements from networks including Global Positioning System stations and campaigns from the Plate Boundary Observatory. Seismic source models incorporate active fault databases like the U.S. Geological Survey Quaternary Faults Database and seismicity-based source zones informed by historic events such as the 1964 Alaska earthquake. Ground motion models leverages empirical datasets from past earthquakes recorded by networks like the California Integrated Seismic Network and synthetic simulations produced with computational resources at centers such as Lawrence Livermore National Laboratory.

National and Regional Implementations

While the national maps provide a baseline, regional adaptations reflect local tectonics and site conditions. State-level authorities in California incorporate state-specific studies into maps used by the California Building Standards Commission, whereas agencies in Alaska and Hawaii integrate considerations from megathrust events in the Aleutian Islands and volcanic settings on Mauna Loa. Metropolitan areas such as Los Angeles, San Francisco, Seattle, and New York City apply refined hazard models in urban planning, often drawing on research partnerships with institutions like Stanford University, University of Washington, and Columbia University.

Applications and Uses

Users of the maps include code bodies such as the International Code Council for seismic provisions, infrastructure owners like the Tennessee Valley Authority for dam safety, and transit agencies including the Metropolitan Transportation Authority for resilient design. Insurance regulators and private reinsurers utilize the maps within catastrophe models developed by firms such as AIR Worldwide and RMS, Inc. Emergency planners in agencies like the Federal Emergency Management Agency and municipal offices reference the maps for hazard mitigation grants and lifeline prioritization. Academics employ the maps to prioritize paleoseismic fieldwork at locations like the New Madrid and Wasatch corridors.

Limitations and Uncertainties

Uncertainties arise from incomplete knowledge of fault geometries, limited paleoearthquake records, and variability in ground motion behavior across geologic settings. Large events such as the 1857 Fort Tejon earthquake exemplify challenges in recurrence estimation for long-return-period hazards. Model epistemic uncertainty is addressed by logic trees and weighted ensembles, yet disagreements persist among experts from institutions like California Institute of Technology and University of Southern California over model weightings. Site-specific amplification due to shallow soils, characterized in studies by the United States Army Corps of Engineers, can produce deviations from mapped values, necessitating local hazard deaggregation for critical facilities such as nuclear plants regulated by the Nuclear Regulatory Commission.

Updates and Revision Processes

Revisions follow multi-year cycles driven by advances in seismic catalogs, geodetic constraints, paleoseismic findings, and ground motion research. The United States Geological Survey organizes scientific working groups and public workshops with stakeholders including the National Institute of Standards and Technology and state seismic commissions to vet proposed changes. Major updates coincide with code adoption cycles of the International Building Code and with guideline revisions by the American Society of Civil Engineers; interim advisory products address emergent findings after significant earthquakes like the 1994 Northridge earthquake and the 2011 Tōhoku earthquake and tsunami, which prompted reassessments of subduction zone hazards.

Category:Seismology