Seismic zones of the United States

Seismic zones of the United States
Name	Seismic zones of the United States
Caption	Major seismic regions and tectonic features of the United States
Region	United States

Seismic zones of the United States describe the spatial distribution of earthquake activity across the United States and adjacent offshore regions, integrating patterns observed in the Ring of Fire, the Gulf of Alaska, and intracontinental provinces; seismic zones inform United States Geological Survey hazard models, Federal Emergency Management Agency mitigation planning, and state-level codes such as the California Building Code and standards influenced by the International Building Code. These zones reflect interactions among plate boundaries, ancient sutures, and active fault systems that affect population centers including Los Angeles, San Francisco, Seattle, Anchorage, Salt Lake City, and Charleston, South Carolina.

Overview

The seismic zoning framework used by agencies such as the United States Geological Survey, the National Earthquake Information Center, and academic programs at institutions like California Institute of Technology and University of Washington integrates instrumental catalogs (e.g., Global Seismographic Network records), paleoseismology findings from sites associated with the San Andreas Fault, and geodetic measurements from networks including Global Positioning System campaigns and Plate boundary Observatory observations. Zoning divides the nation into regions such as the Pacific Northwest, the Alaska Peninsula, the Intermountain West, the Central United States, and the Eastern United States coastal shelves, informing models used by the National Seismic Hazard Model and vendors supporting American Society of Civil Engineers standards.

Major seismic regions

Major seismic regions include the Pacific Northwest subduction zone where the Cascadia Subduction Zone interfaces with the Juan de Fuca Plate and affects cities like Portland, Oregon and Vancouver, Washington; the California transform environment controlled by the San Andreas Fault system that impacts Los Angeles, San Jose, California, and San Diego; the Alaska megathrust and volcanic arc affecting Anchorage and the Aleutian Islands; the Intermountain West with normal fault systems near Salt Lake City and Boise, Idaho; and the New Madrid Seismic Zone in the Central United States influencing Memphis and St. Louis. Eastern regions such as the Charleston, South Carolina seismic zone and the Wabash Valley Seismic Zone produce infrequent but widely felt events that reach New York City and Washington, D.C. via ancient structures such as the Grenville Province and Appalachian orogenic remnants.

Geological causes and fault systems

Seismicity in the United States arises from plate interactions at the Pacific Plate–North American Plate boundary, the subduction interface of the Explorer Plate and Juan de Fuca Plate, and intraplate stress release along reactivated structures like the New Madrid Fault Zone and the Blind Thrust Faults beneath Los Angeles. Major fault systems include the San Andreas Fault, the Hayward Fault, the Cascadia Fault, the Queen Charlotte Fault, and the complex network of offshore faults along the Gulf of Alaska and the Coast Ranges. Volcanotectonic processes at Mount St. Helens, Mount Rainier, and the Aleutian Arc couple seismic and volcanic hazards; glacio-isostatic adjustment in regions formerly covered by the Laurentide Ice Sheet contributes to post-glacial seismicity in parts of the Northeastern United States and Canada.

Seismic hazard assessment and zoning maps

Hazard assessment practices employ probabilistic seismic hazard analysis (PSHA) routines developed by researchers at USGS and partners including Lawrence Livermore National Laboratory and universities like Massachusetts Institute of Technology, integrating paleoseismic data from the USGS Quaternary Fault and Fold Database and intensity records from the Modified Mercalli Intensity scale. Zoning maps such as the USGS National Seismic Hazard Maps and state seismic design maps inform ASCE 7 load cases and insurance models used by the National Flood Insurance Program-adjacent actuaries. Scenario-driven tools—used by FEMA, State Emergency Management Agencies, and utilities like Pacific Gas and Electric Company—simulate ground shaking, liquefaction, and landslide susceptibility for critical infrastructure including ports like Port of Los Angeles and nuclear plants such as Diablo Canyon Power Plant.

Historical significant earthquakes and case studies

Key events shaping understanding include the 1906 San Francisco earthquake and fire, the 1964 Great Alaska earthquake (Prince William Sound), the 1811–1812 New Madrid earthquakes, the 1886 Charleston earthquake, and the 1989 Loma Prieta earthquake. Case studies of the 1994 Northridge earthquake, the 2001 Nisqually earthquake, and recent sequences near Moore, Oklahoma and the Wabash Valley illustrate complexities of shallow crustal rupture, induced seismicity linked to wastewater injection investigated by the United States Environmental Protection Agency and state regulators, and cascading impacts on transportation arteries such as Interstate 5 and Interstate 10.

Monitoring, preparedness, and mitigation

Monitoring relies on networks including the Advanced National Seismic System, the EarthScope array, regional systems run by universities like Southern California Seismic Network, and international collaborations such as the International Seismological Centre. Preparedness programs led by FEMA, state offices like the California Governor's Office of Emergency Services, and non-governmental organizations including the American Red Cross promote drills such as Great ShakeOut and public education rooted in recommendations from CDC emergency guidelines. Mitigation efforts combine early warning systems like ShakeAlert—a collaboration among USGS, Caltech, and University of Washington—with retrofitting programs for bridges, schools, and hospitals, and utility resilience projects by entities such as Bonneville Power Administration.

Land use, building codes, and policy impacts

Seismic zoning drives land-use decisions made by municipal governments in jurisdictions including Los Angeles, San Francisco, and Seattle, and shapes building codes administered under the International Code Council and adopted in state statutes like the California Building Standards Code. Policies influenced by hazard zoning affect insurance markets, post-disaster recovery frameworks under Stafford Act provisions, and infrastructure investment by agencies including the Federal Highway Administration and U.S. Army Corps of Engineers. Integration of seismic risk into urban planning, historic preservation in districts such as Charleston Historic District, and coastal resilience efforts along the Gulf Coast remain active areas of policy, research, and community engagement.

Category:Seismology Category:Geology of the United States Category:Natural hazards in the United States