Reducing Risk Where Tectonic Plates Collide

Most of the world’s 

earthquakes, tsunamis, 

landslides, and volcanic 

eruptions are caused by 

the continuous motions 

of the many tectonic 

plates that make up the 

Earth’s outer shell. The 

most powerful of these 

natural hazards occur 

in subduction zones, 

where two plates collide and one is thrust 

beneath another. The 

U.S. Geological Survey’s 

(USGS) “Reducing Risk 

Where Tectonic Plates 

Collide—A USGS Plan 

to Advance Subduction Zone Science” is 

a blueprint for building 

the crucial scientific 

foundation needed 

to inform the policies 

and practices that can 

make our Nation more 

resilient to subduction 

zone-related hazards.

The Earth’s many tectonic plates can be 

thousands of miles across and underlie both 

continents and oceans. These plates collide, 

slide past, and move apart from each other. 

Where they collide and one plate is thrust 

beneath another (a subduction zone), the most 

powerful earthquakes, tsunamis, volcanic 

eruptions, and landslides occur. The physical 

and economic impacts of these natural hazards 

can be felt worldwide.

The United States has multiple subduction zones. These are found in Alaska, 

Washington, Oregon, and California; the 

commonwealths of Puerto Rico and the 

Northern Marianas; and the territories of 

American Samoa, Guam, and the U.S. Virgin 

Islands. One or more of these regions will 

likely experience a catastrophic subduction 

zone event in the coming decades, as well as 

smaller, more frequent, but still potentially 

destructive events.

A new USGS science plan recognizes 

that making our Nation more resilient to 

these inevitable natural hazards requires a 

solid, scientifically based foundation. The 

Plan to Advance Subduction Zone Science 

(USGS Circular 1428, https://doi.org/10.3133/

cir1428) defines USGS science priorities and 

identifies potential partnerships with other 

organizations involved in related scientific 

research, emergency management, policy 

making, and planning.

Pictured at right: Scientists working 

on an earthquake monitoring 

station in Alaska. Photograph by 

C.F. Waythomas, Alaska Volcano 

Observatory, USGS.

Pictured below: Eruption of Pavlof 

Volcano as viewed from Cold Bay, 

Alaska, 37 miles southwest of the 

volcano. Photograph by Rachel 

Kremer.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3024

June 2017
Recent Subduction Zone Disasters

Since 1964, events both in the 

United States and abroad have highlighted the devastating power of 

subduction-zone hazards. The most 

powerful earthquake in U.S. history, the 

1964 magnitude (M) 9.2 Alaska earthquake, caused tsunamis, 129 deaths in 

3 states, and an estimated $2.38 billion 

in property losses (in 2017 dollars). Most 

deaths and damage resulted from local 

tsunamis caused by undersea landslides. 

The 1980 eruption of Mount St. Helens, 

Washington, killed 57 people, damaged 

more than 185 miles of roads, caused the 

cancellation of more than 1,000 airline 

flights, and resulted in $1.1 to $2 billion 

in economic losses.

The 1991 eruption of Mount Pinatubo, in the Republic of the Philippines, 

was the second largest volcanic eruption 

in the 20th century. Ash from the eruption traveled around the globe, lowered 

temperatures for 2 years, and created 

enormous mudflows of ash and water 

(lahars) on the volcano’s slopes. Because 

the eruption was forecast by USGS 

and Philippine scientists, more than 

75,000 people were evacuated, including 18,000 U.S. military personnel and 

families, and ~$250 million in property 

losses were averted.

In 2004, the M9.1 Sumatra-Andaman earthquake in the Indian Ocean 

and ensuing tsunami claimed more than 

225,000 lives and triggered small earthquakes as far away as Alaska. In 2010, a 

M8.8 earthquake struck Chile, generating 

tsunami waves that killed 525 people and 

caused $15 to $30 billion in damages. 

A year later, the 2011 M9.0 Tōhoku, 

Japan, earthquake triggered large tsunami 

waves, some more than 100 feet high. 

The tsunami inundated low-lying coastal 

areas, caused 15,890 deaths, and cost 

Japan $220 billion, including catastrophic 

damage to the Fukushima Nuclear Power 

Plant that resulted in a national shortage of electrical power and an ongoing 

nuclear containment issue.

Understanding Subduction Zone 

Hazards

Recent advances have enhanced 

subduction zone science, but further work 

is needed to better understand subduction 

zone processes, improve warning systems, minimize community vulnerabilities, and make emergency response and 

recovery activities more effective. The 

USGS Subduction Zone Science Plan 

highlights knowledge gaps and opportunities for improvement in three crucial 

areas—(1) advancing observations and 

modeling of subduction zone processes, 

(2) quantifying subduction zone natural hazards and risk, and (3) providing 

forecasts and rapidly delivered updates 

during and after subduction zone events 

(see table). Implementing the plan will 

make it possible to deliver innovative 

science products that enable the building of more resilient communities and 

infrastructure in regions affected by 

subduction zone hazards.

A Path to a More Resilient Future

USGS research and products will 

lead to a more resilient Nation by providing communities, policymakers, and 

stakeholders with sound, evidence-based 

information with which to make informed 

and effective decisions. The new USGS 

Subduction Zone Science Plan provides a 

blueprint for making advances in subduction zone science a reality. The following 

SOUTH

AMERICAN

PLATE

NORTH AMERICAN

PLATE

NAZCA PLATE

EURASIAN PLATE

PACIFIC PLATE

AUSTRALIA

PLATE

COCOS

PLATE

CARIBBEAN PLATE

PHILIPPINE 

PLATE

JUAN DE FUCA PLATE

ALASKA CASCADIA

MARIANAS

(Northern Marianas,

Guam)

CARIBBEAN

(Puerto Rico,

U.S. Virgin Islands)

TONGA

(American

Samoa)

Subduction zones of the 

United States and its 

territories

EXPLANATION

Map of the subduction zones that are part of the United States and its territories, which are 

shown in red. Subduction zones surrounding the Pacific Ocean belong to the “Ring of Fire.”

Modified from This Dynamic Planet (http://pubs.usgs.gov/imap/2800/).

Melting

Volcano

Ash cloud

Upper plate

Motion Plate

Landslides Tsunami

Subducting plate

Locked interface

Great earthquakes

Schematic cross section of a typical subduction zone. When tectonic plates converge 

(movement indicated by the black arrows), one plate slides beneath (subducts) the upper 

plate. At shallow depths the interface between the plates—a giant fault—may become 

stuck (locked), causing stresses to build until they exceed the fault’s strength and break 

free, resulting in an earthquake. Subduction also creates conditions for landslides, volcanic 

eruptions, and tsunamis.
Potential products for building resilience to subduction zone hazards.

Products Description Scientific Input Application

High-resolution hazard 

maps 

Maps of expected neighborhood-scale 

variations in earthquake shaking and 

ground-failure, tsunami inundation, 

landslide potential, volcanic eruptions and 

lahars

High-resolution topography, onshore 

and offshore; three dimensional (3-D) 

models of Earth’s structure; wellcharacterized faults, unstable slopes, 

active volcanoes

Building design codes, 

prioritized retrofitting, 

urban planning, and 

evacuation routing

Simulations Science-based scenarios conveying 

hypothetical subduction zone events

Geologic field and laboratory studies, 

chronologies of past subduction zone 

events

Improved mitigation 

strategies

Warning systems Advance notice of strong earthquake 

shaking, volcanic eruptions, tsunamis, and 

landslides

Multidisciplinary monitoring systems, 

onshore and offshore 

Rapidly implemented 

life- and propertysaving measures

New types of forecasts Updated projections of aftershocks, landslides 

and ground failures, volcanic lahars and 

ash clouds

Rapidly acquired satellite and surface 

measurements

Safer, faster, and 

more cost-effective 

response and recovery

Novel assessments of 

cascading subduction 

zone events

Likelihoods of landslide-triggered tsunamis; 

earthquake-induced coastal land-level 

changes, flooding and erosion 

Computer models simulating linked 

processes

Rapid and effective 

mitigation, response 

and recovery

examples show how future USGS 

products, developed from advances in 

scientific infrastructure (for example, 

monitoring instruments) and understanding, would help citizens prepare for, react 

to, and recover from subduction zone 

hazards:

• High-resolution hazard and risk 

assessments allow land-use planners, as well as businesses and 

homeowners, to make targeted and 

cost-effective decisions in advance 

of hazardous subduction zone 

events, and to minimize potential losses by situating homes and 

critical infrastructure away from the 

most vulnerable areas. 

• Seafloor monitoring detects small 

offshore earthquakes and changes 

in seafloor topography that indicate 

rapidly building stresses between 

colliding tectonic plates, where the 

most powerful earthquakes and 

tsunamis occur. This information 

allows scientists to notify emergency 

managers, who activate heightened 

readiness procedures.

• Earthquake early warning 

systems detect the onset of a large 

earthquake, estimate the intensity 

of shaking in nearby communities, 

and issue an automated alert, giving 

residents, emergency responders, 

and utility providers crucial seconds 

to prepare. In the following days 

and months, scientists issue updated 

forecasts of where and when aftershocks (earthquakes that follow the 

initial quake) are most likely.

USGS and University of Washington scientists monitoring real-time, shipboard seafloor-mapping 

data in Lake Washington, near Seattle, Washington. The data were being collected to map the 

Seattle Fault and find evidence of past earthquakes and submarine landslides. Photographs by 

Janet Watt and Scott Bennett, USGS.

Vertical exaggeration ×10

EXPLANATION

10

8

6

4

Tsunami wave 

height (meters)

Perspective southeastward view of modeled tsunami wave-height probabilities in 

Seaside/Gearhart, Oregon. Wave heights include the effects of tides. Modified from 

U.S. Geological Survey Open-File Report 2006–1234.

• Advanced notifications of cascading hazards are issued by scientists 

monitoring satellite and groundbased measurements in real-time. 

The scientists detect a massive 

coastal landslide caused when rainsaturated seaside cliffs and mountain 

slopes fail during a subduction zone 

earthquake. Warnings of a landslidegenerated tsunami allow low-lying 

residential areas and local harbors to 

be evacuated.
• Multihazard volcano-monitoring 

systems indicate the start of an eruption at a subduction zone volcano. 

Ash-cloud warnings are issued, 

combining volcano and weather 

observations that guide airplanes 

to new flight paths to avoid engine 

damage. The eruption rapidly melts 

snow and ice at the volcano’s summit, creating a lahar that rushes 

downhill. Monitoring instruments 

trip the lahar warning system, alerting communities to evacuate.

Although scientists cannot predict 

the precise time and place that hazardous 

subduction zone earthquakes, tsunamis, 

landslides, and volcanic eruptions will 

occur, advancing our understanding of 

these events will enable individuals, communities, businesses, and Federal, State, 

and local governments to make more 

well-informed decisions. The USGS Subduction Zone Science Plan describes the 

science needed to further ongoing USGS 

efforts to protect people’s lives and property from natural hazards in order to build 

a safer and more resilient Nation.

For more information, contact:

1-888-ASK-USGS (1-888-275-8747)

https://www.usgs.gov/science/

mission-areas/natural-hazards

Also see USGS Circular 1428 at 

https://doi.org/10.3133/cir1428.

Joan S. Gomberg and Kristin A. Ludwig

Edited by Katherine Jacques, 

Claire Landowski, and James W. Hendley II

Design by Suzanne Roberts

Geologists collect core samples from a coastal marsh in 

Resurrection Bay, Alaska. They are looking at the record of the 

1964 M9.2 earthquake and for evidence of large ancient earthquakes. 

The standing dead trees were killed by submergence in the 1964 

earthquake. Photograph by Peter Haeussler, USGS.

Flooding and damage in Banda Aceh, Sumatra, caused by the 

December 26, 2004, M9.1 earthquake and tsunami. U.S. Geological 

Survey photograph.

Revised March 2013

Kelso

Woodland St. Helens

Ridgefield

Yacolt

Battle Ground

Vader

Toledo

Castle Rock

Toutle

Longview

Silver Lake

Spirit

Lake

Lake 

Merwin

Mount 

St. Helens

Columbia

River

Coweeman River

Lewis River Kalama River

Cowlitz

River

Fork

North

Mount

Adams

Hazard 

Zones Johnston Ridge

Observatory

Regional 

Lava Flow 

Hazard Zone

Cougar Lewis

River Road

Forest Service Road 25

South Fork Toutle River

Kalama Swift 

Reservoir

Yale 

Lake

Toutle River

502

503

503

4

504

505

503

5

30

30

5

0 5 10 Kilometers

0 5 10 Miles

Volcano Hazard Zones

Near–volcano: lava and pyroclastic flows, 

thick tephra, lahars, ballistic ejecta, rock fall

Lahars (volcanic mudflows): potentially 

far-travelled in valleys draining volcano

Regional lava flows: lava flows from small, 

isolated volcanic vents possible

Volcanic ash (not shown): fine fragments 

of volcanic rock carried downwind

Map of ground based volcanic hazards from an eruption of one of the most threatening 

volcanoes in the Cascade Range, Mount St. Helens, Washington. Modified from 

U.S. Geological Survey Open-File Report 95-497.

Landslides cover State Route 1 near 

Ohau Point, New Zealand, after the M7.8 

earthquake in November 2016. Photograph by 

Jonathan Godt, USGS.

ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20173024