Reducing Risk Where Tectonic Plates Collide: The USGS Plan for Subduction Zone Science

Most of the earthquakes, tsunamis, landslides and volcanic eruptions on Earth come from the constant movement of the tectonic plates that make up the planet's outer shell. The most powerful of them happen in subduction zones: places where two plates collide and one is pushed beneath the other. The U.S. Geological Survey's plan to advance subduction zone science, Reducing Risk Where Tectonic Plates Collide, lays out the scientific groundwork that policies and practices need if the country is to withstand these hazards better.

Plates, and where they meet

Tectonic plates can span thousands of miles and lie beneath continents and oceans alike. They move toward one another, slide past one another and pull apart. Where they collide and one is forced under the other, the result is the strongest earthquakes, tsunamis, eruptions and landslides, and the physical and economic damage can be felt around the world.

A typical subduction zone in cross section. As the plates converge (black arrows), one slides beneath the upper plate. Near the surface the boundary between them, a giant fault, can lock; stress builds until it is greater than the fault can hold, the fault breaks free and an earthquake follows. Subduction also sets up the conditions for landslides, eruptions and tsunamis. USGS diagram.

The United States has several subduction zones: in Alaska, Washington, Oregon and California; in the commonwealths of Puerto Rico and the Northern Marianas; and in the territories of American Samoa, Guam and the U.S. Virgin Islands. The fact sheet expects at least one of these regions to suffer a catastrophic subduction zone event in the coming decades, along with smaller events that come more often and can still do harm.

The subduction zones of the United States and its territories, in red: Alaska, Cascadia, the Caribbean (Puerto Rico and the U.S. Virgin Islands), the Marianas (Northern Marianas and Guam) and Tonga (American Samoa). Those around the Pacific Ocean are part of the "Ring of Fire." USGS map, adapted from This Dynamic Planet.

The plan, published as USGS Circular 1428, starts from the view that a country cannot become more resilient to these hazards without a solid scientific basis. It sets the Survey's research priorities and names possible partners among organisations working on related research, emergency management, policy and planning.

Disasters since 1964

• Alaska, 1964. The magnitude (M) 9.2 earthquake, the strongest in U.S. history, set off tsunamis, killed 129 people in 3 states and caused an estimated $2.38 billion in property losses (in 2017 dollars). Local tsunamis raised by landslides under the sea caused most of the deaths and damage.
• Mount St. Helens, Washington, 1980. The eruption killed 57 people, damaged more than 185 miles of road, forced the cancellation of more than 1,000 flights and cost the economy $1.1 billion to $2 billion.
• Mount Pinatubo, Philippines, 1991. The second largest volcanic eruption of the 20th century sent ash around the globe, lowered temperatures for 2 years and produced huge flows of mud made of ash and water, called lahars, on the mountain's slopes. USGS and Philippine scientists forecast the eruption, so more than 75,000 people were evacuated — 18,000 of them U.S. military personnel and their families — and about $250 million in property losses were avoided.
• Sumatra-Andaman, Indian Ocean, 2004. The M 9.1 earthquake and the tsunami that followed killed more than 225,000 people, and the quake set off small earthquakes as far away as Alaska.
• Chile, 2010. An M 8.8 earthquake raised tsunami waves that killed 525 people and caused $15 billion to $30 billion in damage.
• Tōhoku, Japan, 2011. The M 9.0 earthquake produced tsunami waves, some higher than 100 feet, that flooded low-lying coasts. It caused 15,890 deaths and cost Japan $220 billion. That includes catastrophic damage to the Fukushima nuclear power plant, which left the country short of electricity and with a problem of nuclear containment the fact sheet describes as ongoing.

Flooding and damage in Banda Aceh, Sumatra, after the M 9.1 earthquake and tsunami of December 26, 2004. USGS photograph.

Geologists take core samples from a coastal marsh at Resurrection Bay, Alaska, reading the record of the 1964 M 9.2 earthquake and looking for evidence of large earthquakes long before it. The standing dead trees were killed when the 1964 earthquake left them submerged. Photograph by Peter Haeussler, USGS.

What the plan sets out to do

The science has moved forward, but more work is needed to understand how subduction zones behave, to improve warning systems, to make communities less vulnerable and to make response and recovery more effective. The plan names the gaps and opportunities in three areas:

1. observing and modelling the processes at work in subduction zones;
2. measuring their natural hazards and risks;
3. forecasting, and delivering quick updates during and after an event.

Carrying it out, the fact sheet says, would make possible new science products that help communities and infrastructure in these regions become more resilient. It lists five kinds:

• High-resolution hazard maps show how earthquake shaking and ground failure, tsunami flooding, landslide potential, and eruptions and lahars are expected to vary from one neighbourhood to the next. They need detailed topography on land and offshore, three-dimensional models of Earth's structure, and well-described faults, unstable slopes and active volcanoes. They serve building codes, deciding which buildings to retrofit first, urban planning and evacuation routes.
• Simulations are science-based scenarios of hypothetical subduction zone events. They draw on geological field and laboratory studies and on histories of past events, and they lead to better strategies for reducing harm.
• Warning systems give advance notice of strong shaking, eruptions, tsunamis and landslides. They need monitoring networks spanning several disciplines, on land and offshore, and they allow quick action that saves lives and property.
• New kinds of forecasts update the outlook for aftershocks, landslides and ground failure, lahars and ash clouds. They rely on satellite and ground measurements gathered fast, and make response and recovery safer, quicker and cheaper.
• Assessments of cascading events estimate how likely landslides are to set off tsunamis, and how earthquakes change the level of coastal land and the flooding and erosion that follow. They are built on computer models that link processes together, and support fast, effective mitigation, response and recovery.

What it could look like

The fact sheet gives examples of how such products, built on better instruments and understanding, would help people prepare for these hazards, react to them and recover:

• Hazard and risk assessments at high resolution let land-use planners, businesses and homeowners make targeted, cost-effective choices before an event, keeping homes and critical infrastructure out of the most vulnerable places.
• Seafloor monitoring picks up small offshore earthquakes and changes in the shape of the seabed that show stress building quickly between colliding plates, where the largest earthquakes and tsunamis occur. Scientists can then alert emergency managers, who raise their readiness.
• Earthquake early warning detects the start of a large earthquake, estimates how strongly nearby communities will shake and sends an automatic alert, giving residents, responders and utilities vital seconds to prepare. Over the following days and months, scientists update forecasts of where and when aftershocks — the earthquakes that follow the main one — are most likely.
• Warnings of cascading hazards come from scientists watching satellite and ground measurements as they arrive. When rain-soaked sea cliffs and mountainsides give way during a subduction zone earthquake and cause a huge coastal landslide, a warning of the tsunami it may raise lets low-lying neighbourhoods and harbours be evacuated.
• Volcano monitoring across several hazards detects the start of an eruption at a subduction zone volcano. Ash-cloud warnings, combining volcano and weather observations, steer aircraft onto new routes so their engines are not damaged. When the eruption melts snow and ice on the summit and a lahar rushes downhill, instruments set off the lahar warning system and communities are told to evacuate.

USGS and University of Washington scientists watch seafloor-mapping data arrive in real time aboard a ship on Lake Washington, near Seattle. The survey was mapping the Seattle Fault and looking for evidence of past earthquakes and underwater landslides. Photographs by Janet Watt and Scott Bennett, USGS.

A view looking southeast over modelled probabilities of tsunami wave height at Seaside and Gearhart, Oregon, with the effect of tides included. Adapted from USGS Open-File Report 2006–1234.

Hazards on the ground from an eruption of Mount St. Helens, Washington, one of the most threatening volcanoes of the Cascade Range. Adapted from USGS Open-File Report 95-497.

Landslides cover State Route 1 near Ohau Point, New Zealand, after the M 7.8 earthquake of November 2016. Photograph by Jonathan Godt, USGS.

Why it matters

Scientists cannot predict exactly when and where these earthquakes, tsunamis, landslides and eruptions will happen. Understanding them better, the fact sheet concludes, will help individuals, communities, businesses and federal, state and local governments make better-informed decisions. The plan describes the science needed to carry on the Survey's work of protecting lives and property from natural hazards.

Sources

• Joan S. Gomberg and Kristin A. Ludwig, Reducing Risk Where Tectonic Plates Collide, USGS Fact Sheet 2017–3024, June 2017. https://doi.org/10.3133/fs20173024
• The plan itself: USGS Circular 1428. https://doi.org/10.3133/cir1428
• Figures from the fact sheet: photographs by Janet Watt, Scott Bennett, Peter Haeussler and Jonathan Godt, USGS; maps adapted from USGS publications (This Dynamic Planet; Open-File Reports 2006–1234 and 95-497).