U.S. West Coast and Alaska Marine Geohazards

View of John Muir School on Pacific Avenue in Long Beach, California, showing damage from the March 10, 1933 Long Beach earthquake. Photo taken 8 days after the earthquake.

For many people who live near the coastlines, underwater dangers are “out of sight, out of mind.” But in tectonically active regions, such as the west coast of the Americas, the potential lurks for a surge of underwater motion that could disrupt many communities along the coast. 

The 2011 Tohoku earthquake and tsunami were vivid reminders that remote disasters can affect an entire ocean basin. Understanding how and what regions might be affected by faraway disasters is an important, yet complex problem.

Sonar-generated image showing underwater topography and the potential for landslides near the head of Resurrection Bay, Alaska. The terrain looks three times as steep as it occurs naturally. The arrow points to underwater landslide debris from the collapse of a fan-delta following the great Alaskan earthquake of 1964. The town of Seward, which suffered much damage and lost lives due to the quake, had been built on this fan-delta (just above and to the left of the arrow).

In addition to remote threats, local hazards lie just off the shores of the western U.S. Such hazards include shaking by large earthquakes in subduction zones, where one tectonic plate compresses another (Cascadia, Aleutian Trench); or on strike-slip faults, where one tectonic plate moves horizontally past another (central and southern California). Related hazards include tsunamis generated by shifts in the seafloor or by underwater landslides that occur during earthquakes. Landslides can also threaten equipment on the ocean floor such as pipelines, communication cables, and oil platforms.

Mapping along the Queen Charlotte-Fairweather fault required several days aboard the Alaska Department of Fish and Game research vessel Solstice. Here, the boat sits in a marina near Cordova, Alaska.

One barrier to measuring the true seismic risk has been the scarcity of high-resolution maps of the ocean floor. The technology for mapping large parts of the ocean floor with enough detail needed to study offshore faults has only been available for about the last 20 years, long after coastal areas had been densely developed. The USGS Marine Geohazards team applies this technology to the seafloor off several urban regions along the west coast. For example, the San Francisco Bay Area has the highest density of active faults of any urban area in the nation; the densely populated expanse (approximately 20 million people) in southern California is threatened by the nation’s highest level of earthquake risk; and Alaska has had more large earthquakes than the rest of the U.S. combined. In addition, detailed imaging of the ocean bottom has uncovered new evidence of submarine landslides. Creating three-dimensional views of the seafloor down to depths of 12 kilometers has given scientists remarkable ways to examine how a fault works, or how fluids may follow underground paths and possibly trigger landslides.

It’s challenging to know how a fault will behave without seeing its detailed structure: its bends, connections, and branches. To discover a fault’s structure, scientists go to sea to collect streams of data that they turn into comprehensive underwater maps. This type of imaging, along with knowing the age of sediment along faults and measuring other factors such as magnetics and density, can help tell the story of when the fault last ruptured or how fast it’s moving. Since these details are seldom known or easy to calculate for offshore faults, it’s challenging to incorporate these faults into earthquake models and estimate their actual hazard risk.

Reassessing the threat of earthquake, tsunami, and landslide hazards to ports and nuclear power plants on the U.S. west coast can directly impact facility management, emergency-management planning, and plant re-licensing. The data can also affect building codes, the design of highways, bridges, and other large structures, as well as earthquake insurance rates. 

See the USGS Interactive Map of 2014 Fault Sources. This database contains information on faults and associated folds in the United States that are believed to be sources of M>6 earthquakes during 2014.

Sam Johnson explaining details of the Hosgri fault zone at USGS offices in Santa Cruz.

USGS geophysicist Jared Kluesner points at a three-dimensional cross-section of seismic data about 40 kilometers across and several kilometers deep located in the Santa Barbara Channel. This imaging deep below the seafloor allows scientists to visualize and map faults better.