Earthquake Hazards in Southeastern Alaska

While working at sea above the 825-mile-long Queen Charlotte-Fairweather fault, where the Pacific plate slides northwest past the North American plate, USGS geophysicist Danny Brothers could only guess how the new high-resolution imagery would look as USGS instruments beamed it back to the boat; it was the first time anyone had mapped this area with modern technology. What Brothers saw was the most “stunning morphological expression of a strike-slip fault that I had ever seen”—a quintessential fault cutting straight across the seafloor, offsetting seabed channels and submerged glacial valleys, the evidence all perfectly preserved since the last ice age. He remarked on the unusual opportunity to observe how a fault has evolved in the last 20,000 years, because, on land, the rivers and glaciers obliterate much of the record. Brothers believed that working offshore would be instrumental in fully understanding the undersea hazards facing southeastern Alaska. Discovering this beautiful trace of a fault beneath the seabed confirmed just that.

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The Queen Charlotte-Fairweather fault in southeastern Alaska is analogous to California’s San Andreas fault, both in length and type (strike-slip). Both faults form a boundary where two blocks of Earth’s crust—the North American and Pacific tectonic plates—slide horizontally past each other in opposite directions. The Queen Charlotte-Fairweather fault moves about 50 millimeters each year.

The Queen Charlotte-Fairweather fault extends 1,200 kilometers along southeastern Alaska and northern British Columbia, of which 900 kilometers lies offshore. During the past 120 years, the Queen Charlotte-Fairweather fault has generated six earthquakes of magnitude 7 or greater, including a magnitude 8.1 in 1949—Canada’s largest recorded earthquake. A magnitude 7.8 earthquake in 1958 triggered a landslide in Lituya Bay, leading to the largest tsunami run up ever recorded (1,720 feet up a mountainside). The populations of Juneau, Sitka, and other communities throughout southeastern Alaska continue to expand, and more than 1 million tourists visit each year to view and explore the region’s natural wonders, leaving many people vulnerable to its earthquake and tsunami hazards.

In 2012 and 2013, two large earthquakes and associated aftershocks occurred along the southern section of the Queen Charlotte-Fairweather fault, which lies beneath the seafloor. The first, a magnitude 7.8 earthquake near Haida Gwaii, British Columbia (formerly known as the Queen Charlotte Islands), led to tsunami warnings and evacuations in Canada, Alaska, Washington, Oregon, California, and Hawaiʻi. The second, a magnitude 7.5 earthquake, was centered off southeastern Alaska near the town of Craig. These two earthquakes triggered significant concern from the Earth scientists, who realized that relatively little is known about the Queen Charlotte-Fairweather fault and its associated geohazards, largely because it runs offshore. Aside from obtaining critical details about the fault’s structure, it’s also crucial to determine if motion across this major plate boundary is distributed across one, two, or several faults.

What the USGS is doing

In 2015, USGS scientists from the Pacific Coast Marine Science Center and the Alaska Science Center began collaborating with scientists from other institutions to study the offshore portion of the Queen Charlotte-Fairweather fault in U.S. waters—the first systematic effort in more than three decades. The primary goal of these studies was to gain a better understanding of the earthquake, tsunami, and underwater-landslide hazards throughout southeastern Alaska, and also to gather data to develop geologic models that can be applied to other similar plate boundaries around the globe, such as the San Andreas fault system in southern California, the Alpine fault in New Zealand, and Turkey’s North Anatolian fault.

The first phase of these new studies began in May 2015 with a three-week marine geophysical cruise on the Alaska Department of Fish and Game research vessel (R/V) Solstice. A team from the Pacific Coastal and Marine Science Center and the Alaska Science Center collected high-resolution bathymetry (seafloor depth data) using multibeam sonar across approximately 650 square kilometers of seafloor. Along the northern tip of the Queen Charlotte-Fairweather fault near Cross Sound, they also gathered detailed seismic information hundreds of meters beneath the seafloor by towing a cable of hydrophones, or “streamer,” to pick up sound waves that reflect off features beneath the seafloor.

A second cruise in August 2015 aboard the USGS research vessel (R/V) Alaskan Gyre collected additional data in the vicinity of Cross Sound with a chirp sub-bottom profiler, which returns a highly detailed image of features down to 50 meters beneath the seafloor. The scientists subsequently surveyed an area close to Alaska’s capital, Juneau, to identify geologic evidence for recent earthquakes on the nearby Chatham Strait fault and the Coastal shear zone, whose earthquake potential had not yet been investigated with modern mapping systems.

USGS funded and participated in a third cruise in September 2015 led by colleagues at the Geological Survey of Canada and the Sitka Sound Science Center. The Canadian Coast Guard research vessel John P. Tully surveyed several areas along the southern part of the Queen Charlotte-Fairweather fault off Haida Gwaii, British Columbia, and southern Alaska. The scientists deployed a chirp sub-bottom profiler and a deep-water camera system to identify where best to collect sediment near the fault. Then they pushed a 20-foot-long piston corer into the seafloor to retrieve tubes of sediment, the longest of which measured 14 feet. Verifying the age of sediment will help scientists calculate dates of past earthquakes and the rate of movement on the fault. In addition, the team investigated an area off Cape Felix, Alaska, for potential fault branches that extend north into the southeastern Alaskan archipelago.

Researchers from NOAA and the USGS completed the high-resolution mapping with NOAA Ship Fairweather in 2018, collecting multibeam bathymetric data in an area along the U.S. and Canadian international border in water depths ranging from 500 to more than 7,000 feet deep. The 2018 Fairweather survey built on the five previous USGS-led marine geophysical and geological surveys between 2015 and 2017.

What the USGS has learned

Bathymetric images show, for example, the Queen Charlotte-Fairweather fault trace as a nearly straight cut in the seafloor extending for approximately 75 kilometers. Details reveal subtle bends and extensions as the fault evolved, which manifest as small basins or uplifts, including a seafloor ridge that has shifted about 925 meters over the past 19,000 years—a rare, but obvious landmark that helps establish the origin and rate of fault movement. Other details reveal the interplay between fault motion and sedimentation where the fault is most active at the mouth of Cross Sound, just offshore Glacier Bay National Park. The sediment acts as a tape recorder of earthquake history and motion on the fault. For example, movement of the fault over thousands of years may create an opening or basin on the seafloor, which is subsequently filled by sediment. Sampling and imaging sediment in that basin not only provides an age for when the basin formed, but also reveals when earthquakes occurred based on sediment wedges that sloughed off the basin’s sides.

One surprising result from these new studies near the southern tip of Alaska was the discovery of a 250-meter-high cone rising from the seafloor about 10 kilometers west of the fault, near the southern tip of Alaska. On its top was an active fluid plume, which could be seen on sonar records as rising 700 meters into the water column. The deep-water camera system revealed evidence of fluids emanating from the mound, including possible vents, calcium carbonate formations, and chemosynthetic biological communities, which use components of the fluids (such as hydrogen sulfide or methane) as primary energy sources rather than light. The mound was sampled with a grab sampler to collect pieces of the carbonate and unusual biota for further study.

News

“A Closer Look at an Undersea Source of Alaskan Earthquakes” - Eos, August 2017

“Active Mud Volcano Field Discovered off Southeast Alaska” - Eos, November 2015

“Scientists stumble over active underwater volcano in Southeast” - Alaska Public Media, October 2015