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Southeastern Alaska and the adjacent part of northwestern Canada form an important part of the boundary between the Pacific and North American tectonic plates. This region contains a fault system that is like California’s San Andreas fault: the tectonic plates move past each other horizontally at a rate of approximately 50 millimeters/year along the southeastern Alaska coastline. Like the San Andreas fault, the Queen Charlotte-Fairweather fault is right-lateral: to an observer on one side of the fault, the block on the other side is moving to the right.

Along the southern part of this fault margin, the plate boundary is fairly simple, with the right-lateral Queen Charlotte-Fairweather fault accommodating most of the relative motion between the two tectonic plates. In the region northwest of Glacier Bay National Park, however, the distribution of relative motion is not well understood. Relative plate motion in southeastern Alaska appears to be partitioned among several faults, most of which are located offshore. Understanding the partitioning of motion between onshore and offshore faults remains a major scientific problem, as it has significant implications for earthquake hazards throughout the region. If the motion is on one fault, then the hazard is confined to that fault, but if the motion is distributed across several faults over a broad width, then the region of earthquake hazard can be larger.

Earthquakes Prompt Marine Hazards Investigation

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During the last century, the Queen Charlotte-Fairweather fault system has generated six magnitude 7 or greater earthquakes, including a magnitude 8.1 in 1949 offshore British Columbia—Canada’s largest recorded earthquake. A magnitude 7.8 earthquake in 1958 triggered a landslide in Lituya Bay, Alaska, and generated the largest tsunami run-up ever recorded (524 meters/1,720 feet up a mountainside). At risk are the growing populations of Juneau (Alaska’s state capital), Sitka, and other communities throughout southeastern Alaska. Additionally, more than 1 million tourists are drawn to view and explore the region’s natural wonders each year, making many people vulnerable to its earthquake and tsunami hazards. Also at risk are sea-bottom cables that cross the fault system and are critical to the state’s communications.

In 2012 and 2013, a series of large-magnitude earthquakes and associated aftershocks occurred along the southern section of the Queen Charlotte-Fairweather fault system. The first was a magnitude 7.8 thrust-fault earthquake near Haida Gwaii—a group of islands offshore British Columbia near the south end of the Queen Charlotte-Fairweather fault. Just south of this area, the Juan de Fuca plate is subducting (pushing beneath) the North American plate. Compression near a subduction zone commonly produces thrust faulting, in which rock on one side of the fault moves up and over rock on the other side. This earthquake led to tsunami warnings and evacuations in Canada, Alaska, Washington, Oregon, California, and Hawai'i. The second earthquake, magnitude 7.5, was generated by strike-slip faulting (where rock on one side of the fault moves sideways past rock on the other side, typical of the right lateral Queen Charlotte-Fairweather fault). It occurred farther north in U.S. territory, west of the town of Craig, Alaska. These two earthquakes triggered significant concern from the Earth sciences community and led to a general realization that, because of its offshore location, relatively little is known about the Queen Charlotte-Fairweather fault system and the geohazards associated with it. 

In 2015, marine-geohazards researchers at the U.S. Geological Survey (USGS) teamed up with scientists from the Alaska Department of Fish and Game and the Geological Survey of Canada to begin the first phase of a multiyear, onshore-offshore study of the Queen Charlotte-Fairweather fault system. The overarching goal of the study is to better understand the earthquake, tsunami, and submarine-landslide hazards throughout southeastern Alaska and to develop geological models that can be applied to other major strike-slip plate boundaries around the globe, such as the San Andreas fault system of California, the Alpine fault of New Zealand, and the North Anatolian fault of Turkey.

Research cruises conducted in May, August, and September 2015 represent the first systematic efforts to study the offshore Queen Charlotte-Fairweather fault system in U.S. territory in more than three decades. Preliminary results have provided unprecedented imagery of the fault shape and structure, deformation history, and sedimentary processes in the area just offshore Glacier Bay National Park (see enlarged maps, below). The scientists presented many of their findings, maps, and images in a poster session at the American Geophysical Union Fall Meeting in December 2015. Ongoing analysis and comparison with older data are expected to yield additional insights.

First Cruise: Imaging the Seafloor and Layers Beneath the Seafloor

The first phase of fieldwork began in May 2015, with a three-week cruise on the Alaska Department of Fish and Game research vessel (R/V) Solstice to collect marine geophysical data. These included bathymetric data (seafloor depths) and seismic-reflection data (cross-sectional images of sedimentary layers and other features beneath the seafloor). A team of USGS scientists—Danny Brothers, Pete Dartnell, Gerry Hatcher, and Rob Wyland from the Pacific Coastal and Marine Science Center and Peter Haeussler from the Alaska Science Center (see photo, above)—led multibeam bathymetry and multichannel seismic-reflection surveys along the northernmost offshore section of the Queen Charlotte-Fairweather fault, between Cross Sound and Icy Point (see lefthand map, below). North of the survey area, the Queen Charlotte-Fairweather fault takes a westerly bend, producing some shortening between the two plates. The 3,879 meter/12,726-foot-tall, ice-covered Mount Crillon and 4,671 meter /15,325-foot-tall Mount Fairweather are striking examples of the tectonic uplift resulting from that shortening.

During the May 2015 cruise, the team conducted surveys for one to two days, then anchored to catch up on data processing (and sleep) in nearby fjords along the western boundary of Glacier Bay National Park. After 17 days in the study area, the team acquired approximately 650 square kilometers of high-resolution multibeam bathymetry and more than 2,000 kilometers of multichannel seismic-reflection profiles, revealing a textbook example of strike-slip fault morphology (see enlarged map on left, page 3; with detail area shown on right) and evidence for post-glacial (approximately 19,000 years to present) fault movement.

Second Cruise: Looking for Fault Offset and Recent Earthquake Evidence

During a second cruise in early August, Danny Brothers, Jamie Conrad, and Jackson Currie of the Pacific Coastal and Marine Science Center joined Peter Haeussler and Greg Snedgen of the Alaska Science Center on the USGS R/V Alaskan Gyre to achieve two primary objectives:

• Target evidence for Holocene (less than approximately 12,000-year-old) fault offset in the vicinity of Cross Sound by collecting chirp subbottom profiles. These are similar to multichannel seismic-reflection profiles but are produced with higher-frequency sound energy and so provide much greater detail, although they do not extend as deep beneath the seafloor.
• Identify geologic evidence for recent earthquakes along the Chatham Strait Fault and the Coastal Shear Zone (see “Survey Area 2” in overview map).

The team ended up with more more than 250 kilometers of chirp subbottom data and roughly 150 kilometers of multichannel seismic-reflection data.

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Throughout the summer and fall, scientists at the Pacific Coastal and Marine Science Center in Santa Cruz, California (Pete Dartnell, Jared Kluesner, Pat Hart, Alicia Balster-Gee, and Danny Brothers), have been diligently working through the data analysis, including the development of some new, advanced approaches to seismic-reflection data processing. Initial results are phenomenal, showing the Queen Charlotte-Fairweather fault as a nearly straight seafloor lineament for more than 75 kilometers in the bathymetric data. Tears along the fault trace have resulted in a series of small en echelon fault basins and the horizontal offset of seabed features (see enlarged maps, above; and “A Closer Look at an Undersea Source of Alaskan Earthquakes,” in Eos).

Third Cruise: Sampling the Seafloor near British Columbia 

Colleagues at the Geological Survey of Canada (Vaughn Barrie) and the Sitka Sound Science Center (Gary Greene; also emeritus faculty at Moss Landing Marine Labs), led a third cruise, which was funded by the USGS Earthquake Hazards Program and included USGS participants Jamie Conrad and Katie Maier of the Pacific Coastal and Marine Science Center. Conducted in September 2015 aboard the Canadian Coast Guard vessel John P. Tully, this cruise surveyed several areas along the southern part of the Queen Charlotte-Fairweather fault offshore Haida Gwaii, British Columbia, and southernmost Alaska. The crew used a chirp subbottom profiler and a deep-water camera system to pick seafloor areas near the fault for sampling with a 20-foot-long piston corer. They recovered sediment cores as long as 14 feet. Sediment from these cores will be analyzed to provide age data that will help determine information about fault displacements (how far places on either side of the fault have moved relative to each other) and ages of deformation. In addition, a nearshore area off Cape Felix, Alaska, was investigated for possible fault splays (subsidiary faults that branch from the main fault) extending north from the Queen Charlotte-Fairweather fault into the Alexander Archipelago—the islands that make up much of southeastern Alaska.

One surprising result from this cruise was the discovery of a 250-meter-high volcano-like cone at a depth of about 1,250 meters, about 10 kilometers west of the Queen Charlotte-Fairweather fault. On top of this cone was an active fluid plume, which could be seen on sonar records to be rising 700 meters up into the water column. The deep-water camera system revealed abundant evidence of fluids emanating from the mound, including likely vents, formation of authigenic (precipitated in place) carbonate, 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. (See “Active Mud Volcano Field Discovered off Southeast Alaska,” in Eos.)

Onshore Photographic and Lidar Data

Data from the cruises will be combined with new results from fieldwork led by Rob Witter and Peter Haeussler of the Alaska Science Center, Kate Scharer of the USGS Earthquake Hazards Program field office in Pasadena, California, and Chris DuRoss of the USGS Geologic Hazards Science Center in Golden, Colorado. Airborne photography and lidar missions were flown in late August over the onshore section of the Queen Charlotte-Fairweather fault along the western edge of Glacier Bay National Park. Lidar is a remote-sensing technology that uses laser light to make precise measurements of elevation. Combining the onshore lidar data with the seafloor bathymetric data will provide nearly seamless data coverage connecting the onshore topography and offshore bathymetry and allowing us to study this particular section of the Queen Charlotte-Fairweather fault across a wide range of spatial and temporal scales over contrasting geological environments. The lidar data will be used for on-land fault mapping and targeted paleoseismic investigations (studies of evidence for ancient earthquakes) in the summer of 2016.

Also scheduled for 2016 is the beginning of an expanded, comprehensive study of the Queen Charlotte-Fairweather fault system for which the USGS Coastal and Marine Geology Program is currently preparing. The team of scientists from the Pacific Coastal and Marine Science Center and the Alaska Science Center will join forces with Uri ten Brink, Jason Chaytor, and Nathan Miller of the USGS Woods Hole Coastal and Marine Science Center in Woods Hole, Massachusetts. Plans are in the works for a sequence of marine geophysical and geological surveys—stayed tuned!

For more information, please contact Danny Brothers, dbrothers@usgs.gov.

Stunning Images of a Seafloor Fault

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Danny Brothers’ love for revealing Earth’s unseen surfaces is apparent—he’s spent more than 450 days on the water imaging underwater features off the U.S east coast, much of California, and southern Alaska, as well as in the Salton Sea and Lake Tahoe. While Brothers has terrestrial passions, such as mountain biking, nothing makes him smile like discovering the perfect trace of a strike-slip fault on the seafloor. When he and his fellow mappers were bobbing above the 825-mile-long Queen Charlotte-Fairweather fault, one of the fastest-moving strike-slip faults in the world, they had only a faint idea of how the new, high-resolution imagery would look as USGS instruments beamed it back to the boat. It was the first time anyone had used modern technology to map this piece of seafloor off southeastern Alaska. “What we saw was the most stunning morphological expression of a strike-slip fault I had ever seen,” said Brothers, describing the quintessential fault cutting straight across the seafloor, offsetting seabed channels and submerged glacial valleys, the evidence all perfectly preserved since the last ice age. It was an unusual opportunity to observe how a fault has evolved in 20,000 years, he explained, because rivers and glaciers obliterate much of the record on land. It was clear to Brothers that undersea work off Alaska’s shores was essential to truly comprehend the natural hazards facing southeastern Alaska. This discovery also opened up future research possibilities—quite literally—because the scientists found that the moving fault had created scarps, or fresh surfaces, that animals, such as corals, could colonize. Now it’s likely that remotely operated vehicles and camera sleds will soon be added to the mapping team’s quiver of cutting-edge underwater tools.

—Amy West