Expedition along a Hazardous, Fast-Moving Fault off Southeast Alaska—the Queen Charlotte-Fairweather Fault
In July 2017, USGS scientists spent 21 days on the research vessel Ocean Starr collecting imagery of the Queen Charlotte-Fairweather fault off southeast Alaska. We want to understand the earthquake hazards that this large, fast-moving fault poses to communities and tourists in Alaska and Canada.
Like California’s San Andreas fault, the Queen Charlotte-Fairweather separates the Pacific tectonic plate from the North American plate. It is like the San Andreas in terms of its length, its capacity to generate large earthquakes, and the type of plate motion it accommodates. Both faults slip horizontally, making them strike-slip, or transform, plate-boundary faults. In both systems, the two plates move past one another in a “right-lateral” sense: to someone standing on one side of the fault and looking across it, the opposite side moves to the right.
With a slip rate of more than 50 millimeters (2 inches) per year, the Queen Charlotte-Fairweather fault may be one of the fastest-moving strike-slip faults in the world. Because it moves so quickly, the fault can build up lots of stress over a relatively short period of time, and it tends to release that stress in large and frequent earthquakes. During just the past century, the Queen Charlotte-Fairweather fault has ruptured in at least six earthquakes over magnitude 7. These include Canada’s largest recorded earthquake, a magnitude 8.1 event in 1949, and a magnitude 7.8 earthquake in 1958 that caused a landslide to tumble into Lituya Bay, Alaska. The landslide triggered a tsunami that washed 1,720 feet up the opposite slope—the largest tsunami runup ever recorded. Two more-recent earthquakes along the plate boundary—a magnitude 7.8 event in 2012 offshore of Haida Gwaii, British Columbia, and a magnitude 7.5 event in 2013 near Craig, Alaska—spurred renewed interest in understanding earthquake hazards in this region.
Since 2015, the USGS has made a focused effort to collect comprehensive high-resolution geophysical data along nearly 500 kilometers (about 300 miles) of the offshore Queen Charlotte-Fairweather fault north of the U.S.-Canada border. These geophysical data include high-resolution seafloor bathymetry (ocean-floor topography), multichannel seismic-reflection data (cross-sections of sedimentary layers beneath the seafloor), chirp subbottom profile data (very shallow cross-sections showing detailed views of only the most recent sedimentary deposits and deformation), and sonobuoy refraction data (which provide information about the physical properties of sediment and bedrock near the fault).
Together, these data represent the first comprehensive dataset collected along the Queen Charlotte-Fairweather fault since the late 1970s. They will help us evaluate many qualities of the fault, such as exactly how quickly it is moving, whether or not or not there are other faults accommodating plate motion, where earthquake ruptures may end, and which fault strands are actively slipping and pose earthquake hazards today. (An August 2017 Eos article, at https://doi.org/10.1029/2017EO079019 reports highlights of the study so far.)
Smooth sailing on the July 2017 expedition
The July 2017 field expedition was conducted by 10 scientists representing two USGS programs—the Coastal and Marine Geology Program and the Earthquake Hazards Program—and three science centers—the Pacific Coastal and Marine Science Center (PCMSC) in Santa Cruz, California; the Woods Hole Coastal and Marine Science Center (WHCMSC) in Woods Hole, Massachusetts; and the Alaska Science Center (ASC) in Anchorage. The team included chief scientist and Pacific hazards project chief Danny Brothers (PCMSC), co-chief scientist and marine seismologist Nathan Miller (WHSC), navigation and data-management master Jamie Conrad (PCMSC), Alaska earthquake hazards expert Peter Haeussler (ASC), seismic-data-processing guru Jared Kluesner (PCMSC), marine mammal observer and seismic-data processor Alicia Balster-Gee (PCMSC), and Mendenhall Postdoctoral Fellow Maureen Walton (PCMSC), who completed a Ph.D. dissertation in 2016 on the tectonics of the Queen Charlotte-Fairweather fault. The fieldwork could not have been completed without the support of engineering technician Rachel Marcuson (PCMSC), who started her career at the USGS just one year ago; Alex Nichols, another early-career technician from WHCMSC; and the legendary Tom O’Brien (WHCMSC), out for the final field excursion of his 34-year career as a geophysicist with the USGS.
The cruise, which took place aboard Stabbert Maritime’s R/V Ocean Starr (https://www.stabbertmaritime.com/vessels/ocean-starr/), was the fifth multi-week field effort by the USGS since 2015 to map the Queen Charlotte-Fairweather fault in detail using geophysical methods. The Ocean Starr departed on June 28 from Seattle, giving us a chance to get used to the motion of the ship during a 3-day transit to the start of the survey just north of the U.S.-Canada border (near the mouth of the Dixon Entrance). We surveyed northward toward Baranof Island, Alaska, finally ending at the offshore Yakobi Sea Valley on the continental shelf just south and offshore of Glacier Bay National Park and Preserve. We got to enjoy a number of scenic views in the coastal regions, a notable highlight being a 12-hour sonobuoy deployment with the volcano Mount Edgecumbe in the background. The termination port was Juneau, Alaska, where the USGS team received assistance with demobilization from NOAA scientists who were using the Ocean Starr next for their work.
The Ocean Starr survey went just about as well as we could have hoped. The weather was ideal for 90 percent of the fieldwork, with seas generally less than 3 feet, making up for the much rougher weather experienced by the USGS team (including Brothers, Miller, Kluesner, Haeussler, and Balster-Gee) during the previous two Queen Charlotte-Fairweather fault surveys in 2016 (see “Striking New Seafloor Imagery of the Queen Charlotte-Fairweather Fault in the Gulf of Alaska”). Walton was particularly happy not to be thrown from her bunk, as happened the last time she surveyed the Queen Charlotte-Fairweather fault, in April 2013, when she flew across the room mid-nap during a big roll.
In fact, during the July 2017 expedition, life on the Ocean Starr was a lot like the film “Groundhog Day,” with very similar events repeated daily. This is the preferred experience during a seismic survey; boring days typically mean that everything is running smoothly. The calm routine was punctuated by moments of excitement, such as weekly noontime safety drills and a near collision between a sport-fishing vessel and instruments being towed by the ship. These noisy events invariably woke the night crew (including Miller, Walton, Conrad, Nichols, and Marcuson) from a deep sleep.
Towed instruments provide an array of data
The survey equipment included a chirp subbottom profiler and a high-resolution multichannel seismic system, which uses a sparker (electrical) energy source and a long streamer of hydrophones (underwater receivers), all of which are towed behind the vessel during data acquisition. Both instrument systems emit pulses of sound that reflect off boundaries between sub-seafloor layers; the returning echoes are recorded and processed to produce images of what’s beneath the seafloor. (To learn more, visit https://woodshole.er.usgs.gov/operations/sfmapping/seismic.htm.)
For the most part, the equipment functioned well throughout the cruise. We needed to make just one unexpected stop in Sitka to pick up spare parts for a power supply that failed early in the cruise. Our partners at the Sitka Sound Science Center (http://www.sitkascience.org) kindly received the shipped parts and brought them to the dock, enabling us to make a quick (less than 2 hour) stop before resuming our surveying.
The 2017 Ocean Starr expedition focused primarily on collecting high-resolution seismic-reflection profiles with the multichannel system. We collected many of these profiles perpendicularly across the Queen Charlotte-Fairweather fault to gain informative images of fault structures, including the Queen Charlotte-Fairweather fault itself. Early interpretation of the seismic-reflection profiles indicates more than a dozen previously unmapped faults that are not visible in the detailed bathymetric data collected during previous expeditions. We also see evidence for active folding of sediment layers in deep water on the Pacific plate. Landward of the Queen Charlotte-Fairweather fault, on the North American plate, we imaged post-glacial sediment layers deposited in sea valleys—such as the Yakobi Sea Valley—that were excavated by glaciers on the continental shelf during the last glacial maximum, approximately 20,000 years ago.
The pit stop in Sitka enabled us to collect one profile using sonobuoy seismic receivers, which record wide-angle reflections and refracted (bent) seismic (sound) waves. The sonobuoy data provide information about how fast seismic waves pass through the sedimentary layers and underlying bedrock, which is useful for understanding how the sediments, rocks, and fault(s) might deform and slip during an earthquake.
Finally, we conducted a chirp survey in the Yakobi Sea Valley during the final two days of the cruise to image shallow sediment layers throughout the valley and tie them to a radiocarbon-dated core near the continental shelf edge. Tying sedimentary layers to known ages can help us understand the ages of offset features and episodes of deformation, which in turn can help us calculate slip rates and earthquake recurrence intervals. The chirp profiles imaged young earthquake deformation near the epicenter of the 1958 earthquake, which Brothers hopes will link up with onshore work led by Haeussler and other scientists at the ASC.
In total, during the 21-day survey, we collected approximately 3,000 kilometers (1,900 miles) of high-resolution multichannel seismic-reflection profiles, approximately 300 kilometers (190 miles) of subbottom chirp profiles, and one sonobuoy profile spanning approximately 80 kilometers (50 miles).
Brothers led another expedition in September 2017 (see “U.S. and Canadian Scientists Explore Major Undersea Earthquake Fault”) to better understand the age of seafloor features and the sources of seabed gas seeps along the Queen Charlotte-Fairweather fault. Working aboard the Canadian Coast Guard Ship John P. Tully, USGS scientists and colleagues from Natural Resources Canada (https://www.nrcan.gc.ca/home), University of Calgary (http://www.ucalgary.ca/), and the Sitka Sound Science Center (http://www.sitkascience.org/) collected samples of water and seafloor sediment, photographs and video along seafloor transects, and seismic-reflection profiles from Haida Gwaii, British Columbia, to Cross Sound near Juneau, Alaska. They discovered dramatic seafloor features that include methane seeps and chemosynthetic communities along the fault, large underwater landslides, volcanic edifices, and seafloor topography offset by past fault movement.
Back in the office, USGS scientists continued to work on processing and interpretation of the July 2017 data. Walton presented initial interpretations of the 2017 data at the annual American Geophysical Union fall meeting in December 2017 in New Orleans. Cruise scientists look forward to reuniting in late February at a “core party” in Sidney, British Columbia, to continue analysis of the cores and samples.
in this issue:
New Expedition along the Queen Charlotte-Fairweather Fault in Alaska