NOAA Teacher at Sea
Aboard NOAA Ship Fairweather
April 29 – May 13, 2018
Mission: Southeast Alaska Hydrographic Survey
Geographic Area of Cruise: Southeast Alaska
Date: May 2, 2018
Weather From the Bridge
Latitude: 54°41.2 N
Longitude: 134°15.3 W
Sea Wave Height: 5 feet
Wind Speed: 7 knots
Wind Direction: 330°
Visibility: 2 nautical miles
Air Temperature: 9.9°C
Sky: Complete Cloud Cover
Science and Technology Log
NOAA Ship Fairweather is now 46 miles off the southeast coast of Alaska, mapping the ocean floor over a fault. This a transform boundary, so it is a strike slip fault. It is the boundary between the North American and Pacific plates. The United States Geologic Survey (USGS) has hired NOAA to survey the ocean floor in this area called the Queen Charlotte fault. The entire section of the fault is called the Queen Charlotte – Fairweather fault (named for Mount Fairweather, just like the ship’s name.) It runs for over 1,200 kilometers from Yakatat, Alaska to the north and British Columbia to the south. This is a part of a long fault along this plate boundary that is called the San Andreas fault when it is on land in California
The last time this particular area was surveyed was for the creation of navigational charts, between 1900 and 1938, but without accuracy or data density that the multibeam sonar being used today has. Once this portion is surveyed, the entire fault will have been mapped. The mapping has been done by the USGS, the Canadian Geologic Survey, and NOAA.
The photo above shows the features of the sea floor. It is set on top of a navigational chart. You can see the numbers on the old chart that represent depth reading. The data collected today shows depth for the entire area mapped and the features on the sea floor.
Looking at what NOAA Ship Fairweather has already mapped, the fault is very distinct as are the channels that have been offset by past seismic activity. These channels were created from runoff as the glaciers receded from this area 17,000 years ago. Using the offset measurements and the time since the canals where formed, scientists have given a slip rate of 5.5 centimeters per year to this area of the fault. This makes it one of the fastest moving continental – ocean transform boundaries.
NOAA ship Fairweather has sonar that was built for detecting hazards for surface navigation, but it is capable of surveying to several kilometers in depth. The survey team has figured out how map at these great depths up to 2,100 meters. It involves going slowly over the area, and gathering richer data by going over part of the previous survey lines. This is much like painting a wall, where the painter overlaps their brushstrokes so there are not gaps in the coverage. The multibeam solar is also directed in a narrow band, at this depth, for more accurate data.
Why do you think this information is wanted by geologists?
The fault has produced at least seven earthquakes with a magnitude greater than 7. An 8.1 magnitude earthquake was generated from this fault near British Columbia in 1949. To date, it is the largest Canadian earthquake recorded. In 1958, a magnitude 7.8 earthquake above Lituya, Alaska created a massive underwater landslide which produced a tsunami sending water 525 meters (1700 feet feet) up a mountainside. More recently in 2012, a 7.5 magnitude earthquake was measured from this fault, and in 2013, Craig, Alaska was hit with a magnitude 7.5 earthquake.
Scientists want to know more about this fault, which could cause further damage to areas of southeast, Alaska. From the seabed mapping, geologists hope to better understand the slip rate and the intervals between earthquakes.
I have been so impressed with the people on NOAA Ship Fairweather. Everyone has been so welcoming and kind. This small group of people living in small quarters could be difficult for many people, but everyone here is so enthusiastic about the mission and their jobs. They are very open to sharing what they know with me, including explaining the science and technology of the equipment and how the ship functions.
It has been really fun learning about this fault and the surrounding underwater topography. Being able to see the sea bottom as we continue over it is amazing!
I am so happy I will get a chance to share this science with my students. I hope they noticed, as they read this post, the highlighted terms and concepts that we learned this year about faults and earthquakes.
Did you know?
I found a term that was new to me, tectonic geomorphology. It is the study of the interaction between active plates and land process, and how these shape landscapes.
Information used in this post can partly from:
“A Closer Look at an Undersea Source of Alaskan Earthquakes.” Earth and Space Science, vol. 99, no. 2, 2018, pp. 1–6.