Two ocean-bottom seismometers secured on deck of the CGC Maple prior to being deployed offshore Prince of Wales Island.
OBS engineer and crew of the Coast Guard Cutter Sycamore, based out of Cordova, recover one of the seafloor instruments. The yellow and orange plastic encased sphere resembles other instruments that may be afloat offshore Southeast Alaska.
Story last updated at 10/2/2013 - 2:27 pm
Remember that magnitude 7.5 earthquake that shook southern Southeast Alaska back in early January? Coastal travelers should be aware that invaluable data about the aftershocks of that strong tremor might be bobbing on the waves in waters off Prince of Wales Island's west coast. Researchers with the U.S. Geological Survey (USGS) are alerting citizens to keep a look out for missing ocean-bottom seismometers that may be afloat near shore or washed up on land.
The scientific instruments, around two feet in diameter, are glass spheres encased in a yellow or orange plastic shell with a red flag attached to a short pole (although the pole and flag may be missing). The seismometer in its casing weighs around 100 pounds, but should be handled carefully to protect glass and the delicate instruments inside.
Following that significant earthquake last January, which was centered west of Craig along the Queen Charlotte-Fairweather Fault, USGS researchers hastily initiated the ocean-bottom seismometer (OBS) project, specifically focusing on observing aftershocks, in an effort to improve their understanding of recent earthquake activity and associated earthquake and tsunami threats for Southeast Alaskan coastal waters.
USGS scientists aboard the U.S. Coast Guard Cutter Maple worked with the ship's crew to deploy 12 submersible seismometers to the seafloor in a 28 mile stretch where tectonic plates had slipped and aftershocks were most likely expected.
Researcher Dr. Emily C. Roland, Mendenhall postdoctoral fellow at the USGS Alaska Science Center, states that the instruments include three-component seismographs which measure ground motions in vertical and horizontal directions; essentially recording the same kind of vibrations one feels during an earthquake.
Roland states that the study was designed to take advantage of the increase in earthquake activity associated with aftershocks following the magnitude 7.5 earthquake to learn more about the geometry of the plate boundary along the eastern Pacific-western North American tectonic plate boundary that makes up the Queen Charlotte-Fairweather Fault. The aftershock rates are elevated for a period immediately following the quake, which gives scientists a unique opportunity to observe many smaller aftershocks. This may provide information regarding depth distribution along the fault, the number of branching fault strands, and which kind of motion producing earthquake activity is happening. Thrust faulting for example, Roland said, can be more prone to producing tsunamigenic earthquakes.
After the 12 ocean-bottom seismometers had gathered information for about a month at depths from about 600 to 1,400 feet, scientists sent acoustic signals instructing each unit to release its anchor to float back up to the surface for retrieval of the important data. Various obstacles sometimes prevent the process from working properly, such as in conditions where the instrument has become buried in sediment and can't release or if an acoustic transducer (necessary to receive the signal) doesn't function properly.
Luckily, these seafloor seismic instruments have an auto release, which was scheduled to happen on July 20. While the researchers were able to recover seven of the instruments by radio signals, one more has also been located floating around in waters outside of Craig where it was discovered and recovered by the crew of a fishing boat.
Roland said that initial data from the recovered seismometers reveal records of many small magnitude earthquakes, which are aftershocks of the Jan. 5, magnitude 7.5 event.
"It looks as though we should be able to locate tens of events per day during the offshore seismometer deployment period," said Roland, "Some events are of a much smaller magnitude than could be studied using data from the regional seismic array of seismometers that are farther away and on land."
Roland said that the seafloor observations are used to determine more precise earthquake locations, depths and focal mechanisms to help scientists get a sense of the fault slip: transverse strike-slip motion versus convergent thrusting. In the end, the USGS hopes to have a better understanding of the Queen Charlotte-Fairweather Fault system that will lead to an increased understanding of Southeast Alaska's earthquake and tsunami hazards.
Submerged faults can be more difficult to monitor than those on land but, Roland said, "Considering that the Queen Charlotte-Fairweather Fault represents a major fault system (in many ways, on par with the San Andreas Fault in California), relatively little is known about how earthquakes occur along the plate boundary."
The recent earthquake activity and results from this study have inspired discussion on the information that might be acquired by bathymetric mapping of the seafloor along the plate boundary although locating resources to facilitate such studies is always a challenge.
Roland said Southeast Alaska is a very tectonically active corner of the world, and encourages people to learn about the potential hazards of their local area - particularly earthquakes and tsunamis, rare as they may seem.
Any information about the location of the four missing seismometers would be greatly appreciated by the U.S. Geological Survey. If spotted and/or recovered, please contact: Emily Roland at (907) 786-7436 or by e-mail: email@example.com.