U.S. Naval Research Laboratory (NRL) researcher Ben Phrampus recorded the largest U.S. earthquake in nearly half a century when it hit recently off the coast of southern Alaska. The 8.2-magnitude earthquake went mostly unnoticed since it was almost 20 miles below the ocean floor. It briefly, however, sparked evacuations, tsunami warnings in the region, and two aftershocks of 6.2 and 5.6, respectively.
Phrampus, a research physicist in NRL’s geology and geophysics section, was in the right place at the right time while on watch aboard a research vessel when the earthquake hit July 29, and the crew’s instruments recorded the event.
The United States academic community’s national seismic research facility Marcus G. Langseth, operated by Lamont-Doherty Earth Observatory’s Office of Marine Operations at Columbia University, sat west of an island chain called, Haida Gwaii, off the northern Pacific coast of Canada. An acoustic receiver, known as a hydrophone streamer, recorded the noise while it was in the water at the time and showed a massive spike.
“Seeing it live on the data stream was particularly exciting,” Phrampus said. “We were actually seeing the earthquake’s acoustic waves on our instruments in near real time, which is just awesome.”
The earthquake happened near Perryville, Alaska, a small community on the Alaskan Peninsula about 500 miles southwest of Anchorage.
“I was standing watch (on duty) looking at incoming data to make sure everything looked good,” Phrampus said. “I happened to look at the streamer data at the right time to catch the event as it happened.”
The technicians onboard were initially concerned about what they were seeing. “We wanted to be sure there wasn’t an issue with the equipment,” he said. “We quickly recognized it was not an equipment issue. This led to our curiosity to find the source.”
Some quick checking revealed it was an earthquake. They confirmed their findings after viewing the U.S. Geological Survey’s website, which reported the 8.2-magnitude quake.
After the tsunami warnings went off, coastal residents scrambled to higher ground and-or evacuated. The earthquake was the most powerful in North America since a 9.2-magnitude earthquake in Alaska in 1964. Known as the “Great Alaskan earthquake,” it spread across south-central Alaska, ground fissures, collapsing structures, and tsunamis. More than 130 died in the most powerful earthquake recorded in North America. This was the second largest earthquake ever recorded on Earth.
The most recent earthquake may have caused light to moderate damage and moderate shaking, according to preliminary seismic data. The U.S. National Tsunami Warning Center canceled a tsunami warning issued for parts of Alaska after waves of less than one foot arrived onshore. Tremors extended throughout the Alaska Peninsula and Kodiak.
Phrampus said a rupture offshore went all the way to the seafloor along the continental slope. This produced seismic energy that interacted with the sound fixing and ranging channel, or SOFAR, which is an ocean channel that allows sound to carry great distances, according to the National Ocean Service. The ocean consists of many zones, and sound can travel through a zone for hundreds, sometimes thousands of miles. Interacting with the SOFAR converted the energy into acoustic waves, a process called downslope conversion, Phrampus said.
This interaction trapped the acoustic waves in the water column and helped spread them across the Pacific Ocean. Just 20 minutes later, the hydrophone streamer recorded the event, Phrampus said, both surprising and exhilarating the crew.
The research team is assessing tectonics and seismic hazards of earthquakes and tsunamis along the Queen Charlotte Fault, which has produced large earthquakes in the past. Though the recent event was exciting enough, there is still a great deal to learn about the most recent earthquake and the fault itself, he said.
“We’ve only done preliminary analysis to verify that the 8.2 quake is what we are seeing on our equipment,” he said. “We will be diving into this more in the coming months.”
The research cruise ended August 24th. Results will be presented at the American Geophysical Union in New Orleans in December.