Alaska seismologist details Southeast earthquakes

A Jan. 5^th earthquake shook Southeast Alaska, triggering tsunami alerts and evacuations. But it wasn’t the largest temblor to hit the region. A seismologist from the Alaska Earthquake Information Center talked with KRBD about recent and historic seismic activity along the Queen Charlotte Fault.

More than 250 aftershocks have followed the magnitude 7.5 earthquake that hit Southeast Alaska on Jan. 5^th. About 20 of those registered at magnitude 4.0 or greater.

Natasha Ruppert is the seismic network manager at the Alaska Earthquake Information Center in Fairbanks, and she said those aftershocks are just the ones the center’s land-based equipment can detect. There probably have been many, many more, and they likely will continue for a while.

“We’ve done some calculations and based on the rate of seismicity before the January earthquake, it will take at least three years for the aftershock sequence to reach previous background levels,” she said.

The Queen Charlotte Fault has produced several major earthquakes in the past 100 years. The largest known temblor on that fault is a magnitude 8.1 that hit in 1949. Ruppert said that one started in British Columbia, and stretched 300 miles into Southeast Alaska.

“That earthquake produced a very strong shake, and people had a very hard time even standing on their feet in British Columbia and Southeast settlements,” she said.

The Queen Charlotte fault marks the boundary between two tectonic plates: the Pacific Plate to the southwest, and the North American Plate to the northeast. The fault runs up the side of Haida Gwaii in British Columbia, and along the outer coast of Southeast Alaska all the way to Yakutat.

Another large earthquake hit along that fault in 1972. That magnitude 7.6 temblor’s epicenter was closer to Sitka. A much earlier one hit in that same area in 1927, measured at about magnitude 7, but that was too early for seismographic measuring tools to provide much information.

Ruppert said none of the earthquakes in the past 100 years has produced much of a tsunami wave. That’s because of the nature of the fault, which is a strike-slip fault. And what exactly does that mean?

“One side of the fault moves basically horizontally with respect to the other side of the fault,” Ruppert explained. “To produce a tsunami, you need a strong vertical motion of the sea floor. Horizontal displacements of the sea floor, like in these earthquakes in the Queen Charlotte Fault, they don’t produce a large tsunami. They do produce some water disturbances of course, but those are insignificant.”

That’s good news for Southeast Alaska, but it doesn’t mean the region is completely safe from the threat of tsunamis. Large waves and tidal surges still are possible from secondary sources, such as landslides underwater or on land.

“And this was actually illustrated in, I think it was a 1958 earthquake on (the) Fairweather Fault, and there was a huge tsunami in Lituya Bay,” Ruppert said. “This is exactly how it was caused: Strong ground shaking caused the whole flank of the mountain to collapse into the lake, and that caused the tsunami.”

The Jan. 5^th temblor in Southeast followed close on the heels of a magnitude 7.7  earthquake that hit off Haida Gwaii in late October. Ruppert said it’s common for large shocks to occur within months of each other, and more earthquakes will happen.

“Because it’s an active fault, the plates keep moving and the stresses keep accumulating, and eventually the stresses will be released in earthquakes,” she said. “The major earthquakes, of course, are not as frequent, like magnitude 8, but looking at the past history of magnitude 7 earthquakes, every couple of decades, there is a magnitude 7 earthquake somewhere along the Queen Charlotte fault system.”

But when the next mainshock will hit, is not possible to predict.

Ruppert is scheduled to visit Prince of Wales Island on Monday to give a presentation about seismic activity on the Queen Charlotte Fault. The talk, sponsored by the U.S. Forest Service, is set for 6 p.m. Monday at the Craig High School auditorium. She also is scheduled to give a presentation Tuesday in Sitka.