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Seismologist: Possible for major quake to 'stress' faultline

Posted: October 31, 2012 - 9:02pm  |  Updated: November 1, 2012 - 12:11am

Southeast Alaska escaped damage or casualties from a tsunami triggered by a magnitude 7.7 earthquake Saturday night in Haida Gwaii, the Canadian archipelago formerly known as the Queen Charlotte Islands that lies south of the Dixon Entrance.

But the Haida Gwaii temblor occurred along the same Fairweather—Queen Charlotte transform fault system that lies beneath the Alaskan Panhandle, and seismologist Natasha Ruppert said this week that there is no way to tell when or where the fault’s next big quake might be coming.

“Every time you have some major earthquake on a fault, there is a possibility that it will stress the remaining parts of the fault,” said Ruppert, who works at the University of Alaska Fairbanks’ Geophysical Institute, in the Alaska Earthquake Information Center.

Ruppert added, “I can’t say for sure, but it’s always possible that one earthquake makes enough changes so it makes another earthquake more possible.”

Although the Haida Gwaii quake caused no known casualties — even the lightly populated islands themselves did not suffer serious damage — the Fairweather—Queen Charlotte fault system has a history of more spectacular natural disasters, including the deadly 1958 Lituya Bay earthquake in Glacier Bay National Park. That event saw the largest wave on record generated after some 120 million cubic feet of mountainside tumbled into the fjord (http://bit.ly/WZaxeR).

Most of Southeast Alaska’s populated places are located deep within the Inside Passage. Juneau is nestled along the Gastineau Channel, with uninhabited West Douglas Island just one of many natural barriers separating it from the open Pacific Ocean.

“In terms of a tectonic tsunami, the communities that are protected by the islands, they are of course in less danger than the communities that are open to the whole Pacific basin,” said Ruppert. “Those communities might be protected from the tectonic tsunamis … but they might still suffer the effects of triggered tsunamis, tsunamis triggered by the landslides.”

Ruppert said those triggered tsunamis are “harder to evaluate” than typical tsunamis, which are caused by the lowering and raising of the sea floor, in part because their likelihood of occurring depends on how vulnerable a land formation is to sliding into the water.

Tom Mattice, emergency programs manager for the City and Borough of Juneau, offered a rather dark assessment of the possibility of a localized tsunami triggered by a landslide or mudslide.

“You wouldn’t know that it was happening until too late,” said Mattice. He hastened to add, “It is a possible scenario, but it’s not a probable scenario.”

Seismologists are hampered in studying the Fairweather—Queen Charlotte fault system and predicting its activity simply because they have had relatively little time to observe it.

“The history of observation of this seismology is rather short,” Ruppert said. “It’s just a little bit over 100 years.”

In that time, five major earthquakes greater than magnitude 7 have occurred along the fault system: on northern Chichagof Island, in 1927; in what were then called the Queen Charlotte Islands, in 1949; near Lituya Bay, in 1958; near Sitka, in 1972; and in Haida Gwaii, last weekend.

Ruppert said the fault structure is “very similar” to that of the infamous San Andreas Fault in California, responsible for the devastating earthquakes that affected the San Francisco Bay Area in 1906 and 1989.

“It’s the same type of plate boundaries, same type of fault. It’s a strike-slip fault,” said Ruppert.

Earthquakes from strike-slip faults are actually smaller on average than megathrust quakes, such as last spring’s major earthquake in Japan or the massive Indian Ocean quake in 2004, which occur along faults where one tectonic plate is forced down by another plate.

Still, strike-slip fault quakes are potentially very destructive, as in the case of the 1906 event in San Francisco. The earthquake that caused staggering damage in Haiti in 2010 occurred along a strike-slip fault as well.

While Ruppert said she cannot predict when the next big Southeast Alaska earthquake will hit, she said, “It always makes sense to have a plan, to have your emergency supplies available and just to check with your local emergency services where the shelters are.”

Juneau does not have a known history of major earthquake activity, Mattice said, but a big quake is “always a possibility.”

“We always keep it on the radar,” said Mattice.

Mattice said the CBJ does have a draft plan for evacuation routes, though it has yet to be finalized.

“If we had to evacuate a section of the town, or we had to open shelters in a place, we could do that,” Mattice affirmed.

An earthquake is just one of the potential disasters for which it is Mattice’s job to keep Juneau prepared. He said contingency planning for a quake is similar in many aspects to planning for flooding or another such emergency.

“It’s one of those that’s hard to prepare for, those worst-case scenarios,” Mattice admitted. “But when we talk about emergency preparedness … we always look at all-hazards response and how we can be ready for that.”

• Contact reporter Mark D. Miller at 523-2279 or at mark.d.miller@juneauempire.com.

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