Story last updated at 2/6/2013 - 1:39 pm
I was standing next to my front window in Thorne Bay, looking for a book on a shelf when the shaking began. At first I thought it might be a huge wind gust, but it wasn't windy so when the china began jumping around as the whole house shook, it soon became clear that an earthquake was underway. It seemed to last forever in a slow motion sort of way, but finally ceased its underground adjustment in about 20 seconds, leaving me with mixed feelings of relief and apprehension.
A magnitude 7.5 earthquake off the coast of Craig, on the southwest side of P.O.W. Island, around midnight Jan. 4, was felt by most people on the island as well as Canada and even Washington. Numerous people in Craig evacuated low lying areas, waiting nervously for reports from authorities about predictions of further earthquakes or tsunamis that might be expected.
The next day, many people around Prince of Wales Island were interested to learn about exactly what happened and the odds of a repeat performance. Calls poured in to Jim Baichtal, forest geologist with the Tongass National Forest, Thorne Bay District, who then began working with the Alaska Earthquake Information Center (AEIC), the U.S. Geological Survey (USGS) and the State Seismologist to bring a speaker to Southeast Alaska to discuss the issue.
Natalia Ruppert - seismic network manager at the Alaska Earthquake Information Center, Geophysical Institute, University of Alaska Fairbanks - agreed to travel to Craig and Sitka for Forest Service sponsored speaking engagements concerning the recent earthquakes, the sensors and seismic networks in place to gather and distribute timely information and the earth shaking prospects predicted for the future.
The AEIC, a member of the Advanced National Seismic System (ANSS), records data from an integrated network of over 400 seismic stations, locates around 22,000 earthquakes each year. Real-time sensor information is shared with the West Coast and Alaska Tsunami Warning Center, the Alaska Volcano Observatory, the USGS, the National Oceanic and Atmospheric Administration and the Incorporated Research Institutions for Seismology.
Concerned locals packed the Craig High School auditorium on Prince of Wales Island Monday, Jan. 28 for a presentation about earthquakes and how they are related to the tectonics of the Queen Charlotte Fault where recent rumblings have precipitated the attention of Southeast Alaska coastal dwellers.
Ruppert pointed out that three of the 10 largest earthquakes ever recorded in the world have been in Alaska, with the 1964 event in Anchorage claiming title to the second largest at a 9.2 magnitude. On the south edge of Alaska lies the boundary between the North American and Pacific tectonic plates, defined by a series of strike-slip faults known as the Queen Charlotte-Fairweather Fault System. Many of these faults continue through Canada and back into Alaska, creating some of the largest faults in the United States.
The Pacific plate is moving northwestward relative to the North American plate, and is being subducted that plate at an average rate of five to seven centimeters per year. Subduction is defined as the sideways and downward movement of the edge of a plate of the earth's crust into the mantle beneath another plate. Notable earthquakes along the Queen Charlotte-Fairweather Fault include: 1927-magnitude 7.1, 1949-M8.1, 1958-M7.7, 1972-M7.6, 2004-M6.8, 2012-M7.7 and 2013-M7.5.
Using maps, charts and diagrams, Ruppert displayed information showing earthquake and fault positions along the Queen Charlotte-Fairweather Fault along with explanations of the different types of faults and their effects with regard to the potential for a tsunami type wave development.
Aftershocks, occurring along the fault are strongest in the first 10 days after an earthquake. For the recent M7.5 earthquake near Craig the strongest aftershocks were 3 ½ hours (M5.4) and nine days (M5.5) after the earthquake. Within two weeks there were 19 aftershocks with a magnitude of 4 or greater.
Ruppert described how she uses an aftershock decay rate formula to project how long the aftershocks will continue and, with 240 aftershocks already noted, she predicts they will continue for three years from the M7.5 event.
The large group addressed Ruppert enthusiastically with a shower of questions ranging from whether the moon or outside temperatures contribute to earthquake potential to how high one needs to climb to be safe. "The higher the better" Ruppert said finally, bringing a general chuckle through the audience, after explaining there can be no one simple answer for all occasions.
When asked about the 1958 earthquake (M7.7) along the Fairweather Fault that resulted in the Lituya Bay tsunami, Ruppert and Baichtal said in that case the earthquake didn't cause the tsunami but did cause a massive landslide that resulted in a gigantic wave. This led to questions regarding the terrestrial stability of P.O.W. Island and whether there are faults under the island. Baichtal said that there is evidence of old fault lines through P.O.W. Island but nothing is active.
Someone inquired about a rumor that there is a volcano offshore of Craig. Baichtal responded that, yes, indeed there are at least five volcanic vents about 300 feet below the surface and about 20 miles east of the epicenter and fault rupture on the inland side. These inactive volcanoes are believed to have erupted around 14,000 years ago, Baichtal said.
When asked when the next earthquake would happen, Ruppert could only predict probable events based on previous statistics showing the potential for different sized earthquakes over the years, explaining that there is just no way to know exactly when an earthquake will happen and modern type studies have collected data for only about 100 years.
"Be aware, there is a pretty big fault in your backyard," she said.
Alaskan movers and shakers who experience an earthquake are encouraged to report it on the U.S. Geological Survey website at Did You Feel It?