It's Not My Fault! Strike-Slip Movements and Big Earthquakes

“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.” 

This quote was published in a Juneau Empire article on October 31, 2012.  Two months later. a magnitude 7.5 earthquake occurred around midnight on January 4, 2013, on the southwest side of Prince of Wales Island, which bounds the entrance to the Inside Passage in Southeastern Alaska.  This one was felt by most people on the island, as well in British Columbia and as far south as Washington State.

The 1989 Loma Prieta Earthquake that famously rocked San Francisco during a World Series Game, collapsed portions of the Oakland Bay Bridge and the elevated I-880 in Oakland, and burned buildings in the Marina District was a “mere” 7.1 magnitude earthquake.  So, if the 2012 Haida Gwaii quake and the 2013 Prince of Wales Island quake were of greater magnitude, why have few people in the U.S. even heard of them?  The answer is that they occurred in a relatively lightly populated area, not California, and their epicenters were located in the Pacific Ocean on the Queen Charlotte-Fairweather Fault system, not on the well-known San Andreas Fault.

Just like the San Andreas Fault, the Queen Charlotte-Fairweather Fault system is a north-northwest-trending, “transform” fault that bounds the western margin of the North American Plate.  The San Andreas is 810 miles long, running from the Gulf of California to the Pacific Ocean north of San Francisco in the Point Arena-Manchester area.  The Queen Charlotte-Fairweather Fault System is 950 miles long, running from the southern end of the Queen Charlotte Islands to a junction with the St. Elias Fault in Wrangell-St. Elias National Park.

A transform boundary forms between two tectonic plates that are moving horizontally past each other and not moving with one plate subducting beneath the other. The fault associated with a transform boundary is called a “strike-slip fault.”  This means that, if you are looking along the fault, one side of the fault is moving to the right and the other side is moving to the left.

The red line on the map above shows the trace of the Queen Charlotte-Fairweather fault system (map from: http://earthquake.usgs.gov/hazards/products/ak/1999/documentation/).  The Pacific Plate is moving northwest along the fault and the North American Plate is moving to the southeast, so the fault system is a “right-lateral” strike-slip fault.  This designation means that, if you are standing on the fault and looking along its length, the right side is moving toward you and the left side is moving away from you.

The map also shows a long red line within the NA Plate that runs northwest from the Haines area to Mt. McKinley.  This is the Denali Fault System, which forms the northern boundary of both the St. Elias Range and the Alaska Range.  In fact, this fault extends in a great curve from Southeastern Alaska around the northern side of Mt. McKinley, then southwest all the way to the Alaska Peninsula near Bristol Bay.  If you have driven the Alaska Highway from Border City (MP 1222) to Northway Junction (MP 1264), you were driving on top of the Denali Fault, which forms the valley of the Tanana River along this route. 

So, why are these extremely long, transform faults important?  During the relatively short historical record of the Pacific Northwest, the Queen Charlotte-Fairweather Fault System has generated some of the largest earthquakes ever experienced.  Seven major earthquakes have occurred along this fault system in slightly more than a century: 8.2 magnitude near Yakutat in 1899; 7.1 on northern Chichagof Island in 1927; 8.1 in the Queen Charlotte Islands in 1949; 7.7 near Lituya Bay in 1958; 7.1 near Sitka in 1972; 7.7 in Haida Gwaii (the Queen Charlotte Islands) in 2012; and 7.5 near Prince of Wales Island in 2013.

The most spectacular of these quakes wasn’t the largest one, however.  On July 9, 1958, the Fairweather Fault generated a magnitude 7.7 quake that was felt as far south as Seattle and east as far as Whitehorse.  Lituya Bay is a part of the Glacier Bay National Park and Preserve.  The earthquake along the fault loosened about 40 million cubic yards of rock that plunged 3,280 feet into Lituya Bay.  The splash caused 1,300 feet of ice along the entire front of the Lituya Glacier to jump up such that an eyewitness survivor saw it rise into the air and move forward.

 

After the glacier disappeared from view, a wall of water that has been measured at over 1,720 feet (the Empire State Building is 1,470 feet high) washed away everything—6-foot diameter trees, boulders, even the soil right down to the bedrock.  In the 1958 USGS photo above, the rockfall is at the right and the 1,720-foot high wave scar is at the left. 

 

This giant wave swept down the bay and, by the time it swept over Cenotaph Island, it was moving at 600 miles per hour (nearly the speed of sound!).  The 1958 USGS photo below shows Lituya Bay, Cenotaph Island, and the sand spit at the mouth of the bay.  Three fishing boats were anchored near that spit when the wave hit.  Although one boat was destroyed, killing the two-man crew, the other boats rode the wave like surfboards.  The wave carried the two boats 80 feet above the spit and out into the Gulf of Alaska, but the two crews survived.  That 1958 event is still the largest landslide-generated wave ever documented worldwide. 

 

 

However, such earthquake-induced rockfall and landslide events are not uncommon in Alaska, where similar wave scars have been found in Glacier Bay, along the Lynn Canal, near Yakutat, and in Prince William Sound.  And these days, a cruise ship might not "surf" a 600-MPH wave out of Glacier Bay quite as easily as two small fishing boats exited Lituya Bay in 1958.  Instead of a death toll of two, we could see a death toll of 2,000.  And that makes the Queen Charlotte-Fairweather Fault System important, not just to Alaskans, but to everyone who visits the state.

Next time:  We are finally on the road to Alaska!  When I find some interesting geology, I'll post again.