November 5, 2012

Ten years ago Denali shook

Posted by Austin Elliott

Saturday [November 3, 2012]  marked the ten-year anniversary of the largest quake to hit the U.S. since 1964, and the 1906 SF quake before that. The M7.9 Denali earthquake tore a ~250 mile gash through Alaskan glaciers and pine forests along the Denali Fault, which runs beside the eponymous mountain also known as Mt. McKinley, North America’s highest peak.

A detailed topographic survey of the rupture reveals fresh scarps along the sharp linear trace of the fault where it has been offsetting river valleys for millennia, one earthquake at a time. The fault runs from upper left to lower right across this image, and the large deflections of each canyon represent the cumulative result of dozens of 2002-sized earthquakes. This data set is available to view in Google Earth from www.OpenTopography.org

Much like the Haida Gwaii earthquake last week, the Denali quake shook mainly sparsely populated areas, so it was much more probable that anyone feeling it would be on the outer fringes of the shaking from the massive temblor. Indeed footage from a home in Anchorage, 160 miles from the epicenter, shows slow rocking–that telltale sign that some place moderately far from you is really getting hammered. Despite the low frequency of the shaking in this video, its strength is clear, and it lasts a very long time–another sign that you’re on the fringes of a huge earthquake.

One of the many legacies of this earthquake was that it put the trans-Alaska oil pipeline to the test. The pipeline crosses the Denali fault nearly perpendicularly, and was constructed with the knowledge and anticipation of offset along the fault. For ~1,000 feet on either side of the fault, the pipeline’s supports rest in tracks that allow it to shift laterally and bend as the ground beneath it carries the tracks in opposite directions. The structure was designed anticipating an earthquake of magnitude 8.0 with 20 feet of coseismic lateral displacement. At nearly the anticipated size, the Denali quake was a resoundingly successful test of earthquake engineering, which spared us an enormous environmental disaster.

A comparison of the 2002 Denali and 1857 San Andreas fault ruptures. The modern recordings from Alaska help guide our expectations about a contemporary repeat of the southern California quake.

Both its size and its geometry suggest that this rupture may be an excellent modern analogue to the earthquake that ripped along the San Andreas Fault in 1857, before southern California was heavily populated. It thus serves as an excellent source of modern data (seismic recordings, satellite imaging, GPS velocities) to help understand what a repeat of the San Andreas rupture will be like. For example, seismic records from the Denali quake are used in structural engineering tests of seismic safety design, simulating the type of shaking that may be expected around Los Angeles in the next SAF quake.

The USGS hosts a great set of photos from the spectacular Denali fault rupture, including offset glaciers, newly formed waterfalls across the scarp, huge landslides, and my personal favorite: this unfortunate tree that was growing directly atop the fault trace and got sheared in half.

A tree, taking advantage of the groundwater source along the Denali fault, suffers the consequences of its opportunism. Photo credit: Peter Haeussler, USGS.