Fault in Alaska a Tsunami-Maker Candidate
The tsunami that devastated Japan on March 11, 2011, was the largest ever to strike a modern, developed coastline. But probably won’t be the last.
The next great Pacific tsunami may well hail from Alaska. And if it’s anything like the great swells of seawater that region has kicked up in the past, it could cause death and destruction as far away as Hawaii and California.
Recent analysis of Alaska’s potential to generate a future tsunami, highlighted in the May 8 issue of Eos, have turned up a worrisome similarity between the source of the magnitude 9.0 earthquake that generated the Japan tsunami and a particular segment of a major quake-prone fault running along the seafloor just south of Alaska’s Aleutian islands.
That fault, known as the Alaskan-Aleutian subduction zone, marks where the edge of the earth’s tectonic plate carrying the Pacific Ocean plunges beneath another plate to the north. Sudden slip between those plates produced the Good Friday Earthquake (also called the Great Alaska Earthquake) of March 27, 1964, which was the most powerful earthquake in U.S. history. That magnitude 9.2 temblor resulted in 145 deaths, many of them hundreds of miles away—and 90 percent of them due to the resulting tsunamis.
All around the rim of the Pacific Ocean, the edges of tectonic plates are scraping against each other, but it is the vertical motion that occurs at subduction zones that harbors the great potential for tsunami-generating tremors: When one plate slips suddenly past the other, the motion of the seabed displaces the seawater above it.
The magnitude 9.0 Japan quake occurred along a plate boundary known as the Tohoku subduction zone. The resulting tsunami, measuring as high as 33 feet (10 meters), overwhelmed seawalls and other defenses shredded wooden structures nearly to kindling.
Clearly, Alaska has potential for similarly large earthquakes. But that is not what has scientists on alert. The precise location of the earthquake is even more important than its size when it comes to tsunamis. Even a super-strong quake won’t cause more than a ripple if it happens in shallow water, but a moderate earthquake can generate a serious tsunami if it strikes where the water is deep. Indeed, the Tohoku tsunami attained its great height because the portion of the seabed that moved lay beneath five kilometers of seawater.
In Alaska, scientists have zeroed in on one particular section of the Alaskan-Aleutian subduction zone near the Semidi Islands. This locked section of the fault has not ruptured since 1788 (or perhaps even earlier), and geodesy measurements reveal that strain is accumulating rapidly. And just like the portion of Tohoku that slipped last year, this severely strained section Alaska’s subduction zone lies beneath four to five kilometers of water.
Investigations into the source the Tohoku tsunami have given scientists something else to worry about in Alaska. The Tohoku studies revealed that secondary movement along a deep-water branch fault displaced additional seawater, amplifying that tsunami.
So far no one knows about any deep-water splay faults near the Semidi Islands, but scientists are keen to get a closer look at the region’s steep underwater morphology to find out for sure.
PHOTO: Alaska’s Aleutian Islands from space. (Courtesy: NASA/MODIS Rapid Response System)