An Ancestor of the Denali Fault Earthquake
SAN FRANCISCO -- A huge earthquake similar to the November 2002 Denali fault earthquake probably ripped along the same path in the last 650 years, said a scientist who searches for signs of ancient earthquakes. David Schwartz of the U.S. Geological Survey in Menlo Park, California announced results from summer 2003 fieldwork on the Denali fault in Alaska at the annual American Geophysical Union meeting in San Francisco. More than 9,000 scientists who study Earth and space attended the meeting, held Dec. 8-12, 2003.
Schwartz was part of a USGS team that spent 11 days last summer exploring different segments of the 215-mile scar left behind by Earth's largest earthquake in the year 2002. The researchers were looking for better detail of the earthquake's "fault trace," the torn pathway left behind on the ground surface after a large earthquake. The Denali fault earthquake left a dramatic path of cracks through glaciers, and the warmth of summer exposed many jagged lines torn through tundra and forest.
Schwartz and his colleagues searched along the earthquake scar to better define the amount of "slip" on the strike-slip fault, but he was most interested in signs of ancient earthquakes along the fault, a weak area of Earth's crust that slips during earthquakes. Schwartz is a "paleoseismologist," a scientist who looks at ancient clues to decipher how often a certain area has been struck by earthquakes, and how big they were.
Signs of old earthquakes are easy to find along the scar of the recent Denali fault earthquake, Schwartz said. He found streambeds torn in half by the recent earthquake that older earthquakes had pushed out of line centuries before. He found buried trees killed during ancient earthquakes. He found enough evidence in the trenches ripped by the 2002 earthquake to conclude that a large earthquake or earthquakes ripped the same path through the Denali and Totschunda faults between 450 and 650 years ago.
Though the Denali fault is one of Alaska's most visible features--on satellite images it looks like a canal dug through the Alaska Range--scientists know little about its history. Before the earthquake brought researchers from all over the world to Alaska, 30 years had passed since geologists had given the Denali fault a detailed look.
In 1973, Schwartz was among the researchers checking out the Denali fault because it sliced through the route of the trans-Alaska pipeline. Working for a consulting company, he lived in pipeline construction camps for a few weeks while flying a broad swath up and down the pipeline corridor and looking for offset gullies, mismatched rock types, and other clues of past earthquake activity. He was among the researchers that recommended the pipeline be built on sliding rails in the area of the Denali fault crossing. The rails would later allow the pipeline to skid sideways more than 15 feet during the 2002 earthquake.
"It's exciting to have been at a place so early in my career, and then to come back 30 years later," he said. "Last summer, I'd be standing in a place and memories of the past would slowly float in."
Schwartz will be back in Alaska again for three weeks in summer 2004, exploring the parts of the Denali fault that did not rupture in 2002--along the upper Nenana River and west of the Parks Highway, and in eastern Alaska past where the recent earthquake took a right turn down the Totschunda fault near the village of Mentasta Pass. Schwartz is hoping to find whether those sections of the Denali fault not ruptured have given way in recent history, which would relieve some of the present stress on them, or whether they haven't experienced a major earthquake in many hundreds of years.
If the latter is the case, the sections of the Denali fault that didn't rupture on Nov. 3, 2002 are now under extra stress, and may be the sites of future large earthquakes.
