Anchorage Area Fault May Be Due for 6 or 7 Magnitude Quake

A new study by the U.S. Geological Survey (USGS) shows that the Castle Mountain fault in south-central Alaska may be ready to produce a strong magnitude 6 to 7 earthquake. The study, which examined the ancient earthquake history of the Castle Mountain fault, was just published in the October issue of the Geological Society of America Bulletin.

Peter Haeussler, the principal investigator in the study, said his research demonstrated that major earthquakes occurred on this fault on average every 700 years or so in the last 2,700 years, and that the last significant earthquake along the fault occurred about 650 years ago. Therefore, he said, a magnitude 6 to 7 earthquake may occur on this fault in the next 50 to 100 years.

The Castle Mountain fault is the only active fault that comes to the earth’s surface in the Anchorage region, and the eastern part of the fault produced light to moderate magnitude 5.7 and 4.6 earthquakes in 1983 and 1996, respectively. The Castle Mountain fault is part of a larger field of folds and faults in the Cook Inlet region that are capable of producing magnitude 6-7 earthquakes, but because geologists consider the Castle Mountain fault one of the largest and most significant in the region, understanding its earthquake history is important.

To evaluate the earthquake history of the Castle Mountain fault, researchers dug trenches up to 6 feet deep and 100 feet long across the trace of the fault west of Houston, Alaska. The scientists carefully mapped the walls of the trenches, analyzed the relationships between geologic and soil units, and collected samples for radiocarbon dating.

Haeussler and his colleagues found that the fault was not a simple zone where the different geologic sediments were cleanly offset, but rather, “it was a zone of goo” where sediments liquefied and flowed during ancient earthquakes. Unlike similar studies in the lower 48 states, the dense root mat that covers the fault acted like a strong blanket and did not break during ancient earthquakes. As a result, said Haeussler, it was much harder to precisely specify the timing of ancient earthquake events. Also, because the sediments were near the surface and they flowed, the ancient earthquakes must have occurred in the summer months because the sediment was not frozen and able to flow.

The 1964 magnitude 9.2 earthquake that occurred in southern Alaska was related to a slip of the Pacific plate beneath southern Alaska. In contrast, said Haeussler, earthquakes that occur on shallow geologic structures, such as the Castle Mountain fault and the related field of structures in Cook Inlet, may not produce great earthquakes, but they are closer to where people live and work, and as a result may have more impact when they occur. Consequently, this new study of the ancient earthquake history of the Castle Mountain fault greatly increases the understanding of a particular type of earthquake hazard, but ideally there should be studies on all the structures in Cook Inlet to better evaluate how quickly they move, how large are the earthquakes they produce, and how often they occur.

The abstract of this paper, “Paleoseismology at high latitudes: Seismic disturbance of upper Quaternary deposits along the Castle Mountain Fault near Houston, Alaska,” can be found at http://www.gsajournals.org/gsaonline/?request=get-abstract&issn=0016-7606&volume=114&issue=10&page=1296