Earthquakes in the Fairbanks Area are a Puzzle

Owing to growing understanding, over the past twenty years, of what has become known as "plate tectonics" (an outgrowth of what the German geologist Alfred Wegener called "continental drift" in 1912), it is no longer surprising that it is the Pacific coastal areas of Alaska that experience the strongest earthquakes. If the Aleutians are included, there have been at least nine magnitude 8.0 or greater earthquakes produced in these areas during this century. These result when the oceanic Pacific plate grinds against the North American plate along the coastal margins where they abut.

What is less well understood is why central interior Alaska, particularly in the Fairbanks area, produces so many earthquakes (albeit, generally of more modest magnitudes).

The Fairbanks area has a history of seismicity that is both interesting and perplexing. A major magnitude 7.3 earthquake occurred 40 miles southeast of the city in 1937. There have been others of equal or greater magnitude in interior Alaska during this century, but they have occurred at greater distances from Fairbanks, and were probably associated with the Alaska Range and the Denali fault to the south.

Since 1937, however, little else of note happened in the "Fairbanks Seismic Zone" until June of 1967 when a series of magnitude 5 to 6 earthquakes occurred only 10 miles to the southeast. The epicentral zone of this earthquake sequence continues to be active to the present, producing several small earthquakes each day, and a "felt" earthquake approximately monthly. The most recent significant event in this 10-mile wide zone was a magnitude 5.0 earthquake on April 15, 1983, which caused minor damage in the city .

Prior to that, the next largest earthquake since 1967 was a magnitude 5.2 event and an associated earthquake "swarm" which occurred about 20 miles south of the Fairbanks International Airport on December 30, 1981. This also resulted in some damage in the surrounding area.

But perhaps the most interesting aspect of this ongoing seismic activity in the Fairbanks area is the manner in which it has tended (with minor digressions) to progress in a near-lineal fashion to the northwest from the epicentral area activated in 1967 (now known locally--and probably inaccurately--as the infamous "Badger Road fault").

Earthquake swarms lasting from days to upward of a month, and consisting of hundreds of events, have been observed to occur sporadically in sequential fashion along a line extending from the 1937 epicenter, through the 1967 zone, to a point near the Village of Fox, north of Fairbanks.

This is strongly indicative that a hidden fault, passing just east of Fairbanks, is extending its activity to the northwest from the 1937 epicentral zone. Possibly this is occurring in the same manner that cracking in a sheet of metal tends to continue propagation along the zone of weakness where strain is greatest at the end of a crack.

Analytical methods called "focal mechanism determinations" support this hypothesis. But an actual fault, if it exists, has not yet been identified on the ground, despite intense ground, airborne and satellite surveys.