Nenana Mountain Foreshock

• Date: October 23, 2002 (Wed.)
• 3:27 AM (Alaska Time)
• Magnitude: 6.7
• Epicenter: 63.51N, 147.91W (central Alaska)
• Fault: Denali Fault
• Type: Right-lateral strike-slip
• Rupture length: about 30 km
• Depth: 4.2 km

Denali Fault Earthquake (Mainshock)

• Date: November 3, 2002 (Sun.)
• 1:12 PM (Alaska Time)
• Magnitude: 7.9
• Epicenter: 63.52N, 147.44W (central Alaska)
• Fault: Denali Fault
• Type: Striking reverse --> Right lateral strike-slip
• Rupture length: about 300 km
• Depth: 4.86 km
• Fatalities: none!... only one injury reported (a broken arm)

Source: http://www.aeic.alaska.edu/

The 2002 Denali Fault Earthquake is among the biggest North American quakes on record, its magnitude being the same as that of the 1906 San Francisco Earthquake. Having rumbled such a large area of Alaska, it is quite remarkable that the Denali quake caused as little damage as it did.

Interior Alaskans were rudely awakened in the early morning of October 23, 2002 as everything shook vigourously. It was soon over, however, and most rolled over and went back to sleep (Alaskans are accustomed to these things, after all). The earthquake, a 6.7, had occurred in a remote area, and had done very little damage aside from the knocking of grocieries from store shelves near the fault. A cracked ceiling beam was also reported in the Delta Junction library. For comparison, this quake was of the same magnitude as the 1994 Northridge Earthquake.

Naturally, Alaskan geologists became very excited over such a big earthquake, since the Denali Fault had long since been presumed inactive. The University of Alaska’s geological department quickly deployed an extensive network of seismographs along the fault, in hopes of getting in and measuring some of the aftershock action. But twelve days later, it turned out that this strong earthquake had been nothing more than taste of things to come.

I myself had not yet gone to bed at 3:30 in the morning when the Oct. 23 earthquake hit... I had not yet awoken at 1:15 PM when the big one hit on November 3rd. But when the P-waves did roll in, the shaking was such that almost without hesitiation I found that I had reflexively jumped out of bed and (quite futilely, I might add) was standing beneath the doorway to my dorm room. After a number of startling seconds, the violent jolting and jarring had subsided to a brisk quiver (a quiver that didn’t go away for several more minutes), the dust cleared, and I saw that not all that much havoc had been wreaked. A few things had fallen off of their shelves, and there was now a long, thin crack that ran all along the corner of my room. That was about it, although the experience itself had left me a little bit shaken.

Fairbanks as a whole suffered very minimal damage from the quake (being as it was, about 90 miles from the epicenter). Most ironic, perhaps, was the structural damage suffered by UAF’s International Geophysical Institute... It too was really minor though, mostly just some hairline cracks in the cement.

People who were closer to the Denali Fault were not nearly as fortunate. When the seismic waves hit the Tok Cutoff Highway, the road began to shoot full of giant cracks, often three to six feet wide and as much as fifteen feet deep. In the worst of the devastation, it was as though the highway had shattered like a china plate; leaving nothing but large, disconnected blocks of asphault. At least one vehicle, a large semi-truck driven by Bill Miller and Billy Carey, had been cruising down the road at the time this happened. The duo, who’d been hauling some refrigirated groceries, were rather surpised as the road before them was split asunder, shearing off the wheels of their truck and bringing it to rest on its oil pan.

Meanwhile, major damage had been done to settlements in the area, most notably Mentasta Lake. Although very few, if any, houses collapsed (wooden houses tend to fare very well in earthquakes), many suffered severe damage at the hands of the quake’s twisting motions. Liquefaction—soft soil turning to liquid during intense shaking—also caused some damage. During the earthquake, one homeowner was unable to leave his residence after the soil liquified and his house sunk several feet.

Fuel tanks in Mentasta Lake had also been knocked over, and roads leading into the village were unsuitable for travel. This left residents stranded and resource-famished for a number of days until repairs could be made. Mentasta was declared a disaster area, to be dealt with by Alaska Governor Tony Knowles.

Because of the 5m+ lateral movement of the Denali Fault, roads were broken and offset by similar distances where they crossed the fault. One house was known to be straddling the fault; it was all but destroyed by the results of all this lateral motion. The Trans-Alaska Pipeline, which had been engineered with the Denali Fault in mind, fared its first major seismic test remarkably well. A few supports were knocked out near the area where the pipeline crosses the fault, and the pipeline was shut down for the next few days. No major damage or leaks were reported, however.

One impressive feature of the quake was the spectacular geological scars it left behind. Particularly notable was a massive quake-triggered landslide; an enormous portion of the neighboring mountainside was sent tumbling down onto the Black Rapids Glacier. This completely buried much of the mile-wide glacier in an pile of rocky debris.

In the first few weeks after the quake, aftershock action seemed to be almost constant; many of them felt like slow, rolling sort of seismic waves. Even at the at the time this was written (about four months later), I can still feel an aftershock every now and then.