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| Richardson highway crack (UofA GI) | Pipeline support collapse (UofA GI) |
(additional photos at bottom):
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Kara's laptop continued downloading through and after the quake. Afterwards we surveyed the house and found pictures on shelves had tipped over (none had fallen from the walls) and a number of items had fallen off shelves... fortunately only one broken bottle. The most visible sign was a three shelf bookshelf (full of books) which was on top of another bookshelf had moved 3 inches off the wall. After the download completed within half an hour, we checked www.aeic.alaska.edu and earthquake.usgs.gov which had preliminary reports of M7.5 depth of 1km, within an hour that had been revised to M7.9 depth of 10km. The magnitude remained posted as M7.9 and when all seismographs were coordinated the depth was posted as 5km.
There was no structural damage to our house and no significant damage reported in Fairbanks. One injury was reported from Mentasta where a women fell from a porch. Considerable damage was done to roads which crossed the Denali Fault, especially those east of the epicenter (see photos below). The Transa-Alaska Pipeline remained intact, but significant damage to pipeline supports forced oil flow to be shut down for two days. There was significant structural damage to buildings in several villages closer to the epicenter (Mentasta and Slana). There were no deaths. In other parts of the world, a quake of this magnitude could have killed thousands from buildings collapsing, resultant mud slides, fires caused by damaged gas lines, or tsunami's (no tsunami resulted from this quake, too far inland). Fairbanks was close enough to the quake that if any "third world" or non-modern construction practices were used buildings could have collapsed. Alaska also was spared more catastrophic damage due to the sparse population in the immediate vicinity of the epicenter. This was the strongest quake in the world during 2002 through November 3rd. There were 1612 reported earthquake related deaths in 2002 through November 3rd. At least 29 people died in the M5.9 31 October 2002 Southern Italy quake and over 1000 died in the M6.1 25 March 2002 Afghanistan quake. Shallow quakes cause more damage, the Afghanistan M6.1 was only 8km and the Italian M5.9 was only 10km... the Alaska M7.9 was only 5km.
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| Richardson highway crack (UofA GI) | Pipeline support collapse (UofA GI) |
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I have lived in Fairbanks over 35 years. I moved here in 1965, well after the M9.2 quake which devastated Anchorage and Valdez on 27 March 1964. This was by far the strongest I ever experienced. In earthquake magnitude scales, an M7 is ten times stronger than an M6 which is ten times stronger than an M5. On 23 October 2002 an M6.7 quake occurred 50 miles west of the M7.9. The M6.7 was the third strongest I recall. It occurred at 3:27am... more than enough to wake me and shake me some, not enough to cause me to consider getting out of bed to a doorway. During a middle of the night M5.x west of Fairbanks in 2000 I was ready to get out of bed to a doorway, it felt stronger than the M6.7 because it was centered closer to Fairbanks. Kara lived near Seattle during Seattle's M6.8 on 28 February 2001, she catagorized the Seattle quake as comparable but not as severe as the Fairbanks M7.9 experience. Prior to the 2000 the most memorable quake I remember was when I was a boy in the early fall of 1967, that was probably a high M5. Earthquakes are generally memorable by their duration, anything lasting 30 seconds or longer (all the above) really grabs attention.
The following map from USGS shows the M7.9 mainshock as a filled white circle and the 23 October M6.7 foreshoke as an open white circle. Also shown are 415 quakes with a depth of 35km or less of magnitude M2.0 or higher from 22 October until 08 November 2002. The picture below the map shows the Denali fault and relative location of both the M6.7 and M7.9 quakes.
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The earthquake was a magnitude (M) 7.9 and was located along the Denali Fault, 92 miles south of Fairbanks, 176 miles north-northeast of Anchorage, and 43 miles east of Denali Park at a depth of 4.86 km (3 mi). The earthquake is the largest earthquake recorded in the world so far this year (2002). At the epicenter, the earthquake lasted somewhere between 1.5-2 minutes and the resulting rupture was approximately 300 km (186.4 mi.) long. Depending on location, the earthquake’s duration was longer due to the travel rates of the P-Wave and S-Wave generated by the event. In Fairbanks, the earthquake lasted over 3 minutes. The areas that reported the maximum impact were the villages of Slana and Mentasta, where the intensity was gauged to be around seven on a scale of one-to-ten. Other communities who experienced strong intensity included Tok, Paxson, Cantwell and Northway. Fairbanks averaged an intensity of five and Anchorage an intensity of four. There were reports of the earthquake being felt on land as far as Washington, and on water as far as Louisiana and Texas.
The geologists found that measurable scarps indicate that the north side of the Denali fault moved to the east and vertically up relative to the south. Maximum offsets on the Denali fault were 22 feet at the Tok Highway cutoff and were 6.5 feet on the Totschunda fault. The largest offsets were in the region between the Richardson Highway and the Tok Cutoff Highway. This variance will continue to change as aftershocks help the land along the fault fit back together and settle. Frequent aftershocks are expected to occur for the next few months, and will eventually taper off in frequency but continue to occur for up to one year. With an earthquake of this size, it is not unusual to experience aftershocks as large as a M6. As of 7:00 am AKST 06 November, the largest aftershock has been a M5.8 and over 350 aftershocks have been detected.
Seismologists cannot predict exactly when and where the next earthquake will occur, we can expect another significant earthquake in Alaska on any given day. Alaska has more earthquakes than any other region in North America and is by far the most active state in the U.S.; more than 10,000 earthquakes are located on average in Alaska each year. Of those, an average of five per year will be a M6-7, one per year will be a M7-8, and one per 13 years will be larger. Alaska has 11 percent of the world’s earthquakes, and 52 percent of all the earthquakes that occur in the U.S. On March 27, 1964 Alaska experienced an earthquake of M9.2, the second largest earthquake ever to occur in recorded history. Excluding activity in the Aleutian Islands, the recent earthquake of M7.9 was the largest earthquake in Alaska since 1964. Three of the six largest earthquakes in recorded history were in Alaska.
In Alaska, the Pacific plate is actively subducting under the North American plate. There are also several faults in Alaska, including the Denali fault where the M7.9 earthquake recently occurred. These geological factors create frequent earthquakes along the plate boundary in south central Alaska, along the Aleutian chain, and in interior Alaska along seismic zones and faults such as Salcha, Fairbanks and Minto Flats. The M7.9 shock is the largest earthquake on the Denali fault since at least 1912, when a M7.2 earthquake occurred in the general vicinity of the fault more than 50 miles to the east of today’s epicenter. The epicenter of the M6.7 that occurred on 23 October 2002 was immediately west of that of the M7.9 along the Denali fault. The M6.7 ruptured 40 – 50 km (24.9 – 31.1 mi) of land west of the epicenter, and the M7.9 ruptured approximately 300 km (186.4 mi.) of land east of the epicenter toward the trans-Alaska pipeline.
An aerial photo of the Trans-Alaska Pipeline System (TAPS) line near the Denali fault, looking west. This is where the line is supported by rails on which it can move freely in the event of fault offset. Here the line has moved toward the west end of the rails. Alyeska Pipeline Service Company reported no breaks to the line and therefore no loss of oil. Note the transverse crack on the Richardson Highway in lower left. Out of view to the left (south) of this photo is a 2.5 m right-lateral offset of the highway where it crosses the fault.
Richardson Highway road offset (looking south).