Fortunately, the DF earthquake did not cause fatalities or major damage. Only one person reported an injury - a woman who fell in Mentasta Pass and broke her arm. The extent of the damage caused by the earthquake has been widely reported in the literature (EERI, 2003; Yashinsky and Eidinger, 2003). Here we will briefly present some findings specific to this survey.
A vast majority of the buildings reported in this survey are 1- or 2-story wooden houses or log cabins: other types of structures included a few larger buildings, some metal-frame structures, garages, lodges, stores and others. The most common types of damage to the structures were hairline or larger cracks in the walls. Two main factors associated with intensity IX areas were: a) separation of a porch or balcony from the main building, and b) a permanent shift of buildings over foundations. The full statistics of these effects are provided in Appendix B. Examples of other effects include: a few partially or fully collapsed structures or roofs; log houses skewed and twisted; fuel tanks, vending machines, ovens, and other heavy appliances overturned or shifted by inches or feet; parked vehicles shifted several feet in the yard; water wells plugged; burst water lines, etc.
Nearly everyone described the ground shaking as either violent or strong. A person in Slana described the earthquake as “far more violent than in Anchorage in 1964” (i.e. during the MW9.2 Prince William Sound earthquake). The duration of the shaking was estimated anywhere from 10 to 300 sec with an average of about 112±69 sec. Many people reported 1- to 3-feet-high waves on the ground, which is reliable, considering that the maximum vertical displacement at PS10 was 44 cm (Ellsworth et al., 2004). Eighty-four percent of responders had difficulty standing or walking. Most people react to the earthquake by moving to the doorway or running outside (in one case a woman returned back into the house considering it safer than outside!). About 70% of responders were frightened during the shaking (somewhat-36%, very-14%, extremely-21%); the rest felt excitement (mostly male responders). The earthquake was accompanied by strong sound effects described as: “creaking noise,” “cracking sound,” “booming sound,” “very loud roar,” “helicopter sound,” “rambling sound,” ”train noise,” “construction blasting,” and “as if the Alaska Range was ripping apart.”
Some anecdotal reports include:
- Treetops touching the ground (several reports, mainly from Tok and Northway)
- Sounds heard seconds before the shaking started (several reports)
- Two reports from the same location (Tok Cutoff Highway MP65.5) described “booming” or “blasting” sounds that were often heard from the Alaska Range at different times during October 2002. They related these sounds to the Nenana-Denali Fault earthquake sequence since such sounds were not heard at other times.
The results of the macroseismic survey of the DF earthquake, in combination with the USGS CII dataset, allowed establishment of the spatial distribution of the earthquake effects in the form of an isoseismal map. The survey was conducted at more than 99 locations in the near-field zone of the earthquake, including 29 locations of intensity IX, mostly in the communities of Slana and Northway, and along the 40-mile section of the Tok-Cutoff Highway around the Denali fault crossing. The mean isoseismal radii for intensities VIII, VII, VI, V, and IV are 38, 64, 109, 185, and 315 km, respectively. Separation of the zones VIII and IX could not be achieved due to the poor spatial coverage of the data points and large variation of site effects.
There is a good correlation between the intensities and horizontal PGAs observed at the strong-motion sites, with the correlation coefficient of 0.84. Moderate ground shaking (PGA<100 cm/sec2) was felt about one intensity unit stronger than is predicted by the empirical model for western US (Wald, 1999). There is a saturation of intensities in the near-field zone (less than 30 km from the fault). The average intensity in this area is about 8.5±0.7. The attenuation of intensities at greater distances is similar to the western US (Bakun and Wentworth, 1997).
High concentration of the intensity IX locations in the eastern part of the earthquake zone indicate that the ground motion was stronger in these areas compared to the western section. The trend of the intensity-acceleration relationship suggests that the peak horizontal accelerations at Intensity IX locations possibly were more than 0.7 g.
I would like to express my thanks to all survey participants and community officials for making it possible to conduct this survey. I am grateful to Margaret Hopper and David Wald of USGS for useful advice and for the opportunity of using the USGS CII web tool. I also thank Prof. Niren Biswas for the continuous support and numerous discussions during the project. This project was funded by the USGS (Award number: 03HQAG0172, PI: Artak Martirosyan).