MACROSEISMIC SURVEY OF THE M7.9, 2002 DENALI FAULT EARTHQUAKE / A. Martirosyan
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The macroseismic survey of the MW7.9, November 3, 2002 Denali Fault earthquake, conducted by the Geophysical Institute of the University of Alaska Fairbanks in late 2003, provided important data in addition to the US Geological Survey Community Internet Intensity dataset. The intensities of the ground shaking were obtained at more than 99 near-field locations. The largest values, VIII and IX, were obtained at 58 survey locations, mostly in the communities of Slana, Mentasta Lake, Northway and Paxson, and along the sections of the Tok-Cutoff and Richardson Highways adjacent to the fault crossings. The isoseismal map, produced using the combined dataset, portrays the spatial distribution of intensities from IV to VIII/IX. The mean isoseismal radii for intensities VIII, VII, VI, V, and IV are 38, 64, 109, 185, and 315 km, respectively. The average decimal intensity in the near-field zone (less than 30 km from the fault rupture) is 8.5±0.7. There is a good correlation between the intensities and horizontal peak ground accelerations observed at strong-motion sites, with a correlation coefficient of 0.84. Larger concentration of the intensity IX locations in the eastern segment of the earthquake zone indicate that the ground shaking amplitudes were higher than the eastern section, which is consistent with the results of the slip distribution studies.
INTRODUCTION
Seismic intensity is a qualitative measure of earthquake ground shaking at a specific location, determined subjectively during post-earthquake investigations from the nature of earthquake effects and extent of the damage. Isoseismal (equal intensity) maps provide valuable information on distribution of earthquake shaking and are commonly used to characterize the severity of felt earthquakes. The macroseismic studies are especially helpful in seismically active regions where seismic instrumentation is distributed too sparsely in order to provide a reliable spatial coverage. The 2002 Denali Fault (DF) earthquake in Alaska, for example, was recorded at 55 locations by strong-motion instruments, yet only a few of these locations were relatively close to the fault rupture. On the other hand, the US Geological Survey (USGS) obtained more than 3,500 intensity reports for the same earthquake.
Isoseismal maps of past earthquakes are usually based on the historical evidence and damage reports. For present earthquakes the macroseismic surveys are conducted by means of postal questionnaires or actual field surveys. The Internet has provided seismologists with new tools for carrying out macroseismic surveys. The USGS Did You Feel It? web tool makes it possible to collect data from the Internet users immediately after earthquakes and to automatically generate publicly available Community Internet Intensity (CII) maps within hours.
The Modified Mercalli Intensity (MMI) scale (Wood and Neumann, 1931) is one of the intensity scales widely used in many countries, including the US. There are 12 levels in this scale that are noted by Roman numerals from I to XII. In general, MMI depends on numerous factors, including the earthquake magnitude, epicentral distance, local soil conditions and building characteristics. Empirical relationships have been developed that relate the intensity to the earthquake magnitude and epicentral distance, and to the ground-motion parameters, such as peak accelerations and velocities. It is also well established that the attenuation of intensities with the distance varies in different tectonic environments (Bakun and McGarr, 2002).
The MW7.9, November 3, 2002 DF earthquake was the strongest event ever recorded in the interior of Alaska, where it generated a 340-km long surface rupture (Eberhart-Phillips et al., 2003) (Figure 1). The earthquake was digitally recorded at more than 55 strong-motion sites throughout the state at distances up to 280 km from the fault rupture (Martirosyan et al., 2004). At four sites, situated within 60 kilometers from the rupture, the observed PGA values were considerably smaller than those predicted by the ground-motion prediction equations. The closest site to the fault was Pump Station 10 (PS10) of the Trans-Alaska Pipeline System (TAPS), located less than 3 km north of the Denali fault and 85 km east of the epicenter (Figure 1), where the recorded maximum horizontal peak ground acceleration (PGA) was 0.35 g.
Figure 1. Map of the study area. Population points are shown on the background of main roads. Green triangles are the strong-motion sites that recorded the DF earthquake (Click on Figure to see larger version).
The USGS CII dataset for the DF earthquake contains a total of 3,467 reports from 238 zip codes (including from other states) with the maximum reported intensity of VIII. The full statistics and the corresponding intensity map are available from the USGS web site. In the CII methodology each zip code is characterized by a single intensity value; a particular problem with this method in case of the DF earthquake is that most of the zip codes in Alaska cover very large geographic areas due to sparse population distribution. For example, the zip code 99586 (Gakona) has an area of 122x122 km2 and a total population of 328. Overall, nine intensity reports were collected from this zip code for the DF earthquake. Considering that the surface rupture passes through this area, it is clear that the zip-code based mapping of intensities is inadequate for this and other similar areas.
This report provides the results of the macroseismic survey conducted by the Geophysical Institute (GI) of the University of Alaska Fairbanks in late 2003. The survey was aimed to collect additional macroseismic data in the near-field zone of the DF earthquake that was not well represented in the USGS dataset. Such data were collected at more than 99 locations. The two datasets (GI and USGS) were subsequently combined to produce an isoseismal map of the DF earthquake.
METHODOLOGY
We followed the CII methodology (Dengler and Dewey, 1998; Wald et al., 1999) for compatibility with the USGS DF earthquake dataset. The intensities are determined based on an online questionnaire of 18 multiple choice questions, each question addressing a specific aspect of earthquake effects. When a response to the questionnaire is received, the answers are numerically evaluated, and the mean value for each question is calculated from all the responses in the corresponding zip code. Next, a weighted sum is determined for each zip code, which is then transformed to a CII value by a conversion formula (Wald et al., 1999). It is important to note that the CII online questionnaire is somewhat different from the standard postal questionnaire developed for MMI surveys; however, the intensity calculation algorithm is adjusted for comparability with the MMI. The paper version of the questionnaire used in the current survey is provided in Appendix A.
The survey was conducted mostly in the form of personal interviews in the community centers, local businesses (lodges, shops, B&B, etc) and other places. The online Alaska Community Database (http://www.dced.state.ak.us/dca/commdb/CF_COMDB.htm) was helpful for obtaining local contact information. The survey was anonymous in compliance with the requirements of the University of Alaska for the projects involving human participants. The purpose of the survey was explained to the subjects using the following phrasing: The University of Alaska Fairbanks is undertaking a survey to help determine the shaking intensity of the Denali Fault earthquake. You are being asked to participate because you live in an area that was affected by this earthquake. This survey will take approximately 5-10 minutes of your time and will be completely anonymous. By filling out this survey you are agreeing to participate in this study. You are under no obligation to participate and you may stop your participation at any time.
The survey was resumed in two field trips in late October - early November of 2003. The main effort was concentrated on a triangle region bordered by three highways: Richardson, Alaska, and Tok-Cutoff (Figure 1). Special attention was given to the sections of the Richardson and Tok-Cutoff Highways adjacent to the fault crossings. An additional postal survey was mailed to certain residential addresses in the communities close to the fault rupture. Overall, the intensity values were obtained at 99 locations, shown in Figure 2 as circles. The geographic coordinates of the survey points were recorded by a handheld GPS unit. Pictures of selected survey locations, housing types, and earthquake damage photos can be found here.