The November 25, 1988 Saguenay Earthquake: Isoseismal Survey In Ithaca Region, Central New York

Abstract

The Saguenay, Quebec Earthquake of November 25, 1988 was widely felt in southeast Canada and in northeast U.S. (Figure 1a). In Ithaca, New York, 795 km southwest of the epicenter, reports of the intensity of shaking caused by this earthquake varied considerably. Intrigued by these variations, we took advantage of this rare event to construct an isoseismal map of the Ithaca area. The results of our study, detailed below, provide a good example of the variability in the intensity of seismic shaking associated with areas of variable surface topography and lithology. The Saguenay earthquake was an unusually deep crustal event (28 km) of moderate magnitude (M = 6) (Gariel and Jacob, 1989; Basham and Adams, 1989). Its focal mechanism indicates thrusting on north-to-northwest trending, intermediate-dipping planes (Wetmiller et al., 1989). The epicenter was near structures related to a failed Paleozoic rift within the Grenville Basement (Adams and Basham, 1989). Felt areas were reported as far (1100 km) as Washington, D.C. (N.Y. Times, November 26, 1988). That the felt area was exceptionally large for an earthquake of this magnitude was not surprising. It has been previously noted (Nuttli, 1973; Nuttli and Zollweg, 1974) that earthquakes in the eastern United States have much larger felt areas than earthquakes of the same magnitude in the western U.S. Nuttli (1973) reports, for example, that the seismic quality factor, Q, for 1-sec waves is 10 times greater in the east than in the west. Questionnaires were printed in a local newspaper and circulated through Cornell . . .