Site-Dependent Ground Motions from Distant Earthquakes. Revised.
Abstract
This study examines geological and seismological reasons for patterns of guidance alarms in Minuteman Wing V due to the 1975 Pocatello Valley, Idaho and 1979 St. Elias, Alaska earthquakes. Two features are identified which together could cause these patterns of alarms. First is the existence of lightly-damped missile suspension system modes in the period window from 2 to 6 seconds. Second is a 2 to 3 km sedimentary layer underlying the wing, with natural frequencies in this critical period window. Near-vertically-incident P and S seismic body waves which propagate directly from the source through the crust and upper mantle can interact with this structure. A Haskell-Thomson program for body wave propagation in a layered model indicates that peak ground shaking can vary by a factor of 2 at adjacent Wing V flights. Differences are due to variations in sediment thicknesses and wavespeeds, deduced from oil well log and laboratory data. Therefore Wing V alarm patterns appear to be caused by the coincidence of three factors: 1) high gain of the 2 to 6 second suspension system modes; 2) incident body waves in this period range; and 3) sedimentary column resonance periods close to the suspension system natural periods. If the first two are sufficient to trigger alarms, then the third would cause anomalous patterns due to relative amplification between flights.
Document Details
- Document Type
- Technical Report
- Publication Date
- Dec 01, 1980
- Accession Number
- ADA107667
Entities
People
- G. L. Wojcik
- J. Isenberg
- W. S. Dunbar