Accession Number : ADA119180

Title :   Spectral Analyses of High-Frequency Pn/Sn Phases from Very Shallow Focus Earthquakes.

Descriptive Note : Technical rept. 3 Mar 81-2 Mar 82,


Personal Author(s) : Walker,Daniel A

PDF Url : ADA119180

Report Date : May 1982

Pagination or Media Count : 56

Abstract : Total rebuilding of the recording system, rather than planned improvements merely in the filter amplifiers, was necessitated by flooding of the station as a result of typhoon Freda in March of 1981. Improvements included quieter amplifiers greater fidelity produced by improved speed control on the tape drive motors, and an increased dynamic range. The rebuilt system was installed in July of 1981. Since that time numerous seismic phases have been recorded, many being high-frequency Pn/Sn and some being the normal, mantle-refracted P from earthquakes and explosions. A major software advancement was the development of a program that would determine the mean composite spectrum and standard deviations from a number of individual spectrums. No evidence was found for the normal, mantle-refracted P phases at their predicted arrival times within the Pn codes of events at distances of less than 21 degrees, further emphasizing the importance of high-frequency Pn phases in monitoring earthquakes or explosions in an oceanic environment. The levels of noise on the deep ocean bottom were found to be comparable to those found at the quietest continental sites. Explosions were found to have less energy at frequencies below 1.5 Hz and more energy at frequencies above 2 Hz than shallow focus earthquakes of comparable epicentral distance and magnitude.

Descriptors :   *Seismic waves, Pacific Ocean, Earthquakes, Nuclear explosions, Underground explosions, Primary waves(Seismic waves), Secondary waves, High frequency, Spectrum analysis

Subject Categories : Seismology
      Seismic Detection and Detectors

Distribution Statement : APPROVED FOR PUBLIC RELEASE