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Long-Period Seismological Research Program
LAMONT-DOHERTY GEOLOGICAL OBSERVATORY PALISADES NY
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Four long-period, high-gain, three-component seismograph systems were designed, constructed and installed in Alaska, Australia, Israel, and Thailand. A fifth station is being installed in Spain. These instruments have peak response of 500,000 at periods of 35 to 45 seconds. This high sensitivity is achieved by isolating the seismometer from barometric changes, by electronically filtering out the six-second mocroseisms and by shaping the instrument response to correlate with a natural minimum in the earth-to-noise spectrum. Data from Alaska, Australia, and the prototype instrument in New Jersey are sufficient for detecting Rayleigh waves from all earthquakes with a body wave magnitude of from 3.8 at distances of about 0 to 15 degrees to 4.7 at distances greater than 100 degrees. The minimum detectable magnitude is also a function of geographic location. A minimum is found in the spectral amplitude of earth noise between 30 to 40 seconds at all sites. The lowest level of the noise was observed in Thailand. G waves and inversely dispersed Rayleigh waves are commonly observed for events as small as Mb approximately equal to 5. Free oscillations lasting for over 24 hours were observed following an event of magnitude 7.3.
APPROVED FOR PUBLIC RELEASE