A Comprehensive Study of Long-Period (20 to 200 Seconds) Earth Noise at the High-Gain Worldwide Seismograph stations.

摘要

The characteristics of long-period (20 to 200 sec) earth noise were investigated at the 11 high-gain, long-period (HGLP) seismograph stations using as many as 3 years of analog seismograms and digital data. This study provides a significant increase in the number of long-period earth-noise spectra that are available from areas of vastly differing geological and tectonic settings outside the contiguous U.S. This information is important to the study of the long-period portion of the spectra of earthquakes and to discriminating earthquakes from underground explosions. A significant feature of earth-noise spectra, based on data from all the HGLP station, is a minimum between about 25 and 45 sec. This minimum spectra level is most pronounced at sites with more than 200 m of overburden. The level of earth noise generally recorded by vertical-component seismographs at both shallow and deep stations is independent of local meteorological conditions if the seismographs are located on competent bedrock and are subject to rigid environmental controls. The level of earth noise recorded by horizontal component seismographs at HGLP stations, however, is dependent on local meteorological conditions. This dependency decreases with increasing depth of overburden. The decay rate with depth of this horizontal earth noise is such that approximately 150 m of competent overburden is usually sufficient to attenuate about 90% of the earth noise caused by local atmospheric conditions.