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Structures in earth noise beyond twenty seconds—a window for earthquakes

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964

Abstract

The nature of earth noise in the period range 20 to 130 sec has been investigated at the deep (543 m) mine observatory and at a temporary surface installation at Ogdensburg, New Jersey, and its structure, in part, explained. The earth noise spectrum displays a pronounced and stable minimum between 30 and 40 sec, thought to be a transition between the swell-generated microseisms at periods less than 30 sec and nonpropagating ground motion of atmospheric origin at periods greater than 40 sec. We estimate the yearly mean amplitude at 35 sec to be 0.5 mµ/mHz, which is 100 times smaller than estimates of the mean amplitude of the 5- to 9-sec microseisms. At periods greater than 40 sec, the noise spectrum rises at 12 to 14 db/oct. Above 30 sec, the earth noise is quite stable, varying by no more than a factor of 3 over a year with higher levels in the winter and lower levels in the summer. By contrast, microseisms with periods less than 20 sec can change by a factor of 10 to 100 in a few days. The shape and amplitude of the earth noise spectrum beyond 20 sec, as well as its attenuation with depth, can be explained by a model of atmospheric pressure cells loading the Earth on equal-area sectors. There is a secondary source of background disturbance—the long-period coda of earthquakes. These higher-mode free oscillations are excited by relatively small events (m_b = 5.8) and can dominate the background for as much as 10 per cent of the time during active periods.

The structure and stability of the earth noise spectrum at these longer periods allows an important improvement in seismometer design. By shaping the response of the three-component set of long-period high-gain seismographs to resemble the inverse of the mean noise spectrum, the instruments record approximately 10 times as many long-period body and surface waves as the standard long-period seismographs. The elevated signal-to-noise ratio in the 30- to 40-sec window decidedly improves the sensitivity of seismic source-type discriminants.

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