Mercury tiltmeter as an infrasonic detector: Theory, observations, and applications

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Observations of acoustic gravity waves at Eilat by a mercury tiltmeter are reported and interpreted. Waves belonging to the first gravity mode (GR0) and three acoustic modes (S0, S1, S2) that originated at the Chinese nuclear air blast of October 14, 1970, were simultaneously recorded on infrasonic microbarographs and the newly installed MIT (Massachusetts Institute of Technology) tiltmeters. The explosion (yield, about 5 Mt) produced at a distance of 5024 km a surface overpressure of about 300 μb at 240 sec. This overpressure in turn caused a horizontal displacement of 3 μ and a tilt of 3 × 10−10 rad at the same period. Plane-wave theory is used to show that the incident sound wave can excite a surface wave at the recording site through a second-order coupling effect. The induced waves propagate in the earth with the sound phase velocity and decay exponentially with depth. This theory is in good agreement with the observations. It is shown that the surface sound pressure and the induced ground motion are tied by a simple relation from which the mean rigidity of the upper crust at the recording site can be derived. The local rigidity of the granitic upper crust at Eilat is thus found to be 3.22 × 1011 dynes/cm2. Sensitivity of the tiltmeter to infrasonic waves in the period range 20–400 sec is estimated to be 10−12 rad/μb at zero site noise and electronic snr ratio of 1:1, with a detectability threshold at about 20 μb.