Inertial and strain seismometers

There are two basic types of seismic sensors: inertial seismometers which measure ground motion relative to an inertial reference (a suspended mass), and strainmeters or extensometers which measure the motion of one point of the ground relative to another. The wavelength of seismic waves is so large that the differential motion of the ground within a vault is normally much smaller than the motion relative to an inertial reference; strainmeters are therefore generally less sensitive to earthquake signals. However, at very low frequencies it becomes increasingly difficult to maintain an inertial reference, and for the observation of low-order free oscillations of the earth and tidal signals, strainmeters may outperform inertial seismometers. In the presence of gravity, inertial seismometers with a horizontal sensitive axis also respond to tilt, and the better ones are more sensitive to short-term tilt than the majority of dedicated tiltmeters (although inferior in their long-term stability). The principles of operation of horizontal seismometers and tiltmeters are identical. Instruments measuring the angular acceleration, although theoretically required for a complete description of ground motion, have not attained any significance in seismology because the rotational component of seismic signals is in general too small to be directly observed. (Even if the signals were strong enough, existing mechanical sensors would not be able to separate them from the associated large displacements.) We will treat only inertial seismometers in this article. Their theory will be presented as far as it is required for an understanding of specifications, calibration procedures, and operational requirements.

In contrast to most other sensors, inertial seismometers have an inherently frequency-dependent response that must be taken into account when the ground motion is restored from the recorded signal. This is because a suspended mass does not represent a perfect inertial reference. When the ground motion is slow, the mass will begin to follow it, and the output signal for a given ground displacement will therefore diminish. The mechanical system forms a high-pass filter for the ground displacement. Recorders, on the other hand, normally have a constant gain up to some upper cutoff frequency, and contribute only a scale factor to the overall response. We will therefore not discuss their frequency response in detail.