Creep-Enhanced Low-Frequency Sensitivity of Seismometers
  The frequency response of a seismometer is typically assumed to be the textbook case of a viscous damped, simple harmonic oscillator. Real mechanical oscillators are not ideal, and the damping at low frequencies, due to internal friction, is presently too poorly understood to describe from first principles. Even if the low-level motions were smooth (which they are not), the mean position of a seismic mass changes because of creep and creep recovery. This article shows that secondary creep can actually serve to increase the sensitivity of a seismometer at low frequencies. 
Randall D. Peters