Site Correction for Long Period Seismometers, Tiltmeters and Strainmeters

Long-period horizontal seismometers, tiltmeters and strainmeters are influenced by inhomogeneous rock properties in the site region. These inhomogeneities interact with applied stress fields to create local strains and tilts that differ from those averaged over greater distances. These effects have been called strain-strain and strain-tilt coupling. For each instrument, the coupling is defined by three coefficients, so that for observations at a particular point a correction may be defined using a third-order tensor for seismometers and tiltmeters, and a fourth-order tensor for strainmeters. These tensors may be evaluated using instrumental data alone, either at tidal periods, or at seismic periods using properties of Rayleigh waves.