Earth-strain observations made with the Cambridge laser strainmeter

A span of 74 days of data from the 55-m iodine stabilized laser strainmeter in Queensbury tunnel, Yorkshire, England has been analysed in detail. The strain noise level observed by the instrument is similar to that observed by American laser strainmeters at low (tidal) frequencies, but is much greater at higher (seismic) frequencies. This is explained in terms of ocean loading and microseismic noise. A residual time series was obtained by subtracting a best-fit tide from the data, and the larger residuals correlate well with storm surges in the North Sea; this is a further manifestation of ocean loading. The high noise level at seismic frequencies renders the Queensbury site rather unsuitable for normal mode studies, but free oscillations have been observed following an Ms 7.6 earthquake.