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Figure. This map shows the locations of strainmeters that are installed along the San Andreas fault near Parkfield, California. The strainmeters are of two types. Dilatometers that measure changes in volume strain near the fault, and tensor strainmeters that measure horizontal strain in three nonparallel directions. The advantage of the tensor strainmeter is that the amplitudes and directions of the largest and least strains, the shear strains and other strain components, can be calculated. The locations of these stranmeters are shown in relation to the San Andreas fault and the location of the 1966 M6 Parkfield earthquake.
All data are transmitted by satellite to the USGS headquarters in Menlo Park, Ca. Data samples are taken every 10 minutes.
Plots of the changes in strain as recorded on each of these strainmeters are shown. Removal of re-zeros, offsets, problems with telemetry and identification of instrument failures is difficult, tedious and timeconsuming task. In order to have a relatively up-to-date file of data computer algorithms have been written that accomplishe most of these tasks most of the time. Detailed discussion or detailed analysis usually requires hand checking of the data. Flat sections in the data usually denote a failure in the telemetry Gaps denote missing data. All instruments are scaled using tidally generated scale factors.
For any event of interest the 'raw' data are examined to determine the validity of the 'cleaned' data. Tidal prediction, filtering, fourier analysis, power spectra, editing and many other programs are used to analyse these data.
