We have measured changes in water pressure as a function of shear displacement and time during undrained direct shear of flat joint surfaces in saturated quartz monzonite. During the postpeak shear behavior, the water pressure generally decreases, probably owing to opening of new cracks and enlarging of some of the existing cracks and pores. During the stick part of the stick-slip event, there is an accelerated drop in water pressure (preseismic) followed by a momentary rise during the slip (coseismic).

If it is accepted that the slip part of the stick-slip event is a “laboratory earthquake,” then the pore-pressure variations during the stick-slip may have a direct bearing on earthquake prediction through water-level observations in open wells.

It is hereby suggested that for more meaningful measurements of water-pressure variations in wells, pressure transducers with a quick response factor be used with packers, rather than measuring the water-level variations in open wells with appreciable time lag.

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