Sacks-Evertson Strainmeter, Its Installation in Japan and Some Preliminary Results Concerning Strain Steps

Abstract

A liquid-filled resilient cylinder is placed in a borehole and intimately coupled to the surrounding rock. When the strain in the rock changes, liquid is displaced, contracting or extending a thin-walled bellows, into a gas-filled volume, the pressure of which is substantially independent of the rock strain changes. The motion of this bellows is transmitted to a differential transformer as well as to a bimorph-type Piezo-electric crystal. The frequency response of the bimorph sensor is flat from 1 cps down to about a twenty minute period and at lower frequencies is strain rate sensitive. The output from the differential transformer is of course strain sensitive at all frequencies. The operational sensitivities are 2 × 10^-9/mm for the differential transformer and 3 × 10^-10/mm for the bimorph on the recorder (Figs.1 and 2).
Three strainmeters have been installed in the Matsushiro region in central Honshu, Japan, which has a very high seismicity. Two of the instruments are only 300 meters apart, and the third one is 15 kilometers away from the pair. A number of examples of strain steps have been recorded by the 100 m quartz-bar extensometer in the Matsushiro Seismological Observatory. It appears that a number of the steps on the extensometer are probably spurious, for the new strainmeter showed no such steps or much smaller steps (Table 3).
One of the fundamental features of th e strainmeter is its invulnerability to high accelerations which are likely to be encountered during local earthquakes. To test the performance, the strainmeter together with its surrounding rock was subjected to accelerations as high as a few hundred gals, using explosives. No spurious behaviour was detected.