A Portable Strain M E T E R with Continuous Interferos~etric Calibration by J. L. Blayney and Ralph Gilman

A strain meter has been designed embodying techniques not used in earlier models. Minimum use of heavy and permanent auxiliary fixtures has made possible a portable instrument in which no components need be abandoned in order to change location. Provision for frequent or continuous interferometric calibration, and a photographic record thereof, has provided uninterrupted strain measurements with long-term dependability. Also included is the usual continuous low-speed ink recorder for tidal and secular variations, and a highgain band-pass recorder for the observation of long period waves and free oscillations of the earth. Details are given describing the method used for interpreting the interferometer image and its conversion to strain measurement. INTRODUCTION Since a simplified design aud a greatly reduced cost of a linear strain meter (Benioff, 1959, 1963) would make possible the proliferation of such instruments over many active regions, the Seismological Laboratory of the California Institute of Technology has placed in operation a strain-measuring instrument of semi-portable design. Through the generous cooperation of the U. S. Coast and Geodetic Survey, space was made available for this installation in the Kipapa tunnel, site of the World Wide Standard Seismograph Station in Hawaii. This tunnel, driven into the side of Kipapa Gulch I00 feet below the rim, is located on the island of Oahu in a region of weathered basalt. The strain tube is 24.4 meters long, in a direction 61.1 ° west of north and is placed toward the far end of the 220 foot tunnel. In addition to observations of long waves and free oscillations, measurements of mid-oceanic earth tides and a possible correlation between secular strain on Oahu and movements of the magma beneath the island of Hawaii will be of particular interest. Although the fused quartz tube is relatively long and is located in a dry tunnel, features which do not lend themselves to portability nor to an arbitrary choice of sites, the design was planned along lines anticipating a possible shortening of the tube and eventual elimination of the tunnel requirement. Alternate locations may include the basement of an existing building, or a shallow excavation covered by an insulating layer of earth. STRAIN ROD One important change from earlier designs is in the technique for suspending the quartz tube. Rather than hanging the tubing with stainless steel wire from the walls of a concrete channel, a double-hinged support anchored to the floor directly beneath the quartz contains a self-locking clamp to hold the quartz tube firmly to the upper part of the hinge. This inverted pendulum approach was first proposed in 1951 by F. E. Lehner of the Seismological Laboratory staff. Figure 1 shows the hinge and