Measurement of fault displacement by optical parallax

Abstract

Two pairs of lights are being photographed with a camera of 10–meter focal length, one pair on each side of the transform fault between Angel de la Guarda Island and the Baja California peninsula, which is a part of the East Pacific rise. During the winter the average of about 150 photographs defines the angle between the near and far lights with a probable error of 0.01 arc sec (5 × 10−8 rad), which is the customary precision astronomers get for parallaxes of stars with long-focus photographic telescopes. This corresponds to detection with 95% confidence of a 4–mm fault displacement at the distance of the far station, 25 km from the camera, or 0.07 of the annual spreading rate of the gulf averaged over the last 5 m.y. Between March 1970 and March 1972 neither displacement nor strain accumulation has been detected; this result can be interpreted as the locking of the fault in a thick lithosphere. The lights, powered by small solar cell panels, need no attention and are ideal for remote mountain peaks. The camera is automatic and could be serviced once a month. The method can continuously monitor earth displacement in inaccessible terrain better than the geodimeter because a great number of measurements can be made at low cost.