A portable water-tube tiltmeter

Abstract The accumulation of magma within a volcano deforms the ground surface around it. Although pendulum-type tiltmeters are our most sensitive and convenient tool for detecting such deformations, their records are generally difficult to interpret because purely local disturbances obscure the subtle changes produced by the volcano. A new tiltmeter consisting of a permanent tilt base and a portable water-tube leveling system was developed to measure tilting around Kilauea caldera. Because the tilt bases for this instrument are on the ground surface outdoors, the leveling system must be operated under conditions that are far from ideal. If the work is performed at night and if a carefully standardized procedure is followed, leveling can be carried around a circuit consisting of three piers at the vertices of a triangle 2,000 inches on a side with a closure error of less than 10µ (corresponding to an error in the measurement of tilt of less than 0.2 ՠ10-6 radian). Tilting at four new tilt bases around Kilauea caldera between October, 1958, and February, 1959, shows that the summit of the volcano is swelling. An analysis of the tilting around the caldera suggests that magma is accumulating in a reservoir about 4 kilometers beneath the southwest end of the caldera.