Measurement and interpretation of tidal tilts in a small array

Abstract

Strain‐induced tilts caused by variations in material properties can produce large perturbations in the tilt field near a material discontinuity. This is evidenced in data from a closely spaced array of borehole tiltmeters that has been operating at Piñon Flat Geophysical Observatory in southern California since 1987. The array is composed of three borehole tiltmeters located at depths of 24, 36, and 120 m. The tiltmeters have a sensitivity of a few nanoradians. Tidal measurements show differences between the boreholes of up to 50%. The largest percent difference occurs in the semidiurnal band where the tilt field measured in the 36‐m borehole differs by ∼10 nrad at the M2 frequency. This difference has been modeled as a strain‐induced tilt caused by the weathering layer.