High-frequency seismic noise at Lajitas, Texas

Abstract

A preliminary analysis of high-frequency (1 to 20 Hz) ambient seismic noise at a very quiet site in southwest Texas is presented. At frequencies greater than 1 Hz, the displacement noise power decreases at a rate between f⁻⁴ and f⁻⁵. The ambient seismic noise is also characterized by persistent narrow-band peaks (Δf < 0.2 Hz) of unknown origin. The noise appears to be propagating at phase velocities 2.5 to 4 km/sec or greater which suggests higher mode surface or body waves. Estimates of coherence and degree of polarization of three-component array data indicate that the high-frequency ambient noise is relatively well organized over distances of at least 600 to 700 m for a single component but is relatively unorganized in a three-component sense. A principal result of this preliminary analysis is that the use of three-component, high-frequency (1 to 20 Hz) arrays at very quiet sites, coupled with three-component processors, may result in substantial improvement of the thresholds for detection and discrimination of weak seismic events.