Systematic monitoring of millisecond travel time variations near Palmdale, California

Abstract

Repeated measurements of P wave travel time changes have been completed near the San Andreas fault, 25 km west of Palmdale, California. Spanning a 2‐year period from June 1974, these observations were made on 0.1‐ to 8.5‐km base lines and have a worst case accuracy of ±4 ms. The observation area was centered around Bouquet Reservoir where the P wave source, a marine air gun, was secured. Our data were taken approximately every 5 days during June to July 1974, daily during November 1974 to January 1975, and every 2 hours during March to June 1976. Although the area has undergone regional strain at the rate of ∼2·10−7 yr−1 during this period, no net P wave travel time change greater than 1 part in 103 accumulated. We observed instead systematic and continuous arrival time fluctuations that lasted from hours to days and ranged from 1 to 10 ms (Δt/t ≃ 10−3‐10−2). Earth tides and temperature variations, which can produce some signal (Δt/t ∼ 10−3) at diurnal and semidiurnal periods, are at most partially responsible for the observations. We interpret the results in terms of P wave velocity variations in a waterfilled cracked medium in which velocity changes are due to the opening of microcracks during incremental strain. The time scale of anomalous velocity behavior is constrained to be less than the diffusion time of water into newly opened cracks. In this regime little prospect exists of detecting long‐term earthquake precursors.