Seismic Velocity Changes Caused by Water Table Fluctuation in the New Madrid Seismic Zone and Mississippi Embayment

Abstract

AbstractWe use cross correlation of the ambient seismic field to estimate seasonal variations of seismic velocity in the Mississippi Embayment and to determine the underlying physical mechanisms. Our main observation is that the δt/t variations correlate primarily with the water table fluctuation, with the largest positive value from May to July and the largest negative value in September/October relative to the annual mean. The δv/v residuals after water level fluctuation correction correlate with air pressure in the short term and follow the trend of temperature in the long term. The δv/v residuals after water level fluctuation and air pressure correction correlate inversely with the wind speed. The correlation coefficients between water table fluctuation and δt/t are independent of the interstation distance and frequency, but high coefficients are observed more often between 0.3 and 1 Hz than between 1 and 2 Hz because high‐frequency coherent signals attenuate faster than low‐frequency ones. The δt/t variations lag behind the water table fluctuation by about 20 days, which suggests that the velocity changes can be attributed to the pore pressure diffusion effect. The seasonal variations of δt/t are azimuthally independent, and a large increase of noise amplitude only introduces a small increase to the δt/t variation. At close distances, the maximum δt/t holds a wide range of values, which is likely related to local structure. At larger distances, velocity variations sample a larger region so that it stabilizes to a more uniform value. We find that the observed changes in wave speed can be explained by a poroelastic model.