Interaction of Air Pressure and Groundwater as Main Cause of Sub-Daily Relative Seismic Velocity Changes

Abstract

Our study investigates variations of seismic velocity that occur over short time scales of hours. We use coda wave interferometry to inspect 5 months of continuous data from a seismological array in southern Germany. This results in relative seismic velocities (dv/v) that show temporal variations on the order of 10−4. Spectra of the velocity time series contain strong daily and sub-daily behavior indicating that the daily and sub-daily changes in the seismic velocity are primarily caused by atmospheric tides. We also note the influence of temperature changes on daily variations, but as a second-order effect. The explanatory model focuses on depth variations of the groundwater table, linking atmospheric pressure (loading and de-loading the Earth's surface) to variations in seismic velocity. Our results highlight an important environmental influence on seismic velocity that needs to be considered before seismic velocity variations can be used for inspecting fluid and stress variations in situ.