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

Abstract

We 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.

Impacts of Reservoir Water Level Fluctuation on Measuring Seasonal Seismic Travel Time Changes in the Binchuan Basin, Yunnan, China

“…We observe that major precipitation (Figure 12c) occurs at day 200 in the summer. Seismic travel time changes increase to a local maximum at day 230 and lag behind the maximum precipitation for about one month, which is similar to the observation in [12].…”

“…To investigate if the pore pressure affects seismic velocity change, we use a poroelastic model to estimate seasonal variation of seismic travel time in 2016. The authors of [63] proposed a solution for the one-dimension fully coupled diffusion equation, which was used for understanding seismic travel time changes [12,59,64]. Assuming water diffuses in the vertical direction with constant hydraulic diffusivity, we can compute the pore pressure changes P(h, T) at h kilometer depth from the surface on day T as:…”

“…The depth h is set to 500 m, which is the maximum depth sensitivity of surface waves in the frequency ranges of 2-6 Hz. A mathematical transfer function from [12,59,64] to compute a synthetic seismic travel time change: δt t syn (T) = δt t (T) + cov δt t (T), P(T) var(P(T))…”

“…Seismic velocity changes are usually used as a proxy to quantitatively understand crustal deformation. Temporal changes in seismic velocity can reflect fault zone co-seismic damage and post-seismic healing [6][7][8], volcanic eruption [6,9], groundwater level changes [10][11][12], temperature and atmospheric pressure variations [13][14][15][16][17][18], and solid earth and oceanic tidal deformation [17,[19][20][21]. Thus, high temporal resolution monitoring and high precision measurements of seismic velocity changes are necessary.…”

“…Seismic velocity changes have been estimated through measuring travel time or phase difference from active sources including explosion [21][22][23][24][25], electronic hammer [26], airguns [18,27,28], repeating earthquakes [29][30][31][32][33][34], and dephasing of ambient noise crosscorrelations (CCs) [6,35]. Among these methods, temporal changes in ambient noise amplitude and heterogeneous distribution of noise sources [12,17] may introduce a bias to the measurements of seismic velocity changes. The uncertainties in locations of repeating earthquakes might cause concern to the reliability of the measurements of seismic velocity changes.…”