Abstract

Passive image interferometry is applied to monitor seismic velocity changes in Paradox Valley (southwestern Colorado). For this purpose, we analyzed short‐period vertical‐component continuous recordings of seismic ambient noise from the Paradox Valley seismic network from January 2011 to March 2012. Continuous, high‐pressure fluid injection activities at depths of about 4.8 km are occurring in this region. We computed the autocorrelation functions using band‐pass‐filtered data in the frequency ranges 0.2–0.5, 0.5–1, 1–2, and 2–8 Hz. The relative perturbations of the background seismic velocity were estimated using the stretching technique and the moving window cross‐spectral analysis assuming a uniform velocity change in space. A clear seasonal trend of the relative velocity variations is observed, with the higher amplitudes corresponding to the lower frequency bands. The long‐term variations of the relative velocity at low frequencies show an annual period that peaks in winter. For higher frequencies, semiannual seasonal variations are observed. A comparison of our results with the horizontal differential displacements for two pair of Global Positioning System stations, meteorological time series, and water‐table measurements allows us to conclude that hydrologic loading can explain a large fraction of the observed long‐term relative velocity variations in the area. No correlation is observed between the short‐term relative velocity variations and the cumulative number of induced earthquakes in the region, nor with the average brine injection rates. More high‐quality data from a denser network would be necessary to make a detailed interpretation of the short‐term variations in relation to the fluid injection activity in the Paradox Valley region.

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