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Abstract:
The ICDP project "Drilling the Eger Rift" focuses on geodynamic processes in W-Bohemia and Vogtland, such as earthquakes and subsurface fluid flows. Therefore, three boreholes have been drilled and will be instrumented with 3D seismic arrays. The pilot 3D array has 10 borehole geophones with a 10 Hz corner frequency and is surrounded by a surface array with 12 4.5 Hz geophones. The data is recorded by Earth Data Loggers with a sampling rate of 1000 Hz.
The goal is to locate seismic noise sources up to a distance of 15 km that may be linked to fluid migration in W-Bohemia. We analyze continuous seismic noise records of more than 20 stations from regional networks and our seismic array. Differential PSDs were calculated in different frequency ranges to find continuous tremor-like seismic sources before the earthquake swarm in April 2021. Our next step is to locate these sources using regional networks and the 3D seismic array.
The analysis revealed an increase in seismic noise between 4-8 Hz for a 2-hour period on April 5th, 2021, one day before the earthquake swarm started. Seven stations in the region show a median differential PSD level at least three times higher than the differential PSD level for the entire day. These stations are located up to a distance of 12 km to the Nový Kostel focal zone, where most of the earthquakes occurred. The increase is also compared to Radon concentrations measured at Hartoušov (W-Bohemia) to find possible coincidence.
