ALBERT

Description: The network consists of a vertical borehole array equipped with 3C sensors (geophones) for the analysis of swarm earthquakes in the Western Bohemia / Vogtland area located in the German/Czech border region. A surface array is completing the 3D observation of the wave field with 3C sensors (geophones). Waveform data is available from the GEOFON data centre, under network code 6A, and is embargoed until FEB 2035.

Description: The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and multiparameter observation of earthquake swarms and related phenomena, such a laboratory comprises a set of shallow boreholes with high-frequency 3-D seismic arrays as well as modern continuous real-time fluid monitoring at depth and the study of the deep biosphere. This laboratory is located in the western part of the Eger Rift at the border of the Czech Republic and Germany (in the West Bohemia–Vogtland geodynamic region) and comprises a set of five boreholes around the seismoactive zone. To date, all monitoring boreholes have been drilled. This includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline units north and east of the major Nový Kostel seismogenic zone, borehole F3 in the Hartoušov mofette field and borehole S4 in the newly discovered Bažina maar near Libá. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological research. At each of these sites, a borehole broadband seismometer will be installed, and sites S1, S2 and S3 will also host a 3-D seismic array composed of a vertical geophone chain and surface seismic array. Seismic instrumenting has been completed in the S1 borehole and is in preparation in the remaining four monitoring boreholes. The continuous fluid monitoring site of Hartoušov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.

Description: 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.

Description: Within the framework of the Intercontinental Scientific Drilling Programme (ICDP) ‘Drilling the Eger Rift’ project, five boreholes were drilled in the Vogtland (Germany) and West Bohemia (Czech Republic) regions. Three of them will be used to install high-frequency three-dimensional (3D) seismic arrays. The pilot 3D array is located 1.5 km south of Landwüst (Vogtland). The borehole, with a depth of 402 m, was equipped with eight geophones and a fibre optic cable behind the casing used for distributed acoustic sensing (DAS) measurements. The borehole is surrounded by a surface array consisting of 12 seismic stations with an aperture of 400 m. During drilling, a highly fractured zone was detected between 90 m and 165 m depth and interpreted as a possible fault zone. To characterize the fault zone, two vertical seismic profiling (VSP) experiments with drop weight sources at the surface were conducted. The aim of the VSP experiments was to estimate a local 3D seismic velocity tomography including the imaging of the steep fault zone. Our 3D tomography indicates P-wave velocities between 1500 m/s and 3000 m/s at shallow depths (0–20 m) and higher P-wave velocities of up to 5000 m/s at greater depths. In addition, the results suggest a NW–SE striking low-velocity zone (LVZ; characterized by = 1500–3000 m/s), which crosses the borehole at a depth of about 90–165 m. This LVZ is inferred to be a shallow non-tectonic, steep fault zone with a dip angle of about . The depth and width of the fault zone are supported by logging data as electrical conductivity, core recovery and changes in lithology. In this study, we present an example to test and verify 3D tomography and imaging approaches of shallow non-tectonic fault zones based on active seismic experiments using simple surface drop weights as sources and borehole chains as well as borehole DAS behind casing as sensors, complemented by seismic stand-alone surface arrays.