TY - JOUR A1 - Fischer, Tomas A1 - Hrubcova, Pavla A1 - Dahm, Torsten A1 - Woith, Heiko A1 - Vylita, Tomas A1 - Ohrnberger, Matthias A1 - Vlcek, Josef A1 - Horalek, Josef A1 - Dedecek, Petr A1 - Zimmer, Martin A1 - Lipus, Martin P. A1 - Pierdominici, Simona A1 - Kallmeyer, Jens A1 - Krüger, Frank A1 - Hannemann, Katrin A1 - Korn, Michael A1 - Kämpf, Horst A1 - Reinsch, Thomas A1 - Klicpera, Jakub A1 - Vollmer, Daniel A1 - Daskalopoulou, Kyriaki T1 - ICDP drilling of the Eger Rift observatory BT - magmatic fluids driving the earthquake swarms and deep biosphere JF - Scientific Drilling N2 - 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 Novy Kostel seismogenic zone, borehole F3 in the Hartousov mofette field and borehole S4 in the newly discovered Bazina maar near Liba. 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 Hartousov 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. Y1 - 2022 U6 - https://doi.org/10.5194/sd-31-31-2022 SN - 1816-8957 SN - 1816-3459 VL - 31 SP - 31 EP - 49 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Fischer, Tomáš A1 - Hrubcova, Pavla A1 - Dahm, Torsten A1 - Woith, Heiko A1 - Vylita, Tomáš A1 - Ohrnberger, Matthias A1 - Vlček, Josef A1 - Horalek, Josef A1 - Dedecek, Petr A1 - Zimmer, Martin A1 - Lipus, Martin P. A1 - Pierdominici, Simona A1 - Kallmeyer, Jens A1 - Krüger, Frank A1 - Hannemann, Katrin A1 - Korn, Michael A1 - Kaempf, Horst A1 - Reinsch, Thomas A1 - Klicpera, Jakub A1 - Vollmer, Daniel A1 - Daskalopoulou, Kyriaki T1 - ICDP drilling of the Eger Rift observatory BT - magmatic fluids driving the earthquake swarms and deep biosphere JF - Scientific drilling : reports on deep earth sampling and monitoring N2 - 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. Y1 - 2022 U6 - https://doi.org/10.5194/sd-31-31-2022 SN - 1816-8957 SN - 1816-3459 VL - 31 SP - 31 EP - 49 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Hannemann, Katrin A1 - Krüger, Frank A1 - Dahm, Torsten T1 - Measuring of clock drift rates and static time offsets of ocean bottom stations by means of ambient noise JF - Geophysical journal international N2 - Marine seismology usually relies on temporary deployments of stand alone seismic ocean bottom stations (OBS), which are initialized and synchronized on ship before deployment and re-synchronized and stopped on ship after recovery several months later. In between, the recorder clocks may drift and float at unknown rates. If the clock drifts are large or not linear and cannot be corrected for, seismological applications will be limited to methods not requiring precise common timing. Therefore, for example, array seismological methods, which need very accurate timing between individual stations, would not be applicable for such deployments. We use an OBS test-array of 12 stations and 75 km aperture, deployed for 10 months in the deep sea (4.5-5.5 km) of the mid-eastern Atlantic. The experiment was designed to analyse the potential of broad-band array seismology at the seafloor. After recovery, we identified some stations which either show unusual large clock drifts and/or static time offsets by having a large difference between the internal clock and the GPS-signal (skew). We test the approach of ambient noise cross-correlation to synchronize clocks of a deep water OBS array with km-scale interstation distances. We show that small drift rates and static time offsets can be resolved on vertical components with a standard technique. Larger clock drifts (several seconds per day) can only be accurately recovered if time windows of one input trace are shifted according to the expected drift between a station pair before the cross-correlation. We validate that the drifts extracted from the seismometer data are linear to first order. The same is valid for most of the hydrophones. Moreover, we were able to determine the clock drift at a station where no skew could be measured. Furthermore, we find that instable apparent drift rates at some hydrophones, which are uncorrelated to the seismometer drift recorded at the same digitizer, indicate a malfunction of the hydrophone. KW - Time-series analysis KW - Interferometry KW - Broad-band seismometers Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggt434 SN - 0956-540X SN - 1365-246X VL - 196 IS - 2 SP - 1034 EP - 1042 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hannemann, Katrin A1 - Krüger, Frank A1 - Dahm, Torsten A1 - Lange, Dietrich T1 - Oceanic lithospheric S-wave velocities from the analysis of P-wave polarization at the ocean floor JF - Geophysical journal international N2 - Our knowledge of the absolute S-wave velocities of the oceanic lithosphere is mainly based on global surface wave tomography, local active seismic or compliance measurements using oceanic infragravity waves. The results of tomography give a rather smooth picture of the actual S-wave velocity structure and local measurements have limitations regarding the range of elastic parameters or the geometry of the measurement. Here, we use the P-wave polarization (apparent P-wave incidence angle) of teleseismic events to investigate the S-wave velocity structure of the oceanic crust and the upper tens of kilometres of the mantle beneath single stations. In this study, we present an up to our knowledge new relation of the apparent P-wave incidence angle at the ocean bottom dependent on the half-space S-wave velocity. We analyse the angle in different period ranges at ocean bottom stations (OBSs) to derive apparent S-wave velocity profiles. These profiles are dependent on the S-wave velocity as well as on the thickness of the layers in the subsurface. Consequently, their interpretation results in a set of equally valid models. We analyse the apparent P-wave incidence angles of an OBS data set which was collected in the Eastern Mid Atlantic. We are able to determine reasonable S-wave-velocity-depth models by a three-step quantitative modelling after a manual data quality control, although layer resonance sometimes influences the estimated apparent S-wave velocities. The apparent S-wave velocity profiles are well explained by an oceanic PREM model in which the upper part is replaced by four layers consisting of a water column, a sediment, a crust and a layer representing the uppermost mantle. The obtained sediment has a thickness between 0.3 and 0.9 km with S-wave velocities between 0.7 and 1.4 km s(-1). The estimated total crustal thickness varies between 4 and 10 km with S-wave velocities between 3.5 and 4.3 km s(-1). We find a slight increase of the total crustal thickness from similar to 5 to similar to 8 km towards the South in the direction of a major plate boundary, the Gloria Fault. The observed crustal thickening can be related with the known dominant compression in the vicinity of the fault. Furthermore, the resulting mantle S-wave velocities decrease from values around 5.5 to 4.5 km s(-1) towards the fault. This decrease is probably caused by serpentinization and indicates that the oceanic transform fault affects a broad region in the uppermost mantle. Conclusively, the presented method is useful for the estimation of the local S-wave velocity structure beneath ocean bottom seismic stations. It is easy to implement and consists of two main steps: (1) measurement of apparent P-wave incidence angles in different period ranges for real and synthetic data, and (2) comparison of the determined apparent S-wave velocities for real and synthetic data to estimate S-wave velocity-depth models. KW - Time-series analysis KW - Body waves KW - Theoretical seismology KW - Oceanic transform and fracture zone processes Y1 - 2016 U6 - https://doi.org/10.1093/gji/ggw342 SN - 0956-540X SN - 1365-246X VL - 207 SP - 1796 EP - 1817 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hannemann, Katrin A1 - Krüger, Frank A1 - Dahm, Torsten A1 - Lange, Dietrich T1 - Structure of the oceanic lithosphere and upper mantle north of the Gloria Fault in the eastern mid-Atlantic by receiver function analysis JF - Journal of geophysical research : Solid earth N2 - Receiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (approximate to 1year), resulting in a small number of usable records of teleseismic earthquakes. Here we use OBSs deployed as midaperture array in the deep ocean (4.5-5.5km water depth) of the eastern mid-Atlantic. We use evaluation criteria for OBS data and beamforming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5-8km) of oceanic crust. Observations at single stations with thin sediments (300-400m) indicate that a probable sharp lithosphere-asthenosphere boundary (LAB) might exist at a depth of approximate to 70-80km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at approximate to 410km and approximate to 660km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam-formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events. KW - receiver function KW - oceanic lithosphere and mantle KW - ocean bottom seismology Y1 - 2017 U6 - https://doi.org/10.1002/2016JB013582 SN - 2169-9313 SN - 2169-9356 VL - 122 SP - 7927 EP - 7950 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Krüger, Frank A1 - Dahm, Torsten A1 - Hannemann, Katrin T1 - Mapping of Eastern North Atlantic Ocean seismicity from Po/So observations at a mid-aperture seismological broad-band deep sea array JF - Geophysical journal international N2 - A mid-aperture broad-band test array (OBS array DOCTAR) was deployed from June 2011 to April 2012 about 100 km north of the Gloria fault in the Eastern North Atlantic in about 5000 m water depth. In addition arrays were installed on Madeira Island and in western Portugal mainland. For the first time in the Eastern North Atlantic, we recorded a large number of high frequency Po and So waves from local and regional small and moderate earthquakes (M-L < 4). An incoherent beamforming method was adapted to scan continuous data for such Po and So arrivals applying a sliding window waveform migration and frequency-wavenumber technique. We identify about 320 Po and 1550 So arrivals and compare the phase onsets with the ISC catalogue (ISC 2015) for the same time span. Up to a distance of 6 degrees to the DOCTAR stations all events listed in the ISC catalogue could be associated to Po and So phases. Arrivals from events in more than 10 degrees distance could be identified only in some cases. Only few Po and/or So arrivals were detected for earthquakes from the European and African continental area, the continental shelf regions and for earthquakes within or northwest of the Azores plateau. Unexpectedly, earthquake clusters are detected within the oceanic plates north and south of the Gloria fault and far from plate boundaries, indicating active intraplate structures. We also observe and locate numerous small magnitude earthquakes on the segment of the Gloria fault directly south of DOCTAR, which likely coincides with the rupture of the 25 November 1941 event. Local small magnitude earthquakes located beneath DOCTAR show hypocentres up to 30 km depth and strike-slip focal mechanisms. A comparison with detections at temporary mid-aperture arrays on Madeira and in western Portugal shows that the deep ocean array performs much better than the island and the continental array regarding the detection threshold for events in the oceanic plates. We conclude that sparsely distributed mid-aperture seismic arrays in the deep ocean could decrease the detection and location threshold for seismicity with M-L < 4 in the oceanic plate and might constitute a valuable tool to monitor oceanic plate seismicity. KW - body waves KW - earthquake source observations KW - seismicity and tectonics Y1 - 2020 U6 - https://doi.org/10.1093/gji/ggaa054 SN - 0956-540X SN - 1365-246X VL - 221 IS - 2 SP - 1055 EP - 1080 PB - Oxford Univ. Press CY - Oxford ER -