ALBERT

Description: We study the 20 October 2004 M (sub w) 4.4 Rotenburg (Wumme)/Neuenkirchen earthquake, located in a previously aseismic region in the northern German sedimentary basin. We constrain the source parameter by using different techniques. A possible relationship between this event, the regional tectonic setting, and local gas recovery is investigated. Different waveform inversion and modeling approaches constrain the depth of the mainshock between 5 and 7 km. The source mechanism was oblique normal faulting on planes striking roughly north-south. An inversion for kinematic rupture parameters indicates a unilateral rupture propagation toward the north, consistent with the higher macroseismic intensities found toward the north in the region of Hamburg compared with those at a similar distance toward the south in the region of Hannover. Relocations of the mainshock and three of the largest aftershocks indicate that these events occurred within a few kilometers of three major gas fields and at depth close to gas production intervals. Comparison with seismicity triggered in the northern Netherlands by depletion of similar gas reservoirs in a similar tectonic environment suggests that the M (sub w) 4.4 Rotenburg event may be related to gas recovery.

Description: The main cause for mid-period seismic ground distortions are ocean waves generated by atmospheric disturbances. These act upon the earth through different mechanisms. The microseismic wavefield can be divided into primary (T=12-18 s) and secondary (T= 6-9 s) noise. Classical theory tells that the origin of these induced ground distortions depends on the location and the intensity of the low pressure region. A considerable part of the microseismic wave field reaches the GRF-array in southern Germany with high coherency and almost constant amplitudes. Thus it is possible to locate the generating areas using frequency-wavenumber analysis. Five discrete generating areas for secondary microseisms and three generating areas for primary microseisms could be determined in the Atlantic Ocean, the Arctic Sea and the Mediterranean Sea by investigating broadband continuous recordings over four months in winter 1995/96. An essential result is the long-time constancy of the backazimuths of the coherent part of the microseismic wavefield with respect to the origin areas, independent of the location of the moving low pressure zone. Results from a triangulation using additionally broadband data from the NORS AR-array and an independent estimation of the distance of the source region with water wave dispersion data indicate an origin of the secondary microseismic wavefield near the north-Norwegian coast for the strongest source. The array analysis of a temporary network of ten three-component broadband stations in south-east Germany shows that the ratio of energy between coherent Love and Rayleigh waves is much higher for the primary than for the secondary microseismic noise wavefield. This indicates differences in the source mechanisms.

Description: The German Regional Seismic Network (GRSN) comprizes now 16 digital broadband stations equipped with Wieland-Streckeisen STS-2 seismometers, 24-bit dataloggers and a seismological data center at Erlangen. It covers the whole territory of Germany with station-spacings between 80 km to 240 km. The stations are sited in very different environments ranging from near shore at the Baltic Sea coast up to distances of about 700 km from the coast, both within cities and up to about 10 km away from any major settlement, industry or traffic roads. The underground varies from outcropping hard rocks in Hercynian mountain areas, sedimentary rocks in areas of Mesozoic platform cover to up to 1.5 km unconsolidated Quarternary and Tertiary subsoil. Accordingly, seismic background noise varies in a wide range between the upper and lower bounds of the new global noise model. The noise conditions at the GRSN have been investigated systematically by means of displacement power spectral analysis within the frequency range 10-2〈f〈40 Hz. Smoothed power spectra have been calculated by applying the 'average segment method' using record intervals between 4 and 45 min long and between 6 and 25 overlapping segments. Representative samples were taken at different times of the day and the year in order to quantify for all sites the level and degree of variability of seismic background noise. The worst stations of the original GRSN were Berlin (BRLN), Hamburg (HAM) and Liddow (LID), all placed on unconsolidated soft-soil cover, between 50 m (at LID) and about 1.5 km thick (at HAM). But no spectral noise peaks due to wave resonance in this soft-soil cover could be identified because the noise spectra at all these sites are clearly dominated by strong ambient noise sources (traffic, industry and/or coastal surf sea-noise). For all three sites better locations had to be found at not too large distance from the original sites so as to preserve the good overall GRSN network configuration. Suitable alternatives were found at Rüdersdorf (RUE), Bad Segeberg (BSEG) and west of the village of Neunkirchen, Island of Rügen (RGN). RUE and BSEG were placed on locally outcropping sedimentary rock on top of a salt dome and within the cap of a diapir, respectively. The new station RGN was installed only 2.8 km away from the former LID in a huge soil-covered army bunker which provided much better thermal shielding and a more stable basement platform for long-period recordings. The noise power at RUE and BSEG as compared to BRLN and HAM is reduced by about 10 to 50 dB between 0.4Hz〈f〈50 Hz. This corresponds to 1 to 5 orders of magnitude in power spectral density or a factor of 3 to 300 in displacement amplitudes. For some selected, both near and teleseismic events improvements of the spectral signal-to-noise ratio 〉5 for RUE and 〉10 for BSEG have been confirmed for frequencies between about 0.6 Hz〈f〈5 Hz. Thus BSEG has become now a station only somewhat inferior to the good hard rock sites in the central and southern part of Germany while RUE near Berlin is almost comparable with FUR near Munich. RGN, still being inappropriate in the teleseismic detection window around 1 s, is now almost as good as other fine GRSN stations in the long-period range between about 10 s〈T〈50 s, comparable with FUR around f=2 Hz and even better for f〉3 Hz. Strong lateral velocity and impedance contrasts between the outcropping Triassic/Permian sedimentary rocks and the surrounding unconsolidated Quarternary/Tertiary sediments are shown to be the main cause for the strong noise reduction and signal-to-noise ratio improvement at RUE and can account for about 50% of the noise reduction at BSEG.