ALBERT

Description: The Liguro-Provencal-basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block at roughly 32–24 Ma is associated with rifting, shaping the Ligurian Sea. It is highly debated though, whether oceanic or atypical oceanic crust was formed or if the crust is of continental nature, which was extremely thinned during opening of the basin. In order to investigate the velocity structure of the Ligurian Sea a network (LOBSTER) of 29 broadband Ocean Bottom Seismometer (OBS) was installed jointly by GEOMAR (Germany) and ISTerre (France). The LOBSTER array measured continuously for eight months between June 2017 and February 2018 and is part of the AlpArray seismic network. AlpArray is a European initiative to further reveal the geophysical and geological properties of the greater Alpine area. We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea. Because of additional noise sources in the ocean, causing instrument tilt or seafloor compliance, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data to improve the signal-to-noise ratio. Then, we calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. We use teleseismic events by correlating short time windows that include strong events. Those cross-correlations are dominated by earthquake signals and allow to derive surface wave group velocities for longer periods than using AN. Finally, phase velocity maps are obtained by inverting Green’s functions derived from cross-correlation of ambient noise (AN) and teleseismic events. In the course of this ongoing project we target to derive 3D velocity models of the adjacent Alpine belt region and its complex subduction geometry contributing to questions like the prolongation of the Alpine from beneath the Ligurian Sea.

Description: Digital broad-band seismic recordings from station HLG on the small island of Heligoland in the German Bight in the North Sea are available since 2001. Since July 2017, an additional local network of six broad-band seismometers is operational on Heligoland with a focus on investigations of local microseism in the North Sea.In this study, we combine information gathered by passive seismic registration and gravity measurements from a superconducting gravimeter on Heligoland. We utilize 20 years of passive seismic data for long period spectral investigations spanning periods of tens of seconds up to several years. In the period band between 70 to 100 s, H/V ratios reveal strong noise on the horizontal components occurring contemporaneously with strong storms. In addition, distinct peaks in the spectral data are observed at periods of 6, 12 and 24 hours, which can be related to ocean tides and atmospheric pressure variations that are affecting the amplitude of local microseism. We compare the spectral seismic data with three years of gravity spectra from a collocated superconducting gravimeter. They are highly comparable with the seismic spectra and show distinct differences between local tides in the North Sea and solid Earth tides. At even longer periods up to several years, the spectral seismic data reveal a strong signal at one year, accompanied by peaks at 0.5, 2, 3 and 8 years. These variations seem to affect mostly the secondary microseism of the Northern Atlantic recorded at station HLG.

Description: The tectonic evolution of Eastern Arabia comprises three major events that must have modified the entire lithosphere below the present-day passive continental margins. Pan-African plate assembly during the Late Proterozoic established NNE-striking structural trends. Permo-Triassic Pangean breakup and rifting created two rift axes to form the present-day plate geometry. SW-directed obduction of the Semail Ophiolite during the Cretaceous shaped the present-day geology of northern Oman. Recent anisotropic ambient noise tomography, surface wave analysis of earthquake data and receiver function analysis reveal the present-day lithospheric thickness of ∼100 km and the highly variable internal structure of the East Arabian continental crust. A NNE-striking contrast in the lower crust separates felsic from intermediate lithologies in northern Oman and is interpreted as being established during Pan-African orogeny. High VS in the lower crust of northeastern Oman likely relate to Permian mafic intrusions emplaced during Pangean breakup. Mild crustal thickening below the topography is attributed to Late Cretaceous times, where continental subduction led to underthrusting and thickening of the crust and ultimately ophiolite obduction. Since present-day topography emerged only ~30 Ma after obduction ended, we argue that lithospheric thickness, margin perpendicular anisotropy and contemporaneous basanite intrusions are indicative of a relaxed state of stress at that time. Therefore, we propose that marginal erosion of the base lithosphere during Late Eocene facilitated margin-wide emergence of the Oman Mountains. Altogether, the lithospheric configuration in northeastern Arabia preserves imprints from Late Proterozoic Pan-African assemblage, Permian Pangean break-up, Late Cretaceous subduction and obduction, and Cenozoic post-obduction evolution.

Description: Northern Germany is a weak seismicity region with low level of exposure to seismic hazard. Accordingly, seismic hazard assessment has been pursued with low priority in the past. Rather poor observational conditions in the North German Basin are also a limiting factor for seismic monitoring. In a multi-institutional collaboration, we have been able to increase the number of permanent broadband stations in the North German Federal States of Schleswig-Holstein and Mecklenburg-Vorpommern from previously four in the year 2013 to currently fifteen. All data of the network are freely available through the EIDA node at the Federal Institute for Geosciences and Natural Resources (BGR) in Hannover. Three complementary seismic arrays, notably on the islands of Heligoland in the North Sea and on Rügen in the Baltic Sea, improve monitoring capabilities of offshore areas. Data from these seismic arrays also provide an opportunity to investigate properties of oceanic microseism in epicontinental seas. In addition to the development and operation of the seismic network, continuous event detection and analysis routines have been established at Kiel University. Besides low magnitude earthquakes, other natural phenomena like rockslides on cliffs or subrosion events have been repeatedly detected. Known controlled explosions provide an opportunity to quantify detection and location capabilities of the network. The recent destruction of the Nord Stream gas pipelines proved the importance of ongoing network expansion, and the need and reliability of routine seismic monitoring in this region.