ALBERT

Description: Zusammenfasung: In den letzten 20 Jahren ist das globale Netz von Erdbebenstationen (GSN) modernisiert und erweitert worden. Es umfasst jetzt eine grosse Zahl von digitalen Stationen, die die seismischen Signale hochaufloesend ueber einen weiten Frequenzbereich registrieren. Durch diese internationalen Bemuehungen, an denen sich das GFZ im Rahmen des GEOFON-Programms beteiligt, hat sich nicht nur die Qualitaet der Erdbebenueberwachung deutlich verbessert, sondern es ist nun auch moeglich, die Feinstruktur des Erdinnerns mit hoeherer Praezision zu untersuchen. Letzteres wird hier an zwei Beispielen ueber die Tiefenerstreckung von kontinentalen und ozeanischen Strukturen sowie ueber die Feinstruktur der Uebergangszone zwischen oberem und unterem Mantel verdeutlicht. Durch die temporaere Verdichtung seismischer Netze mittels portabler Stationen ist es moeglich, spezifische Fragen zur Struktur der Lithosphaere und das gesamten Erdinnern zu untersuchen. Hierzu werden gewoehnlich Registrierungen von Fernbeben herangezogen, aus denen Strukturbilder in den Tiefen abgeleitet werden koennen, die mit explosionsseismischen Quellen wegen der geringeren abgestrahlten Energie nicht ausreichend durchstrahlt werden koennen. Als Beispiel hierzu zeigen wir Ergebnisse von einem Feldexperiment in Tibet. Abstract Starting about 20 years ago the global network of seismograph stations (GSN) has been upgraded and expanded to a large number of digital stations recording seismic signals with high resolution in a very broad frequency band. This coordinated international effort, with GFZ Potsdam contributing through its GEOFON program, has improved considerably the monitoring capabilities of seismic networks, and it provides the data that allow us to study Earth structure in unpredecedented detail. This is demonstrated for two examples dealing with the depth extent of continental and oceanic structure, and the transition zone between upper and lower mantle. In addition to permanent seismograph stations, portable seismograph networks have been used to temporarily increase the station density in areas of scientific interest, thus enabling detailed studies of both the structure of the lithosphere and the entire globe using data from distant earthquakes. The methods developed for the processing and interpretation of earthquake recorderings have resulted in improved structural images at greater depths that are difficult to probe by explosions because of their limited energy. This is demonstrated in an example of lithospheric and upper mantle studies in Tibet.

Description: We use data from recently installed broad-band seismographs on the islands of Crete, Gavdos, Santorini, Naxos and Samos in the Hellenic subduction zone to construct receiver function images of the crust and upper mantle from south of Crete into the Aegean Sea. The stations are equipped with STS-2 seismometers and they are operated by GFZ Potsdam, University of Chania and ETH Zürich. Teleseismic earthquakes recorded by these stations at epicentral distances between 35° and 95° have been used to calculate receiver functions. The receiver function method is a routinely used tool to detect crustal and upper-mantle discontinuities beneath a seismic station by isolating the P-S converted waves from the coda of the P wave. Converted P-S energy from the oceanic Moho of the subducted African Plate is clearly observed beneath Gavdos and Crete at a depth ranging from 44 to 69 km. This boundary continues to the north to nearly 100 km depth beneath Santorini island. Because of a lack of data the correlation of this phase is uncertain north of Santorini beneath the Aegean Sea. Moho depths were calculated from primary converted waves and multiply reflected waves between the Moho and the Earth’s surface. Beneath southern and eastern Crete the Moho lies between 31 and 34 km depth. Beneath western and northern Crete the Moho is located at 32 and 39 km depth, respectively, and behaves as a reversed crust-mantle velocity contrast, possibly caused by hydration and serpentinization of the forearc mantle peridotite. The Moho beneath Gavdos island located south of Crete in the Lybyan Sea is at 26 km depth, indicating that the crust south of the Crete microcontinent is also thinning towards the Mediterranean ridge. This makes it unlikely that part of the crust in Crete consists of accreted sediments transported there during the present-day subduction process which began approximately 15 Ma because the backstop, i.e. the boundary between the current accretionary wedge of the Mediterranean ridge and the Crete microcontinent, is located approximately 100 km south of Gavdos. A seismic boundary at 32 km depth beneath Santorini island probably marks the crustal base of the Crete microcontinent. A shallower seismic interface beneath Santorini at 20-25 km depth may mark the depth of the detachment between the Crete microcontinent and the overlying Aegean subplate. The Moho in the central and northern Aegean, at Naxos and Samos, is observed at 25 and 28 km depth, respectively. Assuming a stretching factor of 1.2-1.3, crustal thickness in the Aegean was 30-35 km at the inception of the extensional regime in the Middle Miocene.

Description: ORFEUS is the non-profit foundation that coordinates and promotes digital broadband seismology in Europe. Since 1987 the ORFEUS Data Center (ODC) has been its jointly funded data center. However, within the last decade we have seen an exponential growth of high quality digital waveform data relevant for seismological and general geoscience research. In addition to the rapid expansion in number and density of broadband seismic networks this growth is fuelled by data collected from other sensor types (strong motion, short period) and deployment types (aftershock arrays, temporary field campaigns, OBS). As a consequence, ORFEUS revised its data archiving infrastructure and organization, a major component of this is the formal establishment of the European Integrated waveform Data Archives (EIDA). Within the NERIES and NERA EC projects GFZ has taken the lead in developing ArcLink as a tool to provide uniform access to distributed seismological waveform data archives. The new suite of software and services provides the technical basis of EIDA. To ensure that those developments will become sustainable, an EIDA group has been formed within ORFEUS. This founding group of EIDA nodes, formed in 2013, will be responsible for steering and maintaining the technical developments and organization of an effective operational distributed waveform data archive for seismology in Europe. The EIDA Founding nodes are: ODC/ORFEUS, GEOFON/GFZ/Germany, SED/Switzerland, RESIF/CNRSINSU/ France, INGV/Italy and BGR/Germany. These represent EIDA nodes that have committed themselves within ORFEUS to manage EIDA, that is, to maintain and develop EIDA into a stable sustainable research infrastructure. This task involves a number of challenges with regard to quality and metadata maintenance, but also to provide efficient and uncomplicated data access for users. This also includes effective global archive synchronization with developments within the International Federation of Digital Seismograph Networks (FDSN). This ORFEUS – EIDA initiative represents another step towards the construction of the European Plate Observing System (EPOS), and will be a cornerstone of the EPOS services for seismology. The EIDA infrastructure and organization will be presented with a main emphasis on on-going developments and challenges.