ALBERT

Beschreibung: 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.

Beschreibung: P-to-S converted teleseismic waves recorded by temporary broadband networks across Tibet show a north-dipping interface that begins 50 kilometers north of the Zangbo suture at the depth of the Moho (80 kilometers) and extends to a depth of 200 kilometers beneath the Bangong suture. Under northern Tibet a segmented south-dipping structure was imaged. These observations suggest a different form of detachment of the Indian and Asian lithospheric mantles caused by differences in their composition and buoyancy.

Beschreibung: During the TOR-1 passive seismic experiment in 1996/97, a maximum of 139 temporary seismograph stations were operating over the Sorgenfrei-Tornquist Zone (STZ) in an area extending from northern Germany through Denmark to central Sweden. One of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the Trans-European Suture Zone. To achieve this goal,broad-band and intermediate-period (5 s) data of the TOR-1 stations and additional stations of permanent networks (GRSN, GEOFON) were analysed for splitting of SKS and SKKS phases. As a result of the relatively dense station spacing, the method offers good lateral resolution of anisotropy.Preliminary results suggest that the directions of the fast horizontal S wave velocity are affected by the STZ. In central Europe and southern Sweden, far away from the STZ, fast S wave directions are approximately E-W while they turn more northerly closer to the STZ where they are approximately parallel to the trend of the STZ. No significant shear wave splitting was observed north of 57 degr. N and east of 14 degr. E. Small delay times between 0.2 and 0.5 s observed at the northernmost TOR-1 station T40S and T60S may be controlled by anisotropy in a thickened crust. The mantle contribution of horizontal anisotropy within the STZ is probably constrained to an approximately 60-km-thick zone in the depth range between 70 and 300 km. The observations are consistent with a model where azimuthally anisotropy is not governed by present-day mantle flow in the asthenosphere, but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity.

Beschreibung: Mode conversions and reflections at upper-mantle seismic discontinuities may be contained in earthquake seismograms as weak secondary phases that often become visible only after special signal processing techniques are applied to the data. To extract fully the information these secondary phases carry about the three-dimensional structure of the Earth, new observational and interpretational methods have to be developed. However, new sources of possible systematic errors may lead to conflicting results. Studies carried out by various research groups on the thickness of the upper-mantle transition zone, the sharpness of upper-mantle discontinuities and the global existence of a 520 km discontinuity are examples where such discrepancies did arise. Although there is a general consensus that the depths to the 410 km and 660 km discontinuities vary by a few tens of kilometres at most, the question of wither the depth variations of the 410 km and 660 km discontinuities are correlated or ant correlated is still unreso Similarly, different data sets and methods yielded different answers on the sharpness of the upper-mantle discontinuities at 410 km and 660 km depth. Finally, data apparently supporting the global existence of a seismic discontinuity at 520 km depth can be equally well explained by models that do not contain this discontinuity.

Beschreibung: A passive seismological experiment across the northern boundary of the Tornquist Fan was carried out from December 1994 until April 1995 along a 100 km long profile in Denmark and Sweden. Observed PS converted seismic waves indicate an abrupt increase in Moho depth of 4-5 km within 5 km horizontal distance at about 50 km to the northeast of the Øresund. This is a much sharper jump than that inferred from the wide-angle data of EUGENO-S line. 1. Preliminary analysis of seismic shear wave anisotropy indicates asthenospheric mantle flow parallel to the Sorgenfrei-Tornquist Zone. These parallel directions are already observed between the Elbe Line and the Sorgenfrei-Tornquist Zone while further south in central Europe the flow directions follow the trend of the Alpine belt.