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 investigate large-amplitude phases arriving in the P-wave coda of broad-band seismograms from teleseims recorded by the Gräfenberg array, the German Regional Seismic Network and the Global Seismic Network. The data set consists of all events mb ≥ 5.6 from the Aleutian arc between 1977 and 1992. Earthquakes with large-amplitude coda waves correlate with the presence of oceanic crust in the source region. The amplitudes sometimes approach those of the P wave, much larger than predicted by theory. Modelling indicates that phases in the P-wave coda cannot be P-wave multiples beneath the source and receiver, or underside reflections, which precede PP, from upper-mantle discontinuities. Among the events, seismograms are very similar, where the arrival times of the unusual phases agree approximately with the predicted times of S-to-P conversions from the upper-mantle discontinuities under the source. Because the large-amplitude phases in the P-wave coda have little, if any, dependence on event depth and have predominantly an SV-wave radiation pattern towards the receiver, we suggest that they originate as SV and /or Rayleigh waves and are enhanced by lateral heterogeneity and multipathing from the subducting Aleutian slab.

Description: We have imaged the upper mantle discontinuities in a 30 x 20° large region at the active continental margin of the Japan subduction zone and neighboring areas, using jP-to-S converted phases from teleseismic records of permanent broadband stations. The 410 km discontinuity is detected within ± 10 km of its global average position. An interesting exception in its observation is a gap near 135° E, very close to the slab penetration of the 410 km discontinuity and in an area where we have rather high data density. The 660 km discontinuity reaches 700 km depth at two places where it is hit directly by the slab. These data generally show good agreement with tomographic results. Both, the 660 km discontinuity depression and the P-velocity anomalies, suggest about 400-500 K temperature deficit within the slab. However, no downward bending of the 660 km discontinuity is observed in eastern China where the flat lying slab is imaged by tomography. This suggests that the slab does not cool the 660 km discontinuity in this region. Therefore the positive buoyancy required to keep the slab lying flat cannot have been provided by the negative Clapeyron slope of the spinel-perovskite phase transition. Another mechanism is needed, which could possibly be metastable olivine in the cold core of the slab. We have also imaged the shallower portion of the slab down to about 150 km underneath some seismic stations, likely because metastable gabbro is still existing to this depth providing a sufficient velocity contrast. A strong negative converted phase is observed at 150-200 km depth underneath a volcanic region in Japan. If real (observed at one station only), the zone which produces this conversion and which may begin exactly above the slab would require fluid-fed melting.

Description: Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lamé's constant and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lamé's constant with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.

Description: Source parameters of the 1996 Flores Sea and 1994 Fiji-Tonga deep earthquakes are derived from teleseismic body waves recorded by the global seismic network of broadband seismograph stations. Both events consisted of several subevents. Models to approximate the spatial and temporal extent of the source process include point sources, propagating point sources and a combination of these. For the Flores Sea event, rupture lasted about 23 s and terminated some 70 km east of the nucleation point as inferred from the duration of P-wave pulses, in agreement with the findings reported by other investigators. Our preferred model suggests bilateral rupture propagation. It consists time histories, and explains well the large compensated linear vector dipole (CLVD) component inferred from the Harvard centroid moment tensor (CMT) solution. The main moment release in the Fiji-Tonga event lasted only about 15 s. Our best model consists of two point sources with a total moment release of 2.8 x 1020 N m. Rupture propagated subhorizontally from the nucleation point to the north. The termination of rupture was located about 40 km to the north of rupture initiation. The inferred velocity of moment release in the Flores Sea and Fiji-Tonga events was 3-5 km s-1, a value which is higher than that inferred for the great Bolivian earthquake of June 1994. Other derived source parameters (static stress drop, radiated seismic energy and max. seismic efficiency) are also significantly different from those inferred for the 1994 Bolivian event, suggesting that deep earthquake processes do not follow an easily detectable common mechanism.

Description: Observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes. Cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine. Cumulative seismic moment release increases sharply at 530 km depth in both cold and warm slabs, implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change, which would be temperaturedependent. This, together with the observation that deep seismicity predominates near the garnet-rich upper boundary of the subducting plate (former oceanic crust), suggests that the transformation of garnet to perovskite, which may release considerable water, is integral in the cause of earthquakes deeper than 530 km, where most deep earthquakes occur.