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: P- and S-wave modelling of the data obtained during the seismic refraction wide angle reflection experiment of the URSEIS 95 project demonstrate the presence of a 15-18 km thick crustal root beneath the Magnitogorsk-Tagil zone in the central part of the Urals orogen. However, the centre of this crustal root is displaced by 50-80 km to the east of the present-day maximum topography. Also beneath the Magnitogorsk-Tagil island arc zone, an upper crustal body with a high P-wave velocity of 6.3 km s(hoch)-1 at 4-9 km depth can be interpreted as consisting of mafic and/or ultramafic rocks. This, in turn, would help to explain the positive Bouguer gravity anomaly and the surface heat-flow minimum associated with the zone, and would also be consistent with the known surface geology of the zone. Another major feature of the seismic model is the presence of high P- and S-wave velocities (7.5 and 4.2 km s(hoch)-1, respectively) at the base of the crustal root. If the deeper parts of the thickened crust also have high densities (small density contrast of about -0.1 g cm(hoch)-3 with respect to the uppermost mantle) then this helps to explain the absence of a pronounced gravity minimum associated with the root. These high velocities and densities can be most easily explained by mafic rocks or a mix of mafic and ultramafic rocks. Within the structural framework of Berzin et al. (1996) these rocks would belong to the lower Russian plate, which was being subducted beneath the Siberian plate during the Uralian orogeny. It is possible that the crustal root is formed from the remnants of oceanic crust or a mix of oceanic crust and mantle attached to the Russian plate. This would mean that little or no continental crust has been subducted or that subduction, and hence the Uralian orogeny, stopped when there was no more oceanic crust or when an attempt was made to subduct lighter continental crust.

Description: During the Kenya Rift International Seismic Project (KRISP 90) a 450-km east-west seismic profile was shot across the rift in the vicinity of the Equator. Reflectivity modelling of some of the P- and S-phases combined with results derived from the ray-trace forward model enable more information to be extracted from the data set. The Pg phase at the western end of the profile required a different velocity-depth gradient in the Archaean and the Proterozoic terranes crossed by the profile. This, combined with structural and seismic velocity differences derived from the forward model, provides the means for the two units to be distinguished throughout the whole crust. A phase in the PmP coda from shot points within the rift has been successfully modelled as a PmP multiple reflected from the base of or from within the rift infill. The clearest S-phase on the cross-rift profile, SmS, has been reproduced by reflectivity modelling and compared to the same phase from the KRISP 90 flank line: combining these observations with the results from the spectral analysis of the principal P- and S-phases observed from all the shot points on the cross-rift profile supports the contention that most of the seismic attenuation occurs immediately beneath the rift in the upper crust or in the rift infill, with little attenuation occurring in the lower crust.

Description: The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5–10 km in the south and 35–40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle P(tief)n velocities are low, being 7.5–7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40–45 km and 60–65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45–50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle P(tief)n velocity of 7.5–7.7 km/s and crustal thinning of 5–10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P(tief)n velocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal P(tief)n velocities of 8.0–8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15–20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45–50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20–30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5–10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35–40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

Description: The Kenya Rift International Seismic Project (KRISP) seismic refraction-wideangle reflection experiments carried out between 1985 and 1994 show abrupt changes in Moho depths and Pn phase velocities as the rift boundaries are crossed. Beneath the rift flanks, normal Pn phase velocities of 8.0-8.3 km s-1 are observed, except for the Chyulu Hills volcanic field, east of the rift, where it is 7.9-8.0 km s-1. Also to the east, some of the thickest crust (38-44 km) encountered so far beneath Kenya has been observed over a distance of c. 300 km. However, beneath the surface expression of the rift itself, the uppermost mantle velocity of the Pn phase is anomalously low at 7.5-7.8 km s-1 throughout ist length. Beneath the rift itself, there are major differences in crustal thickness, extension and upper mantle velocity structure between the north and the south. Beneath the section from the centre of the Kenya Dome southwards, where the extension is estimated to be 5-10 km, the crust is thinned by c. 10 km to a thickness of 35 km, and the narrow low-velocity zone in the mantle extends to a depth of at least 65 km. However, in the north beneath Turkana, where the extension is 35-40 km, the crust is only c. 20 km thick and two layers with velocities of 8.1 and 8.3 km s-1 are embedded in the low velocity mantle material at depths of 40-45 km and 60-65 km. this mantle velocity structure indicates that the depth to the onset of melting is at least 65 km beneath the northern part of the rift and is thus not shallower than the corresponding depth (45-50 km) in the south. These results, taken together with those from teleseismic studies, petrology and surface geology, have been used to deduce that anomalously hot mantle material appeared below the present site of the Kenya Rift c. 20-30 Ma ago. This led to widespread volcanism along the whole length of the rift and modification of the underlying crust by mafic igneous underplating and intrusion.

Description: The interpretation of seismic refraction/wide-angle reflection data from the 1995 Crustal Investigations off- and on-shore Nazca/Central Andes (CINCA95) project has resulted in the derivation of nine E-W two-dimensional (2-D) velocity cross sections for the region between the Peru-Chile trench and the coast between 19.5°S and 25°S, with three of the cross sections extending a farther 100-200 km inland. These sections define the major lithospheric structures of the upper South American plate and the lower Nazca plate down to uppermost mantle depths of 30-60 km beneath this part of the present-day forearc region. In addition to showing the Nazca plate subducting at an increasing angle of 9-25° down to 30-50 km depth near the coast, these cross sections show a portion of the Moho dipping eastward from 43-50 km near the coast to 55-64 km up to about 240 km inland. Owing to a gap of almost 50 km in the data coverage of the Moho near the coast, it is uncertain from this data set alone whether the Moho defined east of the coast should be correlated with the lower oceanic Nazca plate or the upper continental South American plate. However, a comparison with other seismological data suggests that the Moho identified here east of the coast defines the base of the continental crust of the upper South American plate immediately behind the downgoing Nazca plate. Thus the hypothesis of Delouis et al (1996) that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported. The cross sections also show between 20°S and 22.5°S, a boundary extending from upper crustal levels down to 25-30 km depth subparallel to and 5-10 km above the top surface of the subducting Nazca plate. The discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate.