ALBERT

Description: Seismological models of upper mantle structure provide important constraints on the Earth"s convection system. Resolving the details of the upper mantle discontinuities is important for modelling the composition of the mantle and for understanding the effect that the discontinuities may have on mantle convection. Recently, numerous permanent and temporary seismic stations and networks have been set up around the world. It is possible to get the seismic records for the research needs from data management systems like IRIS, GEOFON, GEOSCOPE, FREESIA, etc. The use of seismograms collected from a large number of stations and earthquakes around the world enable us to study the global and the regional structure of the Earth. In this work, the receiver function technique (e.g. Owens et. Al., 1995) is applied to study the upper mantle structure in the northwest Pacific subduction zone and in the Hawaiian hotspot area. In the northwest Pacific, the Pacific plate is subducted into the upper mantle to more than 600 km depth, indicated by seismicity. In Hawaii, the volcanic edifice of the Hawaiian Islands and seamounts are believed to result from the passage of the oceanic lithosphere over a stationary mantle hotspot (Wilson, 1963; Morgan, 1971; Morgan et. al., 1995). In both regions the upper mantle structure is affected by the cold and warm materials, respectively. To study the extension of the temperature anomaly is important for understanding the Earth"s convection system. The olivine component of the mantle material is intensively studied in laboratories (e.g. Ito and Takahashi, 1989; Irifune, 1987). With increasing temperature and pressure, the olivine crystal undergoes a series of phase transformations which will result in a variation of the seismic structure. The effect of the temperature anomaly on the main upper mantle discontinuities will be discussed in chapter 2. Recently, the receiver function technique is increasingly applied to investigate the upper mantle discontinuities. To isolate the upper mantle conversion phases, newly developed moveout correction and migration methods are applied to separately distributed seismic stations as well as station arrays. The receiver function method used in this study will be introduced in chapter 3. In chapter 4 and 5, receiver function studies in the area of the northwest Pacific subduction zone and the Hawaiian mantle plume are presented. Regional tectonic background and the previous seismological works in these two areas will be first introduced in each chapter, and followed by description of data, processing steps, results and interpretations. In chapter 6, I will summarize the observations of the 410 and 660 topography in the northwest Pacific subduction zone and in the area around the Hawaiian mantle plume.

Description: Noise measurements were carried out at 381 sites in the Cologne area (Germany) using both short period and broad band sensors. The large number of data allowed both assesment of the influence of different sensors in the site response estimation and to compare the widely used H/V technique with the recently proposed Fourier Phase Spectral Method (FPSM). The results show that short period sensors are able to reliably retrieve site effects at frequencies well below their corner frequencies. Moreover, the H/V method should be preferred to the FPSM in determining the fundamental resonance frequency of soils. Finally, a map showing the resonance frequency distribution in the studied area was drawn using the results obtained applying the H/V technique.

Description: The main structures of the mid-European lithosphere, crossing Europe from the Iberian peninsula to the Bohemian Massif, are predominantly formed by the Variscan orogeny. To investigate the anisotropy of the mantle at the transition zone between the two Variscan units, the Saxothuringicum and the Moldanubicum, we carried out a field experiment in SE Germany in 1995-1996: 23 mobile broad-band stations were installed for 6 months in the Vogtland-Oberpfalz-Bavarian Forest area. The station profile crossed the suture zone of the Saxothuringicum and Moldanubicum near the KTB borehole (German Continental Deep Drilling Program). With a mean station spacing of about 10 km? we intended to obtain a high lateral resolution of the anisotropy parameters and to resolve possible changes when passing the suture zone. The analysis of the observed birefringence of SKS phases shows E-W directions for the fast polarization. Therefore, the directions deviate only slightly from the strike of the Hercynian mountain belt and from the direction of the absolute plate motion in that region. Indications for the transition zone come from a rotation of the fast polarization direction from 86 degrees +/- 13 degrees in the northern part of the profile (Saxothuringicum) to 110 degrees +/- 15 degrees in the southern part (Moldanubicum) as well as from strong variations of the splitting parameters with respect to the azimuths of the incoming waves in the middle of the profile. We interpret these variations as an expression of a complex mantle structure formed either by several anisotropic layers with inclined symmetry axes in at least one layer or by a model consisting of inhomogeneous anisotropic layers. A comparison of the azimuthal variations of the splitting parameters in the middle of the profile with those observed at the Grafenberg station GRAl-situated in the central part of the transition zone approximately 100 km to the west-shows remarkable differences, which may reflect lateral variations in the direction and inclination of the symmetry axes in the transition zone even on a small scale. Both observations-the change in the fast polarization direction from the northern to the southern part as well as the variations with respect to different azimuths in the middle of the profile-suggest that the transition zone between the Saxothuringicum and the Moldanubicum continues down into the upper mantle.

Description: The new GFZ/GRGS gravity field models GRIM5-S1 and Cl, current initial models for the CHAMP mission, have been compared with other recent models (JGM 3, EGM 96) for radial accuracy (by means of latitude lumped coefficients) in computations on altimetry satellite orbits. The basis for accuracy judgements are extensive (multi-year) averages of crossover sea height differences from Geosat and ERS 1/2 missions. These data are fully independent of the data used to develop these gravity models. We tested how well these observed differences in all the world's oceans agree with projections of the same errors from the scaled covariance matrix of their harmonic geopotential coefficients. It was found that the tentative (model) scale factor of 5 for the formal standard deviations of the harmonic coefficients of the new GRIM fields is justified, i.e. the accuracy estimates, provided together with the geopotential coefficients, are realistic.

Description: SKS and SKKS shear waves recorded on the INDEPTH III seismic array deployed in central Tibet during 1998 - 1999 have been analysed for the direction and extent of seismic polarization anisotropy. The 400-km-long NNW trending array extended south to north, from the central Lhasa terrane, across the Karakoram-Jiali fault system and Banggong-Nujiang suture to the central Qiangtang terrane. Substantial splitting with delay times from 1 to 2 s, and fast directions varying from E-W to NE-SW, was observed for stations in the Qiangtang trerrane and northernmost Lhasa terrane. No detectable splitting was observed for stations located farther south in the central Lahsa terrane. The change in shear wave splitting characteristics occurs at 32°N, approximately coincident with the transcurrent Karakoram-Jiali fault system but ~40 km south of the surface trace of the Banggong-Nujiang suture. This location is also near the southernmost edge of a region of high Sn attenuation and low upper mantle velocities found in previous studies. The transition between no measured splitting and strong anisotropy (2.2 s delay time) is exceptionally sharp (=15 km), suggesting a large crustal contribution to the measured splitting. The E-W to NE-SW fast directions are broadly similar to the fast directions observed farther east along the Yadong-Golmud highway, suggesting that no large-scale change in anisotropic properties occurs in the east-west direction. However, in detail, fast directions and delay times vary over lateral distances of ~100 km in both the N-S and E-W direction by as much as 40° and 0.5-1 s, respectively. The onset of measurable splitting at 32° N most likely marks the northern limit of the underthrusting Indian lithosphere, which is characterized by negligible polarization anisotropy. Taken in conjunction with decades of geophysical and geological observations in Tibet, the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern Tibet is being squeezed and sheared between the advancing Indian lithosphere to the south and the Tsaidam and Tarim Lithospheres to the north and west, resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability. In northern Tibet, crustal deformation clearly follows this large-scale deformation pattern.