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This study presents results of ambient noise measurements from temporary single station and small-scale array deployments in the northeast of Basle. H/V spectral ratios were determined along various profiles crossing the eastern masterfault of the Rhine Rift Valley and the adjacent sedimentary rift fills. The fundamental H/V peak frequencies are decreasing along the profile towards the eastern direction being consistent with the dip of the tertiary sediments within the rift. Using existing empirical relationships between H/V frequency peaks and the depth of the dominant seismic contrast, derived on basis of the lambda/4-resonance hypothesis and a power law depth dependence of the S-wave velocity, we obtain thicknesses of the rift fill from about 155 m in the west to 280 in in the east. This is in agreement with previous studies. The array analysis of the ambient noise wavefield yielded a stable dispersion relation consistent with Rayleigh wave propagation velocities. We conclude that a significant amount of surface waves is contained in the observed wavefield. The computed ellipticity for fundamental mode Rayleigh waves for the velocity depth models used for the estimation of the sediment thicknesses is in agreement with the observed H/V spectra over a large frequency band
Subduction and collision processes in the Central Andes constrained by converted seismic phases
(2000)
To address one of the central questions of plate tectonics-How do large transform systems work and what are their typical features?-seismic investigations across the Dead Sea Transform (DST), the boundary between the African and Arabian plates in the Middle East, were conducted for the first time. A major component of these investigations was a combined reflection/ refraction survey across the territories of Palestine, Israel and Jordan. The main results of this study are: (1) The seismic basement is offset by 3-5 km under the DST, (2) The DST cuts through the entire crust, broadening in the lower crust, (3) Strong lower crustal reflectors are imaged only on one side of the DST, (4) The seismic velocity sections show a steady increase in the depth of the crust-mantle transition (Moho) from 26 km at the Mediterranean to 39 km under the Jordan highlands, with only a small but visible, asymmetric topography of the Moho under the DST. These observations can be linked to the left-lateral movement of 105 km of the two plates in the last 17 Myr, accompanied by strong deformation within a narrow zone cutting through the entire crust. Comparing the DST and the San Andreas Fault (SAF) system, a strong asymmetry in subhorizontal lower crustal reflectors and a deep reaching deformation zone both occur around the DST and the SAF. The fact that such lower crustal reflectors and deep deformation zones are observed in such different transform systems suggests that these structures are possibly fundamental features of large transform plate boundaries
We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3- D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection