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Summary Local seismic activity has been monitored in the southern part of the Kenya Rift in the area around Lake Magadi. An earthquake recording network consisting of 15 station sites was operated for 8 months from November 1997 to June 1998. During this period, the Magadi area proved to be seismically active. Approximately 10 events per day were detected and found to be equally distributed over the rift floor. The hypocentre depth distribution shows surprisingly large depths of up to 27 km in the south and much shallower depths in the northern part of the area. Apart from the background activity, swarm activity with rates of more than 300 events per day was also recorded. The epicentres are clustered and trace a linear structure lying SSW–NNE over a length of 10 km north of Lake Magadi. Hypocentres in the region of the earthquake cluster are shallow and exhibit a sharp cut‐off at 9 km depth. A surface crack that occurred during the recording period is connected to the earthquake swarm. Analysis of the focal mechanisms of selected earthquakes indicates predominantly normal faulting in response to a WNW–ESE‐directed tensional stress field. This direction corresponds to the general alignment of the southernmost part of the Kenya Rift and to the surface faulting pattern inside the rift. The crustal structure of the area has been investigated using local earthquake tomography and the spatial distribution of hypocentres. The main results of the tomography are a linear positive velocity anomaly following the rift axis and a negative anomaly at shallow depth underneath Lake Magadi. The high velocities can be explained by mafic material that has intruded into the upper crust. The negative anomaly is attributed to highly fractured rocks. Maximum hypocentre depths indicate a body of low shear strength centred at the rift axis and a general deepening of the brittle–ductile transition from north to south.

The observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers. The most suitable method for this determination is Nakamura's technique, which is the ratio of the horizontal-component noise spectrum and that of the vertical component (H/V spectrum). The frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site. Using an extensive database of microtremor measurements carried out in the western Lower Rhine Embayment (Germany), it was possible to show that this correlation is clearly valid for a wide range of thickness, namely, from tens of meters to more than 1000 m. A simple formula was derived that, for the sediments to be found in the area investigated, directly calculates the cover thickness from the frequency of the main peak in the H/V spectrum. A comparison with calculated resonant frequencies suggests the relation derived from the noise measurements depending on the velocity depth function of the shear wave. Classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the resonant frequency of the subsoil. The practical relevance of the investigation is illustrated by means of cross sections, constructed from results of the microtremor analyses, which provide a convincing image of the surficial structure of the areas investigated.

Abstract. Well logs, aquired in the two scientific drill holes of the German Continental Deep Drilling Program (KTB), provide continuous records of physical and chemical data of the metamorphic rocks penetrated. The 4-km-deep pilot hole was almost completely cored, enabling the well logs to be calibrated with regard to rock composition and structural features derived from laboratory analysis of cores. The observed relationships were transferred to the 9101 m deep, nearly uncored, main hole to reproduce in detail the lithology and to estimate physical properties from the logs. Synthetic lithological profiles were constructed for the pilot hole and the main hole by applying the electrofacies concept adapted to the crystalline environment. These profiles provide information on lithostratigraphy, alteration, cataclastic overprint, and petrogenetic features. Cross-hole correlations of these profiles reveal identical rock sequences for large sections of the drilled, metamorphic basement in both holes, in which the primary differences between the protoliths are largely preserved. Multivariate statistical methods were used to determine porosity depth functions from log responses. Linear as well as multilinear regression yielded continuous porosity profiles for both boreholes. Factor analysis was used to extract a parameter interpreted as a fluid and fracture indicator. Comparison of the porosity profiles with lithological information from log, core, and cuttings data revealed two different origins of increased porosity. Rock porosity and permeability are not only related to discrete planar discontinuities such as faults and fractures but also to more extensive zones of intense rock alteration where considerable matrix porosity occurs.