Seismische Bestimmung der bodendynamischen Kennwerte von oberflächennahen Schichten in Erdbebengebieten der Niederrheinischen Bucht

Summary

During the period from 1981 to 1983 seismic down-hole measurements were carried out at 36 selected sites within the Lower Rhine Basin and its vicinity in order to determine systematically the propagation velocities and the amplitude attenuation of compressional waves (P-waves) and shear waves (S-waves) in soft soil deposits of Quarternary and Tertiary age. For comparison, additional velocity measurements were made in Mesozoic and Paleozoic sedimentary rocks.

The seismic P-waves were produced by vertical hammer blows to an iron plate lying on the ground surface. For the generation of horizontally polarized S-waves a pendulum construction with a moving iron mass of some 70 kg’ was used. The mass blows horizontally against a wooden plank which is fixed to the ground. The seismic signals were detected in the borehole at various depths with triaxial geophones and digitally recorded on magnetic tape by means of an 8-channel Lennartz PCM-unit. A vertical stacking was performed in order to improve the signal/noise ratio. The data processing was made with a CBM-4032 microcomputer system.

From seismic propagation velocities and amplitude attenuation curves the elastic and non-elastic properties (dynamic shear modulus, Young’s modulus, bulk modulu’s, Poisson1s. ratio, Q value, non-elastic damping ratio) of the soil layers investigated were calculated. A Statistical evaluation of these data enabled the authors to deduce generalized relationships between dynamic soil properties and soil type, depth, geological age, and overconsolidation.

Based on these results local soil amplification curves for earthquake Vibration in the seismic active regions of the Lower Rhine Basin could be calculated with the frequency domain method of the ‘Steady State Harmonie Oscillations’ (ROESSET 1970). Amplification spectra for selected sites demonstrate that the local earthquake Vibration is strongly affected by the thickness and the dynamic properties of the near-surface-soil deposits.