Use of microtremor measurement for assessing site effects in Northern Belgium - interpretation of the observed intensity during the M-S=5.0 June 11 1938 earthquake
Nguyen, Frederic × Van Rompaey, G Teerlynck, H Van Camp, M Jongmans, D Camelbeeck, T #
Kluwer Academic Publishers
Journal of Seismology vol:8 issue:1 pages:41-56
To provide quantitative information on site effects in the northern part of Belgium, forty-seven H/V microtremor measurements were performed with 5 second seismometers over an area of about 15.000 km(2). Most of the results show a northward regular increase of the fundamental period in agreement with the augmentation of the Mesozoic and Cainozoic soft sediments thickness from a few meters 40 km south of Brussels to 900 m at the Netherlands-Belgium border. The measured resonance frequency values were consistent with theoretical computations performed at different sites on the basis of existing information and shallow seismic experiments. At one particular site (Uccle) where borehole data were available, microtremor measurements using an array of four seismological stations with different apertures allowed to obtain the low frequency part of the Rayleigh wave dispersion curve, extending the range covered by the analysis of surface waves artificially generated. The V-S profile derived from the surface wave inversion corroborates the 1 Hz natural frequency of the site. Comparison of these results with the macroseismic information concerning the M-S=5.0 1938 earthquake which occurred 50 km west of Brussels, confirmed the hypothesis that the geological structure of the Brabant massif is likely to control damage distribution during such an earthquake. Comparison between the intensity map of the 1938 earthquake and the resonance period of sediments obtained by our microtremor study shows a clear relation between the two parameters. During the 1938 earthquake, site effects played a prominent role due to the dimension of the source whose corner frequency was about 1 Hz.