Molron, J.

Master thesis supervised by Lecocq, T.; Van Noten, K. (ULB)

Abstract: The thickness of soft sediments has an important role in the amplification and damping of seismic waves induced by an earthquake. This effect has been observed in central Belgium for some earthquakes (eg Goch 2011 and Ramsgate 2015) where the intensity of shaking was stronger where depth of the Brabant Massif is less than 200 meters. This master thesis aims to calculate the thickness of sedimentary layers above basement rocks by the H/V spectral ratio analysis for the area between Neder-over-Heembeek, Tienen, Gembloux and Hal in central Belgium. This method consists of analyzing seismic ambient noise recorded by three components of movement (one vertical and two horizontal) using broadband seismometer. The H/V spectral ratio is then calculated by dividing the horizontal components over the vertical component and typically gives a peak at the fundamental frequency of soil. In order to define the non-linear regression (powerlaw) between the fundamental frequency of soil and sediment thickness, 175 seismic measurements were carried out above boreholes from which the depth to basement is known. Several powerlaws relationships were constructed according to different criteria including the local geology, the amount of measurements and the geographic distribution of measurements. These different relationships were subsequently applied in three case studies, i.e. Court-Saint-Etienne, La Hulpe and Uccle, in which sediment thickness respectively increases from south to north because of the northwards dip of the top of the Brabant Massif. To determine which powerlaw needed to be used for a given area, a comparison between sediment thickness obtained by the powerlaws and thickness known from local boreholes was performed. Based on this comparison, this master thesis concludes that a local powerlaw, in which the local geology is taken into account, provides the most consistent results in the estimation of the sediment thickness. Four local powerlaws were selected that now can be used at any point in the study area where depth to basement is unknown. This study can now directly be applied for the creation of detailed topographical and geomorphological subcrop maps of the Brabant Massif by using the fast, efficient and inexpensive H/V method. The selected powerlaws are applicable for creating a sediment thickness model of the study area which ultimately can be combined with the existing thickness model of Flanders. This master thesis is a part of the FNRS project led by Koen Van Noten which aims to study the influence of site effects in central Belgium on earthquake strong ground motions.

Keyword(s): HVSR ; Brabant Massif ; mapping ; geophysics