Neefs, Ben ; Van Noten, Koen ; Camelbeeck, Thierry

Talk presented at 7th INTERNATIONAL GOGICA BELGICA MEETING 2021 on 2021-09-17

Abstract: To quickly assess the impact of earthquakes, many countries make use of earthquake intensity prediction equations (IPEs). These empirical equations describe the decay of ground motion in terms of macroseismic intensity and consequently can simulate the damage distribution and damage degree in near real-time. Intensity attenuation is region-specific and is dependent on seismic wave propagation in the crust and its sedimentary cover. To develop IPEs suitable for modelling the impact of Belgian earthquakes, local macroseismic data is thus required. The Royal Observatory of Belgium (ROB) has a rich heritage in collecting felt earthquake and damage reports. This database was extended by intensity data from neighboring countries and is used to develop Belgian IPEs. However, the traditional methodology, i.e. constructing a single intensity attenuation law for large areas, is not fit for the complex geological situation in Belgium. Macroseismic intensity distribution patterns of past Belgian earthquakes do not cohere to the common assumption of an isotropic intensity decrease with increasing epicentral distance. Intensity attenuation is strongly influenced by crustal and cover geological features that heavily control and limit seismic wave propagation and cause an anisotropic display of intensity decay. Geological features that have influenced past intensity distribution patterns and attenuation in Belgium are: - The strongly compacted and deformed Lower Palaeozoic Anglo-Brabant Massif, which easily transfers ground motion along its core axis in a WNW-ESE direction. - The increasing thickness of soft sedimentary Cenozoic strata covering the Anglo-Brabant massif towards the Belgium-Dutch border, effectively filtering high-frequency ground motions. - The Lower Rhine Embayment boundary faults, impeding effective ground motion transfer and serving as seismic mirrors. - The Midi-Eifel Fault which separates the Ardenne allochthon from its foreland and acts as a seismic barrier. - The shallow Walloon Carboniferous coal basins along a thin band across the country with a fast intensity attenuation. The quantitative effect of these geological regions on macroseismic intensity are best to be modelled separately, with different IPEs for each characteristic region in Belgium. But given the small size of both Belgium and these regions, local earthquakes are prone to hit multiple areas with different attenuation properties at once. This urges the necessity to subselect macroseismic data out of different earthquakes to be able to model IPEs independently. Furthermore, modelling anisotropic intensity decay requires an additional dimension, e.g. such as bedrock depth, to be added to the prediction equation, which is uncommon in literature and further complicates this goal. The complex Belgian geology thus requires an entire new methodology to be developed to be able to construct Belgian IPEs, which will be elaborated in detail in this presentation.

Keyword(s): Macroseismology ; Regional Attenuation ; Intensity Prediction Equation
Funding: PhD ROB funding/PhD ROB funding/PhD ROB funding