Camelbeeck, Thierry ; Van Noten, Koen ; Hendrickx, Marc ; Lecocq, Thomas

Talk presented at 7th Geologica Belgica International Conference, Tervuren, Belgium on 2021-09-17

Abstract: In the Carboniferous coal area of the Hainaut province in Belgium, a century of shallow seismic activity occurred from the end of the 19th century until the late 20th century. This seismicity is the second largest source of seismic hazard in NW Europe. In this sequence, five seismic events (Mw~4.0), that occurred on 3 June 1911, 3 April 1949, 15 December 1965, 16 January 1966, and 28 March 1967, locally caused moderate damage to buildings to maximum intensity VII on the EMS-98 scale. For decades, the natural causality of this seismicity and its potential link with coal extraction is disputed due to uncertainties in earthquake location and depth. To tackle this discussion in the near future, we reviewed a century of intensity data collected by official macroseismic surveys held by the Royal Observatory of Belgium, press reports, and contemporary scientific studies. In this contribution, we present methodological advances on how we updated the magnitude, epicentral intensity and location, and depth of 123 Hainaut seismic events only using macroseismic data. We discuss the impact and damage of this unique seismicity on the building infrastructure and people. Our study highlights the capability of shallow (< ~6km), small-magnitude earthquakes to generate damage. The intensity dataset shows that inside the Carboniferous coal basin (that extends from Mons to Liège) intensity attenuates much faster than in the surrounding Paleozoic Brabant and Ardenne basements due to unique characteristics of the coal basin. Using the improved intensity dataset, we modelled a new Hainaut intensity attenuation law and created relationships linking magnitude, epicentral intensity and focal depth. The new attenuation model suggests that current (international) hazard maps overestimated ground motion levels in the Hainaut area due to the use of inadequate ground motion prediction equations. Hence, the model should be used to evaluate the potential impact of current and future, e.g. geothermal energy, projects in the Hainaut area and other regions with a similar geological configuration.

Keyword(s): Macroseismology ; Historical seismology ; Intensity