Van Noten, Koen ; Lecocq, Thomas ; Hinzen, Klaus ; Camelbeeck, Thierry

Talk presented at 5th International Geologica Belgica Conference, Mons, Belgium on 2016-01-27

Abstract: Since 1938, when an earthquake occurs, the Royal Observatory of Belgium (ROB) sends out national earthquake inquiries to each municipality to create intensity maps of the felt area. Since 2002, only few hours after the ML 4.9 Alsdorf earthquake, the paper inquiry was complemented by an online “Did You Feel It?” (DYFI?) inquiry on the seismology.be website to gather witness reports. Since 2010, the ROB has established a partnership with the Bensberg Seismic Network (University of Cologne) to acquire real-time transfrontier macroseismic data. Analysis of macroseismic DYFI? data contributes to seismic hazard calculation and the construction of real-time shake maps in the near future. In this presentation, nearly 14 years of DYFI? data will be evaluated. Macroseismic analysis of felt earthquakes is important as the perception of people can be used to detect local/regional site effects without covering the whole country with seismometers. Investigations on (strong) ground motions generated by earthquakes provide evidence that the regional/local geological structure of a site, especially the presence of soft sediment layers, modify the characteristics of seismic shaking at the surface. Especially the thick sediment cover above the Brabant Massif basement rocks dominate how moderate-magnitude earthquakes are felt in central and northern Belgium. The 1938 ML 5.6 Zulzeke-Nukerke, 2002 ML 4.9 Alsdorf (Germany), 2011 ML 4.3 Goch (Germany) and 2015 ML 4.1 Ramsgate (UK) events all resulted in a particular non-concentric macroseismic distribution in which higher intensities are farther reported in an E-W direction than in a N-S orientation. This peculiar intensity distribution cannot be explained by circular geometrical amplitude attenuation alone, relating energy decay to increasing epicentral distance, but rather illustrates a low-pass filtering effect of the cover sediments above the Brabant Massif resulting in the absence of sensible higher frequencies in seismic waves arriving at the surface for these earthquake magnitudes. Another interesting aspect is the sound earthquakes produce at the surface. Since 2009, the ROB added a question to the DYFI? questionnaire inquiring the sound perception of people. If a sound was heard, people are asked if it was either light (brief or prolonged) or strong (brief or prolonged), hereby hinting on the audible frequency content of seismic waves arriving at the surface. The sound reported after ML > 3 earthquakes typically show an increasing intensity (louder and longer) with decreasing epicentral distance. However, also hypocentral depth plays an important role with a shallower sources resulting in a higher percentage of heard responses.

Keyword(s): Did You Feel It? ; macroseismology ; mapping ; Geology