Path and site effects deduced from transfrontier internet macroseismic data of two recent M4 earthquakes in NW Europe
Koen Van Noten1, Thomas Lecocq1, Christophe Sira2, Klaus-G. Hinzen3, and Thierry Camelbeeck11Royal Observatory of Belgium, Seismology - Gravimetry, Ringlaan 3, B-1180 Brussels, Belgium 2French Central Seismological Office, University of Strasbourg, Rue René Descartes 5, 67084 Strasbourg Cedex, France 3University of Cologne, Bensberg Erdbebenstation,Vinzenz - Pallotti - Stasse 26, D-51429 Bergisch Gladbach, Germany
Received: 03 Nov 2016 – Accepted for review: 07 Nov 2016 – Discussion started: 09 Nov 2016
Abstract. The online collection of earthquake testimonies in Europe is strongly fragmented across numerous seismological agencies. This paper demonstrates how collecting and merging “Did You Feel It?” (DYFI) institutional macroseismic data strongly improves the quality of real-time intensity maps. Instead of using ZIP code Community Internet Intensity Maps we geocode individual response addresses for location improvement, assign intensities to grouped answers within 100 km2 grid cells, and generate intensity attenuation relations from the grid cell intensities. Grid cell intensity maps are less subjective and illustrate a more homogeneous intensity distribution than the ZIP code intensity maps. Using grid cells for ground motion analysis offers an advanced method for exchanging transfrontier equal-area intensity data without sharing any personal’s information. The applicability of the method is demonstrated on the DYFI responses of two well-felt earthquakes: the 8 September 2011 ML 4.3 (MW 3.7) Goch (Germany) and the 22 May 2015 ML 4.2 (MW 3.7) Ramsgate (UK) earthquakes. Both events resulted in non-circular distribution of intensities which is not explained by geometrical amplitude attenuation alone but illustrates an important low-pass filtering due the sedimentary cover above the Anglo-Brabant Massif and in the Lower Rhine Graben. Our study illustrates the effect of increasing bedrock depth on intensity attenuation and the importance of the WNW-ESE Caledonian structural axis of the Anglo-Brabant Massif on seismic wave propagation: seismic waves are less attenuated – high Q – along the strike of the massif but are stronger attenuated – low Q – perpendicular to this structure, especially when they cross rheologically different seismotectonic units that are separated by crustal-rooted faults.
Van Noten, K., Lecocq, T., Sira, C., Hinzen, K.-G., and Camelbeeck, T.: Path and site effects deduced from transfrontier internet macroseismic data of two recent M4 earthquakes in NW Europe, Solid Earth Discuss., doi:10.5194/se-2016-150, in review, 2016.