Macroseismic effects in Germany of the 1992 Roermond earthquake and their interpretation

Abstract

The Roermond earthquake of April 13, 1992, with a local magnitude of ML = 5.9 , belongs to the largest earthquakes which have been observed in the Lower Rhine Embayment in historical time. It was felt in central and western Europe over an area of about 600 000 km2. The German territory forms much of the eastern part of that area. The most distant reports in Germany came from Kiel (450 km), Berlin (540 km) and Munich (520 km). The epicentral intensity is observed in the German-Netherlands border region was VII on the MSK-scale. For an ML = 5.9 earthquake this I0 is unusually low as compared to other large earthquakes in the Lower Rhine Embayment. Two factors are assumed to be mainly responsible for the low epicentral intensity: (1) the focal depth which is deeper than normal, and (2) the unusually strong absorption of seismic energy by a more than 1500 m-thick layer of soft Tertiary and Quaternary sediments within the Roer Valley Graben near the epicenter. More than 2000 macroseismic reports from 600 different localities have been interpreted, resulting in detailed isoseismal maps for Germany. The following mean isoseismal radii have been determined: r7 = 6 km, r6 = 42km, r5 = 102 km, r4 = 179 km, r3 =322 km, r2 = 440 km. Based on these isoseismal radii the macroseismic focal depth has been determined with an iterative computer program based on the method of Sponheuer (1960). The uncorrected observed I0 = VII gives a focal depth of about 26 km. However, if we correct I0 for the influence of the sedimentary graben fill, resulting in a value of VII-VIII, the obtained depth is about 17 km. This corresponds better with the focal depth as determined instrumentally by various working groups.