THE ROERMOND EARTHQUAKE OF 13 APRIL 1992, THE NETHERLANDS - GEOLOGICAL ASPECTS

The epicentre of the Roermond earthquake is located near the western b oundary fault of the Roer valley trough, one of the deepest and the mo st active in the Quaternary part of the Lower Rhine graben. The Late P leistocene and Holocene activity of the trough is manifested by offset s of the main (Mindel) and the lower (Riss and Late Pleistocene) terra ces along the boundary faults. Surface fractures have been observed in an area of more than 50 km2. 2.5-3.5 km to the north of the town of R oermond, at 0.8 km to the south of the village of Herkenbosch and in t he southeastern part of the village of Montfort. Three types of ruptur es were differentiated: scarps up to 50 cm high along open fractures n ear the Maas River; open fractures (continued by scarps in some places ) and open fractures with a liquefaction of the Quaternary alluvium sa nds. The last type is predominant. The location of the ruptures depend s on the landscape and water-table of the region. While they could be produced solely by hydraulic shock during the earthquake (increased by the wet spring season), the majority of the ruptures strike N50-degre es-W +/- 10-degrees, i.e. parallel to the main trough faults, or N55-d egrees-E + 10-degrees, along 'neotectonic lines', parallel to the Maas valley and deduced from space imagery. Thus, the ruptures could be th e secondary surficial effect of the earthquake, linked indirectly with the active tectonics of the region.