TECTONOGEOMORPHIC AND PALEOSEISMIC CONSTRAINTS ON ACTIVE INTRAPLATE FAULTS IN THE LOWER RHINE GRABEN, CENTRAL EUROPE

Identification of active faults in low-strain intraplate regions is hampered by the spatiotemporal scattering of large earthquakes and by barely detectable strain accumulation. In populated humid regions, both hillslope and anthropogenic processes further challenge the recognition of seismogenic structures. The Lower Rhine Graben in central Europe is a prime example of a seismically active low-strain rift situated in a humid and densely populated region. The approximate location of potentially active faults in this region is well known, but knowledge of the recurrence of large earthquakes and of the dominant fault slip mode is still rudimentary. The current debate ranges from slip dominated by repeated large earthquakes to slip dominated by aseismic creep.

In this study we examine the signs of coseismic deformation in the geological record, and the surface expression of active fault segments in the central and southwestern sectors of the Lower Rhine Graben.

We carried out a LiDAR study in the central part of the Lower Rhine Graben to examine the preservation potential of active fault scarps in populated, humid settings. Results illustrate that the fault scarps are subject to severe degradation due to fluvial erosion, hillslope processes and intensive anthropogenic overprint.

A trench excavation at the Schafberg fault in Holocene sediments yielded a broadly distributed fault zone with a net vertical displacement of 1 ± 0.2 m. Analysis of the faulted strata and radiocarbon ages of event horizons reveal evidence of at least one historical rupture event. The complex deformation pattern in the trench included a range of features such as liquefaction, gravel rotation, and a peculiar abundance of fractured clasts in the fault zone. Detailed analysis of the fractured-clasts allows new insight into the complex coseismic rupture and fracture processes in unconsolidated gravel deposits near the surface. Results of this study show that faults in the Lower Rhine Graben can produce large surface-breaking earthquakes. Results further imply that specific patterns of fractured clasts in fault zones may be a detector of coseismic rupture.