Active tectonics of the Lower Rhine Graben (NW Central Europe)

Identification of active seismogenic faults in low-strain
intraplate regions is a major challenge. The understanding of
intraplate earthquakes is hampered by the spatiotemporal scattering
of large earthquakes and by barely detectable strain accumulation.
In populated humid regions, both hillslope and anthropogenic
processes are important challenges to the recognition of
potentially active faults. The Lower Rhine Graben is the NW segment
of the European Cenozoic Rift System. It 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
fault segments 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.
The purpose of this thesis is to determine whether the Lower Rhine
Graben is an exception to the usually observed deformational
behaviour of the upper crust, whereby active faults fail by brittle
behaviour. The thesis addresses the Holocene, historical and
present-day tectonic activity of the Lower Rhine Graben. It
examines the signs of coseismic deformation in the geological
record, and the surface expression of active fault segments. I
analyzed high-resolution LiDAR terrain models of segments of the
Erft and Wissersheimer faults, in order to understand the
preservation potential of active fault scarps in populated, humid
settings. Results of the LiDAR analysis illustrate that the central
part of the Lower Rhine Graben is characterized by severe
degradation and modification of suspected seismogenic structures.
Degradation is due to fluvial erosion, hillslope processes and
anthropogenic overprint. This analysis shows, also for the first
time, the severity of surface modification of the region resulting
from aerial bombing during World War II. A large trench excavation
at the Schafberg fault in Holocene sediments yielded a broadly
distributed fault zone with a peculiar abundance of fractured
clasts. A particular question at this site is whether or not the
fault ruptured in the 1756 AD Düren event. The excavation reveals
the first evidence of historical seismogenic faulting in the Lower
Rhine Graben. Coseismic deformation at this site is expressed by a
net vertical displacement of 1 ± 0.2 m and complex gravel
fracturing. Analysis of the faulted strata and radiocarbon ages of
event horizons reveal evidence of at least one, possibly two
coseismic events since the Holocene. The youngest of them overlaps
with the 1756 AD Düren earthquake. The complex deformation pattern
in the trench included a range of features such as liquefaction,
rotated, and fractured clasts in the fault zone. I developed a new
analysis technique based on “fractured-clasts”, which allows
insight into coseismic rupture and fracture processes in
unconsolidated gravel deposits. Results of this paleoseismic study
show that faults in the Lower Rhine Graben do not move dominantly
by aseismic creep. They further support the observation that faults
in low-strain intraplate rifts can produce large surface-breaking
earthquakes. The results of this thesis further imply that specific
patterns of fractured clasts in fault zones may be a detector of
coseismic rupture, and could in principle be used to calculate the
energy involved in the rupture process.