EVIDENCE OF COSEISMIC SURFACE RUPTURING IN THE LOWER RHINE EMBAYMENT: A POSSIBLE SOURCE FOR THE 1756 DüREN EARTHQUAKE

Intraplate earthquakes pose a significant hazard in populated regions like central Europe. While the locations of potentially active faults are well known, our data base about the recurrence of earthquakes on these faults is rudimentary. The current debate ranges from slip dominated by large seismogenic ruptures to slip dominated by aseismic creep. Field evidence in support of the former is sparse, and hence, some authors concluded that many faults in central Europe deform by slow aseismic creep rather than by large, ground rupturing earthquakes.

We report new results from a paleoseismic study across the Schafberg fault in the area of Germany's largest historical earthquake (1756 AD, M_L 6.2 ± 0.2) that clearly revealed field evidence of significant damage of the earth's near-surface layers due to seismogenic faulting. At the trench site the fault is covered by < 5 m-thick Holocene fluvial gravel and flood deposits overlaying Lower Devonian shale. Our mapping revealed a surface offset of ~ 1m and a narrow zone of localized deformation expressed by abundant fractures with aligned and broken clasts extending vertically throughout the entire gravel unit. We mapped 237 fractured clasts and the long-axis orientation of ~ 10.000 clasts, which defines a ~ 10 m-wide deformation zone that coincides with the surface offset.

We interpret these features as the result of coseismic deformation at the near-surface end of the rupture, and we rule out slow deformation due to aseismic creep as governing process to cause rupturing of pebbles this close to the surface. Preliminary radiocarbon data bracket the event horizon to Latest Holocene age, which may correspond to the 1756 event. Further analyses are in progress.

We identified coseismic deformation at the Untermaubach site, because special conditions produced a number of features not normally observed in other fault exposures. The thin sedimentary cover (< 5 m) above basement rocks and the high water table may have played an important role in producing this unusual deformation pattern. Our results imply that seismogenic surface ruptures in continental interiors may be more common than previously thought.