Magnitudes and surface rupture lengths of paleo-earthquakes at the NW-part of the Peel Boundary fault zone, Roer Valley Rift System

The Peel Boundary Fault zone (PBFZ) is the 125 km long, seismically active, northern bounding fault zone of the Roer Valley Rift System (RVRS). The last damaging earthquake along the PBFZ was the Roermond earthquake of 1992. It had a magnitude of Mw 5.3 and no surface rupture. Previous results from two trenching studies located in the central and southeastern parts of the PBFZ provided evidence for two surface rupturing paleo-earthquakes. The largest earthquake had an estimated magnitude of Mw ~6.8 and a surface rupture length of at least 35 km. As it took place around the Late Pleniglacial – Late Glacial transition a link to glacio-isostatic motions is likely. Results from a new trench situated at the northwestern part of the PBFZ shows evidence for three to four paleo-earthquakes, of which three were surface rupturing. These comprise two normal faulting- and one, younger transtensional displacement. The normal faulting events have ~1 m vertical displacements each, which translate into magnitudes of Mw ~7. Like the previous results, they occurred during the Late Pleniglacial-Late Glacial transition, at ~15 ka and ~ 14 ka. The younger trans-tensional event occurred sometime during the Holocene, pre-dating an unaffected, 13th century man-made paleo-channel on the hangingwall. The potential fourth, non-surface rupturing earthquake is indirectly evidenced by loading deformations of a sand layer and a collapsed brick-wall in the infill of the paleo channel. Comparison of our trenching results to those from the two previous studies, which were located farther to the southeast along the PBFZ, shows that for one event a correlation is possible. The correlation would indicate a surface rupture length of at least 55 km. Combined, all trenching results indicate that the characteristic maximum rupturing displacement is ~1 m, and thus that Mw ~7 is the maximum magnitude of paleo-earthquakes along the PBFZ.