Earthquake-induced deformation structures in lake deposits: A Late Pleistocene to Holocene paleoseismic record for Central Switzerland

Abstract

Traces of three larger historic and at least seven prehistoric earthquakes during the last 15,000 years were found in the sedimentary record of four lakes in Central Switzerland. The Swiss historic earthquake catalogue of approximately the last 1000 years reports three larger earthquakes in Central Switzerland with moment magnitudes varying between Mw = 5.7 and Mw = 6.2 (1964 AD Alpnach, 1774 AD Altdorf, 1601 AD Unterwalden) and the nearby catastrophic Mw = 6.9 event close to Basel in 1356 AD. In order to determine the recurrence intervals of such events and thus, the seismic hazard and risk, paleoseismic investigations were carried out in four different lakes of Central Switzerland (Lungerer See, Baldegger See, Seelisberg Seeli, Vierwaldstätter See) using high-resolution seismic data and sediment core analyses. Depending on lake basin geometry, sediment type and local ground shaking earthquake- induced deformation structures comprise large-scale mass movement deposits and small-scale in-situ deformation features. The signatures of historic earthquakes show that lacustrine sediments are only affected by seismic shocks of a minimum magnitude of ~Mw = 5.7 and within or close to the isoseismal line of intensity VII. Traces of at least six prehistoric events in Central Switzerland of similar size and magnitude than the Mw = 6.2 1601AD Unterwalden earthquake and one prehistoric 1356AD Basel-type earthquake are determined by evaluating the type of deformation and the basin-wide as well as the regional distribution of deformations. One further event in the Basel region and four events in Central Switzerland are less well defined in the sedimentary record. Compared to the Mid Holocene earthquake frequency in Central Switzerland seems to be enhanced during Late Pleistocene/Early Holocene time and during the last 4000 cal y BP, which can be related to isostatic rebound after the ice retreat starting at about 15,000 cal y BP and a periodic activation of a seismogenic zone in the Alps during recent times.