Magnitude and source area estimations of severe prehistoric earthquakes in the western Austrian Alps

Abstract

In slowly deforming intraplate tectonic regions such as the Alps only limited knowledge exists on the occurrence of severe earthquakes, their maximum possible magnitude, and their potential source areas. This is mainly due to long earthquake recurrence rates exceeding the time span of instrumental earthquake records and historical documentation. Lacustrine paleoseismology aims at retrieving long-term continuous records of seismic shaking. A paleoseismic record from a single lake provides information on events for which seismic shaking exceeded the intensity threshold at the lake site. In addition, when positive and negative evidence for seismic shaking from multiple sites can be gathered for a certain time period, minimum magnitudes and source locations can be estimated for paleo-earthquakes by a reverse application of an empirical intensity prediction equation in a geospatial analysis. Here, we present potential magnitudes and source locations of four paleo-earthquakes in the western Austrian Alps based on the integration of available and updated lake paleoseismic data, which comprise multiple mass-transport deposits on reflection seismic profiles and turbidites and soft-sediment deformation structures in sediment cores. The paleoseismic records at Plansee and Achensee covering the last ∼10 kyr were extended towards the age of lake initiation after deglaciation to obtain the longest possible paleoseismic catalogue at each lake site. Our results show that 25 severe earthquakes are recorded in the four lakes Plansee, Piburgersee, Achensee, and potentially Starnbergersee over the last ∼16 kyr, from which four earthquakes are interpreted to have left imprints in two or more lakes. Earthquake recurrence intervals range from ca. 1000 to 2000 years, with a weakly periodic to aperiodic recurrence behavior for the individual records. We interpret that relatively shorter recurrence intervals in the more orogen-internal archives Piburgersee and Achensee are related to enhanced tectonic loading, whereas a longer recurrence rate in the more orogen-external archive Plansee might reflect a decreased stress transfer across the current-day enhanced seismicity zone. Plausible epicenters of paleo-earthquake scenarios coincide with the current enhanced seismicity regions. Prehistoric earthquakes with a minimum moment magnitude (M_w) 5.8–6.1 have occurred around the Inn valley, the Brenner region, and the Fernpass–Loisach region and might have reached up to M_w 6.3 at Achensee. The paleo-earthquake catalogue might hint at a shift in severe earthquake activity near the Inn valley from east to west to east during postglacial times. ShakeMaps highlight that such severe earthquake scenarios do not solely impact the enhanced seismicity region of Tyrol but widely affect adjacent regions like southern Bavaria in Germany.