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 Eastern Alps based on the integration of available and updated lake paleoseismic data. The paleoseismic records at Plansee and Achensee covering the last ~10 kyrs 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 kyrs, from which four earthquakes are interpreted to left imprints in two or more lakes. Earthquake recurrence intervals range from ca. 1,000 to 2,000 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 (MW) 5.8–6.1 might have occurred around the Inn valley, the Brenner region and the Fernpass-Loisach region, and might have reached up to MW 6.3 at Achensee. The paleo-earthquake catalogue might hint at a shift of severe earthquake activity near the Inn valley from east to west to east during Postglacial times. Shakemaps highlight that such severe earthquake scenarios not solely impact the enhanced seismicity region of Tyrol, but widely affect adjacent regions like southern Bavaria in Germany.
Highlights
35 Lake paleoseismic studies in different tectonic settings have demonstrated that seismic shaking can leave specific traces in lacustrine sedimentary archives e.g. by basin-wide mass wasting or in-situ soft-sediment deformation above a certain intensity threshold at the lake site (Avşar et al, 2016; Doughty et al, 2014; Howarth et al, 2014; Monecke et al, 2004; Petersen et al, 2014; Praet et al, 2017; Strasser et al, 2013)
When positive and negative evidence for seismic shaking from multiple sites can be gathered for a certain time period, 15 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
Despite the uncertainties of potential temporal changes in the intensity threshold and a potential incomplete earthquake record, the expansion of continuous paleoseismic records to their longest-possible extent is especially valuable in intraplate tectonic settings where recurrence rates are long and paleoseismic event number is typically low, and little knowledge is available on recurrence patterns of severe earthquakes. 420
Summary
35 Lake paleoseismic studies in different tectonic settings have demonstrated that seismic shaking can leave specific traces in lacustrine sedimentary archives e.g. by basin-wide mass wasting or in-situ soft-sediment deformation above a certain intensity threshold at the lake site (Avşar et al, 2016; Doughty et al, 2014; Howarth et al, 2014; Monecke et al, 2004; Petersen et al, 2014; Praet et al, 2017; Strasser et al, 2013). The second method calculates potential source faults, the rupture length and the magnitude in a forward modelling approach, which handles positive/negative shaking evidence and considers the uncertainty of the IPE (Vanneste et al, 2018). This method 55 requires an accurate map of potentially active faults, which is complicated in the Alps due to the vast amount of faults within a confined space for which only limited knowledge exists on their subsurface continuation and interrelation to other faults. We provide possible earthquake parameters of paleo-earthquakes that can explain the observational sedimentological evidence in the lakes and discuss our results in the context of seismicity in intraplate tectonic settings
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