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

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 10 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 15 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 20 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 25 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 30 southern Bavaria in Germany. https://doi.org/10.5194/nhess-2021-281 Preprint. Discussion started: 19 October 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction
Lake paleoseismic studies in different tectonic settings have demonstrated that seismic shaking can leave specific traces in 35 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). As glacigenic lakes are often characterized by continuous sedimentation since deglaciation, they can have continuously recorded earthquakes since then. Especially in slowly deforming, intraplate tectonic settings like the Alps, recurrence rate of severe earthquakes often exceed the timespan of 40 historical earthquake documentation (Beck, 2009;Brooks, 2018;Kremer et al., 2017;Moernaut, 2020;Simonneau et al., 2013). Therefore, long-term lake paleoseismic records provide valuable data to better constrain the recurrence rate and the maximum possible magnitude of severe earthquakes. However, the epicenter location, magnitude and rupture extent of the event mostly remains unknown solely based on a lacustrine paleoseismic archive. Moreover, on-fault evidence are hardly preserved in the Alps due to penetrative anthropogenic landscape modification, gravitational slope processes and relatively 45 high erosion rates (Ustaszewski and Pfiffner, 2008). The so far unraveled active faults are only discovered in special environments, such as displaced Roman archeological remains (Galadini and Galli, 1999), tectonically damaged speleothems (Plan et al., 2010) and in the sedimentary infills of lakes (Fabbri et al., 2017(Fabbri et al., , 2021Gasperini et al., 2020;de La Taille et al., 2015;Oswald et al., 2021a). To date, two different ground motion modelling methods have been developed to overcome these problems. The first method calculates potential magnitude-epicenter assemblages over a grid based on 50 positive/negative shaking evidence of a certain intensity threshold using a backward application of an intensity prediction equation (IPE; Strasser et al., 2006;Kremer et al., 2017after Bakun & Wentworth, 1997. The second method calculates potential source faults, the rupture length and the magnitude in a forward modelling approach, which equally handles positive/negative shaking evidence and considers the uncertainty of the IPE (Vanneste et al., 2018). However, this method requires an accurate map of potentially active faults, which is complicated in the Alps due to the vast amount of faults within 55 a confined space for which only limited knowledge exists on their subsurface continuation and interrelation to other faults.
Recent lacustrine paleoseismological studies in the western Eastern Alps retrieved three continuous paleoseismic archives covering the Holocene (Oswald et al., 2021a(Oswald et al., , 2021b. This study aims to extend these existing paleoseismic records in the western Eastern Alps towards the timing of lake initiation where applicable. We compare the timing and relative imprint size of earthquake-related deposits. Based on this we evaluate potential single earthquake events with multiple lake imprints and 60 calculate recurrence statistics on the paleo-earthquake catalogue of the region. Furthermore, 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. https://doi.org/10.5194/nhess-2021-281 Preprint. Discussion started: 19 October 2021 c Author(s) 2021. CC BY 4.0 License.
Brenner fault (Eisbacher and Brandner, 1996;Mancktelow et al., 2015;Ortner et al., 2006). These faults were mainly active together with the Periadriatic fault south of the study area in Cenozoic times caused by NNW-ward indentation of the Adriatic microplate in the south and slab-rollback in the eastern Pannonian basin leading to lateral escape tectonics (Ratschbacher et al., 1991;Rosenberg et al., 2004). GPS measurements show that the Adriatic microplate is still pushing northward with about 1-2 mm/a (Métois et al., 2015) causing moderate seismicity with dominant N-S thrust faulting 75 mechanisms focused in a 100 km wide E-W zone in the Inn valley ( Fig. 1; Reiter et al., 2018). This zone of enhanced seismicity also hosted a few severe historically documented earthquakes (Hammerl, 2017;Hammerl et al., 2012;Stucchi et al., 2013). Recent studies established three paleoseismic records in the study area spanning the last 10 kyrs by hydroacoustic imaging of the lake subsurface and sediment core analyses in the lakes Plansee, Piburgersee and Achensee ( Fig. 1; Oswald et al., 2021a;b). Earthquake imprints are expressed as seismic-stratigraphic horizons of multiple, coeval mass-transport 80 deposits (MTDs) directly overlain by a (mega-)turbidite in seismic data of Achensee and Plansee, whereas in Piburgersee seismic shaking caused in-situ soft-sediment deformation structures (SSDS).

Extension of the paleoseismic records and event-scaling
The paleoseismic records of Plansee and Achensee are extended further back in time by i) the investigation of outstanding 85 turbidite deposits within the glaciolacustrine sediments by visual analyses of X-ray computer tomographic (CT) and core image data in deeper core sections (only Plansee), and by ii) extending the age-depth models beyond the reach of the core towards initiation of glaciolacustrine sedimentation interpreted on seismic data, which allows the timing of multiple, coeval MTD horizons to be estimated. The sediment depth between the base of the core and the interpreted horizon of the onset of glaciolacustrine sedimentation is calculated using the two-way travel time (TWT) in seismic data and a mean acoustic 90 velocity of the glaciolacustrine seismic unit (~1610 m/s) derived from p-wave velocity measurements on the 12-15 m subsurface depth interval of the cores that recovered the upper part of the glaciolacustrine unit. For the age-depth modelling, we used the broad age range 15-17 ka BP as input age for the onset of glaciolacustrine sedimentation, based on the agebracketing of glacier readvancement (~17 ka BP) and their final moraine termination (~15 ka BP; Ivy-Ochs et al., 2008).
This results in the extension of the pre-existing age-depth models (Oswald et al., 2021a(Oswald et al., , 2021b covering the entire sequence 95 of glaciolacustrine and lacustrine sedimentation from the deglaciation of the basin to the present day. Age-depth models from the long cores Plan18-L1 at Plansee and ACH19-L3 at Achensee (Oswald et al., (2021a;b) are based on a combination of short-lived radionuclides (210Pb and 137Cs) and radiocarbon dates (Table S1) and calculated using the R-package Bacon version 2.5.0 (Blaauw and Christen, 2011). For the age-depth modelling, event deposits with thickness of >5 cm are deleted to obtain an event-free sediment depth. In order to obtain also an event-free sediment depth 100 for the calculated sediment depth below the reach of the core, we assume a constant sedimentation rate and comparable amount and thickness of event deposits as in the cored section of glaciolacustrine sediments. In principle, an earthquake causes a specific sedimentary imprint when a certain, lake specific seismic intensity threshold is exceeded (e.g. Monecke et al., 2004;Van Daele et al., 2015). Above this threshold, measurements of the sedimentary imprints such as outstanding turbidite thickness (Moernaut et al., 2014) or the occurrence of postseismic landscape response 105 (Howarth et al., 2016) might actually indicate much higher intensities. Therefore, the inferred earthquake events are relatively scaled above the lake's individual intensity threshold based on their imprint size. For Piburgersee, SSDS are scaled based on three development stages ranging from folded layer, incipient breccia to intraclast breccia with increasing intensity (Molenaar et al., 2021;Oswald et al., 2021b;Wetzler et al., 2010) including the deposit thickness and a normalization to the range 0 to 1. The largest imprint in each record corresponds to a value of 1. At Plansee and Achensee, events are relatively 110 scaled based on the MTD volume (V) and turbidite thickness (T) as follows: In a first step, V is divided by the slope area and T is divided by the ratio of the slope area (Aslope) to basin area (Abasin) to make the imprints of several lakes with different morphometric characteristics comparable (equations 1 and 2).

=
(1) = ⁄ (2) 115 V is obtained from mapping MTDs in reflection seismic data, Aslope and Abasin are derived from spatial analysis of bathymetric data and T is measured from the sediment core. The total MTD volume is strongly dependent on the slope area but does not change with basin area. The turbidite thickness is dependent on both, the slope and the basin area, because e.g. a small depocenter surrounded by large slopes would result in a relatively thicker turbidite. To calculate a scaled sedimentary imprint SEDscaled, the morphometrics-corrected MTD volume (Vcorr) and turbidite thickness (Tcorr) are normalized to the 120 range 0 to 1, where the largest Vcorr and Tcorr corresponds to a value of 1, and summed up. Along the Alpine Fault in New Zealand, postseismic landscape response was documented to indicate 1-2 intensity levels higher than events that lack such response (Howarth et al., 2016), but in our case study there is no quantitative data to confidently determine this value for incorporation in the event scaling. In order to also add the occurrence of postseismic landscape response (L) as this third parameter, we define L = 1 for events with postseismic landscape response and L=0 for events without landscape response. 125 In a last step, SEDscaled of the individual records are normalized to the range of 0 to 1 for a better comparison between different paleoseismic records (equation 3).

Recurrence statistics
The earthquake recurrence interval (in years) is calculated on the interevent times obtained on each lake record. where an imminent event in the following year is assumed (cf. Griffin et al., 2020). In this way, we also consider the elapsed time since the last event as minimum value for the latest interevent time. Additionally, insights into the recurrence pattern of 135 earthquakes (i.e. aperiodicity) of the lake records are obtained from the calculation of burstiness, which is directly transformed from the coefficient of variation (CoV) representing the mean-normalized standard deviation of the interevent times (Goh and Barabási, 2008). In analogy to the recurrence interval, burstiness and CoV are calculated on interevent times derived from all age-depth model simulations of each record (Kempf and Moernaut, 2021). We subdivide recurrence patterns, where a paleoseismic record with burstiness -1 to -0.33 (CoV < 0.5) is strongly periodic, -0.33 to -0.05 (CoV 0.5-140 0.9) is weakly periodic, -0.05 to 0.05 (CoV 0.9-1.1) is aperiodic and 0.05 to 1 (CoV > 1.1) is bursty. It has to be noted that our approach does not incorporate the uncertainty in the estimation of burstiness due to the limited event number, i.e. there is a tendency to underestimate the burstiness of the true population mean (Kempf and Moernaut, 2021;Williams et al., 2019).
However, we consider this approach appropriate for first order estimations, especially given the fair amount of paleoseismic events (8-11) in each record and the record continuity. In addition, we calculated recurrence interval and burstiness of the 145 regional event record, where the individual events of all three lakes, each comprising all ages of the individual age-depth model simulations are sorted by their mean age. This has the advantage that the stratigraphic order is also retained for the recurrence rate within the individual records, but samples somehow randomly between the different records.
Hence, this sampling technique does not artificially overestimate the recurrence intervals of subsequent events within an individual record, which is the case for a pure random sampling between event ages of several records. However, recurrence 150 intervals might be slightly underestimated when shifting from one record to another, as the sampling might not be completely random due to the prior defined event order.

Source area and magnitude reconstruction of paleo-earthquakes
Reconstruction of potential source areas and magnitudes of paleo-earthquakes is based on a two-step geospatial grid-search analysis (Strasser et al., 2006(Strasser et al., , 2013Kremer et al., 2017 after Bakun andWentworth, 1997) reversely applying the empirical 155 IPE developed for deep earthquakes in the Swiss Alps which is also applicable for >MW 5.5 earthquakes (Fäh et al., 2011).
The input parameters consist of the intensity threshold for generating lacustrine mass wasting or soft-sediment deformation structures (SSDS), along with site coordinates of positive and negative evidence for seismic shaking that exceeds this threshold. In this study, we consider earthquake-related deposits i.e. enhanced mass wasting or SSDS as potentially coeval for the calculation of paleo-earthquake scenarios when more than 40% of the 95% probability density functions (PDFs) of 160 the event ages overlap. If a lake recorded earthquake-related deposits at the observed event period, the intensity threshold was exceeded and the lake shows positive evidence for seismic shaking. In contrast, when no earthquake-related deposits are present in the lake record at the observed time, it is assumed that the intensity threshold was not reached, resulting in negative evidence for seismic shaking as input for the calculation of plausible magnitudes and source areas. For the studied lakes in Tyrol, the intensity threshold is defined to seismic intensity (SI) ~VI¼ on the European macroseismic scale (EMS-165 98;Grünthal, 1998) representing the mean value of the lake's individual threshold intensities (Oswald et al., 2021a(Oswald et al., , 2021b. This intensity threshold is also similar to the threshold value VI⅕ established for swiss peri-alpine lakes (Kremer et al., 2017), which further justifies to apply the estimated threshold value SI ~VI¼ to all studied lakes for the reconstruction of paleo-earthquake scenarios. In a first step of the grid-search analyses, minimum magnitudes are calculated for each cell based on the positive earthquake evidence. In a second step, each cell is evaluated for the sites with negative evidence and 170 rejected if the calculated site intensity from the magnitude of the cell exceeds the intensity threshold of a site with negative earthquake evidence (see also Kremer et al., 2017).

Calculation of paleo-earthquake shakemaps
For a deterministic presentation of the paleo-earthquake effects, we created shakemaps to show the expected seismic intensity (EMS-98) in the broader study area for selected paleo-earthquake scenarios. Therefore, we used the empiric 175 intensity (I)distance (d) relation (equation 4) for earthquakes in Central Europe by Sponheuer (1960): with an absorption coefficient (0.002 for earthquakes in Central Europe) and the expected maximum intensity I0 from the empiric relation by Rudloff & Leydecker (2002)  As these relations do not describe site effects that contribute to the expected intensity in the study area, we used the macroseismic data from regional earthquakes of central Europe recorded between 2013 and 2020 to determine site parameters a and b for each point x that describe the median intensity difference from the empirical relation (equation 6). In a second step, we used the intensities of historical earthquake with MW >5 in Tyrol (Stucchi et al., 2013) to estimate the 185 difference Id of the maximum intensity to the empirically calculated I0. The resulting equation has the form: While parameter a is depending on earthquake parameters, b relates to local topographic effects taken from a 1 km resolution digital elevation model. Therefore, uncertainties can be expected for topographic features < 1 km especially narrow valleys.

Extension of the paleoseismic records at Plansee and Achensee
Seismic data of Plansee holds eleven seismic-stratigraphic horizons of coeval, multiple MTDs (A-K in Fig. 2A), of which the upper five each correspond to an amalgamated turbidite in the sediment core (Oswald et al., 2021b). As this assemblage https://doi.org/10.5194/nhess-2021-281 Preprint. Discussion started: 19 October 2021 c Author(s) 2021. CC BY 4.0 License. of observational evidence for horizon A, is linked to the severe, historic MW 5.3 Namlos earthquake, also horizons B-E were 195 interpreted as earthquake-induced mass-wasting events (Oswald et al., 2021b). Towards greater depths six additional horizons of coeval, multiple MTDs occur in seismic data (F-K in Fig. 2A), each containing 3 to 5 MTDs in the main basin of Plansee (see supplementary information of Oswald et al. 2021b).
In the sediment core, two main types of turbidites thicker than 5 cm are observed in the glaciolacustrine clays and mapped throughout the core (Fig. S2). The first turbidite type is characterized by brown to light grey colored, normal graded deposits 200 with a distinct light-colored clayey silt cap on top and contains terrestrial macro-organic remains (Fig. 2b). In analogue to the lacustrine sedimentation, these graded turbidites are interpreted to result from lake-external detrital sediment transported into the lake during hydrological events (Kiefer et al., 2021). The second type consists of overall grey homogeneous turbidites with only a thin graded base and a distinct clayey-silt cap on top and without any macro-organic remains (Fig. 2c). These homogeneous turbidites are interpreted as sediment remobilization of lake-internal slopes. The turbidite of event J contains a 205 5 cm, angular dolomite gravel within a homogeneous silt matrix in the uppermost part. This gravel potentially indicates a dropstone transported within a snow avalanche on a frozen lake (cf. Vasskog et al., 2011). Additionally, based on core-toseismic correlation, each of the seismic-stratigraphic horizons H-J correspond to a homogeneous turbidite in the 15 m long sediment core (Fig. 2a). Therefore, we interpret these homogeneous turbidites derived from lake-internal enhanced mass wasting as earthquake-related in analogue to the earthquake proxy in the Holocene lacustrine sediments in Plansee (Oswald 210 et al., 2021b) and similar lake settings (e.g. Strasser et al., 2013). In contrast, seismic-stratigraphic horizons F and G correlate to a single or a sequence of graded and much thinner turbidites separated by background sediment (Fig 2a-b). The interpretation of seismic stratigraphic horizons F and G is not unique and they could either represent earthquakes with potentially lower intensities at the lake or hydrological events that caused onshore mass wasting within a short period. Therefore, event horizons F and G are not further used for the paleoseismic catalogue of the region. 215 The pre-existing age-depth model covering the Holocene is extended towards the base of glaciolacustrine sediments by adding an additional radiocarbon age in the Late Glacial at 7.5 m event-free sediment depth (Table S1; Fig. S2) and an inferred deglaciation age at circa 15-17 ka BP at 13.5 m (Ivy-Ochs et al., 2008). This results in paleo-earthquakes at circa 12.7, 13.4, 14.6 and 16.0 ka BP with 95% age ranges of 12.5-12.9, 13.3-13.6, 14.3-14.9 and 15.6-16.4 ka BP, respectively (Table 1). 220 Achensee contains twelve seismic-stratigraphic horizons with multiple MTDs, delta collapse deposits and (mega-)turbidites (A-L in Fig. 3a; Oswald et al., 2021a). Similar to Plansee, sedimentary earthquake imprints are expressed by the occurrence of multiple, coeval MTDs in seismic data and a corresponding amalgamated turbidite was previously correlated to the historical severe MW ~5.7 Hall earthquake in 1670. Therefore seismic-stratigraphic horizons B-L were interpreted as earthquake-related, but only a rough age estimate (11 -18 ka BP) was provided for the event horizons K and L which are 225 below the reach of the sediment cores. In contrast, the youngest event horizon A is not earthquake-related and represents a period of enhanced shoreline erosion due to anthropogenically induced lake level changes especially at the early phase of hydropower generation at Achensee. In reflection seismic data of Achensee, lake strata is offset by subaqueous surface ruptures in prolongation of an onshore mapped fault. The upper terminations of these surface ruptures occur at three different stratigraphic levels, where each 230  Table S1) and an assumed lake initiation age (~15-17 ka BP) derived from the regional deglaciation age ( termination is directly overlain by multiple coeval MTDs (event hoirzons G, K and L). This surface rupture-MTD association was previously interpreted as combined on-fault and off-fault paleoseismic evidence for three local severe earthquakes (Oswald et al., 2021a). The other event horizons with multiple coeval MTDs and a corresponding turbidite were interpreted as the record of remote earthquakes exceeding the intensity threshold at Achensee SI ~VI. So far, eight of the eleven seismic-stratigraphic horizons with multiple MTDs (B-I) and one surface-rupturing horizon (G) are cored and dated 235 by two 11 m long sediment cores. The extension of the age-depth model results in two local surface-rupturing events at ~11.0 and ~11.5 ka BP (K and L) and a remote earthquake at ~10.8 ka BP with 95% age ranges of 10.6-11.5, 11.0-11.9 and 10.4-11.2 ka BP, respectively (Table 1).

The paleo-earthquake catalogue of Tyrol and its surrounding regions
The sedimentary archives of Plansee, Piburgersee and Achensee represent the first continuous records of prehistoric 240 earthquakes in the western Eastern Alps (Tyrol and its surrounding regions) and cover the last 16,000 years (Fig. 4a, Table   1). For each event, a qualitative intensity scaling is applied above the intensity threshold for recording earthquakes based on sedimentological criteria, such as MTD volume, turbidite thickness, development stage and thickness of SSDS and occurrence of postseismic landscape response ( Fig. 4a; Oswald et al., 2021a;b). In contrast to Plansee and Achensee, the paleoseismic record of Piburgersee is limited to the Holocene lacustrine sediments due to the lack of earthquake-induced 245 soft-sediment deformation structures in the glaciolacustrine sediments and due to the missing seismic penetration (< 0.003 s TWT) to potentially link outstanding turbidites to multiple coeval MTDs.
The Plansee paleoseismic record (event number n=9) shows the most regular recurrence pattern of the investigated records being at the boundary of 'weakly periodic' and 'strongly periodic' with a mean burstiness of -0.29 (range -0.40 to -0.22; Fig.   3b). It also has the largest mean recurrence interval of circa 2,000 years. The Achensee paleoseismic record (n=11) is 250 'weakly periodic' (mean burstiness -0.20; range -0.32 to -0.11) with a mean recurrence interval of circa 1,100 years. For the Piburgersee record (n=8), aperiodicity and recurrence intervals are calculated in two ways to account for the relatively long earthquake quiescence since ~3.0 ka BP, once considering only the closed interevent times and a second time with assuming an imminent event next year. Both methods show a 'weakly periodic' to 'aperiodic' recurrence pattern (mean burstiness of -0.04 / -0.06; range -0.16 to 0.03 / -0.13 to 0.00) but vary in the recurrence interval with circa 1000 and 1250 years for the 255 closed intervals and the imminent event next year, respectively. The 'real' recurrence interval at Piburgersee is potentially even longer, as the imminent event will probably be further in the future. Note that the smaller scatter of burstiness and recurrence interval in the Piburgersee dataset considering an imminent event next year is artificial due to the small age error of the assumed event in 2022 compared to the event ages derived from the individual age model simulations. Evaluating these recurrence statistics in light of the geographical position of the records within the Alpine orogen, the more externally 260 situated Plansee shows the longest recurrence interval and most periodic recurrence pattern, whereas the more orogeninternal records (Piburgersee and Achensee) tend to aperiodic recurrence behavior with about only half of the recurrence interval at Plansee. Additionally, we combine the event ages of all earthquake-related sedimentary imprints derived from the three paleoseismic records and consider the interpreted coeval events at circa 3.0, 4.1 and 6.8 ka BP only once (Oswald et al., 2021b;Fig. S3). This results in 25 severe earthquakes that occurred in the study area every 630 years on average with a 265 weakly periodic recurrence pattern.
We examined the herein presented paleo-earthquake catalogue of Tyrol (Table 1) for further overlapping event ages, as this potentially indicates that a single event left traces in multiple lakes and thus impacted an area of several tens of km circumference. However, for overlapping event ages two or more less strong events at different locations and within a shorter time period than the error of the age-depth model are also conceivable (Kremer et al., 2017;Oswald et al., 2021b). 270 For this event-age synchronicity evaluation, we incorporated the pre-existing surrounding lacustrine paleoseismic archives at   al., 2021b) and Achensee (this study and Oswald et al., 2021a) also including the potential earthquake-related mass wasting events at Starnbergersee (Daut, 1998). Earthquake symbols are relatively scaled based on their imprint size above the EMS-98 intensity threshold for recording earthquakes at the lakes (cf . Oswald et al., 2021b). Duration of lacustrine and glaciolacustrine sedimentation is indicated by continuous and dashed horizontal grey lines, respectively. The likelihood assessment that these periods represent single earthquakes with an impact in multiple lakes (green colour scale) is based on age overlaps of the individual event age PDFs (Fig. S3) and sedimentological evidence further described in the main text. B) Recurrence interval-burstiness diagram of the three records calculated on all individual age-depth model simulations at the respective lake resulting in >5400 data points for each record. The recurrence interval varies from circa 1000 to 2000 years for the individual records and is circa 680 years for all three records combined. The burstiness shows weakly periodic to aperiodic recurrence patterns for all data sets. Note that Piburgersee is considered as two data sets given the long open end since the last event: one data set where only the closed interevent times were considered and a second data set where an imminent event is assumed in the next year.  (Schwestermann, 2016), Zürich (Strasser et al., 2013;Strasser and Anselmetti, 2008), Walen (Zimmermann, 2008), Silvaplana (Bellwald, 2012), Iseo (Lauterbach et al., 2012) and Ledro (Simonneau et al., 2013) for a potential event age overlap, but found only negative evidence for overlapping event ages with the three Tyrolean lakes. 275 Moreover, the sedimentary archive of Ammersee, which was primarily investigated for its flood record, holds no paleoseismic events in the available core data of the last ~5.7 kyrs (Czymzik et al., 2013). At Starnbergersee, limnogeological investigations found enhanced mass wasting in seismic and core data at around 4 ka BP (3.7 to 4.7 ka BP) and in the Early Atlantic (circa 6.5 ± 0.5 ka BP; Daut, 1998). As the establishment of a continuous paleoseismic record was not the focus of the study at Starnbergersee, we only consider the two mass wasting events at circa 4 and 6.5 ka BP as 280 potential evidence for seismic shaking (Fig. 4a), but do not extract negative evidence from this record. The paleoseismic event catalogue of the western Eastern Alps holds six periods with overlapping event ages in two records at circa 3.0, 6.4, 6.5, 6.6, 6.8 and 8.3 ka BP and one period at circa 4.1 ka BP with overlapping event ages in three records (Fig. 4a). A single severe event with impact in multiple lakes is very likely for the events at ~3.0 (Plansee, Piburgersee) and ~4.1 ka BP (Plansee, Piburgersee and potentially Starnbergersee), because the 95% PDFs of the event age ranges at Plansee and 285 Piburgersee match well with 55% and 45%, respectively (Oswald et al., 2021b;Fig. S3). Additionally, potential terrestrial secondary paleoseismic evidence is documented during these periods by the occurrence of several large prehistoric rockslides (up to 1 x 10 9 m³ rockslide deposit volume; Prager et al., 2008;Oswald et al., 2021b). Furthermore, a single earthquake at circa 6.8 ka BP is indicated by a 68% PDF event age overlap between lakes Piburgersee and Achensee (Fig.   4a). In the time period of ~8.3 ka BP the PDF event age overlap at the lakes Achensee and Plansee is low (19%) and also the 290 observational evidence contradicts the possibility of a single event at this time. In detail, the rupture location is restricted to the Achensee area based on primary paleoseismic evidence. Within the same period, seismic shaking beyond the EMS-98 intensity threshold is indicated at Plansee by extensive mass-wasting and the occurrence of postseismic landscape response, but no earthquake-related sedimentary imprint is found at Piburgersee. We also propose to reject the possibility of a single earthquake with an imprint in multiple lakes at circa 6.4 (Piburgersee-Plansee) and 6.6 ka BP (Plansee-Achensee) due to the 295 poor PDF event age overlap of 6% and 4%, respectively.

Potential scenarios of paleo-earthquakes
Based on the interpreted coeval earthquake evidence in multiple lakes at circa 3.0, 4.1, 6.5, and 6.8 ka BP, we investigate assemblages of potential source area and magnitude for these events under the assumption of a single earthquake event based on a reverse application of the IPE in a geospatial analysis (Fig. 5). For the following scenarios, the surrounding 300 paleoseismic archives of the lakes Constance, Zürich, Walen, Iseo, Ledro, Silvaplana and Ammersee (for the last ~5.7 kyrs) were applied as negative earthquake evidence due to missing event-age overlap or no earthquake imprints at all in the case of Ammersee.

Scenario A at ~3.0 ka BP
Positive earthquake evidence is present in Plansee and Piburgersee at ~3.0 ka BP, whereas Achensee, Ammersee and the Swiss and Italian paleoseismic archives show negative evidence (Fig. 5b). The grid-search analysis provides solutions of potential earthquake sources in the western study area with a minimum magnitude source area in the middle between Plansee 310 and Piburgersee around the village Nassereith with MW 5.7 (Fig. 5b). The large prehistoric rockslides at Tschirgant and Haiming indicated as violet triangles in Fig. 5b also fall within the time range of this event and were interpreted to be earthquake triggered (Oswald et al., 2021b). These rockslides are located slightly south of the minimum magnitude area and much closer to Piburgersee than to Plansee. Additionally, the relative scaling of the sedimentary imprint size (0.98 at Piburgersee and 0.11 at Plansee; Table 1) indicates a much higher shaking intensity at Piburgersee than at Plansee. 315 Therefore, we interpret the epicenter of scenario A to be more likely located south of the halfway distance between the two lakes (white line in Fig. 5b) with a minimum magnitude of MW 5.8. Hereinafter we consider MW 5.8 as minimum magnitude for scenario A and use this value for further considerations and analyses (sect. 4.3).

Scenario B at ~4.1 ka BP
At ~4.1 ka BP, positive earthquake evidence is found in Plansee and Piburgersee and potentially in Starnbergersee, whereas 320 Achensee, Ammersee and the Swiss and Italian lakes show negative evidence for seismic shaking (Fig. 5c). The rockslides at Fernpass, Eibsee and Stöttelbach (violet triangles in Fig. 5c) were previously related to this earthquake (Oswald et al., 2021b). Moreover, results from structural analyses and rock slope failure modelling in response to seismic shaking also suggest a seismic trigger for the Fernpass rockslide (Lemaire et al., 2020). The derived minimum-magnitude and source area lies north of Garmisch-Partenkirchen with a magnitude of MW 6.1. This event had the largest sedimentary imprint at Plansee 325 (relative imprint scale = 1) but had only an intermediate imprint size in Piburgersee (relative imprint scale = 0.63; Table 1).
This observational evidence further supports that the localization of the already small region of plausible source areas and minimum magnitudes from the grid-search analysis (Fig. 5c). It must be noted, that this analysis strongly relies on the less constrained evidence at Starnbergersee. Not considering Starnbergersee as a datapoint would result in the same magnitudesource area distribution as in scenario A (Fig. 5b), but resulting in a minimum magnitude estimate of MW 6.0 around Plansee 330 and the rockslides at ~4.1 ka BP considering the sedimentary imprint size and the locations of the earthquake-triggered rockslides (see argumentation above).

Scenario C at ~6.5 ka BP
Plansee and potentially Starnbergersee have positive evidence for seismic shaking at ~6.5 ka BP and negative evidence is found in the other available records (Fig. 5d). The region north of Garmisch-Partenkirchen is a potential source area with a 335 minimum magnitude of MW 5.9, centrally located between the two lakes, or increasingly higher, if the epicenter would have been located further towards the north, west or south. Better constraining the epicenter based on the sedimentary imprint is not feasible for this event, as there is no data on the imprint size at Starnbergersee. However, the low imprint size in Plansee (0.11; Table 1) hints at an earthquake which occurred farther away. Therefore, we assume the identified minimum magnitude area to be the most likely epicenter location for this event. Similar to scenario B, the grid-search analysis strongly relies on the less constrained evidence in Starnbergersee. In the case of a false interpreted evidence in Starnbergersee for this event, the minimum magnitude area would shift towards Plansee.

Scenario D at ~6.8 ka BP 345
At ~6.8 ka BP positive earthquake evidence is found in Achensee and Piburgersee, whereas negative evidence is derived from Plansee (Fig. 5e). The minimum magnitude source area is located around Innsbruck with a minimum magnitude estimate of MW 6.0 and with increasing magnitudes towards the east and southeast. As both Achensee and Piburgersee have only minor sedimentary imprints sizes for this event (0.11 and 0.38, respectively; Table 1), an epicenter location farther away from both record like the area of Innsbruck or south of it is most likely. 350

Effects of selected paleo-earthquakes
The derived magnitudes from the modeled paleo-earthquake scenarios are well above what has been calculated from instrumental earthquake records of the last century or from historical documentation in the study area. To get a first-order idea of the consequences of such rare high-magnitude events, we explore the regional shaking effects of four selected paleoearthquake scenarios, which are reasonably constrained by our lacustrine paleoseismic data. The epicenter location of the 355 earthquake scenarios for the circa 3.0, 4.1 and ~6.8 ka events are estimated based on the minimum magnitude source area as revealed from the grid-search analysis. In a second step, the epicenter locations are considered more plausible closer to earthquake-triggered rockslides and lakes with higher sedimentary imprint scales (see sect. 4.2). For the estimated epicenter area, we take the nearest city and derive the magnitude calculated from the inverse application of IPE in the grid-search analysis for the respective paleo-earthquake scenario. Besides the exploration of regional shaking effects for plausible 360 earthquake scenarios at circa 3.0, 4.1 and ~6.8 ka BP, we test for the maximum possible magnitude of the surface-rupturing earthquake at Achensee at ~8.3 ka BP, that did not exceed the EMS-98 intensity threshold at Piburgersee and Plansee. A focal depth of 10 km is assumed for calculating the shakemaps presented in Fig. 6, which lies within the 5-15 km depth 365 range of recent seismicity (Reiter et al. 2018). The respective calculated seismic intensities for selected towns in the broader study area are listed in Table 2.

Effects of a MW 5.8 earthquake at Imst (Scenario A at ~3 ka BP)
A MW 5.8 earthquake scenario at Imst would have an epicentral intensity of VIII¼ and the earthquake would affect large 370 parts of Tyrol and also areas of southern Bavaria, Baden-Württemberg and Vorarlberg (Fig. 6a). Especially western Tyrol and the northern Alpine foreland from Kempten to Starnbergersee would be strongly affected (SI: VI to VII) in a total area of circa 9,000 km². The very strongly affected area with intensities >VII (~550 km²) is found around the epicenter.

Effects of a MW 6.1 earthquake at Garmisch (Scenario B at ~4.1 ka BP)
A MW 6.1 earthquake scenario at Garmisch would have an epicentral intensity of VIII ¾ and would affect large parts of 375 Tyrol and southern Germany (Fig. 6b). In detail, the earthquake would be strongly felt in an area of circa 20,000 km² ranging from most of Tyrol up north to Augsburg and from lake Constance to Chiemsee. The very strongly affected area (SI: VII-VIII) would be expected in an area of circa 5,200 km² located in the Alpine foreland from Kempten to almost Tegernsee, up north to Ammersee, and also in the Inn and the Wipp valley towards the south. Severe seismic shaking is expected around the epicenter in an area of circa 270 km². 380

Effects of a MW 6.0 earthquake at Innsbruck (Scenario D at ~6.8 ka BP)
A MW 6.0 earthquake scenario at Innsbruck would have an epicentral intensity of VIII½ and would be felt in whole western Austria, Bavaria and South Tyrol (Fig. 6c). An area of circa 14,300 km² are expected to be strongly affected by seismic shaking (SI: VI-VII) especially in the Inn valley, but also in the Alpine foreland from Kempten to Rosenheim. Within an area of 1,300 km² in central Tyrol would experience very strong seismic shaking (SI: VII-VIII) and severe seismic intensities 385 (SI >VIII) are expected in the Inn valley in the area of the epicenter (~65 km²).

Investigation of the maximum possible magnitude for the surface-rupturing earthquake at Achensee at ~8.3 ka BP
As the ~8.3 ka BP surface-rupturing event at Achensee is not recorded in Piburgersee or Plansee, we estimate its maximum possible magnitude by considering that the threshold intensities at Piburgersee and Plansee were not exceeded. These conditions are met for an earthquake at Achensee with a magnitude of Mw 6.3, which defines the maximum possible 390 magnitude for the ~8.3 ka surface-rupturing event. Such an event would have an epicentral intensity of VIII ¾ and would  1 ka BP, c) near Innsbruck at ~6.8 ka BP. In contrast, d) presents the maximum possible magnitude of MW 6.3 for a surface-rupturing earthquake at Achensee without exceeding the seismic intensity threshold at Plansee and Piburgersee. The shakemaps are based on empirical relations of intensity-to-distance (Sponheuer, 1960) and magnitude-to-epicentral intensity (Rudloff & Leydecker, 2002), modified by macroseismic data of recorded earthquakes and epicentral intensities of historical events, and corrected for topographic effects. The calculated seismic intensities at major towns for the different paleo-earthquake scenarios are provided in Table 2. https://doi.org/10.5194/nhess-2021-281 Preprint. Discussion started: 19 October 2021 c Author(s) 2021. CC BY 4.0 License. affect large parts of western and central Austria, Bavaria and parts of South Tyrol (Fig. 6d). In total about 30,000 km² are expected to be strongly affected (SI: VI-VII). In large parts of the Inn valley and southern Bavaria (circa 8,500 km²) very strong seismic intensities (SI: VII-VIII) are expected and severe shaking (SI >VIII) would occur in circa 540 km² located in the Achensee region and the adjacent Inn-and Ziller valleys. Surface ruptures are commonly present for MW >6.0 395 earthquakes (Stirling et al., 2002), which provides a minimum magnitude estimate for the surface-rupturing events at Achensee (Oswald et al., 2021a). In combination with the maximum possible magnitude from the shakemap (Fig. 6d) the magnitude range for the ~8.3 ka BP surface-rupturing event at Achensee is bracketed to MW 6.0-6.3.

Extension of paleoseismic records to Late Glacial times 400
In both Plansee and Achensee, the different lithologies and sedimentation rates of the glaciolacustrine and the lacustrine sediments may also affect the sensitivity of the lake acting as a natural seismograph. The earthquake proxy of multiple MTDs with a corresponding turbidite cannot be validated by historical events onsite for the glaciolacustrine sediments from Late Glacial times. However, similar signatures have been successfully validated as proxies for seismic shaking in actual proglacial lakes in Alaska (Van Daele et al., 2019;Praet et al., 2017), which could be seen as analogues for the Late Glacial-405 Achensee and -Plansee. Increased sedimentation rates as present in the glaciolacustrine sediments might increase the sensitivity of the subaqueous slopes to failure (Chassiot et al., 2016;Wilhelm et al., 2016), which implies a lower intensity threshold to record seismic shaking (Praet et al., 2017). This in turn could lead to a misinterpretation of increased seismicity within Late Glacial times, as cause for the high event frequency in the Late Glacial Plansee record (circa 820 years recurrence rate). It can be assumed that seismicity in the western Eastern Alps was enhanced in Late Glacial-Early Holocene 410 times due to post-glacial unloading and following isostatic rebound, as interpreted in other formerly glaciated regions (Beck et al., 1996;Bellwald et al., 2019;Brooks and Adams, 2020;Strasser et al., 2013). In contrast, sedimentary imprints of earthquakes might be misinterpreted as hydrological event layers in the glaciolacustrine sediments due to the failure of slopes predominantly influenced by clastic sedimentation. This makes it challenging to distinguish turbidites that originate from in-lake remobilization versus external fluvial sources (Praet et al., 2020). Despite the uncertainties of potential temporal 415 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 thus little knowledge is available on recurrence patterns of severe earthquakes.

Intensity values used for the paleo-earthquake scenarios
For the calculation of potential source areas and magnitudes of paleo-earthquakes an IPE is reversely applied in a grid-search analysis using a single, uniform positive and negative intensity threshold solely on the lake records with a presumed intensity threshold of SI VI¼. This is due to the fact that the intensity thresholds at the individual lakes could only be determined with 430 maximum one positive evidence of a historical earthquake (Oswald et al., 2021a(Oswald et al., , 2021b. Additionally, the magnitudes of such historical earthquakes can have already relatively large uncertainties, as it is the case for the 1670 Hall earthquake (Stucchi et al., 2013), which in turn gets transferred to the intensity threshold estimation. For Piburgersee, the intensity threshold of SI VI½ to VII was estimated from negative evidence of the 1930 Namlos earthquake (Oswald et al., 2021b), which also corresponds to intensity thresholds obtained in similar lake settings (Monecke et al., 2004). Moreover, due to the 435 absence of a well-documented historical earthquake that left evidence at multiple lake sites, we are not able to test the performance of the grid-search analysis using the presumed intensity threshold (Kremer et al., 2017). The earthquake-related rockslides are not included as input data points for the grid-search analysis due to their unknown and potentially very variable intensity threshold. When simply applying the environmental seismic intensity scale (ESI scale; Serva et al., 2016), rockslides with deposit volumes >10 7 m³ would indicate a seismic intensity of X, which would contradict with the negative 440 evidence found at Achensee in only 50 to 65 km distance to the potential source areas of the ~4.1 and ~3.0 ka BP events.
This negative evidence constrains the maximum intensity at the rockslide sites to circa SI VIII ½. Such a lower threshold for rockslide triggering can be explained by the relatively long-lasting interplay of hydromechanical and seismic preconditioning weakening the rock slope stability (Oswald et al., 2021b;Prager et al., 2008).

Discussion of the individual paleo-earthquake scenarios 445
The calculation of plausible source areas and minimum magnitudes of paleo-earthquakes assumes a single event with impact in multiple lakes. However, especially in the intraplate tectonic setting of the Alps earthquake successions or periods with enhanced seismicity are common, as exemplified by some of the strongest historical earthquakes that have occurred as clusters e.g. six events with magnitudes ranging from MW 5.1-6.4 within five months in the Friuli region in 1976 (Carulli and Slejko, 2005) and two events of MW 5.5 to 5.7 (± 0.4) in central Tyrol in 1670 and 1689 (Stucchi et al., 2013). The time 450 periods between these historical earthquake successions are far below the age uncertainty of radiocarbon-based age-depth models and would remain undifferentiated unless multiple consecutive earthquake imprints can be distinguished within the sedimentary record (Wils et al., 2021). Therefore, it cannot be excluded that one or the other of the presented paleoearthquake scenarios actually represent two or more closely spaced and consecutive earthquakes with slightly smaller are present and also severe historical earthquakes are documented (Hammerl, 2017;Stucchi et al., 2013). Therefore, we propose that the scenarios A and D are likely to have happened in these regions near Imst and Innsbruck, respectively with the calculated minimum magnitudes of MW 5.8 and 6.0, respectively (Fig. 6). With increasing distance and magnitude, the 460 resulting magnitudes and source area from the grid-search analysis cannot be further evaluated and narrowed down due to missing data points, which is especially the case for scenario A (towards the south; Fig. 5b) and scenario D (towards east and south; Fig. 5e). Plausible source areas of scenarios B (~4.1 ka BP) and C (~6.5 ka BP) are located at the northern part of the Fernpass-Loisach region (see considerations in sect. 4.2), which is characterized by slightly increased seismicity within the generally seismically less active Alpine front ( Fig. 5a; Reiter et al., 2018). It is conceivable that enough deformation is 465 transferred towards the north and localized at this inherent, large-scale fault structure to generate severe earthquakes.
However, the lack of present-day seismicity in the northern study area (Reiter et al., 2018) complicate the assessment of other potential active fault zones in the Alpine foreland despite the presence of major Alpine thrusts successively approaching the surface. The shakemaps of selected paleo-earthquake scenarios provide first-order information that severe earthquakes within the 470 western Eastern Alps also strongly affect large areas in the northern Alpine foreland in southern Germany. Several perialpine lakes are located in southern Germany, which are not yet studied for their potential paleoseismic records but could essentially help in identifying more earthquake scenarios with multi-lake impact and help in better constraining epicenter locations and magnitudes of the herein proposed scenarios. A previous study at Ammersee found no earthquake imprints for the last 5.7 kyrs (Czymzik et al., 2013). This contradicts the results of the shakemaps of the paleo-earthquake scenarios (Fig.  475 6), where at least two events have exceeded SI VII, which is above the upper threshold value to record seismic shaking in comparable, Swiss peri-alpine lakes (Monecke et al., 2004). Either the sediments of Ammersee have a different susceptibility to fail during seismic shaking or event horizons of multiple mass wasting remained undiscovered due to the lack of reflection seismic data or inadequate core siting to capture earthquake-related turbidites.
By taking the proposed source areas of the paleo-earthquake scenarios A, B and C (see argumentation in sect. 4.2), we 480 observe a spatio-temporal migration of paleo-earthquakes from NE to SW in the area of the Fernpass-Loisach fault system (Fig. 7). When assuming these events ruptured on this fault system, this could indicate a progressive failure along this major fault structure similar to what is observed in some transform plate boundary regions e.g. the North Anatolian fault (Stein et al., 1997). Such stress redistribution patterns are not limited to active tectonic regions with high deformation rates, but are also documented for the Friuli 1976 earthquake sequence in the Alps (Slejko, 2018). Yet, the interpretation of a progressive 485 failure at the Fernpass-Loisach fault system might be biased by the fact that we can only estimate source areas for paleoearthquakes that are recorded in at least two lakes. This interpretation ignores at least four paleo-earthquakes recorded only in Piburgersee between the 6.5 and the 4.1 ka BP events. However, these events are likely produced by a different fault source than the Fernpass-Loisach fault system, considering the missing coeval earthquake evidence in Plansee.

Potential Implications of the paleoseismic catalogue of the western Eastern Alps 490
The three presented paleoseismic archives are located at different positions within the Alpine orogeny in terms of current seismicity (Fig. 5a), which is also reflected in their earthquake recurrence rates and patterns. We interpret the relatively shorter recurrence rates within the more inneralpine records Piburgersee and Achensee to be related with enhanced tectonic loading around the Inn valley also in prehistoric times. Hence, longer recurrence rates in the more external Plansee potentially reflect a decreased stress transfer across the current-day enhanced seismicity zone in the south. Additionally, 495 recurrence behavior of the inneralpine records is slightly more aperiodic than the more external Plansee. This could reflect different processes increasingly affecting and modulating seismicity with increasing topography and elevation in the area of the inneralpine records, i.e. glacial isostatic adjustment, topographic potential energy or erosion (Mazzotti et al., 2020).
Especially a change of lithospheric stress due to post-glacial unloading and isostatic rebound has been previously discussed to increase seismicity within the western Alps during Late Glacial to Early Holocene times (Beck et al., 1996;Kremer et al., 500 2017;Strasser et al., 2013). Without being able to resolve the processes modulating seismicity, increased seismicity related to post-glacial unloading could potentially be indicated by the increased event frequency (822 years recurrence rate) between 12 and 16 ka BP at Plansee.
In comparison to the overall aperiodic recurrence patterns of other lacustrine paleoseismic records within the Alps (Lakes Cadagno, Ledro, Bohinj, Savine, Lucerne, Annecy compiled by Moernaut, 2020), the paleoseismic records of Tyrol are 505 slightly more periodic, but with longer recurrence rates, except lake Lucerne (recurrence interval = 2034 years, burstiness = -0.09) which is comparable to Plansee. The slightly more periodic recurrence behavior in the Tyrolean records could be partly artificial, as records with low event numbers tend to underestimate the burstiness and thus, tend towards a more periodic recurrence behavior (Kempf and Moernaut, 2021;Williams et al., 2019). However, the more regular earthquake recurrence pattern in Tyrol could indicate active deformation localization within one or only a few fault sources, whereas deformation 510 in the other alpine paleoseismic records is distributed over a complex system of interacting fault sources, which is more typical for intraplate tectonic settings (Liu et al., 2011). In Tyrol, the circa 20 km broad and 100 km long zone of currently enhanced seismicity around the Inn valley is likely to mostly occur on a single active fault source or strongly interrelated fault sources linked in connection with activity at the sub-Tauern ramp (Ortner et al., 2006;Reiter et al., 2018). Additionally, constant tectonic loading of these faults is caused by ongoing 1-2 mm/year N-ward pushing of the Adriatic microplate 515 (Métois et al., 2015) located directly south of the study area.
The paleoseismic catalogue also provides insights in the spatio-temporal patterns of strong earthquake occurrence from the two inneralpine records Achensee and Piburgersee (Fig. 4a). A high event frequency (590 years recurrence rate) occurred at ~12 to ~8 ka BP at Achensee during which only one event is documented at Piburgersee. In contrast, the period of ~7 to ~3 ka BP is characterized by a high event frequency (540 years recurrence rate) at Piburgersee and only two events in Achensee. 520 Since then, three events are recorded in Achensee but none in Piburgersee. This observation potentially indicates a spatial shift of paleoseismicity from east to west to east in postglacial times. Strikingly, the proposed periods of enhanced seismicity at Achensee and Piburgersee both end with an outstanding severe earthquake, as documented by the size and amount of their sedimentary imprints (Fig. 4a). Whether such a particularly severe earthquake significantly altered the local stress field by releasing a significant portion of energy accumulated by tectonic loading in the respective area, or this observation is just 525 coincidence cannot be resolved by the available amount of paleoseismic archives.
A maximum credible earthquake magnitude of MW 6 (+0.5 if a strike slip mechanism was involved) was previously assessed based on macroseismic data in this region (Lenhardt et al., 2007). The presented paleo-earthquake scenarios provide databased observational evidence that an earthquake with a maximum credible magnitude e.g. the MW 6.0-6.3 at ~8.3 ka BP at Achensee happened several times in the study area during the past 16 kyrs. In particular, this has implications for the seismic 530 demand in the building code of critical infrastructure such as dam structures, which are constructed based on the assessed maximum credible earthquake (Zenz and Obernhuber, 2002). In contrast, these findings do not have implications for the seismic design of general buildings and infrastructure (ÖNORM B 1998(ÖNORM B -1, 2011, as the recurrence interval of all three records combined (~630 years) is larger than the therein considered recurrence interval (475 years or 10% probability of exceedance within 50 years). However, it remains to future earthquake hazard assessments and policy makers to find 535 https://doi.org/10.5194/nhess-2021-281 Preprint. Discussion started: 19 October 2021 c Author(s) 2021. CC BY 4.0 License. adequate strategies to implement such low frequency-high impact scenarios, evaluate available mitigation and response strategies in case of such a cross-border event and prepare the society also in intraplate tectonic regions for a possible worstcase scenario.

Conclusions
The first lacustrine paleoseismic archives of the western Eastern Alps revealed in total 25 sedimentary imprints of severe 540 seismic shaking expressed as multiple, coeval MTDs, SSDS or subaqueous surface ruptures. At the individual studied lake sites, seismic shaking with SI > VI¼ occurs every 1,000-2,000 years in a weakly periodic to aperiodic recurrence behavior.
Severe paleo-earthquakes with minimum magnitudes of MW 5.8-6.1 might have occurred in the present-day zone of enhanced seismicity around the Inn valley, Brenner region and Fernpass region, but also in the present-day low seismicity zone of the upper Loisach region. A surface-rupturing earthquake at Achensee is constrained to MW 6.0-6.3. NE-SW 545 migration of paleo-earthquakes between circa 6.5 to 3.0 ka BP might hint at a progressive failure along the Fernpass-Loisach fault. Moreover, a shift of paleoseismicity from east to west to east in the area of the Inn valley during Postglacial times might be indicated by the regional paleo-earthquake catalogue. The shakemaps of paleo-earthquake scenarios highlight that severe earthquakes in the study area could strongly affect the northern foreland regions. Complementary paleoseismic investigations especially north and east of the study area are required to evaluate and further improve these initial results. 550

Data availability
Data and codes are available from the authors upon request.

Author contribution
JM, MS and PO designed the study. PO, JM and MS acquired reflection seismic and core data, calculated earthquake 555 recurrence statistics, generated the paleo-earthquake scenarios and interpreted the results. JS wrote the code of the shakemaps. PO wrote the manuscript and produced the figures with input from all co-authors.