Combined On-Fault and Off-Fault Paleoseismic Evidence in the Postglacial Infill of the Inner-Alpine Lake Achensee (Austria, Eastern Alps)

The Eastern European Alps are characterized by slow active deformation with low- to moderate seismicity. Recurrence rates of severe earthquakes exceed the time span of historical documentation. Therefore, historical and instrumental earthquake records might be insufficient for seismic hazard assessment and high-quality paleoseismic data is required. However, primary geological observations of postglacial fault activity are scarcely found, because major faults are buried below thick sedimentary sequences in glacially overdeepened valleys. Moreover, high erosion rates, gravitational slope processes and penetrative anthropogenic landscape modification often obscure geomorphic features related to surface ruptures. Here we present one of the rare paleoseismic data sets showing both on-fault evidence as subaqueous surface ruptures and off-fault evidence as multiple coeval mass-transport deposits (MTDs) and megaturbidites within a single high-resolution seismic-stratigraphic framework of the inner-alpine lake Achensee. Co-occurrence of on-fault and off-fault paleoseismic evidence on three stratigraphic levels indicates seismic activity with inferred moment magnitudes MW ∼6–6.5 of the local, lake-crossing Sulzgraben-Eben thrust at ∼8.3 ka BP and twice in Late Glacial times. Additional eight stratigraphic levels with only off-fault paleoseismic evidence document severe seismic shaking related to the historical MW ∼5.7 earthquake in Hall (CE 1670) and seven Holocene earthquakes, which have exceeded a local seismic intensity of ∼VI (EMS-98) at Achensee. Furthermore, we discuss natural and methodological influencing factors and potential pitfalls for the elaboration of a subaqueous paleoseismic record based on surface ruptures and multiple, coeval MTDs.

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Combined on-fault and off-fault paleoseismic evidence in the postglacial lacustrine sediments of Achensee (Austria, Eastern Alps)

10.5194/egusphere-egu21-555 ◽

2021 ◽

Author(s):

Patrick Oswald ◽

Jasper Moernaut ◽

Stefano Fabbri ◽

Marc De Batist ◽

Irka Hajdas ◽

...

Keyword(s):

High Resolution ◽

Seismic Data ◽

Sediment Cores ◽

The Other ◽

Maximum Magnitude ◽

European Alps ◽

Landscape Modification ◽

Reflection Seismics ◽

Major Fault ◽

Surface Ruptures

Intraplate tectonic regimes such as the European Alps are characterized by low crustal deformation rates and thus long recurrence rates of severe earthquakes. High-quality paleoseismic archives are required to overcome our limited perspective of earthquake recurrence and maximum magnitude. However, especially on-fault paleoseismic evidence is scarcely found because of high erosion rates, gravitational slope processes and penetrative anthropogenic landscape modification, which often obscure geomorphic features related to surface ruptures.Here, we present the inneralpine lake archive of Achensee in the Northern Calcareous Alps (6.8km&#178; area; 133m water depth) cross-cut by a major fault and potentially holding a continuous paleoseismic archive since the last deglaciation at ~18 ka BP. This major fault is a Cretaceous-Paleogene relatively steep-dipping thrust, with at least 15km length and several hundreds of meters geological offset, located within the current area of enhanced seismicity and oriented to be preferentially re-activated in the current stress field. We used a high-resolution multi-beam bathymetry, a combination of a very dense grid of 3.5kHz &#8220;pinger&#8221; subbottom profiler and single-channel high-frequency (~0.8-2.0kHz) &#8220;sparker&#8221; reflection seismics to investigate the postglacial infill with high-resolution and image the deeper structures (e.g. the glacially scoured valley). The seismo-stratigraphic interpretation was ground-truthed and 14C-dated by five, up to 11m long sediment cores from the two main subbasins.We discovered at least eight strong earthquakes hitting the region in the past 11,000 years by off-fault paleoseismic evidence expressed by coeval, multiple mass-transport deposits (MTDs) and co-genetic turbidites. These earthquakes must have reached seismic intensity of >VI (EMS-98) at the lake site calibrated with the strongest known historical earthquake of the region (ML 5.2 in Hall CE1670). MTD size and extent corresponding to the CE1670 earthquake compared to the other earthquake imprints let us infer that at least four of the paleo-earthquakes reached higher intensities at Achensee.Strikingly, Achensee has also recorded on-fault evidence expressed by steeply-dipping to vertical faults offsetting the lacustrine stratigraphy. These stratigraphic offsets can be traced downwards to the acoustic basement, which hints at faulting originating in the bedrock. For at least two stratigraphic levels, these faults are directly overlain by multiple MTDs indicating that fault activity and slope failures have occurred quasi-simultaneously. The faults observed on the seismic data, affecting the sedimentary infill of the lake, are located above the inferred trace of the major fault where it crosses the lake. Based on this rather unique combined on-fault and off-fault evidence we propose strong paleo-earthquakes documenting activity of this major thrust at ~8.5 ka BP and in the Late Glacial period (below reach of sediment cores). We suggest that these earthquakes have reached ML~5.5-6, which is within the magnitude capability of this thrust and at the lower limit of generating surface ruptures according to worldwide magnitude-surface rupture relationships. The other six event horizons lacking in on-fault evidence either represent earthquakes sourced from another fault in the region, earthquakes with a smaller magnitude not capable of surface rupturing like the ML5.2 earthquake in Hall CE1670 or on-fault evidence is blurred in seismic data by subsequent stacking of MTDs.

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Seismic control of large prehistoric rockslides in the Eastern Alps

Nature Communications ◽

10.1038/s41467-021-21327-9 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Patrick Oswald ◽

Michael Strasser ◽

Christa Hammerl ◽

Jasper Moernaut

Keyword(s):

Eastern Alps ◽

European Alps ◽

Rock Slopes ◽

Seismic Control ◽

Temporal Cluster ◽

Multiple Datasets ◽

Moderate Seismicity ◽

Spatio Temporal ◽

Epicentral Intensity

AbstractLarge prehistoric rockslides tend to occur within spatio-temporal clusters suggesting a common trigger such as earthquake shaking or enhanced wet periods. Yet, trigger assessment remains equivocal due to the lack of conclusive observational evidence. Here, we use high-resolution lacustrine paleoseismology to evaluate the relation between past seismicity and a spatio-temporal cluster of large prehistoric rockslides in the Eastern Alps. Temporal and spatial coincidence of paleoseismic evidence with multiple rockslides at ~4.1 and ~3.0 ka BP reveals that severe earthquakes (local magnitude ML5.5–6.5; epicentral intensity I0VIII¼–X¾) have triggered these rockslides. A series of preceding severe earthquakes is likely to have progressively weakened these rock slopes towards critical state. These findings elucidate the role of seismicity in preparing and triggering large prehistoric rockslides in the European Alps, where rockslides and earthquakes typically occur in clusters. Such integration of multiple datasets in other formerly glaciated regions with low to moderate seismicity will improve our understanding of catastrophic rockslide drivers.

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Morphology and spatio-temporal distribution of lacustrine mass-transport deposits in Wörthersee, Eastern Alps, Austria

Geological Society London Special Publications ◽

10.1144/sp500-2019-179 ◽

2019 ◽

Vol 500 (1) ◽

pp. 235-254 ◽

Cited By ~ 2

Author(s):

Christoph Daxer ◽

Maddalena Sammartini ◽

Ariana Molenaar ◽

Thomas Piechl ◽

Michael Strasser ◽

...

Keyword(s):

Mass Transport ◽

Temporal Distribution ◽

Late Glacial ◽

Eastern Alps ◽

Excess Pore Pressure ◽

European Alps ◽

Fine Grained ◽

Spatio Temporal ◽

Morphological Differences ◽

Mass Transport Deposits

AbstractIn lakes, landslides can be studied in high resolution due to their accessibility and limited size. Here, we investigate mass-transport deposits in glacigenic Wörthersee (Eastern European Alps) by integration of seismic, sediment core and multibeam bathymetric data. Two outstanding landslide events were revealed: the first occurred in the Late Glacial, leading to multiple deposits of up to 15 m thickness; they consist of sandy turbidites and mudclast conglomerates, which are overlain by a 2.5 m thick megaturbidite. The extensive, likely earthquake-triggered failure linked to this event was preconditioned by rapid sedimentation of fine-grained glaciolacustrine sediments and associated build-up of excess pore pressure. The second event was presumably triggered by a major earthquake (Mw≈7) in AD 1348 and comprises a mass-transport complex and several landslides, which led to a c. 30 cm thick turbidite. In total, 62 landslides are imaged in the multibeam map, 6 of which are most likely human-induced. Some of these show horseshoe-type compressional ridges and frontal breaching, whereas others exhibit an extensive zone of rafted blocks. We attribute these morphological differences to four main factors: (1) slope gradient and changes therein; (2) preconditioning of the impacted zone; (3) volume of remobilized sediment; and (4) type of impactor.

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The use of Monte Carlo simulations for seismic hazard assessment in the U.K.

Annals of Geophysics ◽

10.4401/ag-3617 ◽

2000 ◽

Vol 43 (1) ◽

Author(s):

R. M. W. Musson

Keyword(s):

Monte Carlo ◽

Seismic Hazard ◽

Hazard Assessment ◽

Seismic Hazard Assessment ◽

Probabilistic Seismic Hazard Assessment ◽

Logic Tree ◽

The United Kingdom ◽

The Monte Carlo Method ◽

Moderate Seismicity ◽

Seismicity Model

The input required for a seismic hazard study using conventional Probabilistic Seismic Hazard assessment (PSHA) methods can also be used for probabilistic analysis of hazard using Monte Carlo simulation methods. This technique is very flexible, and seems to be under-represented in the literature. It is very easy to modify the form of the seismicity model used, for example, to introduce non-Poissonian behaviour, without extensive reprogramming. Uncertainty in input parameters can also be modelled very flexibly - for example, by the use of a standard deviation rather than by the discrete branches of a logic tree. In addition (and this advantage is perhaps not as trivial as it may sound) the simplicity of the method means that its principles can be grasped by the layman, which is useful when results have to be explained to people outside the seismological/engineering communities, such as planners and politicians. In this paper, some examples of the Monte Carlo method in action are shown in the context of a low to moderate seismicity area: the United Kingdom.

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A SSHAC Level 3 Probabilistic Seismic Hazard Analysis for a New-Build Nuclear Site in South Africa

Earthquake Spectra ◽

10.1193/060913eqs145m ◽

2015 ◽

Vol 31 (2) ◽

pp. 661-698 ◽

Cited By ~ 40

Author(s):

Julian J. Bommer ◽

Kevin J. Coppersmith ◽

Ryan T. Coppersmith ◽

Kathryn L. Hanson ◽

Azangi Mangongolo ◽

...

Keyword(s):

South Africa ◽

Seismic Hazard ◽

Epistemic Uncertainty ◽

Hazard Analysis ◽

Probabilistic Seismic Hazard Analysis ◽

Seismic Hazard Analysis ◽

Probabilistic Seismic Hazard ◽

Recurrence Rates ◽

Moderate Seismicity ◽

Level 3

A probabilistic seismic hazard analysis has been conducted for a potential nuclear power plant site on the coast of South Africa, a country of low-to-moderate seismicity. The hazard study was conducted as a SSHAC Level 3 process, the first application of this approach outside North America. Extensive geological investigations identified five fault sources with a non-zero probability of being seismogenic. Five area sources were defined for distributed seismicity, the least active being the host zone for which the low recurrence rates for earthquakes were substantiated through investigations of historical seismicity. Empirical ground-motion prediction equations were adjusted to a horizon within the bedrock at the site using kappa values inferred from weak-motion analyses. These adjusted models were then scaled to create new equations capturing the range of epistemic uncertainty in this region with no strong motion recordings. Surface motions were obtained by convolving the bedrock motions with site amplification functions calculated using measured shear-wave velocity profiles.

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A high‐resolution speleothem proxy record of the Late Glacial in the European Alps: extending the NALPS19 record until the beginning of the Holocene

Journal of Quaternary Science ◽

10.1002/jqs.3255 ◽

2020 ◽

Vol 36 (1) ◽

pp. 29-39

Author(s):

Hanying Li ◽

Christoph Spötl ◽

Hai Cheng

Keyword(s):

High Resolution ◽

Late Glacial ◽

European Alps ◽

Proxy Record ◽

The Holocene

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Glaciation's topographic control on Holocene erosion at the eastern edge of the Alps

Earth Surface Dynamics ◽

10.5194/esurf-4-895-2016 ◽

2016 ◽

Vol 4 (4) ◽

pp. 895-909 ◽

Cited By ~ 7

Author(s):

Jean L. Dixon ◽

Friedhelm von Blanckenburg ◽

Kurt Stüwe ◽

Marcus Christl

Keyword(s):

Eastern Alps ◽

European Alps ◽

Glacial Erosion ◽

Erosion Rates ◽

The Alps ◽

Glacial Processes ◽

Cosmogenic 10Be ◽

Topographic Forcing ◽

Eastern Edge

Abstract. What is the influence of glacial processes in driving erosion and uplift across the European Alps? It has largely been argued that repeated erosion and glaciation sustain isostatic uplift and topography in a decaying orogen. But some parts of the Alps may still be actively uplifting via deep lithospheric processes. We add insight to this debate by isolating the role of post-glacial topographic forcing on erosion rates. To do this, we quantify the topographic signature of past glaciation on millennial-scale erosion rates in previously glaciated and unglaciated catchments at the easternmost edge of the Austrian Alps. Newly measured catchment-wide erosion rates, determined from cosmogenic 10Be in river-borne quartz, correlate with basin relief and mean slope. GIS-derived slope–elevation and slope–area distributions across catchments provide clear topographic indicators of the degree of glacial preconditioning, which further correlates with erosion rates. Erosion rates in the easternmost, non-glaciated basins range from 40 to 150 mm ky−1 and likely reflect underlying tectonic forcings in this region, which have previously been attributed to recent (post 5 Ma) uplift. By contrast, erosion rates in previously glaciated catchments range from 170 to 240 mm ky−1 and reflect the erosional response to local topographic preconditioning by repeated glaciations. Together, these data suggest that Holocene erosion across the Eastern Alps is strongly shaped by the local topography relict from previous glaciations. Broader, landscape-wide forcings, such as the widely debated deep mantle-driven or isostatically driven uplift, result in lesser controls on both topography and erosion rates in this region. Comparing our data to previously published erosion rates across the Alps, we show that post-glacial erosion rates vary across more than 2 orders of magnitude. This high variation in post-glacial erosion may reflect combined effects of direct tectonic and modern climatic forcings but is strongly overprinted by past glacial climate and its topographic legacy.

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Cortical deformation in the Aguacaliente-Navarro fault system (Central Valley, Costa Rica) from Geodetic data (GNSS and InSAR)

10.5194/egusphere-egu2020-12015 ◽

2020 ◽

Author(s):

Juan José Portela Fernández ◽

Alejandra Staller Vázquez ◽

Marta Béjar Pizarro

Keyword(s):

Costa Rica ◽

Seismic Hazard ◽

Seismic Hazard Assessment ◽

Central Valley ◽

Fault System ◽

Moderate Seismicity ◽

Cumulative Strain ◽

Gnss Data ◽

The City ◽

Turrialba Volcano

The Central Valley, Costa Rica, is subject to moderate seismicity, related to the Central Costa Rica Deformation Belt: a region with diffuse deformation, where Caribbean, Cocos and Nazca Plates, as well as the Panama Micro-plate, interact. &#160;The Eastern part of the valley is dominated by the Aguacaliente-Navarro fault system. The city of Cartago was destroyed by an earthquake Ms 6.4 in 1910, associated with the rupture of the Aguacaliente fault. Volcanic unrest &#8211;mainly in Turrialba Volcano, with recent activity reported- is present in the area, thus resulting in a very complex interaction zone, where seismic hazard studies are crucial.In this context, we process GNSS observations from five different campaigns -2012, 2014, 2016, 2018 and 2020- in 13 stations in the area, in order to estimate their Caribbean-fixed velocities, hence the regional cumulative strain. Additionally, we use both InSAR and GNSS data to measure volcanic deformation, aiming to refine the computed velocities by removing volcanic deformation from the tectonic signal.The refined velocities allow us to asses a more precise cumulative strain for the Aguacaliente-Navarro fault system, which is useful to improve seismic hazard assessment in Cartago, one of the most important cities in the region.

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