EGU2020-10802
https://doi.org/10.5194/egusphere-egu2020-10802
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Quantitative paleoseismology in Carinthia, Eastern Alps: Calibrating the lacustrine sedimentary record with historical earthquake data

Christoph Daxer1, Christa Hammerl2, Maria del Puy Papi-Isaba2, Stefano Claudio Fabbri3, Patrick Oswald1, Jyh-Jaan Steven Huang1, Michael Strasser1, and Jasper Moernaut1
Christoph Daxer et al.
  • 1Institute of Geology, University of Innsbruck, Austria (christoph.daxer@uibk.ac.at)
  • 2ZAMG - Central Institute for Meteorology, Vienna, Austria
  • 3Institute of Geological Sciences, University of Bern, Switzerland

In intraplate settings with moderate seismicity, recurrence intervals of strong earthquakes (Mw >6) typically exceed the short time span of instrumental and historical records. To assess the seismic hazard in such regions, lake sediments are increasingly used as earthquake archives: they can record strong seismic shaking as mass transport deposits (MTDs), turbidites or sediment deformations, preserved over several thousands of years. To provide information on paleo-earthquake size, however, the sedimentary imprints need to be thoroughly calibrated with independent information on seismic shaking strength.

In Carinthia (Eastern Alps, Austria), numerous lakes have experienced several devastating historical earthquakes with local seismic intensities (SI) ranging from V-XI (EMS-98 scale), although being located in an intraplate environment. Given that i) these events are well-spaced in time (AD1201, AD1348, AD1511, AD1690, AD1857 and AD1976), ii) due to historical earthquake research, an exceptional historical documentation exists, and iii) accurate shakemaps can be built based on a local Intensity Prediction Equation (IPE), we can examine the relationship between seismic intensity and the type, size and spatial distribution of sedimentary imprint in the lakes.

Here, we present investigations on two large lakes – Wörthersee and Millstätter See – by a dense grid of reflection seismic profiles (~640 km overall), 80 short (~1.5 m) sediment cores and multibeam bathymetry. The lakes consist of several sub-basins with potentially different intensity thresholds for the generation of sedimentary imprints. Mapping of MTDs, their scarps and associated turbidites as well as accurate dating (radiocarbon and varve counting on sediment thin sections) shows that the AD1348 earthquake (Mw ~7) led to extensive slope failures in both lakes. The AD1511 (Mw ~6.9) and AD1690 (Mw ~6.5) events, which exhibited lower local intensities (~VII) compared to those of AD1348 (VIII), are recorded as minor MTDs and turbidites. Quantitative description of earthquake-related event deposits (cumulative turbidite thickness, volume of mass transport deposits/megaturbidites) suggests a linear correlation with the respective local intensities in both Wörthersee and Millstätter See.

The AD1976 earthquake (Mw ~6.5; SI V-VI at the lakes) is not evidenced in the sedimentary record and therefore can be used for constraining the minimum threshold intensity for seismically-induced event deposits. By applying a grid-search approach using an empirical intensity-attenuation relationship, we can narrow down possible earthquake scenarios. Our data suggests that the highly debated epicentre of the AD1348 earthquake was much closer to the Austrian-Italian border than the epicentre of the AD1976 Friuli earthquake, possibly originating from the Periadriatic lineament. The AD1511 event probably had its epicentre southeast of our study area in Slovenia, and therefore further east than previous studies suggested. The AD1690 earthquake, however, is most likely of a local origin.

Our study reveals that investigating one lake, let alone one sediment core, is insufficient to reconstruct a region’s seismic history. Due to the exceptional setting of Carinthia, however, we can constrain the intensity pattern and localise the most likely epicentral region and fault source of past earthquakes. In an ongoing interdisciplinary study, we use this calibration to construct long calibrated lacustrine records for the last 14 ka.

How to cite: Daxer, C., Hammerl, C., del Puy Papi-Isaba, M., Fabbri, S. C., Oswald, P., Huang, J.-J. S., Strasser, M., and Moernaut, J.: Quantitative paleoseismology in Carinthia, Eastern Alps: Calibrating the lacustrine sedimentary record with historical earthquake data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10802, https://doi.org/10.5194/egusphere-egu2020-10802, 2020

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Presentation version 2 – uploaded on 11 May 2020 , no comments
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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Comment on EGU2020-10802, Christoph Grützner, 04 May 2020

    Very interesting! So for the AD1690 earthquake, your modelling suggests that the entire fault ruptured, right? What magnitude would that correspond to and how does this fit with the magnitude derived from macroseismic data?

     

    Thanks!

  • AC1: Comment on EGU2020-10802, Christoph Daxer, 04 May 2020

    Hi Christoph!

    Yes, probabilities of exceedence of the threshold intensity are highest for a rupture along the whole fault - but you see our problem there: when we only have positive evidence in our lakes, the probabilites are always highest for max-length ruptures (= maximum magnitude allowed). In our case, a full-length rupture of this fault corresponds to a magnitude of ~7, overestimating the magnitude derived from macroseismic data by 0.5.

    Deriving the most likely fault along which the rupture most likely occured, however, seems to work. This makes me realize that by stacking the probabilities in the visualization, the epicentral region could potentially be refined.

    Cheers

    Christoph

    • CC2: Reply to AC1, Christoph Grützner, 04 May 2020

      Great, thanks for the explanation!

  • CC3: Comment on EGU2020-10802, Katleen Wils, 08 May 2020

    Hi Christoph!

    Very interesting work, it's nice to see that the method we proposed for fault identifcation also seems to work in other settings! The results look very promising, I'm looking forward to see how it evolves further. 

    Concerning your issue on maximum probabilities obtained for full-fault ruptures, negative evidence is of course an important factor on limiting the maximum magnitude. Are the probabilities significantly higher for a full rupture compared to those for a quasi-full rupture? If they are, I would still consider the fact that previous estimates might be a bit too low. However, I can imagine that they reach some kind of plateau at some point, which would indeed indicate that you cannot distinguish the really highest-magnitude earthquakes. A possible way to work around this could be to consider MTD volumes in different parts of the lakes rather than considering them as one point? I'm sure that shaking in AD 1348 was much stronger than in e.g. AD 1690 considering the thickness and basin-wide distributions of these turbidites.

    Anyway, if you want to discuss on this, feel free to contact me and/or Kris Vanneste, I'm sure he would be interested as well to see your results!

    Cheers,
    Katleen

    • AC2: Reply to CC3, Christoph Daxer, 08 May 2020

      Hi Katleen!

      Yes I am very happy about the first result, and I have to say the approach you developed in Chile is just brilliant!

      So far, I only modelled ruptures with a rather limited length and magnitude (Mw max ~7) due to steps and gaps in the fault model. I am updating the fault model at the moment and will try to carry out a sensitivity analysis with the new model, in order to get an idea of how well or not we can distinguish large-magnitude earthquakes. But I have partly worked with having more than one datapoint per lake, which certainly narrows done possible fault ruptures, especially because the lakes also cover some distance (18 km in case of Wörthersee).

      Thanks also for the offer of discussion, I am a 100% sure that I will take up on that as soon as I have a set of specific questions - which will be in the following weeks, hopefully!

      Cheers and take care!

      Christoph

      • CC6: Reply to AC2, Katleen Wils, 08 May 2020

        Sounds great! Looking forward :)

  • CC4: Comment on EGU2020-10802, Giuliana Rossi, 08 May 2020

    Dear Christoph,

    nice work!

    I work on seismicity in Friuli and surroundings. I found very interesting the estimate for 1348 and the intensity threshold you can derive for the absence of 1976 recordings. Could you also give the reference of Rapuc et al. 2018 for Bohini lake?

    Thank you in advance

    Giuliana Rossi

    OGS, Italy

    Institute of Oceanography and Applied Geophysics -OGS

    grossi@inogs.it

  • AC3: Comment on EGU2020-10802, Christoph Daxer, 08 May 2020

    Dear Giuliana,

    Thank you very much for your comment!

    Yes, according to our data, intensities at our lake sites were much higher during the AD1348 event than during AD1976. You can find the reference of Rapuc et al. 2018 here:

    All the best

    Christoph

    • CC5: Reply to AC3, Giuliana Rossi, 08 May 2020

      no reference...

      • AC4: Reply to CC5, Christoph Daxer, 08 May 2020

        Dear Giuliana,

        I am sorry, apparently it is not possible to post it as a link. And I only now realised that the reference is missing in the display - thanks for pointing it out, I will update the display straight away.

        The reference is here: www.onlinelibrary.wiley.com/doi/full/10.1111/sed.12446

        I hope it is working like this.

         

  • CC7: Comment on EGU2020-10802, Giuliana Rossi, 08 May 2020

    Dear Christoph,

    Perfect!

    Thanks and many wishes for your work.

    Kind regards

    Giuliana