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7000 years of earthquake record in Julian ALps (Lake Bohinj, Slovenia)
William Rapuc, Pierre Sabatier, Maja Andric, Christian Crouzet, Fabien Arnaud, Emmanuel Chapron, Andrej Smuc, Anne-Lise Develle, Bruno
Wilhelm, François Demory, et al.
To cite this version:
William Rapuc, Pierre Sabatier, Maja Andric, Christian Crouzet, Fabien Arnaud, et al.. 7000 years of earthquake record in Julian ALps (Lake Bohinj, Slovenia). International Meeting of Sedimentology, Oct 2017, Toulouse, France. 2017. �hal-01773779�
7000 years of earthquake record in Julian ALps
(Lake Bohinj, Slovenia)
Environnements, Dynamiques et Territoires de la Montagne
Laboratoire
EDYTEM
EDYTEM
1 : EDYTEM, Université Savoie Mont Blanc, CNRS, Le Bourget du Lac, william.rapuc@hotmail.fr ; 2 : ZRC SAZU, Institute of Archaeology, Ljubljana, Slovenia ;
3 : ISTerre, Université de Savoie, CNRS, bat. Belledonne, Le Bourget du Lac ; 4 : GEODE, Université Jean Jaurés, Toulouse ; 5 : ISTO, Université d’Orléans, Orléans; 6 : Department of Geology, University of Ljubljana, Slovenia ; 7 : LTHE, Université Grenoble, Grenoble ; 8 : CEREGE, Université Aix-Marseille, CNRS, IRD,
Aix-en-Provence ; 9 : LSCE, Université de Versailles Saint-Quentin, CNRS, Gif-sur-Yvette ; 10 : Institut für Geographie, Jena, Germany
William RAPUC
1, Pierre SABATIER
1, Maja ANDRIC
2, Christian CROUZET
3, Fabien ARNAUD
1, Emmanuel CHAPRON
4,5, Andrej SMUC
6, Anne-Lise
DEVELLE
1, Bruno WILHELM
7, François DEMORY
8, Edouard REGNIER
9, Jean-Louis REYSS
1, Gerhard DAUT
10& Ulrich VON GRAFENSTEIN
9INTRODUCTION
MATERIALS & METHODS
DISCUSSION
RESULTS
CONCLUSION
REFERENCES
• Julian Alps are characterised by a moderate to high
seismic activity that has included several destructive earth-quakes, such as the 1511 AD “Idrija” earthquake (MSK = X). Accurate assessment of the seismic hazard is required to reduce the high vulnerability of this region.
• Earthquake shaking may destabilise previously deposited sediment, which slides to the bottom of the lake and formed seismically induced mass wasting deposits (MWD), such as turbidites or homogenites.
• This study focused on Lake Bohinj (Fig. 1), in the Julian Alps (Slovenia). During the Quaternary, this area was affec-ted by major NW-SE striking faults (Camassi et al., 2011). • To reconstruct the earthquake chronicle, Lake Bohinj
sediments were mapped with seismic reflection survey and cored in 2012 to provide a 12-m-long sedimentary record. • A multiproxy analysis associating sedimentological and geochemical measurements was performed to reconstruct the long-term earthquake record.
• 4 generations of MWDs are identified across eastern sub-basin (Fig. 1D) at the edges of the subaqueous slopes. MWDs evolved laterally into transparent acoustic facies developing onlaps, typical signature for muddy turbidites or homogenites. • Lake Bohinj sediment core is subdivided into 7 different
sedimentary units. Most of the sedimentary record is Unit IV (443-998 cm, Fig. 2) composed of a coarse sandy thick erosive base overlaid by a homogeneous silty facies and a clayey cap. • The upper part of the sedimentary record (0-443 cm) is interbedded by two types of graded-beds:
o Type I is composed of (i) a coarse sandy base (Fig. 3), (ii) a central part with homogeneous silt, (iii) a thin light clayey cap. Typical characteristics of Homogenite-type deposits,
confirmed by higher values of Foliation (Chapron et al., 2016). o Type II is characterized by a gray graded bed with a coarse and erosive silty base becoming slightly finer until a white clayey cap (Fig. 4). Characteristic of turbidite-type
deposits (Sturm and Matter, 1978)
• The age depth model was created associating short-lived
ra-dionuclides profiles and 21 calibrated 14C ages. 7 outliers were
unused because sampled in instantaneous deposits (Fig. 5).
• Several mechanisms can trigger MWDs: e.g., earthquakes, spontaneous delta collapses, and considerable lake-level changes. The 15-m-high moraine ridge (Fig. 1) prevents sediment contribution of hyperpycnal flow from the delta and delta collapses to reach the coring location.
• Earthquakes record by MWDs depends on the sensitivity of a lake to record a seismic event. To characterize the sensitivity of Lake Bohinj, earthquake-sensitivity threshold Index (ESTI) method was applied (Fig. 6; Wilhelm et al., 2016).
• Seismic trigger hypothesis for the three distinct types of deposits:
o Three most recent T1 correspond to three strong historic earthquakes (Fig. 6, 1348, 1511 and 1690 AD earthquake)
o Between 3500 and 2000 yr cal BP major destabilizations occur in the watershed by human activities (Fig. 7) that led to increases in sedimentation rate and thus increase ESTI (more earthquakes should have been recorded). High T2 deposits number identified could be the consequence of sensitivity increase.
o Unit IV occurred coevally with geomorphological changes in the area (Bovec Terrace, Fig. 1 , Marjanac et al., 2001)
• The Bohinj earthquake chronicle presents several time intervals with no local seismic activity recorded by homogenite-type deposits. These periods could be linked to the lack of major seismic events or to ESTI decrease
• This is the first earthquake chronicle over the last 7000 years in the Julian Alps with 29 homogenite-type deposits.
• The regional seismic activity recorded by a lake is highly connected to
earthquake settings, distance and the lake’s sensitivity to recording a seismic event (ESTI), which is related to the
sedimentation rate.
• When sedimentary sequence presents a variable ESTI, it should be prohibited to present a mean return period for seismic
activity recorded by the lake system. Thanks to the LSM, ARSO, Robert Jensterle, D. Valoh, J. Dirjec, S. von Grafenstein, S. Kuharič, M. Zaplatil and to ARRS Research program.
• Camassi, R., et al. (2011) Journal of Seismology, 15, 191–213. • Chapron, E., et al. (2016) In: Submarine Mass Movements and
their Consequences, pp. 341–349. Springer.
• Marjanac, T., et al. (2001) Geologija, 44, 341–350. • Moulin, A., et al. (2014) Tectonophysics, 628, 188–205.
• Rovida, A., et al. (2016) Istituto Nazionale di Geofisica e
Vulcanolo-gia (INGV).
• Sturm, M. and Matter, A. (1978) Wiley Online Library.
• Wilhelm, B., et al. (2016) Journal of Geophysical Research: Earth
Surface, 121, 2–16. Sava Fault Idrija Ljubljana Bled Kranj Tolmin Bovec Udine Montfalcone Ravne Fault Rasa Fault Idrija Fault Mt Triglav >10 m > 20 m > 30 m >35 m >40 m >45 m inlet outlet 1000 m Surface Geology: Scree Unconsolidated Moraine Amonitico rosso Limestone Mid Jurassic-Low. Cretaceous Upper Triassic Limestone lity Upper Triassic Massive Limestone Watershed shape
Main and secondary Stream, Waterfall
Thrust Fault, Fault
1750
1500
2000
500m
Savica Lake Bohinj
525m
1000
1500
1750
Glacio-lacustrine clay
Sveti Duh (525.89 m asl)
-35 m -40 m -45 m -50 m -55 m MWD-B Depth blf (m) 100 m N S MWD-D BO12 MWD-D MWD-C MWD-A top base 1348 MSK=IX-X 1511 MSK=X A C B D 1942 MSK=VI 1690 MSK=VIII-IX 1976 MSK=IX-X
Major Strike-slip Fault Lake Bohinj 20 km
13°E 13°30’E 14°E 14°30’E
Major Historical Earthquakes Secondary Strike-slip Fault
46°20’ N 46°10’ N 45°50’ N 45°40’ N 46°N 46°30’ N
FIG. 1
Fig. 1- General presentation of the study site. (A) Location of the study area with the major strike-slip faults and major
historical earthquakes, modified from Moulin et al., 2014. (B) Geological map of the Lake Bohinj watershed.
(C) Bathymetric map of Lake Bohinj. (D) Seismic profile oriented across the eastern basin showing the coring site location and four generations of mass wasting deposits (MWD).
10 6 9 8 7 Cl St Sd Pb 0 BO12 1 2 3 4 5 11 12 I II III IIIb IV V VI VII Depth (m ) I I II 0 2000 4000 6000 8000 Br (cps) 0 10 20 30 LOI 550° (%) 0 1000 2000 Ca (cps x103) 0 10 20 30 40 LOI 950° (%) 0 20000 40000 Ti (cps)
FIG. 2
Fig. 2- Main sedimentological and geochemical results. Lithological and geochemical results (Ti, Br and
Ca contents) associated with the BO12 sequence Lost On Ignition (LOI) 550° (dark-gray dots) and LOI 950° (light-gray dots). MAG: 520 x HV: 20.0 kV WD: 26.1mm 100 µm 700 µm MAG: 59 x HV: 20.0 kV WD: 26.0mm T1 Coarse Base Continuous Sedimentation 30.5 30.7 Depth (cm) Grain Size (µm) Depth (cm) 0 10 20 30 40 50 60 70 Type 1 deposits A Foliation Inc K3 Lineation C B B C 0 2 4 6 8 1 1,02 1 1,02 0 10000 20000 0 10 20 30 40 30 60 90 10-1 100 101 102 103 % Ti (cps) 2 2.5Sorting3 3.5 4 Median (µm)
FIG. 3
Fig. 3- Detailed results for T1 deposits. (A) AMS and grain size. EDX cartography in (B) T1 deposit and (C) continuous
sedimentation. 1 1,02 Lineation 140 145 150 155 Depth (cm) 1 1,02 1,04 1,06Foliation 10-1 100 101 102 103 0 2 4 6 8 Type 2 deposits 0 5000 10000 Ti (cps) Grain Size (µm) A B MAG: 160 x HV: 20.0 kV WD: 26.0mm 70 µm T2 Coarse Base Continuous Sedimentation 151.71 151.73 151.75 151.77 151.79 151.81 Depth (cm) B % 30Inc K360 90 2 Sorting2.5 3 Median (µm) 0 10 20 30
FIG. 4
Fig. 4- Detailed results for T2 deposits. (A) AMS results, Ti content and grain size. (B) EDX cartography at the boundary
of a T2 deposit and the continuous sedimentation.
Depth (m ) 14C Dates Short-lived radionuclides (210Pb,137Cs) Rejected 14C Dates 4000 8000 12000 0 Age (cal. BP) 10 0 1 2 3 4 5 6 9 8 7 11 12 BO12
Hiatus
Fig. 5- Age-depth models from radiocarbon and short-lived radionuclide dates.
FIG. 5
IV VI VIII X 1 10 100 1348 1511 1942 1976 1690 Epicentral MSK Intensit y Distance lake-epicentre (km)A
B
Earthquakes that not generated mass movements Earthquakes that generated mass movements LILSedimentation rate (mm.yr-1)
ESTI Index
BOH
BOHT2
Fig. 6- ESTI estimation for Lake Bohinj. (A) Lake Bohinj sequence results associated with CPTI15 data. Green line represents the limit of lake Bohinj
sensitivity to earthquakes calculated for the historical period. The red dotted line represents the hypothetical limit of lake Bohinj sensitivity with an ESTI of 0.18 calculated for the mean sedimentation rate over the period of 3500 to 2000 yr cal BP, unique period with T2 deposits. (B) The ESTI versus sedimentation rate for Lake Bohinj (green) over the last centuries with previous results (black) from Wilhelm et al. (2016). The red cross corresponds to the ESTI and
sedimentation rate for the period of T2 deposits (3500 to 2000 yr cal BP). This diagram suggests that a significant increase (decrease) of the sedimentation rate appears to be the dominant factor resulting in an increase (decrease) of earthquake deposits in the lake (Wilhelm et al., 2017).
FIG. 6
I II III I IIIbIV
0 1 2 3 4 5 6 7 Age (k. yr cal BP.)
Sedimentation rate (mm.yr
-1 ) + -Sedimentation 0 1 2 3 0 1 Br (cps ) Dynamics of organic inputs + -0 2000 4000 6000 0 1 2 3 4 5 6 7 Age (k. yr cal BP.) Units
Low sedimentation rate
Good record of seismic activityErosion
Opening A forest of spruce, beech and pine surround Lake Bohinj
Progressive rise of Grasses and Pines
Sedimentation Evolution of watershed Anthropogenic impact 6 1690 AD 1511 AD 1348 AD T2 Deposit s (nb/30yrs) 0 12 T1 Deposit s
Low sedimentation rate Good record of seismic activity
Opening
Earthquake Chronicle
Fig. 7- Lake Bohinj earthquake chronicle. Comparison between the organic input dynamic (Br), T2 and T1
frequency with major historical earthquakes and the mean sedimentation rate. Gray shadings correspond to intervals with potential higher ESTI.
