Reconstructing Historical Seismicity from Lake Sediments (Lake Laffrey, Western Alps, France)

doi: 10.1111/j.1365-3121.2005.00620.x Reconstructing historical seismicity from lake sediments (Lake Laffrey, Western Alps, France)

Je´roˆ me Nomade,1,2 Emmanuel Chapron,3,4 Marc Desmet,5 Jean-Louis Reyss,6,7 Fabien Arnaud5 and Vincent Lignier5 1Laboratoire de Geódynamique des Chaıˆnes Alpines, UMR CNRS 5025, Universite´ Joseph Fourier, Grenoble, France; 2Laboratoire de Geódynamique des Chaıˆnes Alpines, UMR CNRS 5025, Universite´ de Savoie, Savoie, France; 3Institut des Sciences de la Terre d’Orleáns, UMR CNRS 6113, Universite´ d’Orleáns, Orleans, France; 4Geological Institute, ETH Zu¨rich, CH-8092 Zu¨rich, Switzerland; 5EDYTEM, UMR CNRS 5204, Universite´ de Savoie, Savoie, France; 6Laboratoire des Sciences du Climat et de l’Environnement, UMR 1572 CEA-CNRS, Gif/Yvette, France; 7Laboratoire Souterrain de Modane, CEA-CNRS, Modane, France

ABSTRACT Sediment archives from a mountain lake are used as indicators a constrained age-depth model. Over the last 250 years, five of of seismotectonic activity in the Grenoble area (French western such debris flow deposits could be related to historical Alps, 45°N). Sedimentological analysis (texture and grain-size earthquakes of MSK intensities greater than VI over an area characteristics) exhibits several layers resulting from instanta- of <60 km. One debris flow deposit triggered at the beginning neous deposits in Lake Laffrey: six debris flow events up to of the last century can be related to an historical landslide 8 cm thick can be attributed to slope failure along the western possibly triggered by the artificial regulation of the lake level. flank of the basin. Dating with 210Pb and 137Cs gamma counting techniques and the reconnaissance of historical events, provide Terra Nova, 17, 350–357, 2005

E5E 6 E7 E 8 Introduction Since a few decades lacustrine sedimentary records are used to infer palaeoseismic activities in areas where Geneva 1905 large events occurred (Sims, 1975; N 46 Ben-Menahem, 1976; Doig, 1986; 1822 Marco and Agnon, 1995; Inouchi Lake Anterne et al., 1996). First studies dating palaeoseismological events using radio- Lake le Bourget nuclide (137Cs and 210Pb) and AMS Chambéry 14C techniques in lake sediments were done in Quebec (Doig, 1986, 1990, 1754 1998). Recently, in several French Grésivaudan paleo-lake alpine lakes of contrasted size and Grenoble 1881 setting these techniques highlighted 1782 that some lake sediments could be N 45 1962 good recorders of earthquakes, even (ML=5.3) in moderate seismicity areas (Chapron 1963 Lake Laffrey et al., 1999; Lignier, 2001; Arnaud (ML=4.9) et al., 2002). Those studies have shown that sedimentary structures triggered by earthquakes (i.e. slope failure and seiche deposits) can be Trièves paleo-lake easily detected and characterized with high-resolution laser grain size meas- N 44 urements. Lebrun et al. (2001), demonstrated Historical earthquakes that a significant site effect exist in the 80 km MSK Correspondence: Je´roˆ me Nomade, Labo- Scale VI VI-VII VII VII-VIII ratoire de Geólogie des Chaıˆ nes Alpines, LGCA UMR CNRS 5025, Maison des Fig. 1 Location of Lake Laffrey in the French Alps. Main historical earthquakes Geósciences, BP 53, 38041 Grenoble Ce- discussed in the text are indicated by circles, with corresponding MSK intensities at dex, France. Tel.: +33 (0) 4 79 75 81 02; epicentres and local Richter magnitude (ML). Lakes Le Bourget and Anterne, as well fax: +33 (0) 4 79 75 87 77; e-mail: jerome. as the extension of the Gre´sivaudan and the Trie` ves palaeolakes (Monjuvent, 1978; [email protected] Nicoud et al., 1987) are also located.

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A B 10 1754

re BBF 20 è VI-VII Is VII 30 LAF 0103

re 1881 è Is Laffrey land slide Grenoble 39 Lake Laffrey 1782 1 VI

e 30 ch n a 20 m o 10 R 5

1962 500 m VII-VIII Lake Pétichet M = 5.3 L c 2

D 3 PET 1 4 PET 3 10 1963 PET 2 5 km VII 10

ML = 4.9 Lake Pierre-Châtel

10 1- Lake Robert (2000m a.s.l) BBF Belledonne border fault Grenoble urban area (active strike-slip fault) PC 1 2- Lake Laffrey (909m a.s.l) PC 2 Major fault Historical earthquake 3- Lake Petichet (926m a.s.l) VI (with MSK intensities) 4- Lake Pierre-Chatel (929m a.s.l) Major thrust Local magnitude M = 4.9 5- Lake Blanc d'Huez (2500m a.s.l) River L (Richter)

Fig. 2 Seismotectonic setting of the study area and location of investigated lacustrine sedimentary infill. (A) Structural setting modified after Grasso et al. (1992) and Thouvenot et al. (2003). (B) Bathymetric maps (isobaths: 5 m) of lakes Laffrey, Pe´tichet and Pierre Chaˆ tel (Delebecque, 1898) highlight the development of littoral platforms near LAF0103 coring site and the Laffrey landslide. While in lake Robert the sedimentation rate of 0.1 mm yr)1 over the last 7000 years (Nomade, 2001) prevents the recognition of historical earthquakes, in lakes Petichet and Pierre-Chaˆ tel, preliminary studies including radionuclide dating on short cores (PET 1, 2, 3 and PC 1, 2) highlight significant bioturbation, and are thus not suitable for palaeoseismic investigations (Fain, 2004). A clear recognition of earthquake-triggered sedimentary events in the proglacial lake Blanc d’Huez is complicated by the strong human impact (in this ski resort area) and by a low sedimentation rate (0.5 mm yr)1) over the last 500 years (Fain, 2004). On the contrary, limited bioturbation and high sedimentation rate at site LAF0103 provide the best conditions for palaeoseismological investigations in this seismotectonic area.

Grenoble urban area (French western study describes the characteristics as mainly cluster around estimated Alps) and that the amplification of well as the chronology of the first magnitudes 4–5 and stronger events ground motion in this large alpine metre of sediments accumulated in the (such as the Corrençon event in 1962) valley can cause large risk in some deep basin of Lake Laffrey, and dis- were above but below Mw 6. The city part of the city. Lake Laffrey located cusses the sensitivity of this site to of Grenoble in the Ise` re glacial valley 20 km south of Grenoble (Figs 1 and record the regional seismicity. is built on a thick quaternary infill 2), was selected among other sur- (Monjuvent, 1978; Nicoud et al., rounding lakes for coring and multi- 1987) and a major risk exists in the Geological setting disciplinary sedimentological studies centre of the basin for an Mw in order to test the seismic database of Large tectonic features such as base- 5.5 earthquake located at <20 km the area that extends over the last ment thrusts and strike–slip faults (Cotton et al., 1999; Lebrun et al., 500 years (Lambert et al., 1996, 1997; characterize the western Alps south 2001). Evidences for strong palaeo- BRGM, IPSN, EDF, 2002). The sedi- of Grenoble. This tectonically active earthquakes were described in gla- mentary infill of this lake is the most region (Fig. 2A) is affected by ciolacustrine to lacustrine clayey suitable because of its location close to moderate magnitude earthquakes formations associated with the Gre´s- well-known significant historical (Lambert et al., 1997; Thouvenot ivaudan and the Trie` ves palaeo-lakes earthquake epicentres (Fig. 2). This et al., 2003). Historic earthquakes (Fig. 1) of post-Riss and Wu¨ rmian

2005 Blackwell Publishing Ltd 351 Historical seismicity in the NW Alps • J. Nomade et al. Terra Nova, Vol 17, No. 4, 350–357 ......

Percentage of Sorting A clays, silts, sands 1.8 2 2.2 2.4 2.6 0 20 40 60 0 200 400 600 800 0 0% 50% 100%

1 20 2

3 40 Clays s s ilt d S Silts

4 n a Sands S 60

80 5

nt thickness (cm) 6

Sedime 100 Facies 1 Facies 2

Facies Sorting Clay (%) Silt (%) Sand (%) CaC03 (%)

Mean value 23 2.1 195.5 13 78.8 8.1 1 32 Standard deviation 3.7 0.06 29 2 1.9 2.9

Mean value 34.7 2.3 281.7 12.3 71.6 16.2 2 33 Standard deviation 15 0.2 123.7 2.4 5.4 6.42

Mean value 27.9 2.2 219.9 13.1 74.2 12.8 2 top 29 Standard deviation 4.6 0.11 35.5 2.5 2.6 3.6

Mean value 45.2 2.4 371.6 11 67.5 21.4 2 bottom 37 Standard deviation 19.1 0.26 154.1 2 6.1 6.4

Fig. 3 Lithology and grain-size characteristics of LAF0103. Compared to gravity coring devices broadly used to sample the first metre of lacustrine sediments, UWITEC piston coring (a 3 m long piston corer) induces almost no compaction during sampling and a well-preserved mud-line for upper cores. This core can be subdivided in two main facies based on grain-size measurements as well as macroscopic and microscopic descriptions. (A) Six layers (facies 2) labelled from 1 to 6 are contrasting (high values of Q99 and rather bad sorting) with the background sedimentation (facies 1). (B) Facies 1 and 2 characteristics are summarized. ages, respectively (Beck et al., 1992; tributary of the catchment drains first platform with steep platform slopes Lignier et al., 1998). into the nearby Lake Pe´tichet surrounding the central basin Lakes Laffrey, Pe´tichet, Pierre-Chaˆ - (Fig. 2B). Lake Laffrey is oligothro- (Fig. 2B). At 20 m depth in the nor- tel and Mort are small glacial lakes phic, dimictic and it has a limited thern part of the basin, Pourchet et al. damned by successive frontal moraine clastic supply (Delaquaize, 1979). Its (1989) suggested the occurrence of belts. These lakes were formed during water level is mainly resulting from a some mixing in the recent sediments, the Wu¨ rm glaciation by the retreat on ground water flow in the glacial based on 137Cs measurements. the Matheysin plateau of a small deposits and since its artificial regula- branch of the large Romanche glacier tion at the beginning of the twentieth Methods (Monjuvent, 1978). Lake Laffrey century; the lake level has a mean (2.7 km long, 0.6 km wide and 39 m annual variation of 2 m. The bathy- The core LAF0103 corresponds to the deep) is the largest lake of the Math- metric map of Lake Laffrey performed first metre of sediment sampled at eysin plateau. It has a nivo-pluvial by Delebecque (1898) shows no signi- 4500¢53¢¢N and 546¢40¢¢Eby38m catchment area of 17.4 km2 culmin- ficant delta development, but high- water depth (Fig. 2B). This core was ating at 2135 m a.s.l. and the main light steep western slopes and a littoral retrieved in 2001 using UWITEC

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Error bar

Fig. 4 Radioactive gamma measurements from core LAF0103 vs. core depth (A) and model thicknesses (B, C). When error bar is invisible, the error is inferior to point size. 210Pb activity is less important in debris flow deposits than in facies 1 and support the idea that debris flow deposits are sedimentary events remoulding older lacustrine sediments. When the values associated with debris flow deposits are excluded, unsupported 210Pb profile (B) is more linear and a mean AR of 2.2 mm yr)1 can be calculated (based on 27 measurements of similar grain-size samples from 20 to 24 lm). Although a first interpretation based only on 137Cs measurements would suggest that the upper peak at 1.8 cm could be related to the 1986 Chernobyl accident, they should be interpreted carefully because: (i) of the occurrence of mixing between the 60¢ and the 80¢, and (ii) of the detection of 241Am in subsurface samples. Therefore, 137Cs data can only be used here to date the onset of first nuclear tests in 1954. These data suggest a variable mean AR of 2.8 mm yr)1 since 1954, but a variable AR in the upper 10 cm of the core is also suggested by fluctuations in the 210Pb profile. devices and represents the upper sec- located in the underground laboratory white millimetric laminas made of tion of a 14 m long coring site (Des- of Modane (LSM) in the French Alps clastic particles (quartz, feldspar, cal- met et al., 2003). (M.I.M 96). Methods are described in cite) and biogenic ones (diatoms, Sedimentological investigations on Reyss et al. (1995). Supported 210Pb in authigenic calcite, fragment of LAF0103 involved detailed litholo- each sample was assumed to be Arthropods). Facies 2 is a matrix- gical descriptions, high-resolution in equilibrium with the in situ 226Ra. supported graded bed, with a sharp video captures, smear slides analyses, Unsupported 210Pb was calculated by base and a gradational upper limit grain size and carbonate content subtracting 226Ra activity from total characterized by a dark blue colour. measurements (Fig. 3). Laser grain 210Pb. Smear slide microscopic observations size measurements were realized every revealed that these layers have nearly 5 mm using Malvern Mastersizer the same composition than facies 1, Data presentation and S. Carbonate contents were also meas- although their base have higher car- interpretation ured with the same instrument, using bonate content (up to 37%) essentially the comparison of obscuration on made of cauliflower-like forms of Sedimentology total sediment samples and on decar- carbonate concretions. Six layers of bonated samples, as described by facies 2, up to 8 cm thick, are visible Core description and lithology Trentesaux et al. (2001). on the core and labelled from 1 to 6 in The chronology of the core is based Core LAF0103 is subdivided into two Fig. 2. on 210Pb and 137Cs dating techniques. distinct facies (Fig. 3A): a fine-grained 210Pb, 241Am, 137Cs and 226Ra were faintly laminated pattern (facies 1) Grain-size measurements measured in dried sediment by direct interbedded with thick coarser- gamma assay in a large, low back- grained layers (facies 2). Throughout Facies 1 is relatively homogenous, ground, well-type germanium detector the core, facies 1 consists of dark and characterized by clayey-silt deposits

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CalculatedHistorical Local Intensity 2000 1900 1800 1700 Following the nomenclature of Mul- 0- ageeathquake in Laffrey (MSK) der and Cochonat (1996), the occurrence of inverse-grading in these mass flow deposits is interpreted as due to - the development of debris flows. In DF 1 VI the coarse fraction of facies 2, the 1957 1963 (M = 4.9) L occurrence of carbonate concretions 20- DF 2 1962 VI that are typical from the littoral plat- 1957 (M = 5.3) L form bench according to Magny - (1992), suggests a source area for the debris flows located in the steep west- DF 3 1904 (II) ern slopes of the lake surrounding our 1902 1901 (III-IV) 40- coring site (Fig. 2B). Because domin- ating N–S winds are only developing significant fetch at the extremities of - 1881 V-VI the lake basin, these debris flows DF 4 1868 1866 (IV-V) cannot be related to storms events (i.e. tempestites). 60-

Chronology -

cm) Radionuclide measurements DF 5 1787 1782 V ess ( 80- The 210Pb unsupported activity curve (Fig. 4A) is non-linear in a semi-log hickn graph and the activity in the debris

nt t - DF 6 1753 1754 ? flow deposits is less important than in facies 1. Values of 137Cs radioactivity vs. thickness (Fig. 4A) highlight a Sedime 100- progressive increase starting at Facies 1 Facies 2 Age-depth model (with error bar: +/– 13 yrs) 25 cm, and a significant increase till 10 cm followed by a plateau and 1787 V-Recorded earthquakes (IV-V) -Unrecorded earthquakes ) 1866 finally a clear peak above 300 Bq kg 1 )1 at 1.8 cm, whereas young sediments Fig. 5 Age-depth model for LAF0103. A mean AR of 2.8 mm yr since 1954 and of )1 )1 137 210 have about 200 Bq kg . The progres- 2.2 mm yr for older sediments was used based on Cs and Pb values in facies 1. 137 )1 This age-depth model discussed in the text is strongly supported by the resolution sive rise in Cs up to 60 Bq kg and (±13 years) of the correlation of DF deposits with five strong earthquakes over the the formation of a plateau from 5 to last 250 years (strong regional earthquakes with local MSK intensities close to VI in 10 cm indicate a significant mixing of 1963, 1962, 1881, 1782 and 1754), and the Laffrey landslide triggered between 1900 sediments contaminated by the atmo- and 1910. The exact origin of this landslide is not clear, but to some extent, might be spheric nuclear weapon tests that cul- related to the artificial elevation of the lake level in the early twentieth century and to minated in 1963. changes in water circulation within glacial deposits (R. Soupiset, personal communication). Modern age-depth model Following Arnaud et al. (2002), we with a mean grain size of 23 lm, a deposits formed by a rain of particles propose to exclude the 210Pb and 137Cs sorting of 2.1 with both low standard not sorted by bottom currents (Pass- values associated with debris flow deviations (Fig. 3B). Facies 2 is signi- ega and Byramjee, 1969). This facies is deposits and to establish a model of ficantly less sorted than facies 1 and thus interpreted as the result of a sediment thickness involving only fa- ranges from sandy-silt to clayey-silt continuous lacustrine sedimentation cies 1 deposits. Removing the facies 2 deposits from the bottom to the top, pattern essentially resulting from the events makes the unsupported 210Pb respectively. In detail, these layers are settling of clastic and biogenic sedi- profile more linear (Fig. 4B) and matrix-supported deposits character- ments in the water column. On the allows us to calculate a mean accu- ) ized by a coarsening upward sequence contrary, facies 2 highlights grain-size mulation rate (AR) of 2.2 mm yr 1 or )2 )1 2 at the base to a fining upward characteristics that are typical from 0.078 g cm yr (R ¼ 0.9) since AD sequence at the top. graded suspension deposits sorted by 1850 using a simple 210Pb decay model the settling out of a bottom current (CFCS) as described by Goldberg (Passega and Byramjee, 1969). These (1963) and Appleby and Oldfield Sedimentary processes matrix-supported layers are also sug- (1978). Removing the debris flow The grain-size characteristics of facies gesting that the transport energy was deposits (DF 1 and 2) makes the 1 (Q99 and mean grain size, Fig. 3A) supplied by sediment weight (i.e. a 137Cs profile clearer (Fig. 4C): the are typical from uniform suspension mass flow) rather than water current. progressive increase from 13 to

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Fig. 6 Distance from epicentre vs. MSK intensity showing the sensibility of the Lake Laﬀrey sedimentary record to regional seismicity: only events with MSK intensities ¼VI, located at <60 km are recorded. Largest regional historical earthquakes of the NW Alps area (1822 and 1905) recorded in other lakes and other earthquakes discussed in the text (1866, 1901, 1904) are also indicated in this graph.

10 cm (model thicknesses) and plateau ments and (ii) their extrapolation mentary event with a well-documen- from 10 to 5 cm are related to the down-core. In Lake Laffrey, the two ted historical earthquake (Doig, 1986; occurrence of first nuclear tests start- stacked debris flow deposits DF 1 and Siegenthaler et al., 1987; Chapron ing in 1954 and to the mixing of DF 2 at 12.5 cm (model thickness) et al., 1999; Arnaud et al., 2002; Schn- contaminated sediments after 1963. occurred after 1954 based on the ellmann et al., 2002). In this study, This interpretation is comforted by values of 137Cs. Assuming a mean two stacked debris flow deposits (DF the first occurrence of 241Am in the AR of 2.8 mm yr)1 since 1954, these 1 and 2) dated to have occurred in sediments at 12 cm (Appleby et al., two events were triggered around 1957 are correlated with the Corre- 1991) (see Fig. 4 for details). 1957. DF 1 and DF 2 can thus nçon-en-Vercors earthquake in 1962 reasonably be correlated with the and the Monteynard earthquake in strongest regional historical earth- 1963, which both shook the Grenoble Discussion quakes during the last century located urban area and damaged buildings close to Lake Laffrey (1962 and 1963 (Lambert et al., 1996, 1997; BRGM, Age-depth model accuracy events; Figs 1 and 4). The mean AR of IPSN, EDF., 2002). Near Lake Laff- According to Pourchet et al. (1989), 2mmyr)1 till 1850 deduced from our rey, these two earthquakes had local 137Cs mixing in the sediments of Lake 210Pb data and its extrapolation intensities VI (MSK). The grain size Laffrey before the accident of Cher- down-core within facies 1 deposits is and textural characteristics of the nobyl is due to a rapid diffusion similarly strongly supported by the related stacked sedimentary events process (i.e. bioturbation). The chan- correlation of DF 4, 5 and 6 with are clearly different from the back- ges in modern AR deduced from our strong historical earthquakes (Fig. 5). ground sedimentation and their char- radionuclide measurements together acteristics are similar to the ones of with the sporadic occurrence of 137Cs DF 3, 4, 5 and 6. Based on our age- Earthquake-triggered debris flows mixing may therefore be the result of depth model (Fig. 5), and on the local changes in the trophic state of the lake Palaeoseismology studies using lake intensity required in 1962 and 1963 to system due to changes in land use, as sediments are based on the detailed trigger DF 1 and 2, DF 4 can be already documented by Delaquaize characterization of sedimentary events related to the well-documented histor- (1979) and presently monitored by and on the establishment of a precise ical earthquake of 1881 (local MSK fishing state agencies (R. Soupiset, chronology. Because mass wasting intensity V–VI) rather than the one of personal communication). In such a deposits such as DF could also be 1866 (local MSK intensity IV–V). setting, the reconnaissance of recent produced by other processes (such as Following the same procedure, DF 5 historical data in sediment cores is lake level changes or overloading), the matches the earthquake of 1782 (local useful to further constrain (i) the AR best argument to assess a seismic MSK intensity V) and DF 6 the one deduced from radionuclide measure- triggering is to correlate such sedi- of 1754 of unknown intensity. The

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calculated age of DF 3 not only thus required to extend palaeoenvir- versant des lacs de Laffrey et de Pe´tichet, matches two earthquakes with local onmental and palaeoseismic recon- Unpublished Doctoral Dissertation, MSK intensities

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