Sedimentary Geology 213 (2009) 121-135

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Fluidization of buried mass-wasting deposits in lake sediments and its relevance for paleoseismology: Results from a reflection seismic study of lakes Villarrica and Calafquén (South-Central Chile)

Jasper Moernauta *, Mare De Batista, Katrien Heirman a, Maarten Van Daele a, Mario Pino b, Robert Brümmer b, Roberto Urrutiac a Renard Centre of Marine Geology (RCMG), Ghent University, Krijgslaan 281 (S8), 9000 Ghent, Belgium b Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile c Centro EULA, Universidad de Concepción, Casilla ISO-C, Concepción, Chile

ARTICLE INFO ABSTRACT

Article history: A dense grid of very-high resolution seismic profiles on Lake Villarrica provides a quasi-3D view on Received 5 May 2008 intercalated lenses of low-amplitude reflections, which are connected by acoustic wipe-out patches and Received in revised form 2 December 2008 fractures to an underlying voluminous mass-wasting deposit. The lenses are interpreted as being created by Accepted 12 December 2008 earthquake-triggered liquefaction in this buried mass-wasting deposit and subsequent sediment fluidization and extrusion at the paleo-lake bottom. These sediment volcanoes are mapped in detail. They have a rather Keywords: uniform circular geometry and show a linear relationship between apparent width and maximum thickness Fluidization structures Lake sediments on a seismic section. The largest sediment volcanoes are up to 80 m wide and 1.9 m thick. Their slope angles Reflection seismic profiling designate a syn- to post-depositional sagging of most sediment volcanoes. Sediment volcano detection and South-Central Chile mapping from nearby Lake Calafquén further strengthen the revealed geometrical relationships. Locally, Paleoseismology some of the sediment/fluid escape structures extend to a higher position in the stratigraphy, which points to a polyphase escape process associated with multiple multi-century spaced strong earthquakes. Thickness and morphology of the source layer seem to exert a dominant control in the production of sediment/fluid extrusions. This study shows that reflection seismic profiling allowed recognizing 4 different seismic events in the studied stratigraphie interval, which are evidenced by mass-wasting deposits and/or fluidization features. © 2008 Elsevier B.V. All rights reserved.

1. Introduction outcrop availability often hamper the possibilities of studying the spatial and geometrical aspects o f this sediment extrusion process in Fluidization structures, such as sand blows, are a common effect of more detail. However, successful attempts have been made for severe seismic shaking in continental settings and their use in visualizing sand-blow craters, vents and source layers using ground- paleoseismological research is widely acknowledged (Audemard and penetrating radar (GPR) techniques (Liu and Li, 2001 ; Al-Shukri et al., De Santis, 1991 ; Obermeier, 1996; Hibsch et al., 1997 ; Green et al., 2004; 2006; Maurya et al., 2006). Obermeier et al., 2005; Mörner, 2005; Castilla and Audemard, 2007). Lacustrine sedimentary sequences are generally uniform over The processes behind liquefaction/fluidization of soft sediments are longer distances than those in continental settings. Fluidization studied w ith laboratory experiments, by means of which various kinds structures in lacustrine deposits are, therefore, often used for of soft-sediment deformation structures have been produced (Nichols paleoseismic analyses (e.g. Beck et al., 1996; Rodríguez-Pascua et al., et al., 1994; Owen, 1996; M oretii et al., 1999 and references therein). 2000; Singh and Jain, 2007; etc.). However, point observations (cores) Comprehensive mapping and measuring of fluidization structures in or 2D logging of available lacustrine outcrops often still remain the field allowed to define empirical relationships between quantita­ insufficient to accurately study the geometry and spatial distribution tive earthquake parameters, such as magnitude and/or epicenter of earthquake-induced sediment blows. Reflection seismic techniques location, and sand-blow dimensions (Castilla and Audemard, 2007 and of appropriately high resolution can be applied in order to overcome references therein). Unfortunately, erosional processes, strong lateral these typical limitations and to provide a (pseudo) 3D image of the variability of sediment layers, anthropogenic disturbance and limited fluidization features. Even though most lacustrine paleoseismological studies using reflection seismics focus mainly on the recognition of basin-wide

* Corresponding author. Tel: +32 9 2644637; fax: +32 9 2644967. slope-failure events (Schnellmann et al., 2002; Karlin et al., 2004; E-mail address: [email protected] (J. Moemaut). Strasser et al., 2006), some examples o f liquefaction-related structures

0037-0738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2008.12.002 122 ]. Moemaut et al. / Sedimentary Geology 213 (2009) 121-135 have also been identified on high-resolution seismic records. In the present study, we will illustrate the application of closely- Structures caused by vertical sediment mobilization with minimal spaced very-high-resolution reflection seismic profiles for the analysis horizontal displacement have been interpreted in various lacustrine of fluidization structures in the lacustrine infill of Lake Villarrica in sedimentary records, and have in most cases been attributed to a South-Central Chile. We w ill further discuss and document the origin, seismic origin (Shilts and Clague, 1992; Shilts et al., 1992; Hofmann geometry and spatial distribution of these structures as well as their et al., 2006). Diapir-like structures, which were possibly earthquake- paleoseismological implications, and we will compare them with triggered, have also been observed on subbottom echosounder similar structures in the subsurface o f nearby Lake Calafquén. profiles (Clague et al., 1989). In Lake Puyehue (South-Central Chile, 40.7°S), Moernaut et al. (2007) interpreted vertical fluid-escape 2. Study area structures, which were attributed to the dewatering of a buried mass-wasting deposit. Temporal coincidence between the top of these Lake Villarrica and Lake Calafquén (resp. 39.25°S; 214 masi and structures and basin-wide mass-wasting deposits points towards a 39.55°S; 204 masi) are large glacigenic lakes (ca. 20x10/5 km) common genesis, most likely a strong earthquake. It has not been located at the foot of the Cordillera de Ios Andes. The lake basins established yet whether such indications of fluid-escape events in originated from glacial valley overdeepening and the formation of lacustrine sedimentary records can effectively be used as trustworthy large frontal moraine ridges during the Late Quaternary glaciations paleoseismological tools. (Laugenie, 1982). The landscape around the lakes is dominated by

^Trancura River É^Lake Villarrica

Villarrica Pu con

Villarrica Volcano

Lahar pathway Lake Pullinque Main river

Lake Pellaifa Moraine

Fig. 1. Morphological setting of Lake Villarrica and Lake Calafquén. Lake surrounding topography derived from SRTM data. Slope shader illumination from the North. Moraine locations (“Llanquihue” moraine belt) based on Laugenie (1982). Lahar pathways derived from Laugenie (1982) and satellite pictures. Lake Villarrica: Bathymetric contours every 10 m. Deepest part: 167 m below lake level (based on SHOA, 1987). Lake Calafquén: Bathymetric contours every 20 m, derived from Volland et al. (2007) combined with our seismic dataset. Deepest part: 212 m below lake level. Inset figure: location of the study area in South-Central Chile. ]. Moernaut et al. / Sedimentary Geology 213 (2009) 121-135 123

10km

Lake Villarrica

10km 0______1 2km Calculated bathymetry SW part of Lake Villarrica

Pinger survey lines Sparker survey lines Lake Calafquén

Fig. 2. A) Seismic survey lines on Lake Villarrica. B) Seismic survey lines on Lake Calafquén. C) Bathymetry for the SW part of Lake Villarrica calculated from reflection seismic data. Isobaths every 5 m.

Villarrica Volcano, one of South America's most active volcanoes has a surface of about 2650 km2 and comprises Villarrica Volcano (W itter et al., 2004). Destructive lahars have frequently occurred (2847 masi), Quetrupillán Volcano (2360 masi) and Sollipulli during the past decades and sometimes propagated into the eastern Volcano (2282 masi). Its main tributary is the Trancura River, the part of the lakes. The western part of Lake Villarrica is morpholo­ course and alluvial plain of which have been strongly influenced by gically protected from laharic inflow by the presence of the moraine lava flows and lahars from the Villarrica Volcano. Toltén River, which ridges. Morphologically, Lake Villarrica consists of a single, deep cross-cuts the moraine ridges, constitutes the outflow of the lake central basin (up to 167 m depth), and a shallower area with more towards the Pacific Ocean. morphological variability in the SW part of the lake (Figs. 1 and 2C). A The landscape of South-Central Chile is strongly impacted by small rocky island (Allaquillén Island) marks the transition from the recurrent, large megathrust earthquakes, such as the giant AD shallower area to the central basin. Lake Villarrica's catchment area 1960 Valdivia earthquake (Mw: 9.5), which originate at the interface

Slide scar

Mass-wasting deposit Studied interval

Uniform drape

Complex infill with Jmited acoustic erosional unconformities penetration / Rock basement

±50m

Fig. 3. Sparker seismic profile showing the total sedimentary infill in the W part of Lake Villarrica. TWT: two-way travel time. 124 ]. Moemaut et al. / Sedimentary Geology 213 (2009) 121-135 between the subducting Nazca Plate and the overlying South (“pinger") were executed on Lake Villarrica in December 2001 and American Plate. Seismic ground-shaking reached a Modified Mercalli February 2007, and on Lake Calafquén in February 2007. Intensity of Vll to XI in the Chilean Lake District (Duke and Leeds, We first used The CENTIPEDE multi-electrode sparker (300 J, main 1963), triggered numerous landslides, both subaerially as suba- frequency: 400-1500 Hz) as seismic source and a single-channel high- queously (W right and Mella, 1963; Chapron et al., 2006; Moernaut resolution streamer as receiver in order to get a general overview of et al., 2007) and caused widespread infrastructural damage due to the lake basins sedimentary in fill (Fig. 3). These acoustic instruments liquefaction o f susceptible soils (Duke and Leeds, 1963). Historical were towed by the Huala 11, the small research vessel from the evidence and paleoseismological studies in South-Central Chile Universidad Austral de Chile. For a more detailed image o f the revealed that several earthquakes with strength comparable to the subsurface, we used the “pinger" source/receiver, which was mounted one in AD 1960 occurred throughout the Holocene with a century to on a Cataraft system and towed by the Huala 11. The acoustic signal, multi-century recurrence interval (Lomnitz, 2004; Cisternas et al., w ith a central frequency o f 3.5 kHz, penetrated up to 25-30 ms TWT 2005; Moernaut et al., 2007; Blumberg et al., 2008; Behrmann et al., (±20 m) of the sedimentary fill with a vertical resolution of about 15- 2008). 20 cm and shot-point spacing of about 1-1.5 m. Navigation and positioning were done by GPS. All data were 3. M ethods recorded digitally on a TR1TON-EL1CS Delph-2 acquisition system. Data processing, including band-pass filtering, scaling and spiking decon­ Reflection seismic surveys with a high-resolution CENTIPEDE volution, was carried out on a LANDMARK ProMax system and seismic sparker and very-high resolution GEOPULSE subbottom profiler stratigraphie interpretation was done using SMT's Kingdom Suite

Frontal S Failed Eroded slope Slide scar

Post-failure sediment drape

VGC02B

Villarrica

Slope break

VSCTEST2 I I Mass-wasting deposit b Slidingsurface Mass-wasting deposit a (a1-a2-a3) 100m

'Eroded slope M------► Unfailed slope < — ► VSCTEST2 Failed slope

0.060-

0.070- Mass-wasting deposits

0.080- ►

0.090

0.100-

0.1101 Sliding surface MWD-a1

0.120 Older MWD’s

0.130Í

0.140-

0,1501 Frontal ramp 0.160Í

Fig. 4. A) Mapped mass-wasting features in the SW part of Lake Villarrica. Isobaths every 10 m. Location of seismic profile in (B), (C) and Fig. 6 indicated by dashed lines. B) Slope failure features. C) Downslope evolution of the slope failure features towards the mass-wasting deposits (MWD's). ]. Moernaut et al. / Sedimentary Geology 213 (2009) 121-135 125 package. Water and subsurface depths were calculated using a mean constructing a more detailed bathymetric map for the SW part o f Lake acoustic velocity o f 1500 m/s. Villarrica (Fig. 2C). The reconnaissance survey in 2001 showed that the deep basin of Additionally, a few short cores (up to 0.31 m; Figs. 4 and 5) were Lake Villarrica has a very lim ited acoustic penetration, possibly due to taken in Lake Villarrica w ith a UWITEC gravity corer from a zodiac: gas blanking and/or the episodic supply of coarse material transported VSCTEST2, in 22 m water depth, and VGC02BIS, in 121 m water depth by lahars and/or underflows originating at the main tributaries. The (Fig. 4). Core analysis is still in progress, but the cores were already penetration in the shallower, SW part of the lake is much better described, photographed and grain-size measurements were per­ (Fig. 3). Seismic data from this area reveal the presence o f voluminous formed every 1 cm on bulk sediment using a laser diffraction particle mass-wasting deposits with indications of vertical subsurface fluid analyser (Malvern Mastersizer 2000). Distribution parameters were and sediment mobilization (Fig. 4). For these reasons, the 2007 survey calculated following Folk and Ward (1957) using GRADISTAT software. focused essentially on this SW region (Fig. 2A). The spacing between These initial analyses provide a general insight in the recent pinger seismic profiles during the 2007 survey was -500 m, in order depositional processes in the lake. to map the mass-wasting features. In 3 small areas an even denser The reconnaissance seismic survey on Lake Calafquén also revealed pinger grid spacing of -5 0 m was achieved (grid 1-3; Fig. 9), in an several mass-wasting deposits and indications o f subsurface sediment attempt to accurately map the subsurface sediment mobilization mobilization. Due to less morphological complexity and a denser and structures. The totality of seismic data (2001 and 2007) also allowed more extensive survey grid in Lake Villarrica, we w ill principally focus

VSCTEST2 VGC02BIS Mean (|jm) Sand:silt:clay Mean (pm) Sand:silt:clay A ) 10------20 ------30 ------40 20% 40% 60% 80% 0 10 20 30 20% 40% 60% 80%

10 10 Ê o

Q. 0 Q

20 20

30

100% \ vsc TEST2 >

\\ %7U \ \ \ \\A \ \ m \ 5 ^ X % V\ V %\ \ % \

0% 1000 100 10 Grain-size (pm)

Fig. 5. A) Grain size parameters of short core VSCTEST2 and VGC02BIS. B) Average grain-size distribution of VSCTEST2 (source sediments of MWD-al ) compared to Fig. 8 in Obermeier (1996) based on Tsuchida and Hayashi (1971). 126 ]. Moemaut et al. / Sedimentary Geology 213 (2009) 121-135 our study on this lake, but we w ill use the data o f Lake Calafquén for 4.2. Mass-wasting, sediment volcanoes and fluid escape comparison. Seismic-stratigraphic analysis in Lake Villarrica reveals that lens­ 4. Results shaped sediment bodies with a characteristic transparent-to-chaotic seismic facies are present in the uppermost seismic un it (Figs. 3 4.Î. General seismic stratigraphy, sediment characterization and 4). These lenses, which are up to 3 km long and 5 m thick, show upward-concave geometries and in downslope direction they wedge The seismic sparker profiles image the entire lacustrine in fill (100- out in between the continuous parallel-stratified reflectors of this 150 m thick) in the SW part o f Lake Villarrica, down to the rock seismic unit. More upslope, abrupt and steep transitions occur basement (Fig. 3). The uppermost seismic unit generally consists of between the transparent-to-chaotic facies o f the lenses and the parallel and continuous reflectors with low-to-high reflection ampli­ generally parallel-layered facies. These lenses bear all the typical tudes, which are uniformly draped on top of i) an irregular basement seismic characteristics of deposits resulting from the failure of morphology and ii) a complex basin infill characterized by lateral submerged lacustrine slopes, as described and sedimentologically seismic facies variability and major erosional unconformities. In this ground-truthed in several other studies (e.g. Schnellmann et al., paper, we w ill focus on the upper half of the uppermost seismic unit 2005), and we therefore interpret them as lacustrine mass-wasting and, therefore, do not aim to establish a detailed seismic stratigraphy deposits (MWD's). of the entire basin infill. Such a comprehensive study on the total Several o f such mass-wasting deposits were identified in the sedimentary infill should allow unraveling the evolution of the western part of Lake Villarrica: i.e. MWD-al, MWD-a2, MWD-a3 and sedimentary environment in this glacigenic lake from the onset of MWD-b. Spatial mapping o f MW D-al towards its upslope source area the last deglaciation until the present (e.g. Charlet et al„ 2008). In reveals that it resulted from the failure o f an underwater slope (Fig. 4). general, the lower seismic units are believed to have been rapidly This is evidenced by the identification of i) a 7-m-high scarp in the deposited in a sub- to pro-glacial environment, while the uppermost seismic stratigraphy and lake-floor morphology (i.e. the slide scar), seismic unit has been deposited much more slowly (typically located where the paleo-slope changes in slope angle from 0.2° to 2.3°, sedimentation rate 0.5-1.5 mm/yr.; Bertrand et al„ 2008b) in an and of ii) a continuous high-amplitude reflector that correlates with open, post-glacial lake. The timing of deglaciation of the southern the base o f the MWD (i.e. the basal sliding surface) (Fig. 4B). By Chilean Lake District has been set around 17,500-17,150 cal a BP mapping the slide scar, we observed that it is located at water depths (Mcculloch et al„ 2000). However, the lacustrine record of Lake that range between 12 and 80 m, that it affects the lacustrine Puyehue (40.7°S) indicates a significantly older tim ing o f glacier sediments of the uppermost seismic unit and that its most westward retreat from the lake basin, i.e. around 24,750-28,000 cal a BP (Charlet point is located only 300 m east of Villarrica's marina. A step-up (in et al„ 2008). This means that we cannot simply attribute an age to the downslope direction) o f the MWD's basal surface towards the paleo- lower limit of the uppermost seismic unit in Lake Villarrica, nor lake bottom occurs at a water depth o f 100 m (Fig. 4A/C). In map view, accurately estimate sedimentation rates from seismic stratigraphy it shows a convex geometry and links up with the abovementioned alone. slide scar. We therefore interpret this feature as a frontal ramp Macroscopic observations and smear slide analysis on the short structure (e.g. Frey-Martinez et al., 2006). cores show that the olive brown/gray sediments of VSCTEST2 and The seismic data show that another voluminous mass-wasting VGC02B1S are composed of biogenic particles (diatoms), organic deposit, w ith associated slide scar and frontal ramp (MWD-a2) and a matter, amorphous clays, volcanic glasses and crystallized minerals. In small mass-wasting deposit (MWD-a3) are located at exactly the same VSCTEST2 (at 18-19 cm depth), a dark, fine sandy, tephra layer is stratigraphie level (LÍ) as MWD-al. Together they constitute the detected, within a fine matrix. Sediments in short core VGC02B1S are sedimentary record of a mass-wasting event (MWE), which indicates macroscopically laminated while VSCTEST2 is homogeneous. The that multiple slope failures (involving different segments of the lake grain-size distribution of short core VSCTEST2 shows that it is slopes) took place simultaneously. During this event, up to 9.9 * IO6 m3 composed o f poorly sorted coarse silt w ith an average mean of of entrained lacustrine sediments was deposited in these three 25.0 pm. Average clay content (<2 pm) is 3.8% and average sand MWD's. Higher up in the stratigraphy, another mass-wasting deposit content (>63 pm) is 23.1 %. VGC02B1S is also composed o f poorly sorted (MWD-b) w ith a volume of 1.0 * IO6 m3 occurs at stratigraphie level L3 silts, but w ith a slightly lower mean of 18.3 pm. Average clay content (Figs. 4A and 10B). Evidence for older MWD's is also found deeper in (<2 pm) is 4.9% and average sand content (>63 pm) is 11.8%. The cores the stratigraphy (e.g. Fig. 4C). display only limited variations in grain-size and lithogenic content In conclusion, we interpret MWD-al, MWD-a2 and MWD-b as (Fig. 5). frontally emergent landslides (sensu Frey-Martinez et al., 2006), in Due to the setting of Lake Villarrica in an area of active volcanism, which a sequence of sub-lacustrine slope sediments failed along a intercalations of tephra deposits can be expected in the uppermost bedding-parallel slip plane (up to 7° dip for MWD-al), ramped out its seismic unit (e.g. Lake Puyehue: Bertrand et al., 2008a,b), as shown by original basal shear surface and flowed over the contemporaneous the sandy tephra layer in core VSCTEST2. However, the seismic data lake bottom following the steepest slope gradients on the paleo-lake indicate that thick (>~0.5 m) sandy tephra layers are probably absent floor (0.5° dip for MWD-al ). Deposition (Mulder and Alexander, 2001 ) in the studied seismic-stratigraphic interval, because they would have took place when the mass flow decelerated as it reached a lower slope totally hampered seismic penetration of the 3.5 kHz seismic signal. gradient (0.2° for MWD-al). Based on the grain-size data of the In analogy with studies on the seismic stratigraphy and sediment available short cores, we infer that -apart from occasional thin tephra cores o f nearby Lake Puyehue (40.7°S; Charlet et al., 2008; Bertrand layers- there are no major grain-size variations in the studied et al., 2008b), which exhibits a quite similar lake setting (i.e. Andes seismic-stratigraphic interval. Considering the locations of the short piedmont, nearby active volcano, frontal moraine belt in the west), we cores, we surmise that the composition and grain size of the interpret the uppermost seismic unit as representing a post-glacial sediments involved in the mass-flow deposit are probably comparable accumulation of autochtonous lake sedimentation (suspension set­ to those of short core VSCTEST2, although the failed sedimentary tling of mainly biogenic particles) when the glacier had already sequence is located at a deeper stratigraphie interval than the one retreated far into its catchment area. We will further use the term sampled by the core. Therefore, we tentatively classify the MWD's as “background sedimentation" for both autochtonous lake sedimenta­ silty m ud-flow deposits (M ulder and Alexander, 2001; Canals et al., tion and episodic, thin tephra layer deposition during the time interval 2004). However, since the exact transport mechanism of the mass of the uppermost seismic unit. movement is not known and downslope transformations could have ]. Moernaut et al. / Sedimentary Geology 213 (2009) 121-135 127 taken place (Mulder and Alexander, 2001), we prefer to use the more amplitudes just above the lenses. The top o f these amplitude wipe-out general term “mass-wasting deposit" in the present study. effects is located at stratigraphie level L3. In literature (e.g. Schnellmann et al., 2005), sediment cores in the We interpret these lenses as sediment volcanoes (or sediment distal parts of lacustrine mass-wasting deposits typically show an blows), which were induced by i) a sudden increase of pore-fluid amalgamation of both relatively small blocks of folded and remoulded pressure until liquefaction took place in the buried mass-wasting strata, and a more disintegrated mudclast conglomerate. These distal deposit (i.e. the source layer), ii) subsequent hydraulic fracturation in parts are commonly overlain by a turbidite, deposited from the the overlying drape of background sedimentation with/without a suspension cloud generated by mass-movement and/or associated mass-wasting-associated turbidite at its base (i.e. the cap layer), iii) tsunami/seiche action. sediment fluidization and iv) extrusion at the contemporaneous lake Remarkable is the occurrence of several decameter-scale lenses bed (i.e. stratigraphie level L2). We w ill strengthen this interpretation (Fig. 6) that appear to be emanating from the MWD's. These lenses in the next paragraphs through detailed mapping and measuring of have a transparent to low-amplitude seismic facies and they occur at these sediment volcanoes and the source layer. The acoustic wipe-out stratigraphie level L2. The lenses are connected to the underlying patches are interpreted to represent traces of fluid migration without MWD's trough transparent/low-amplitude patches in a 0.5-m-thick sediment expulsion (e.g. Chapron et al„ 2004) through a hydraulically medium- to high-amplitude layer with abundant low-offset sub­ fractured drape of background sedimentation above the sediment vertical faulting. The lenses are buried by a drape o f continuous, volcanoes. This second phase o f fluid expulsion also occurred as a parallel high-amplitude reflections, with locally reduced reflection single event, i.e. at stratigraphie level L3.

Sediment volcanoes

Stratigraphie levels of fluid/sediment escape

Mass-wasting deposit a1

Sediment volcano Acoustic wipe-out Sediment volcano

.1 0 m ;" Draping slope

Sagging slope

Mass-wasting deposit Mass-wasting deposit p

Fig. 6. Pinger profile showing mass-wasting deposit al (MWD-al) with associated fluid/sediment escape features. Zooms: detailed visualization of sediment volcanoes and fluid escape features (wipe-out). LÍ = stratigraphie level 1, etc. 128 ]. Moemaut et al. / Sedimentary Geology 213 (2009) 121-135

4.3. Geometry of sediment volcanoes 4.4. Multi-phase sediment extrusion

The maximum thickness and apparent width of all sediment Locally, some sediment volcanoes occur at the stratigraphie level volcanoes were measured on the seismic profiles (in 2D) (Fig. 8A). L3, which is the same stratigraphie level as the one on which MWD-b Most volcanoes have a w idth o f 10-50 m and a thickness o f 0.2-1.6 m, was deposited and as the top of most wipe-out patches described but volcanoes up to 80 m wide and 1.9 m thick were also identified. above (Fig. 6, 9 and 10A). These L3 sediment volcanoes are located Volcanoes which are less than 0.2 m thick fall below the vertical above the L2 sediment volcanoes and connect w ith these by acoustic resolution of the seismic images. Using densely spaced seismic wipe-out patches. Some o f the L3 volcanoes are located above a zone profiles (Fig. 9), we managed to map the full geometry of individual of strongly disrupted sediments in which no acoustic layering is volcanoes (in pseudo-3D). They form circular or slightly elongated visible. These L3 sediment volcanoes indicate that a polyphase structures. Fig. 8A shows that a clear linear relationship exists sediment expulsion took place from the source layer (MWD-a) during between the apparent width and maximum thickness of the 2 successive events, separated by a time-equivalent of about 0.7-0.8 m volcanoes on a 2D cross-section. This correlation is further strength­ of background sedimentation. ened by integrating the maximum thickness and diameter measure­ Other sediment volcanoes and fluid-escape patches were also ments on the volcanoes mapped in pseudo-3D. observed at the L4 stratigraphie level (Fig. 10B). They all occur above Similar measurements on sediment volcanoes observed on seismic MWD-b, which indicates that MWD-b was also subjected to sudden profiles from nearby Lake Calafquén (Figs. 1, 7 and 8C) also reveal a sediment/fluid escape processes. At a few places, fluid-escape patches clear linear trend between apparent width and maximum thickness. at L4 occur above sediment volcanoes of L3. Moreover, the trend lines o f the Lake Villarrica and the Lake Calafquén sediment volcanoes have a very similar slope value (Villarrica: 0.0232 4.5. Spatial distribution of sediment volcanoes versus Calafquén: 0.0233) and intersect near (0;0). The slope values (Fig. 8C-D) o f the sediment strata directly above Fig. 9 shows that all L2 sediment volcanoes are located above MWD-a, (i.e. the draping sediments) and underneath (i.e. the breached but with large variations in sediment volcano distribution. Using densely sediments) the outer edges of the sediment volcanoes were also spaced seismic profiles, we managed to map the spatial distribution of measured. We observe that the upper lim it of a volcano with a flat the larger volcanoes in the detailed grids 1,2 and 3. The distance between base has a -3 ° slope at its outer edges. A negative linear relationship is their centres is about 30-60 m in the areas w ith abundant sediment revealed between these slope values, but w ith noteworthy scatter, volcanoes. Grid 1 volcanoes are more uniform in dimensions and spacing possibly due to the measurement accuracy. Measurements on than those in grids 2 and 3. MWD-a is thicker at grid 1 (range: 2.9-3.7 m) sediment volcanoes in Lake Calafquén also indicate this negative than at grid 2 and 3 (range respectively 1.1-2.4 m and 0-2.5 m), and linear trend, but with a slightly steeper trend-line slope (Villarrica: shows less variability in thickness at grid 1. For grid 2, less sediment -1.3824 versus Calafquén: -1.4658). We will discuss these linear volcanoes (and w ith smaller dimensions) are located in the area where relationships below. MWD-a is thinnest (1.1-1.5 m).

Thin MWD’s

Thick MWD Sediment volcanoes • .. .. —:

Fig. 7. Pinger profile showing mass-wasting deposits (MWD's) and sediment volcanoes in the SW part of Lake Calafquén. Inset figure: bathymetry of Lake Calafquén (isobath every 20 m); white line: location of shown seismic profile; thick black line: slide scar; transparent cover: outline of the thick MWD. J. Moernaut et al. ¡ Sedimentary Geology 213 (2009) 121-135 129

A

3.5 E 1.4 CO CO cu y = -1.3824X + 4.1891 c 0 .2 .5 o R2 = 0.806 JZ Villarrica I- O) X y = 0.0232X + 0.0027 2? 15 CD O) R2 = 0.9425 A2D

0.4 • 3D

0.5 0.2 < seismic resolution

0 10 20 30 40 50 60 70 80 o 0.5 1 1.5 2 2.5 3 3.5 Apparent width (m) Draping slope (°)

3.5 v1 .6 y = -1.4658X + 4.376 cr> R2 = 0.8442 cu 1.2 Calafquén

X y = 0.Û233X + 0.0021 O) 0.6 O) ^ R2 = 0.9531 CD tí) 0.4 0.5 0.2 < seismic resolution 0 20 30 40 60 70 80 o 0.5 1 1.5 2 2.5 3 3.5 Apparent width (m) Draping slope (°)

Fig. 8. A-B) Uniform geometry of individual sediment volcanoes: linear relationship between maximum thickness and apparent width for respectively Lake Villarrica and Lake Calafquén. Triangles: measured on 1 section; circles: mapped in 3D on multiple sections. C-D) Syn- to post-depositional sagging: slope measurements at the outer edges of sediment volcanoes (draping: upper limit; sagging: lower limit) for respectively Lake Villarrica and Lake Calafquén.

The spatial distribution of L3 volcanoes was also mapped and grid grained cohesive sediments (i.e. background sedimentation) with a 1 provides a pseudo-3D view on the distribution of these structures. relative lower permeability and subsequent seismic loading could Sediment volcanoes at L3 mostly occur where the underlying MWD is have increased pore-fluid pressure in the mass-flow deposit until thickest (e.g. MWD-a2; grid 1 ) or close to large variations in thickness hydraulic fracturation of the capping layer took place, followed by of the MWD. fluid migration through a network of closely spaced fractures. Diviacco Detailed observations on seismic sections show that, in some cases et al. (2006) propose that fluids can also be expelled from sediments (Fig. 10C), the stronger disruptions -caused by fluid/sediment escape being suddenly overloaded by the deposition of a mass flow. This processes- occur above slope breaks in the top o f the MWD (steepening phenomenon on its own, however, cannot create sediment volcanoes downhill: 0.2° to 1.2° and 0.2° to 1.1 °). Below, we w ill discuss the different on stratigraphie levels that are separated from the MWD by periods o f factors possibly controlling the spatial distribution of the observed background sedimentation, as is the case in the present study. sediment volcanoes. Therefore, we regard the MWD itself as the main source layer for the fluid and sediment escape, w ith a possible enhancement through 5. Discussion expelled fluids from the overridden lake floor sediments. Above, we inferred that the studied mass-wasting deposits 5.1. Source layer, cap layer and spatial distribution of sediment blows probably mainly consist of coarse silty sediments (core VSCTEST2). The grain-size distribution curve (Fig. 5) for these cored sediments Previous reflection-seismic studies already showed the existence plots at an empirically derived lim it between “most liquefiable" and of vertical fluid-escape structures originating from buried mass- “potentially liquefiable" distributions (Fig. 8 in Obermeier, 1996 — wasting deposits (Baltzer et al., 1998; Chapron et al., 2004, Trincardi based on Tsuchida and Hayashi, 1971). If the percentage of et al., 2004, Diviacco et al., 2006). In some o f these studies, as in the sediments <5 pm is above 15%, seismic liquefaction is not likely to present study, it is assumed that the phenomenon o f fluid expulsion is occur (Wang, 1979). In the studied core (VSCTEST2 and VGC02B1S), caused by the dewatering of a fluid-rich, underconsolidated and this fraction is respectively 10.6% and 13.4% for the averaged sample. loosely packed mass-wasting deposit. This underconsolidation is Such a high amount of fine particles could have significantly believed to originate from the entrainment of fluids during mass- decreased the liquefaction susceptibility of the MWD. However, movement followed by a rapid mass-flow deposition, resulting in a the long run-out distance of the lacustrine mass-movement loose packing of the grains. Such rapid deposition of fine-grained (±6400 m for MWD-a1) could indicate a downslope transformation sediments can also allow the development of excess pore-fluid of a more cohesive mass movement into a fluid-rich, reduced pressures, which are likely to be sustained for long periods o f time density flow (Canals et al., 2004). This would lead to partial particle (Maltman, 1994, pp. 97; listad et al., 2004). Progressive burial by fine­ fall-out within the flow so that a significant sorting could have 130 J. Moemaut et al. / Sedimentary Geology 213 (2009) 121-135

MWD-a isopachs (m): - Multiphase 1 2 _3 4 5 sediment extrusions - Thick MWD Pinger survey line L2 sediment volcano 3 sediment volcano Outline MWD-b GRID1

- Locally: few/small sediment volcanoes - Thin MWD

GRID2 GRID3 GRID1

GRID2 GRID3

100m

100m 100m Identified sediment volcano (1 profile) ^ Mapped sediment volcano (>1 profile) — Mapped thickness of sediment volcano (black-white: 0-2m)

Fig. 9.Spatial distribution of sediment volcanoes at L2 and L3. Isopach map of MWD-a (source layer). Dense survey grids 2 and 3 allowed mapping of individual sediment volcanoes at L2 level. Grid 1 allowed mapping of sediment volcanoes at L2 and L3 level. For illustrative purposes, the sediment volcanoes identified on a single profile are shown as perfect circular bodies. occurred during mass movement (Mulder and Alexander, 2001), lower sand content in VGC02B1S than in VSCTEST2) (Fig. 5). We increasing the liquefaction susceptibility of such a graded mass- expect a thin (<0.3 m; below seismic resolution) turbidite/homo- movement deposit. Thus, taking into account the abovementioned genite on top of the MWD, deposited from an associated turbidity sorting process during mass movement, we postulate that loosely current (suspension flow) on top of the dense mass flow (Locat and packed, coarse silty MWD sediments are intrinsically susceptible to Lee, 2002). Such a normally-graded event deposit with fine-grained seismic liquefaction. top might act as a more effective permeability barrier than the Lateral variability in thickness and properties of the cap layer is sequence of “normal" background sedimentation (represented by known to exert a dominant control on the distribution of fluidization VGC02B1S). It is presented in the literature that bioturbation structures (Nichols et al„ 1994; Hildebrandi and Egenhoff, 2007). In processes can create preferred pathways in the cap layer (Audemard the deep lacustrine environm ent of our study area, however, seismic and De Santis, 1991 ), but no indications o f bioturbation were found in stratigraphy suggests that the cap layer was uniformly draped over the laminated sediments of short core VGC02B1S. Due to this lack of the source layer. The short cores show that the background pre-existing anisotropies (weak zones), other factors should become sedimentation in the depositional area of the mass flows is slightly dominant in controlling the distribution and dimensions of sediment finer than the sediments in the failure area (i.e. lower mean and volcanoes. J. Moernaut et al. ¡ Sedimentary Geology 213 (2009) 121-135 131

(/V)Fluid escape wipe-out 2 events of sediment extrusion L4 sediment volcanoes

MWD-b

MWD-a1 Mass-wasting deposit a1

200m

Strongly disturbed stratigraphy

Slope break ", s „ W ! - AT -V ft Mass-wasting deposit b 200m i<3==> Lateral fluid migration Mass-wasting deposit a (a1-a2-a3)

Fig.10. Pinger profiles. A) Multiphase escape: examples of fluid escape (left) and sediment extrusion (right) at L3, above the L2 sediment volcanoes. B) The sediment volcanoes at L4 originated in MWD-b. C) The stratigraphy is most strongly disturbed at slope breaks, supporting the hypothesis of lateral fluid migration at the top of MWD-a towards the slope bréales. Dashed line: MWD outlines before fluid/sediment escape. D) Location of the shown pinger profiles in respect to MWD-a and MWD-b.

It has been shown for continental settings, where groundwater has dipping surface (in this case: 0.5°). Overpressure and fluid escape a free phreatic water surface, that the thickness o f the source layer is through more steeply inclined sediment layers would favour lateral also an important factor determining the susceptibility to liquefaction spreading processes over pure hydraulic fracturation, and in extreme and the thickness of cap layer that can be breached (Obermeier, 1996; cases could lead to the formation o f the headscarp o f a large slide w ith Obermeier et al„ 2005). In Lakes Villarrica and Calafquén, but also in the top o f the MWD as basal sliding plane (e.g. Chapron et al., 2004). nearby Lake Puyehue (Moernaut et al„ 2007), it seems that only the Due to the lack of long sediment cores trough the MWD and cap thickest MWD's (>±1.5 m) are capable of producing extensive fluid/ layer, the relative contribution o f the abovementioned processes sediment-escape structures that are visible on the seismic sections. cannot be conclusively determined. Sedimentological characterization Our observations indicate the existence of a possible relationship of source and cap layer and in-situ geotechnical measurements are between source layer thickness and amount of extruded sediment: i.e. recommended to better understand and quantify the liquefaction Fig. 9 (grid 2) suggests that a thicker source layer is able to produce susceptibility, permeability barriers and pore-fluid overpressures of more closely spaced or larger sediment volcanoes. Moreover, it MWD's. appears that multiphase sediment extrusions principally take place were the source layer is thickest (>3 m for MWD-b). It has to be noted 5.2. Depositional mechanisms o f sediment volcanoes that these correlations are not as conclusive when viewing the entire study area of Fig. 9, which means that other controlling factors should A linear relationship has been observed between the maximum be considered as well. Lateral fluid migration at the upper lim it of the thickness and the width of the sediment volcanoes in Lakes Villarrica MWD, a process already suggested by e.g. Chapron et al. (2004), could and Calafquén (Fig. 8A-B). This implies that these features are have directed fluid overpressure towards places where a slope break governed by a common depositional mechanism: i.e. the extruded (-1°) focused lateral fluid migration and allowed build-up of sediments could freely flow at the lake bottom in all directions. additional overpressure (Fig. 10C). Local disruption o f the cap layer Furthermore, the almost identical linear trend values between by fluid-escape processes severely complicates the reconstruction of Villarrica's and Calafquén's measurements indicate that comparable the pre-extrusion topography of the MWD. source sediments were involved in the sediment extrusion process, i.e. A field o f sediment volcanoes, as the one mapped in Fig. 9, can only mass-wasting deposits originating from sub-lacustrine slope failure in be created when the source layer is located on a horizontal to weakly areas dominated by background sedimentation. No indications for 132 ]. Moernaut et al. / Sedimentary Geology 213 (2009) 121-135 down-slope flow of the extruded sediments over the gently dipping structures in Lake Villarrica were created by episodes o f sudden gas (±0.5° in case o f Lake Villarrica) paleo-lake bottom were found. The escape. Moreover, as the MWD's result from failed lacustrine uniform geometry of the sediment volcanoes eliminates the possibi­ sequences, there is no reason why they would have had a higher lity o f sill-type emplacement of mobilized sediments (e.g. Obermeier, biogenic gas content than sequences o f background sediments. 1996), which should result in much more geometrical variability. - We found no indications for sudden gravity loading of the source The negative linear relationship between the slopes of the layer as potential trigger for liquefaction (e.g. M oretii et al., 2001 ). overlying and underlying layers at the sediment volcano edges The source layer is draped by a laterally uniform sequence of (Fig. 8C-D) can be interpreted as a result o f sagging o f the sediment background sediments with/without turbidite at the base, in which volcanoes. The more the cap layer collapses (increasing sagging slope), we do not expect strong lateral variations in density. However, the the less prominent the volcano can form a positive relief on the lake rapid deposition of MWD-b (up to 3 m thick) could have locally bottom (decreasing draping slope). Our measurements indicate a wide increased pore-water pressures in MWD-al, enhancing the range in the amount o f sagging, from totally non-sagged structures to liquefaction/fluidization processes. However, there are no clear a sagging slope o f 3°. However, the totally sagged end-member -w ith indications in the spatial pattern and dimensions of sediment a horizontal upper surface- observed in the experiments of Nichols volcanoes to support this (Fig. 9). Moreover, MWD-b deposition et al. (1994) was not detected in our data. We postulate that the did not take place above MWD-a2, although most L3 (=level of revealed volcano sagging could be caused by the accumulating MWD-b) sediment volcanoes have been mapped above MWD-a2. extruded sediment load on the cap layer (Nichols et al., 1994) and/or - The deposition of MWD-b, the formation of some of the sediment by the created sediment deficit in the source layer near the volcanoes and the fluid escape processes took place at the same fluidization conduits (Davies, 2003). Furthermore, the revealed time, i.e. at stratigraphie level L3, which indicates a common negative trend provides information about the timing of sediment triggering mechanism, affecting these deposits at a basin-wide volcano sagging. If the sagging process occurred entirely after depo­ scale. sition, the slope o f the trend line should be -1, i.e. passing trough points (3;0) (no collapse) and (0;3) (volcano build-up totally com­ Earlier paleoseismological studies already postulated that a pensated by sagging). As most of the data points are located above Modified Mercalli Intensity (MM1) of Vll is the lower lim it at which this line, a significant amount of syn-depositional sagging must have liquefaction becomes a common phenomenon, both in continental taken place during most sediment extrusions, instantly counter­ (Obermeier, 1996) and in lacustrine (Monecke et al., 2004) settings. acting the creation of positive relief on the contemporaneous lake Reported damage and effects caused by the AD 1960 Valdivia floor. earthquake show that such megathrust earthquakes are capable of generating MM1 values greater than Vll, even at more than 100 km 5.3. Earthquake triggering distance from the rupture plane. Such earthquakes have a long duration of shaking, high values of ground acceleration, and more Our data and seismic-stratigraphic analysis illustrate that a basin- energy in the low-frequency spectrum. These are all factors that wide event, involving multiple simultaneous underwater slope fail­ favour the creation of excess pore-water pressure in susceptible ures, took place at the tim e o f deposition of stratigraphie level LÍ. The sediments, which can lead to liquefaction (Leynaud et al., 2004; occurrence of simultaneous slope failures in areas of undisturbed Biscontin and Pestaña, 2006). Giant megathrust earthquakes, such as background sedimentation points towards a strong triggering e.g. the 1964 Cascadia earthquake (Mw: 9.2), are known to have mechanism of regional importance. Taking into account South-Central liquefied beds that are as shallow as 0.5 m subsurface (-comparable to Chile's record of historical and prehistorical giant earthquakes, it is this paper), and gave rise to abundant and large fluidization structures most likely that such mass-wasting events were triggered by strong (Obermeier, 1996). seismic shaking, as was already proposed and discussed in detail for More local earthquakes, such as continental-crust earthquakes other lakes in the region (Chapron et al., 2006; Moernaut et al., 2007, (Barrientos and Acevedo-Aranguiz, 1992) or intra-subducting slab Volland et al., 2007; Bertrand et al., 2008a). In these studies, other earthquakes (Beck et al., 1998), should also not be excluded yet as a possible triggering mechanisms (e.g. rapid sedimentation, wave- potential liquefaction trigger, although no such events have been action, lake-level fluctuations and sudden gas discharge) that could historically reported near Lake Villarrica. Dating and correlating create m ultiple coeval slope instabilities in the glacigenic lakes in lacustrine paleoseismological records from a wide latitudinal range South-Central Chile were carefully ruled out. Moreover, it was should allow discerning between local or megathrust earthquakes as demonstrated that the recent most mass-wasting event in Lake the triggering mechanism of sub-lacustrine liquefaction and slope Puyehue (Moernaut et al., 2007), Lake Nahuel Huapi and Reloncavi failures. Fjord (Chapron et al., 2006) was induced by the w ell-know n giant AD 1960 earthquake. 5.4. M ulti-phase fluid/sedim ent escape and paleoseismological Several arguments indicate that the fluid and sediment extrusions implications documented in the present study were also triggered by earthquake shaking: Our seismic data indicate that two events of sediment extrusion took place (i.e. at stratigraphie levels L2 and L3) with MWD-a as - Most of the fluid escapes and sediment extrusions happened source layer, and that the L3 sediment extrusions were located above during distinct events (i.e. at stratigraphie levels L2, L3 and L4). the buried sediment volcanoes of L2. In continental settings, This means that the upward-directed hydraulic force was suddenly paleoliquefaction studies and historical reports show that liquefac­ applied and of short duration, and therefore that a strong and tion has a strong tendency to reappear at the same location and can abrupt trigger was required. repeatedly use the same conduit for fluid/sediment venting (Ober­ - The escape structures are located at 100-120 m water depth, which meier, 1996). This process has been studied in detail in laboratory eliminates the possibility o f wave-induced liquefaction (e.g. Sumer tests (e.g. Oda et al., 2001). We postulate that during a new et al., 2006). earthquake, at the time of deposition of stratigraphie level L3, - The positive relief of the sediment volcanoes clearly differs from reliquefaction of the source layer took place and that previously- the concave crater-like depressions (pockmarks) commonly found created fluid-escape fractures and sediment volcanoes acted as at ocean margins and related to focused gas or liquid flow (Hovland preferential pathways for focused fluid flow. This explains why most et al., 2002). This eliminates the possibility that the studied escape wipe-out patches at stratigraphie level L3 are located above buried J. Moernaut et al. ¡ Sedimentary Geology 213 (2009) 121-135 133

(^Background sedimentation EQ1 Background sedimentation

Turbidite?0

MWD-a Sediment extrusion Fluid escape

^p^Background sedimentation Mass-wasting (Ê) EQ3 Turbidite?

^ Sediment extrusion

Ftuid escape Few sediment extrusions Fluid escape íü ® l Fractured layer M W D-b Few sediment extrusions EQ=Earthquake

Fig. 11. A-G : Schematic illustration of how the alternation of earthquake-induced event deposits (light gray) and periods of background sedimentation (dark gray) constituted the studied stratigraphie sequence (see text for details). Earthquake (EQ) 1, 2, 3 and 4 took place when respectively LÍ, L2, L3 and L4 formed the paleo-lake bottom.

sediment volcanoes. The potential o f creating fluid/sedim ent escape 285 yrs inferred from historical records and paleo-tsunami records in decreases over tim e due to compaction and loss o f excess pore-fluid South-Central Chile (Cisternas et al., 2005). Evidently, the revealed pressures during fluid/sediment-escape events causing the MWD to paleo-earthquake events should be accurately dated using long attain a “ norm al" consolidation state. sediment cores through the event deposits. Fig. 11 summarizes how the present-day stratigraphie sequence The above scenario illustrates that paleoseismic reconstructions has developed by the alternation between periods of earthquake that are solely based on the identification of lacustrine mass-wasting shaking and periods of background sedimentation: deposits (e.g. Schnellmann et al., 2002) can underestimate the frequency of paleo-earthquake recurrences, especially in areas of (A) Earthquake 1 triggered a basin-wide event of slope instability. high seismicity. Applying that approach to our case, only earthquakes This event produced voluminous mass-wasting deposits 1 and 3 would have been identified, w hile earthquakes 2 and 4 -w hich (MWD-al and MWD-a2) at the stratigraphie level LÍ, which did not produce any mass-wasting deposits- would have been are possibly overlain by a thin associated turbidite. overlooked. This characteristic “under-recording", which has also (B) A period of background sedimentation uniformly buried these been discussed for major active faults in a deep marine setting (Sea o f mass-wasting deposits. Marmara: Beck et al., 2007), results from the time that is needed for (C) Earthquake 2 triggered liquefaction in these buried mass-wasting creating sufficient static loading on a “weak layer" of the slope deposits and subsequent sediment fluidization and extrusion at sedimentary sequence to become susceptible to failure during strong the contemporaneous lake bottom. These processes created shaking (e.g. Strasser et al., 2007). On the other hand, fluid/sediment numerous sediment volcanoes at stratigraphie level L2 and extrusion structures can only be created when a source layer w ith intensively fractured the cap layer on top o f the MWD. excess pore-fluid pressures, such as a MWD, is present under a cap (D) A period of background sedimentation uniformly buried the L2 layer o f sufficiently impermeable sediments. Both processes (mass sediment volcanoes. wasting and fluid/sediment extrusion) thus suffer from limitations in (E) Earthquake 3 triggered a local slope failure, producing MWD-b, earthquake recording capacity. Fortunately, these limitations occur in and triggered some renewed fluid and sediment extrusion out different periods during the depositional history of a stratigraphie o f MWD-a. This fluid/sedim ent escape process used the sequence. The most complete paleoseismic record w ill therefore be formerly created conduits and sediment volcanoes as migration obtained by using the combined earthquake recording capacity of pathways. mass wasting and fluid/sediment extrusion. (F) A period of background sedimentation uniformly buried the L3 sediment volcanoes, fractures and MWD-b. 6. Conclusions (G) Earthquake 4 triggered some sediment extrusions out of buried MWD-b and locally renewed fluid escape out of MWD-a. After This study reveals the geometry, spatial distribution and origin of this event, only background sedimentation has taken place large-scale lacustrine sediment volcanoes based on seismic-strati­ without new fluid/sediment extrusions. graphic interpretations of very-high-resolution reflection seismic profiles in Lake Villarrica. The sediment volcanoes have a uniform In this scenario, four earthquake events are recorded in the studied circular geometry with a linear relationship between apparent width stratigraphie interval. The thickness of background sedimentation in and maximum thickness on a seismic section. Slope measurements on between the seismic events ranges between 0.5 m and 0.8 m. Volland the outer edges of the sediment volcanoes indicate that syn- to post- (2006) dated a short sediment core (1.14 m) taken 1 km east of depositional sagging o f several volcanoes took place. These empirically Allaquillén Island in the deep central basin, which revealed an average derived relationships are confirmed and supported by measurements sedimentation rate of about 1.2 mm/yr. If we assume a similar on similar sediment volcanoes in nearby Lake Calafquén. We postulate sedimentation rate for our studied stratigraphie interval, this would that these fluidization structures result from earthquake-induced yield an earthquake recurrence interval of about 420-670 yrs. Such a liquefaction of an underconsolidated buried mass-wasting deposit and multi-century recurrence interval for giant earthquakes is roughly in subsequent sediment fluidization and extrusion at the paleo-lake the same order of magnitude -albeit still significantly larger- than the bottom. Our results indicate that the thickness and morphology of the 134 ]. Moernaut et al. / Sedimentary Geology 213 (2009) 121-135 source layer exert a dominant control on the dimensions and spatial Bertrand, S., Charlet, F., Charlier, B., Renson, V., Fagei, N., 2008b. Climate variability of Southern Chile since the Last Glacial Maximum: a continuous sedimentological distribution of sediment volcanoes, and illustrate the ability of the record from Lago Puyehue (40°S). Journal of Paleolimnology 39,179-195. source layer to re-liquefy during multiple earthquake events. Biscontin, G., Pestaña, J.M., 2006. Factors affecting seismic response of submarine Widespread fluid-escape features were also identified. Together slopes. Natural Hazards and Earth System Sciences 6, 97-107. Blumberg, S., Lamy, F., Arz, H.W., Echtler, H.P., Wiedicke, M., Haug, G.H., Oncken, O., with the mass-wasting deposits and the sediment volcanoes, they 2008. Turbiditic trench deposits at the South-Chilean active margin: a Pleistocene- represent the sedimentary record of 4 paleoseismic events in the Holocene record of climate and tectonics. Earth and Planetary Science Letters 268, studied stratigraphie interval. This study indicates that fluidization 526-539. structures can be used as an additional paleoseismological tool. These Canals, M., Lastras, G., Urgeles, R., Casamor, J.L., Mienert, J., Cattaneo, A., De Batist, M., Haflidason, H., Imbo, Y., Laberg, J.S., Locat, J., Long, D., Longva, O., Masson, D.G., provide the possibility to extract paleoseismic information from the Sultan, N., Trincardi, F., Bryn, P., 2004. Slope failure dynamics and impacts from lacustrine sedimentary archive for periods following an earlier event seafloor and shallow sub-seafloor geophysical data: case studies from the COSTA of earthquake-induced slope instability, when the generation of a new project Marine Geology 213, 9-72. Castilla, R.A, Audemard, F., 2007. Sand blows as a potential tool for magnitude slope failure by seismic shaking is unlikely. estimation of pre-instrumental earthquakes. Journal of Seismology 11,473-487. Sedimentological ground-truthing, in-situ geotechnical measure­ Chapron, E., Ariztegui, D., Mulsow, S., Villarosa, G., Pino, M., Outes, V., Juvignié, E., ments and further detailed mapping -also in other types of lacustrine Crivelli, E., 2006. Impact of the 1960 major subduction earthquake in Northern Patagonia (Chile, Argentina). Quaternary International 158, 58-71. environments- is required in order to better understand the processes Chapron, E., Van Rensbergen, P., De Batist, M., Beck, C., Henriet, J.P., 2004. Fluid-escape governing such large-scale lacustrine fluidization structures, and to features as a precursor of a large sublacustrine sediment slide in Lake Le Bourget, effectively link them to well-documented historical earthquakes to NW Alps, France. Terra Nova 16, 305-311. Charlet, F., De Batist, M., Chapron, E., Bertrand, S., Pino, M., Urrutia, R., 2008. Seismic examine if they can be used for (semi-) quantitative paleoseismolo­ stratigraphy of Lago Puyehue (Chilean Lake District): new views on its deglacial and gical analysis. Holocene evolution. Journal of Paleolimnology 39,163-177. Cisternas, M., Atwater, B.F., Torrejon, F., Sawai, Y., Machucha, G., Lagos, M., Eipert, A., Youlton, C., Salgado, I., Karnataki, T., Shishikura, M., Rajendran, C.P., Malik, J.K., Rizal, Y., Acknowledgments Husni, M., 2005. Predecessors of the giant 1960 Chile earthquake. Nature 437,404-407. Clague, J.J., Shilts, W.W., Linden, R.H., 1989. Application of subbottom profiling to We thank Alejandro Peña and Koen De Rycker for their logistic and assessing seismic risk on Vancouver Island, British Columbia. Geological Survey of technical support during the seismic surveys of 2001 and 2007. We Canada, Current research, part E. Paper, vol. 89-1E, pp. 237-242. Davies, R.J., 2003. Kilometer-scale fluidization structures formed during early burial of a thank François Charlet and Emmanuel Chapron for their contribution deep-water slope channel on the Niger Delta. Geology 31 (11), 949-952. during the seismic survey in 2001. We thank Willem Vandoorne for Diviacco, P., Rebesco, M., Camerlenghi, A., 2006. Late Pliocene mega debris flow digitizing the bathymetric map of Lake Villarrica. This work was deposit and related fluid escapes identified on the Antarctic Peninsula continental margin by seismic reflection data analysis. Marine Geophysical Researches, 27(2), financially supported by the Special Research Fund of the Universiteit 109-128. Gent, through a 4-year research project and a project of bilateral Duke, C.M., Leeds, D.J., 1963. Response of soils, foundations, and earth structures to the scientific cooperation w ith Chile. Jasper Moernaut acknowledges the Chilean earthquakes of 1960. Bulletin of the Seismological Society of America 53, 309-357. support of the Institute for the Promotion of Innovation through Folk, R.L., Ward, W.C., 1957. Brazos River bar: a study in the significance of grain size Science and Technology in Flanders (IWT-Vlaanderen) and Katrien parameters. Journal of Sedimentary Petrology 27, 3-26. Heirman and Maarten Van Daele o f the Fund for Scientific Research of Frey-Martinez, J., Cartwright, J., James, D., 2006. Frontally confined versus frontally emergent submarine landslides: a 3D seismic characterization. Marine and Flanders (FWO-Vlaanderen). The contributions of Mario Pino and Petroleum Geology 23, 585-604. Roberto Urrutia were respectively supported by the Chilean projects Green, R.A, Obermeier, S.F., Olson, S.M., 2004. Engineering geologic and geotechnical FORECOS P04-065F from the Millennium Initiative (MIDEPLAN) and analysis of paleoseismic shaking using liquefaction effects: field examples. Engineering Geology 76, 263-293. 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