# Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska)

Nore Praet, Jasper Moernaut, Maarten van Daele, Evelien Boes, Peter J. Haeussler, Michael Strupler, Sabine Schmidt, Michael G. Loso, Marc de Batist

► To cite this version:

Nore Praet, Jasper Moernaut, Maarten van Daele, Evelien Boes, Peter J. Haeussler, et al., Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska). Marine Geology, 2016, 384, pp.103-119. 10.1016/j.margeo.2016.05.004. hal-04714312

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1 Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska)

2

3

4 Nore Praet¹,*, Jasper Moernaut²,³, Maarten Van Daele¹, Evelien Boes¹, Peter J. Haeussler⁴, Michael

5 Strupler¹,³, Sabine Schmidt⁵, Michael G. Loso⁶,⁷ and Marc De Batist¹

6

7 1) Renard Centre of Marine Geology, Ghent University, Krijgslaan 281 (S8 WE13), Ghent, Belgium

8 2) Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Campus Isla Teja, Valdivia, Chile

9 3) Geological Institute, ETH Zürich, Sonneggstrasse 5, Zürich, Switzerland

10 4) U.S. Geological Survey, Anchorage, 4210 University Drive, Anchorage, United States of America

11 5) EPOC, UMR CNRS 5805, Univ. Bordeaux, Allée Geoffroy St Hilaire, Pessac 33615, France

12 6) Department of Environmental Science, Alaska Pacific University, Anchorage, AK 99508, United States of America

13

14 7) Wrangell-St. Elias National Park and Preserve, Copper Center, AK 99573, United States of America

15

16 *Corresponding author: nore.praet@ugent.be

17

18 Abstract

19

20 Sublacustrine landslide stratigraphy is considered useful for quantitative paleoseismology in low-

21 seismicity settings. However, as the recharging of underwater slopes with sediments is one of the

22 factors that governs the recurrence of slope failures, it is not clear if landslide deposits can provide

23 continuous paleoseismic records in settings of frequent strong shaking. To test this, we selected

24 three lakes in south-central Alaska that experienced a strong historical megathrust earthquake (the

25 1964 M<sub>w</sub>9.2 Great Alaska Earthquake) and exhibit high sedimentation rates in their main basins (0.2

26 cm yr⁻¹ -1.0 cm yr⁻¹). We present high-resolution reflection seismic data (3.5 kHz) and radionuclide
27 data from sediment cores in order to investigate factors that control the establishment of a reliable
28 landslide record. Seismic stratigraphy analysis reveals the presence of several landslide deposits in
29 the lacustrine sedimentary infill. Most of these landslide deposits can be attributed to specific
30 landslide events, as multiple landslide deposits sourced from different lacustrine slopes occur on a
31 single stratigraphic horizon. We identify numerous events in the lakes: Eklutna Lake proximal basin
32 (14 events), Eklutna Lake distal basin (8 events), Skilak Lake (7 events) and Kenai Lake (7 events). The
33 most recent event in each basin corresponds to the historic 1964 megathrust earthquake. All events
34 are characterized by multiple landslide deposits, which hints at a regional trigger mechanism, such as
35 an earthquake (the synchronicity criterion). This means that the landslide record in each basin
36 represents a record of past seismic events. Based on extrapolation of sedimentation rates derived
37 from radionuclide dating, we roughly estimate a mean recurrence interval in the Eklutna Lake
38 proximal basin, Eklutna Lake distal basin, Skilak Lake and Kenai Lake, at ~ 250 yrs, ~ 450 yrs, ~ 900 yrs
39 and ~ 450 yrs, respectively. This distinct difference in recording can be explained by variations in
40 preconditioning factors like slope angle, slope recharging (sedimentation rate) and the sediment
41 source area: faster slope recharging and a predominance of delta and alluvial fan failures, increase
42 the sensitivity and lower the intensity threshold for slope instability. Also, the seismotectonic setting
43 of the lakes has to be taken into account. This study demonstrates that sublacustrine landslides in
44 several Alaskan lakes can be used as reliable recorders of strong earthquake shaking, when a multi-
45 lake approach is used, and can enhance the temporal and spatial resolution of the paleoseismic
46 record of south-central Alaska.

47

48

**Keywords:** sublacustrine paleoseismology; landslide record; south-central Alaska

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50

# 1. Introduction

51
The sedimentary record of subaquatic landslide deposits in lakes has been used as a proxy of past earthquakes in paleoseismological studies in a variety of tectonic settings. The reasoning behind this approach is that seismic shaking in lake basins can cause widespread and coeval (i.e. in different locations of the lake at the same time) failure of sediment-covered slopes. In Swiss perialpine lakes (intraplate collisional setting) this approach was successfully applied to reconstruct a regional seismic history (Schnellmann et al., 2002; Monecke et al., 2004; Strasser et al., 2007; Strasser et al., 2011). It has also been applied in lakes in subduction zone (Chile; Moernaut et al., 2007; Bertrand et al., 2008), continental transform fault (southern Patagonia; Waldmann et al., 2011), intraplate graben (Canada; Doughty et al., 2013) and half-graben (Lake Tahoe; Smith et al., 2013) settings.

Some of these studies (e.g. Moernaut et al., 2007; Moernaut et al., 2009) demonstrate that the sublacustrine landslide deposit record in some high-seismicity regions is incomplete and may not have captured all large past earthquakes. This is attributed to the low sedimentation rates (~ 1 mm yr⁻¹) in these lakes, which prevent the accumulation of sufficient sediment on a potential sliding surface in between two seismic events (e.g. average recurrence rate of ~ 140 years for M<sub>w</sub> > 7 ¾ events in south-central Chile). This observation raises the question whether sublacustrine landslide deposit records from lakes with higher sedimentation rates could be better suited for paleoseismological studies in high-seismicity regions.

In this paper we investigate which factors define the quality and completeness of the sublacustrine landslide deposit record for paleoseismicity in three lakes in south-central Alaska. These three lakes were selected because i) they are located in a high-seismicity subduction zone setting, ii) they experienced a strong historical megathrust earthquake (the 1964 M<sub>w</sub>9.2 Great Alaska Earthquake), the effects of which are well-known and have been reported in detail in literature (e.g. Plafker, 1965), iii) they cover a wide range of morphologies and sedimentary regimes, and iv) they exhibit overall significantly higher modern sedimentation rates (~ 0.5 cm yr⁻¹; Rymer and Sims, 1976) in comparison with the Chilean lakes studied by Moernaut et al. (2007, 2009), possibly reducing the effect of under-representation.
This study investigates the distribution of landslide deposits and megaturbidites attributed to the historic 1964 megathrust earthquake in the three lakes and compares their distribution to prehistoric landslide events using seismic stratigraphy analysis. Variable slope angles and sedimentation rates allow investigation of the influence of these factors on the generation of subaquatic landslides, and discussion of the landslide deposit record for each lake. In this paper, we show that a multi-lake approach will allow construction of a landslide record that can be used for paleoseismological purposes in south-central Alaska.

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## 2. Setting

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### 2.1 Tectonic setting and earthquake history

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South-central Alaska is located on top of the Alaska-Aleutian megathrust, a 4000-km-long convergent plate margin where the Pacific Plate subducts beneath the North American Plate at approximately 5.5 cm yr⁻¹ in a northwesterly direction (343°) (DeMets et al., 1990). On March 27, AD 1964, a M<sub>w</sub> 9.2 megathrust earthquake ruptured an 800-km-long segment of the megathrust, extending from Prince William Sound (PWS) to Kodiak Island (Plafker, 1969) (Figure 1A and Figure 1B). It remains the largest instrumentally recorded earthquake in North America and the second largest worldwide. The resulting land-level changes created a region of subsidence and a parallel region of uplift, both with a SW-NE orientation (Figure 1B). Coseismic uplift of the sea floor produced a large and destructive near-field tsunami along the adjacent Alaska coast and a trans-Pacific tsunami that propagated southeastward towards California, Pacific Northwest and Hawaii (Plafker, 1969). The earthquake involved a complex rupture pattern with an along-strike variation in coseismic slip. The areas beneath PWS and Kodiak Island (orange area on Figure 1A) showed a significant coseismic slip, up to 20 m and 15 m, respectively. In contrast, the area beneath Cook Inlet, offshore the Kenai Peninsula (blue area on Figure 1A), experienced a relatively low amount of slip of <5 m (Christensen and Beck,
1994; Holdahl and Sauber, 1994; Johnson et al., 1996; Santini et al., 2003; Suito and Freymueller, 2009). However, GPS measurements of the postseismic deformation indicate a locked interface beneath southern PWS and Kodiak Island, while the area offshore the Kenai Peninsula, is currently creeping (Freymueller et al., 2008). For the PWS region, the 1964 earthquake is the only great megathrust event described in historical records. Coastal paleoseismic records date the penultimate great earthquake along this segment as AD 1020-1150 (Combellick, 1993; Carver and Plafker, 2008; Shennan et al., 2014). This region also experiences upper plate earthquakes, originating from nearby fault systems (e.g. Castle Mountain Fault; Haeussler et al., 2002) and intermediate-depth (or intraslab) earthquakes, especially in the region around Cook Inlet.

## 2.2 Study area - Anchorage area and Kenai Peninsula

The study area comprises three proglacial lakes in south-central Alaska (59°00′-61°50′N, 147°50-152°00′W): Eklutna, Skilak and Kenai lakes (Figure 1C). Eklutna Lake is located 45 km northeast of Anchorage, in the Chugach Mountains. Skilak and Kenai lakes are situated 85 km south of Anchorage, on the Kenai Peninsula, which is bounded by Cook Inlet to the west, Prince William Sound to the northeast and the Gulf of Alaska to the south.

The Kenai Peninsula is composed of two parallel geomorphological regions: the Kenai Lowland to the northwest and the Kenai Mountains to the southeast. This mountain range, comprising Jurassic-Cretaceous rocks of the Valdez Group and McHugh Complex (Burns et al., 1991; Bradley et al., 2003), is aligned with the Chugach Mountains to the north. Kenai Lake is situated within the Kenai Mountains, while Skilak Lake straddles the western edge of the mountains and the flatter Kenai Lowlands, which consist of glacial, fluvial and alluvial deposits (Reger et al., 2007).

Repeated glaciations during Pleistocene time scoured all the lakes of the Kenai Peninsula and Chugach Mountains. In Skilak and Eklutna lakes repeated glaciations/deglaciations created several deep and shallower sub-basins that were dammed by a series of Pleistocene moraines (Karlstrom,
1964; Reger et al., 2007). During the Holocene, several periods of glacier advances (Neoglaciation) occurred. The first major glacier expansions occurred by 3.0 ka BP. The second major advance happened from ~ AD 550 to ~ AD 720, called the "first millennial AD" advance. The Little Ice Age (LIA) started around AD 1180-1320 and culminated with two advance phases, respectively around AD 1540-1710 and around AD 1810-1880 (Barclay et al., 2009).

Eklutna Lake (Figure 2A) has an average width of 1.4 km and a length of 10.5 km. It consists of two sub-basins: a larger and deeper (max. depth 57 m) proximal sub-basin, which is closest to the headwaters, and a smaller and shallower (max. depth 49 m) distal sub-basin. The lake is mainly fed by the West Fork and East Fork streams, which carry melt water from the West Branch Glacier, Eklutna Glacier and several smaller glaciers into the lake basin (Rymer and Sims, 1976; Brabets, 1993). Deposition of sediment at the mouths of these tributaries formed a delta at the southeastern end of the lake. Several alluvial fans along the shores of the lake bring sediment from the surrounding mountains into the lake. In AD 1929, a storage dam was constructed as an enlargement of a natural dam (terminal moraine) in the northern part of the lake. The infrastructure converted the naturally dammed lake into a reservoir (Logan, 1967).

Skilak Lake (Figure 2B) is 24 km long and reaches a width of 7 km in its western part. A submerged platform separates the lake basin into a proximal and a distal sub-basin: the proximal deep basin (max. depth: 194 m) gradually transitions into a shallower distal basin (max. depth 140 m) towards the west. Water flowing out from Kenai Lake is transported by the Upper Kenai River to the eastern end of Skilak Lake. However, the majority of the Skilak Lake's water and sediment comes from the termini of the Skilak and North Goat glaciers. Both inflow systems build a large delta at the eastern end of the lake (Rymer and Sims, 1976; Perkins and Sims, 1983; Sonett and Williams, 1985).

Kenai Lake (Figure 2C) is a narrow (maximum width of 2.5 km) fjord-like lake within the Kenai Mountains. It stretches along three almost perpendicular segments (total length: 38 km) that consist of several sub-basins. The central, east-west oriented segment is composed of two deep sub-basins with maximum depths of 174 m and 168 m. The catchment of Kenai Lake comprises an extensive
156 area of ranges and valleys within the Kenai Mountains, transporting glacial melt water and
157 overflowing water from upstream lakes towards the Kenai Lake basin (Rymer and Sims, 1976; Reger
158 et al., 2007). Along the shoreline, sediments from tributaries have accumulated in several large delta
159 systems and alluvial fans that represent the main points of sediment influx into the lake. During the
160 1964 earthquake large delta systems in Kenai Lake failed: the Lakeview, Rocky, Lawing, Meadow
creek, Ship creek and Porcupine deltas (Figure 2C) all collapsed, causing destructive far-shore and
161 backfill waves with run-up heights up to 22 m and 9 m, respectively. Also seiching was observed with
162 at least nine distinct waves having periods between 1.40 and 36.36 minutes (McCulloch, 1966).
163
164

165 **3. Methods**

167 **3.1 Reflection seismic data**

169 In the summer of 2012, a survey of all the lakes was conducted, during which reflection seismic data
170 and short sediment cores were collected. High-resolution reflection seismic data (Figure 2) were
171 acquired using a 3.5 kHz GEOPULSE pinger source, mounted on an inflatable cataraft, towed
172 alongside a GPS-positioned vessel (U.S. Geological Survey 25-foot research vessel *Moose Dancer*).

173 The pinger signal reached a maximum penetration depth of approximately 120 ms two-way travel
174 time (TWT) (~ 90 m), 75 ms TWT (~ 55 m) and 60 ms TWT (~ 45 m) in Skilak, Eklutna and Kenai lakes,
175 respectively. The seismic data have a theoretical vertical resolution of 10 cm-20 cm (Rayleigh
criterion). 176

177 The seismic and GPS data were recorded on a TRITON ELICS Delph2-system and later converted into
178 SEG-Y file format. After applying a band-pass filter to eliminate high- and low-frequency noise,
179 seismic stratigraphic interpretation was performed using Seismic Micro-Technology's Kingdom Suite
180 package (version 8.8). Bathymetric maps were constructed by interpolation of lake-floor horizon
181 picks on the seismic data. An acoustic velocity ~ 1450 m s⁻¹ (sound velocity in freshwater of ~ 10°C;
182 Chen and Millero, 1977) was used for time-to-depth conversions in the water column, from which a
183 bathymetry map was constructed.

184

185 **3.2 Terminology**

186

187 In this study, we focus on sublacustrine landslide deposits that can be identified, interpreted and
188 mapped using our seismic data. On pinger seismic profiles, landslide deposits appear as wedge-
189 shaped units with a positive relief and a transparent-to-chaotic seismic facies, that are intercalated
190 between acoustically stratified infill (e.g. Schnellmann et al., 2002; Moernaut and De Batist, 2011).

191 These units are typically thicker at the base of the slope and thin out away from the slope. Often they
192 are also associated with morphological features characteristic of failure of the adjacent slope
193 sediments (Figure 3), i.e. a slide scar, often with clear head scarp. We will use the generic term
194 "landslide deposit" (more or less equivalent to the term "mass-transport deposit") and, in the
195 absence of sediment cores through most of them, refrain from using any other terminology that may
196 have connotations in terms of sedimentary structure and/or transport process (e.g. slumps, slides,
197 debris-flow deposits).

198 We will also discuss the presence and distribution of landslide-derived turbidites that are discernible
199 on seismic data. For these we will use the term "megaturbidite" (Bouma, 1987), because their large
200 thickness exceeds the resolution of the seismic profiles. On pinger seismic profiles, a megaturbidite is
201 characterized by a transparent seismic facies and a lens-shaped, ponded geometry, in which they fill
202 in paleodepressions, thin out laterally and onlap onto other units towards the basin margins (e.g.
203 Fanetti et al., 2008; Van Daele et al., 2015). Their interpretation as a turbidite is also supported by
204 cores. We cannot discuss the presence and characteristics of other thinner turbidites, because these
205 could not be detected with the pinger data we used.

206 We performed a geomorphological analysis of the shoreline of the lakes, involving the identification
207 of deltas and alluvial fans and/or the presence of rivers and streams using satellite images from
208 Google Earth, ASTER Global Digital Elevation Model (ASTGTM2) (LP DAAC, 2009) and observations in
209 the field. In this study, we classify slope sediments either as tributary-sourced (delta and alluvial fan)
210 or as hemipelagic-sourced slope sections. Landslide deposits occurring below a delta or alluvial fan
211 (blue arrows, Figure 2) are considered to be the result of "delta and alluvial fan failures," which we
212 further describe as "tributary landslide deposits." Failures of a hemipelagic slope section are further
213 referred to as "hemipelagic slope failures" and the deposits as "hemipelagic landslide deposits."

214

215 **3.3 Seismic stratigraphy**
216

216

217 The top of a landslide deposit defines a seismic stratigraphic horizon, while the base of a landslide
218 deposit represents the lower limit of deformation (i.e. unconformity). The top of a (landslide-derived)
219 megaturbidite, which is stratigraphically on top of this landslide deposit, can also be linked to that
220 same horizon, as we assume that both the landslide deposit and the resulting turbidite are deposited
221 by the same "instantaneous" event. Picking these isochrons and correlating them throughout the
222 lake basin allows us to establish an event stratigraphy and verify the relative chronology of the
223 different landslide events in different parts of the basin. A single landslide in these lakes can result
224 from a large range of processes, such as spontaneous failure of delta slopes due to strong
225 sedimentary loading, or potentially even (extreme) flooding. However, synchronicity of landslide
226 events, as evidenced by landslide deposits sharing the same event horizon, suggests a regional
227 trigger, such as an earthquake, i.e. the synchronicity criterion (Schnellmann et al., 2002). In this
228 study, we infer an earthquake trigger if we identify two or more coeval landslide deposits. Thus, we
229 do not present a comprehensive catalog of all landslide deposits in these lakes.

230 The identification of the lowermost seismic event horizon is limited by the penetration of the seismic
231 signal. Only landslide deposits that were clearly resolvable on the seismic data, i.e. in theory thicker
232 than 20 cm, were identified for further discussion. Moreover, since the seismic data are limited by a
233 theoretical resolution of minimum 10 cm, the minimum age resolution we can discern between two
234 (or more) "coeval" deposits is ~ 10 yrs-20 yrs of difference in depositional age, applying a
sedimentation rate of ~ 1 cm yr⁻¹ or ~ 0.5 cm yr⁻¹, respectively.

235
236 Volumes of landslide deposits and megaturbidites were calculated by multiplying the average thicknesses of the deposits (calculated with a sound velocity of 1500 m s⁻¹; Pinson et al., 2013) with the surface area over which they occur.

239

240 **3.4 Coring, core opening and sedimentological description**

241

242 Short cores (30 cm-175 cm) were collected with a hammer-driven gravity corer in Eklutna (five cores), Skilak (six cores) and Kenai (nine cores) lakes. The cores were split, photographed and described. This allowed us to make a first-order distinction between background sedimentation, reflecting the normal, steady-state accumulation of sediments, and intercalated event deposits, reflecting anomalous events that create deposits with a characteristic sedimentological signature. As no organic macroremains were found within the mainly clastic background sediments, no ¹⁴C dating could be performed on these sediments.

249

250 **3.5 Radionuclide dating**

251

252 In order to illuminate the last ~ 100-150 year history, we performed ²¹⁰Pb and ¹³⁷Cs dating on sediment samples from cores EK12-01 (Eklutna Lake, Figure 4A) and SK12-03 (Skilak Lake, Figure 4B).
253
254 Selected samples were freeze dried and ²¹⁰Pb, ²²⁶Ra, ²³²Th and ¹³⁷Cs measurements were performed using a low-background, high-efficiency, well-shaped detector (CANBERRA) at the University of
255
256 Bordeaux (Schmidt and De Deckker, 2015). Calibration of the γ- detector was achieved using certified reference materials (IAEA-RGU-1; SOIL-6). Measured activities are expressed in mBq/g and errors are calculated, based on 1 standard-deviation counting statistics.
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258
Sediment ages were calculated using excess activity of ²¹⁰Pb (²¹⁰Pbₓs) (half-life: 22.3 yrs), which is incorporated rapidly into the sediment from atmospheric fall-out and following water column scavenging. ²¹⁰Pbₓs was determined by subtracting ²²⁶Ra from the total measured ²¹⁰Pb activity. Errors in ²¹⁰Pbₓs were calculated by combining errors in ²¹⁰Pb and ²²⁶Ra activities. ²¹⁰Pbₓs was normalized considering ²³²Th in order to reduce impact of changes in sediment grain size (Stupar et al., 2014).

In order to estimate depositional ages and sedimentation rates, a Constant Flux Constant Sedimentation model (Oldfield and Appleby, 1984) was applied for dating sediments in core SK12-03, while a Constant Initial Concentration model (Robbins and Edgington, 1975) was used for dating sediments in EK12-01. For the construction of the radionuclide dated age-depth model, all instantaneous event deposits were removed (Arnaud et al., 2006; Von Gunten et al., 2009).

The ²¹⁰Pb-based age model was verified by the artificial radionuclide, ¹³⁷Cs, also measured in the sediment. This artificial radionuclide has well known pulse inputs in the environment as a byproduct of nuclear weapon testing in the AD 1960s. Usually the maximum fallout of AD 1963 is very well preserved in lake sediments in the northern hemisphere (Pennington et al., 1973).

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## 3.6 Sedimentation rates

275

The identification of the ¹³⁷Cs peak and the ²¹⁰Pb age model in EK12-01 and SK12-03, allows certain marker horizons in the sediment cores to be linked to historical events. For example, there is a distinct marker horizon, which is located very near to the ¹³⁷Cs peak (Figure 4) and can therefore be inferred to date back to AD 1964. This marker horizon can be observed in, and correlated among, all short cores. Using this correlation, background sedimentation rates can thus be calculated for short cores on which no radionuclide dating was performed. We divided the sediment thickness above the marker horizon by the amount of years passed between AD 1964 and the retrieval of the cores in AD 2012. The thickness measurements are “event-free,” meaning we excluded deposits that would have been deposited nearly instantaneously.

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Mapping an arbitrary stratigraphic interval in the upper part of the seismic data gives an estimate of the relative variations of the sediment thickness across the basin. This seismic interval is not disturbed by the presence of megaturbidites or landslide deposits, however, smaller-scale turbidites, which are not resolvable on the seismic profiles, were not taken into account. Sedimentation rates for the entire basin were modeled by first performing a linear regression between the thickness (in ms) of the stratigraphic interval and the sedimentation rate at the coring locations, and subsequently applying the resulting relationship to the thickness map of the mapped stratigraphic interval.

Sedimentation rates discussed here only concern the basin floor environment, as no cores were collected on slopes.

294

## 4. Results

296

### 4.1 Sedimentary facies

298

The background sedimentary infill of the studied Alaskan lakes consists of laminated very fine to fine silt and mud. In detail, an individual lamina (0.5 cm-1 cm) is composed of a coarser grey base and a finer lighter-colored top (see EK12-05 in Figure 5 and EK12-01 in Figure 6), probably representing a clastic varve, which typically forms in proglacial lake systems, such as Eklutna and Skilak lakes (Perkins and Sims, 1983).

Occasionally interrupting these background sediments are graded layers (1 cm-60 cm), characterized by a silty to sandy base (dark brown to grey) and a lighter-colored muddy to clayey top (see SK12-01 in Figure 7 and KE12-05 in Figure 8). Sometimes the base appears erosive and often contains organic macroremains. These graded units are interpreted as turbidites, which can be deposited by a variety of processes (e.g. Sturm and Matter, 1978; Van Daele et al., 2015; Wilhelm et al., 2015b). Since the sedimentary regimes of these proglacial lakes are strongly controlled by the dynamics of adjacent glaciers, hyperpycnal flows associated with melt water discharge from these glaciers or with floods

310
due to heavy precipitation are a possible cause. Turbidity currents can also be the result of delta collapses, onshore landslides, debris or mud flows flowing into the lake. Also failure of hemipelagic slope sediments within the lake, a tsunami, or a seiche, can create turbidity currents. All of the above processes can be attributed to a local event or an earthquake.

315

## 4.2 Basin morphology and seismic stratigraphy of the sedimentary infill

317

The Eklutna Lake proximal basin is bounded by moderately steep slopes (10°-18°), covered by a very thin sedimentary drape (~ 2 m maximum). The steepest slopes (18°) are located in the southwestern part of the proximal sub-basin. Slope gradients in the distal basin show southeast to northwest decreasing values ranging between 15° and 5°. The basin infill consists of a relatively uniform succession of deposits with a horizontally stratified seismic facies, which represents the lake's background sedimentation. It comprises laterally continuous reflections, usually with low to moderate amplitude, but in places also high-amplitude reflections are present (Figure 5). This acoustically well-stratified basin fill is locally interrupted by landslide deposits that are on average 1 m-2 m thick. Above these landslide deposits, narrow vertical zones with reduced reflection amplitudes are present (Figures 5 and 6), probably representing fluid-escape structures (Moernaut et al., 2009).

At Skilak Lake, slopes on opposite sides of the proximal basin show significantly different gradients: northern slope values range between 10° and 15° with a sediment cover reaching a thickness of ~ 40 m, while southern slopes show slope values, up to 25°, with no discernible sediment drape. Slopes in the distal basin are no steeper than 12.5° and are covered by a maximum ~ 12 m of sediment. The gradient of the slope between the basin and intrabasinal platform varies between 5° and 15°. This slope accumulates sediments with thicknesses up to ~ 30 m in some areas. The overall characteristics of the sedimentary infill of Skilak Lake are comparable to those of Eklutna Lake. Skilak Lake has uniform, horizontally stratified background sedimentation, which is locally interrupted by the
occurrence of landslide deposits (Figure 7). In addition, and unlike in Eklutna Lake, a series of lens-shaped units that are confined by high-amplitude reflections can be observed at certain stratigraphic levels in the deepest part of the basin. We interpret these units as landslide-derived megaturbidites, e.g. the 1964 megaturbidite (inset Figure 7).

In Kenai Lake, slope gradients can reach up to 25°-30° in the central east-west section of the lake.

The seismic data do not reveal any significant sedimentary cover. The north-south oriented basins are bounded by slopes with gradients up to 20°, bearing a sedimentary cover with a thickness up to 7.5 m. Overall, the characteristics of the sedimentary infill of Kenai Lake are comparable to those of the two other lakes. Thick megaturbidites are present in the two deeper sub-basins (Figure 8). The landslide deposits show stronger absorption of the acoustic signal than in the other lakes. The quality of the seismic data around Quartz Creek delta was not sufficient to distinguish any deposits.

## 4.3 Event horizons

In Eklutna Lake, the seismic stratigraphy reveals 14 seismic event horizons (Events Ek-1 to Ek-14) in the proximal basin (Figures 5 and 9) and 8 events (Events Ek-A to Ek-H) in the distal basin (Figures 6 and 9), which cannot be well correlated across the basins. The distribution of ¹³⁷Cs reveals one clear peak at a depth of 21.5 cm in EK12-01 (Figure 4A), which corresponds to the maximum fall-out in AD 1963. This is also confirmed by the ²¹⁰Pb model. A turbidite located just at the level of this peak, and thus inferred to date back to AD 1964, can be correlated throughout the whole basin at a core depth between ~ 20 cm-50 cm. This 1964 turbidite is not visible on the seismic profiles (Figures 5 and 6), because it is < 10 cm thick (Rayleigh criterion). However, the uppermost event horizon in the proximal basin seismic stratigraphy (Event Ek-1 and Ek-A) is marked by landslide deposits that are buried under < ~ 1 ms (< 72.5 cm) of sediment. Therefore, Event Ek-1 and Event Ek-A very likely correspond to the 1964 earthquake. This historic event generated 15 individual landslide deposits (Figure 9) distributed over both sub-basins, with a total volume of approximately 6.8 x 10⁶ m³ (Table
1). Seven tributary landslide deposits account for a total volume of 2.4 x 10⁶ m³ and can be attributed to delta and alluvial fan failures (in green, Figure 9). In the proximal basin, 13 pre-1964 events generated multiple landslide deposits ranging from two to nine coeval landslide deposits with total volumes between 8.6 x 10⁴ m³ and 4.6 x 10⁶ m³, respectively (Table 1). In the distal basin, seven pre-1964 events are indicated by three to six landslide deposits, with volumes ranging between 1.2 x 10⁵ m³ and 2.2 x 10⁶ m³.

The seismic stratigraphy of Skilak Lake comprises seven event horizons (Figures 7 and 10) with multiple landslide deposits (Events Sk-1 to Sk-7). The uppermost event (Event Sk-1) on the seismic data is characterized by a megaturbidite that covers the main basin (Figure 7). Seismic-to-core correlation allows identification of this megaturbidite (~ 50 cm thick), especially as no turbidites with comparable size are present in the upper meters of the sedimentary infill. This turbidite shows a fining upward trend with a medium to coarse sand base evolving to a clayey top (Figure 7). The sediments above the megaturbidite cannot be resolved by the seismic data, moreover they are obscured by the strong lake bottom reflector (“ringing effect”). Radionuclide dating on short core SK12-03 shows that this turbidite was deposited in AD 1964, as the ¹³⁷Cs peak is located just below this turbidite, at a depth of 14.25 cm (Figure 4B). Therefore, Event Sk-1 most likely represents the 1964 megathrust earthquake. The 1964 event generated 23 landslide deposits (Figure 10) with a total volume of 9.9 x 10⁷ m³ (Table 1). Of these 23 landslide deposits, a (tributary) landslide deposit, with a volume of 1.5 x 10⁷ m³, can be attributed to delta failure in the eastern part of the basin (in green, Figure 10). A megaturbidite, with a volume of 2.7 x 10⁶ m³, covers the deep basin (in orange, Figure 10). Six Pre-1964 events show the presence of 14-23 coeval landslide deposits, with volumes ranging between 8.8 x 10⁶ and 7.2 x 10⁷ m³. Events Sk-2, Sk-4, Sk-5 and Sk-7 also show megaturbidites in the deep basin, with most having larger volumes than the 1964 megaturbidite (up to 4.8 x 10⁶ m³) (Table 1).

The seismic stratigraphy of Kenai Lake reveals seven event horizons (Events Ke-1 to Ke-7; Figure 8 and 11). The uppermost event (Ke-1) has two megaturbidites in the deep sub-basins, which are also
identified in the cores as graded layers (~ 60 cm thick) with a medium sand base and clay top (Figure 8). In line with observations in Skilak Lake, these turbidites can most likely be attributed to the 1964 megathrust earthquake. In the absence of geochronological data from sediment cores, this interpretation is also based on the fact that all reported delta failures during the 1964 earthquake (McCulloch, 1966) can be linked to landslide deposits of Event Ke-1, identified on the seismic data: i.e. Lakeview delta and Rocky delta in the eastern north-south segment; the Lawing delta, Meadow Creek delta and Ship Creek delta in the east-west segment; the Porcupine delta in the western north-south segment (in yellow, Figure 11). There is no evidence of multiple delta failures since AD 1964, supporting our interpretation. In addition to the landslide deposits from these large delta failures, 16 other tributary landslide deposits can be attributed to the same event (in green, Figure 11). Only one landslide deposit (in red, Figure 11), with a volume of $3.9 \times 10^6 \text{ m}^3$ appears to have been generated by hemipelagic slope failure. In total, the 1964 event resulted in 23 landslide deposits with a volume of $8.5 \times 10^7 \text{ m}^3$ (Table 1). The two Ke-1 megaturbidites correlated to the 1964 event have a combined volume of $2.1 \times 10^6 \text{ m}^3$. The seismic stratigraphy reveals six seismic event horizons prior to Event Ke-1. Several of these seismic events generated megaturbidites (Events Ke-2, -3, -5 and Ke-7). The combined volume of the two megaturbidites related to Event Ke-7 ($3.3 \times 10^6 \text{ m}^3$) exceeded the volume of the 1964 megaturbidites (Table 1).

## 4.4 Sedimentation rates

The sediment cores EK12-01 and SK12-03 have inferred background sedimentation rates of $0.43 \text{ cm yr}^{-1}$ and $0.25 \text{ cm yr}^{-1}$, respectively (Figure 4). In all cores, for which no radionuclide dates were available, the (mega)turbidite attributed to the 1964 earthquake was used as a basal time marker in order to estimate post-1964 background sedimentation rates. Conversion of seismic intervals in two-way travel time with these sedimentation rates provided an estimation of an average post-1964 sedimentation rate in each lake basin.
The average post-1964 sedimentation rate in Eklutna Lake increases from the distal basin towards the delta in the southeastern part of the lake from ~ 0.2 cm yr⁻¹ to ~ 1 cm yr⁻¹. The average post-1964 sedimentation rate in Skilak Lake varies from ~ 0.2 cm yr⁻¹ in the distal basin to ~ 0.5 cm yr⁻¹ in the proximal basin. In Kenai Lake, the average post-1964 sedimentation rates are ~ 0.25 cm yr⁻¹ in the western north-south oriented segment and increase towards the east-west oriented segment, which are ~ 0.5-0.6 cm yr⁻¹. The sedimentation rates are the highest in the eastern north-south oriented segment, and are up to 1 cm yr⁻¹.

422

## 5. Discussion

424

### 5.1 1964 megathrust earthquake

426

In terms of both the number and of volume of landslide deposits, the 1964 megathrust earthquake represents the largest event in each basin (Table 1). Although the landslide deposits associated with this event have not undergone compaction, and they thus may appear larger, similar observations were made in Alaskan fjords near Valdez and Whittier (Ryan et al., 2010; Haeussler et al., 2014). This large 1964 imprint was explained by the very long preceding interseismic interval of ~ 900 years (Carver and Plafker, 2008) and enhanced sedimentary influxes (e.g. Diedrich and Loso, 2012), after the discharge of material, built up during the LIA.

The 1964 earthquake generated different effects in all of the studied lake basins. The event produced numerous landslide deposits in Skilak, Kenai (both with 23 landslide deposits) and Eklutna lakes (15 landslide deposits). Skilak Lake and Kenai Lake experienced voluminous failures, with volumes of individual landslides in Skilak Lake (4.3 x 10⁶ m³) and Kenai Lake (3.7 x 10⁶ m³) being an order of magnitude larger than those in Eklutna Lake (4.4 x 10⁵ m³) (Table 1).

Both hemipelagic- and tributary-sourced slope sections failed during the 1964 event. However, the ratio of landslide deposits corresponding to each type of sediment source varies in every lake basin.
441 The majority of landslide deposits in Kenai Lake represent tributary landslide deposits and can be
442 attributed to delta and alluvial fan failures (95 % of total volume, Table 1 and Figure 11). In Eklutna
443 Lake, both types of landslides are present, with approximately equal numbers (7 of the 15 landslide
444 deposits attributed to delta and alluvial fan failures, Figure 9). However, tributary landslide deposits
445 account for only 33 %-39 % of the total accumulated volume (Table 1). In Skilak Lake, most landslide
446 deposits resulted from hemipelagic-sourced slope failures (Figure 10). Tributary landslide deposits, as
447 a result of delta failure in the eastern part of Skilak Lake, only account for 15 % of the total landslide
448 volume (Table 1).

449

450 **5.2 Megaturbidites**

451

452 Strong shaking in AD 1964 not only generated multiple individual landslide deposits on various slope
453 sections, but also created megaturbidites that were deposited in the deepest parts of Skilak Lake
454 (Figure 7 and 10) and Kenai Lake (Figure 8 and 11). The seismic and core data reveal no
455 megaturbidite in Eklutna Lake (Figure 5, 6 and 9). In Skilak and Kenai lakes the failed sediment
456 volumes are one or two orders of magnitude larger than landslide deposit volumes in Eklutna Lake
457 (Table 1). This suggests that the volume of remobilized sediments for a particular area of deposition
458 partly controls megaturbidite formation (i.e. thick enough to be resolved by the seismic data). Also,
459 not every landslide event that occurred prior to AD 1964 shows evidence of a megaturbidite.
460 In AD 1964, several seiches were reported in Kenai and Skilak lakes, likely caused by a combination of
461 tilting of the lake basins, i.e. westward tilting (90 cm) in Kenai Lake and eastward tilting (50 cm) of
462 Skilak Lake, and the movement of large amounts of remobilized sediment (Grantz et al., 1964;
463 Plafker, 1965; McCulloch, 1966; Waller, 1966). This implies that the formation of a megaturbidite is,
464 at least partly, shaped by the occurrence of seiche waves, similar to observations in alpine lakes
465 (Chapron et al., 1999; Schnellmann et al., 2006) and Chilean lakes (Van Daele et al., 2015).
466
467 **5.3 Generation of a landslide record**

468

469 Previous studies of the susceptibility of subaquatic slopes to failure (e.g. Morgenstern, 1967;

470 Hampton et al., 1996; Canals et al., 2004; Masson et al., 2006; Strasser et al., 2007; Strasser et al.,

471 2011; Wiemer et al., 2015) have shown that slope instability is governed by the ratio between

472 destabilizing loading forces acting on slope sediments (e.g. overburden weight, slope angle, ground

473 motion) and sediment properties resisting downslope movements (e.g. shear strength,

474 presence/absence of “weak” layers). In general, this susceptibility of sediments for failure on a slope

475 segment increases with time until slope failure takes place (the susceptibility is “reset”).

476 The generation of successive landslide deposits is not only a function of the susceptibility of the

477 adjacent slopes to failure, but also depends on the recharging capacity of the slopes, that is, the ratio

478 between sediment accumulation rate and failure recurrence through time. The number (and volume)

479 of successive landslide deposits varies significantly within the basins of the studied lakes (Figure 12A).

480 In this section we will discuss which preconditioning factors influence the initiation of landslides

481 (during seismic triggering) and therefore also the landslide record. No geotechnical data are available

482 from these slopes, hence, differences in slope failure susceptibility controlled by sediment strength,

483 could not be assessed. We therefore discuss the varying numbers and volumes of landslide deposits

484 in Eklutna, Skilak and Kenai lakes mainly in terms of slope angle and overburden weight, which is

485 controlled by the sedimentation rate. As we did not collect cores from delta or alluvial fan slopes and

486 thus have no information about sedimentation rates on such slopes, we will focus our discussion on

487 hemipelagic slope sections.

488 In the Eklutna Lake proximal basin (Figure 12A), the highest number of successive (hemipelagic)

489 landslide deposits occurs on the eastern lateral slope and in a small area in the middle of the western

490 lateral slope. In these parts, slope angles are 10°-15° (Figure 12B). Slopes that are only a few degrees

491 steeper (up to 18° in the southwestern part) show significantly lower numbers of landslide deposits.

492 This suggests that the slope angle represents a significant parameter influencing slope instability in
the proximal part of Eklutna Lake. Also, these areas with a higher number of successive landslide deposits are found on slopes facing a tributary, characterized by locally higher sedimentation rates.

In the Eklutna Lake distal basin (Figure 12A), the number of successive landslide deposits decreases towards the most distal part of the lake, consistent with decreasing values in sedimentation rate (reduction of 50%).

In Skilak Lake (figure 12A), the northern and southwestern intrabasinal slopes bordering the deep central basin show the most complete succession of landslide deposits. In this area the slope values range between 5° and 15° (Figure 12B) and the sedimentation rate is relatively high, which is twice the sedimentation rate of the distal basin (Figure 12C). In the southern part of the deep basin, sedimentation rates are similar, but slopes are steeper (~ 20°), and barely any landslide deposits are present. Small-scale spontaneous failure of these slopes probably occurs before sediment can accumulate. The present-day sediment cover also confirms this observation with a thick sediment drape (up to ~ 40 m) on the northern and southwestern slopes while barely any sediment is present on the southern slopes. Lower slope values in the distal sub-basin (<10°) favor the presence of thick landslide deposits. However, the number of successive landslide deposits is lower (Figure 12A) than in the proximal basin, probably due to a lower sedimentation rate, which is ~ 1/2 of the sedimentation rate in the proximal basin, because of a lack of tributary streams (Figure 12C).

The lack of successive hemipelagic landslide deposits in Kenai Lake (Figure 12A) is most probably related to the surrounding steep, rocky slopes (up to 30°, Figure 12B), inhibiting sediment accumulation before slope instability occurs. The landslide record in the eastern north-south oriented segment and the east-west oriented segment of Kenai Lake (Figure 12A) can mainly be attributed to delta and alluvial fan failures. This is in line with other observations of landslide deposits and turbidites in basins with steep slopes (Håkanson and Jansson, 2002; Moernaut and De Batist, 2011; Strasser et al., 2011; Van Daele et al., 2015).

The study of hemipelagic slopes in Eklutna, Skilak and Kenai lakes indicates that slope gradients between ~ 5°-20° are required in order to generate landslide deposits, because these slope values
allow sufficient sediment accumulation before small-scale spontaneous slope failure occurs. Within this slope gradient interval, slopes with lower gradients can hold thicker sediment cover than those with higher gradients. Therefore these slopes fail less frequently over time, while generating more voluminous successive landslide deposits. Areas with slope gradients > 20° (unstable conditions) are constantly susceptible to failure; however, they do not generate landslides, due to insufficient sediment accumulation before failure. Areas with slope values <5° (stable conditions) are not generating a landslide record in the studied lakes, even if the sedimentation rates are high. If the slope gradient is favorable (~ 5°-20°), the slope stability depends on the overburden weight, as faster loading of the slopes will increase the slope instability. Therefore, higher sedimentation rates will increase the number of successive landslide deposits in the record. The landslide record of Eklutna, Skilak and Kenai lakes is thus dependent on the slope angle and sedimentation rate, which is in agreement with conclusions from studies in other lakes, such as in the Alps (Strasser et al., 2007; Strasser et al., 2011; Wilhelm et al., 2015a) and Chile (Moernaut et al., 2007).

532

## 5.4 Landslide record potential to act as a seismometer for past (megathrust) earthquake events

534

If the sedimentation rates calculated for the last ~ 50 years are extrapolated further back in time, the landslide record in the Eklutna Lake proximal basin, Eklutna Lake distal basin, Skilak Lake and Kenai Lake reveals 14 earthquake events in ~ 3000 yrs, 8 events in ~ 3500 yrs, 7 events in ~ 5500 yrs and 7 events in ~ 3000 yrs, respectively. These age ranges should be regarded as a first-order approximation only, as a simple extrapolation of recent sedimentation rates does not take into account compaction or possible variations in sedimentation rate over the studied millennial-scale record. Previous studies have shown that sedimentary fluxes possibly increased during the LIA (Diedrich and Loso, 2012; Ólafsdóttir et al., 2013) and older Neoglaciations (Leonard, 1986, 1997), which implies that the calculated age ranges probably represent maximum ages. Also, the dam construction in the lower end of Eklutna Lake, may have altered modern sedimentation rates in the
distal basin. Thus, in the following paragraphs we only compare the order of magnitude in which
545
events are recorded in the different lake records.

The 1964 megathrust earthquake caused multiple slope failures in all studied basins, affecting both
547
tributary- and hemipelagic-sourced slope sections. Pre-1964 events in each basin also show multiple
548
slope failures. Applying the synchronicity criterion (Schnellmann et al., 2002) to the mapped
549
deposits, strong earthquake shaking probably triggered all discussed events, suggesting that the
550
landslide record in each basin represents a record of past earthquakes. However, the confidence
551
level of our interpretation that the events are seismically-triggered, increases with a higher number
552
of landslide deposits (Table 2).

Delta slopes often fail more easily by gradual overloading, without the need for a seismic trigger (e.g.
554
Lake Brienz; Girardclos et al., 2007). The landslide record of Kenai Lake might thus be regarded as
555
less meaningful for assessing (megathrust) earthquake recurrence. However, multiple coeval delta
556
and/or alluvial fan failures hint to a more regional triggering mechanism that can only be explained
557
by strong earthquakes, similar to the multiple delta failures that occurred during the 1964
558
earthquake. This implies that records of delta and alluvial fan failures may be useful for establishing
559
paleoseismic records in high-seismicity settings, because seismic triggering occurs prior to
601
spontaneous failure due to overloading.

All the examined lake basins show a different landslide record. Based on our first-order estimation of
562
the record age, the Eklutna Lake distal basin and Kenai Lake show comparable mean recurrence
563
intervals of ~ 450 yrs, similar to records of great megathrust earthquakes based on records in coastal
564
marshes (Carver and Plafker, 2008; Shennan et al., 2014). The recurrence interval in Skilak Lake is
565
significantly longer, as it spans almost double the time for approximately the same number of events,
566
which is ~ 900 yrs. The record in the Eklutna proximal basin shows the shortest recurrence interval of
567
~ 250 yrs.

This discrepancy in event recording can be explained by a different sensitivity of the subaquatic
569
slopes to strong earthquake shaking, which depends on preconditioning factors such as the
570
sedimentation rate. For example, the Eklutna proximal basin recorded almost double the amount of events, in a smaller time span than the Eklutna distal basin, which we attribute to significantly higher (double) sedimentation rates (Figure 12C). As both sub-basins experienced the same earthquake history, this means that higher sedimentation rates (faster slope recharging) increase the sensitivity to earthquake-triggered slope failure and thus decrease the intensity threshold for slope instability.

Therefore the proximal basin of Eklutna Lake probably records earthquakes of lower macroseismic intensity. Skilak Lake has a similar sedimentation rate as in the Eklutna distal basin, but it recorded a similar number of events in double the time span. Hence, factors other than the sedimentation rate must be influencing the initiation of failures and therefore the sensitivity of the record, such as variations in sediment type, or the presence of weak layers (e.g. Strasser et al., 2007; Strasser et al., 2011; Wiemer et al., 2015). In basins with steep slopes, like Kenai Lake, the landslide deposits are attributed to delta and alluvial fan failures. This partly explains the lower number of failures in Skilak Lake for the same time interval as in Kenai Lake, because deltas are more prone to slope failure than hemipelagic slopes (e.g. Strasser et al., 2013; Moernaut et al., 2014; Van Daele et al., 2015).

Also the different location of the lakes (seismotectonic setting) has to be taken into account, since the same earthquake event can locally exhibit different macroseismic intensities, and thus also affect the landslide record in each basin. There is a recognized change in locking behavior from the Anchorage area (e.g. Eklutna Lake) to the Kenai Peninsula (e.g. Skilak Lake and Kenai Lake) based on modern geodetic measurements (Zweck et al., 2002). Therefore it is possible that the lakes experienced a slightly different (megathrust) earthquake history. Also, the area around Eklutna Lake may be more prone to shallow earthquakes from surrounding fault systems, such as the Castle Mountain Fault (Haeussler et al., 2002), due to its more inland location.

This study of landslide records in Alaskan lakes indicates that a multi-lake approach is required in order to construct a paleoseismic record of megathrust earthquakes. Using multiple lakes with a different seismotectonic setting also has the advantage to infer spatial variations of megathrust earthquakes and to identify the effects of local seismic sources.
597

598

## 6. Conclusions

599

600

High-resolution seismic reflection profiling shows that horizontally stratified background

601

sedimentation in three south-central Alaskan lakes is occasionally interrupted by landslide deposits

602

and megaturbidites. In the Eklutna Lake proximal basin, Eklutna Lake distal basin, Skilak Lake and

603

Kenai Lake, seismic stratigraphy analysis reveals 14, 8, 7, and 7 successive landslide events,

604

respectively.

605

Radionuclide dating and seismic-to-core correlation confirms that the youngest event in Eklutna and

606

Skilak lakes, and very likely in Kenai Lake, corresponds to the historic 1964 Alaska earthquake. This

607

event produced the most numerous and voluminous slope failures of observed sequences.

608

In each lake, the number and volume of successive landslide deposits, and therefore the landslide

609

record, is dependent on the slope angle and sedimentation rate. In order to generate a landslide

610

record in the studied lakes, slope gradients between ~ 5°-20° are required. If the slope gradient is

611

favorable, then higher sedimentation rates result in faster slope recharging and a higher number of

612

successive landslide deposits.

613

All events show evidence of multiple coeval slope failures, including delta and alluvial fan failures.

614

This implies that the landslide record in the Eklutna Lake proximal basin, Eklutna Lake distal basin,

615

Skilak Lake and Kenai Lake represents a record of past seismic events, with a mean estimated

616

recurrence interval of ~ 250 yrs, ~ 450 yrs, ~ 900 yrs and ~ 450 yrs, respectively. The significantly

617

different earthquake record between studied basins can be explained by preconditioning factors,

618

such as variations in slope recharging and sediment sources. High sedimentation rates and a

619

predominance of tributary slope failures increase the sensitivity for recording the paleoseismic

620

record and thus lower the intensity threshold for slope instability. Moreover, depending on the

621

setting of each lake, a slightly different earthquake history will also affect the paleoseismic record.
Multiple delta or alluvial fan failures can be useful for establishing paleoseismic histories in high-seismicity settings, because seismic triggering can take place before spontaneous failure due to overloading. This is in contrast with studies in regions with infrequent seismic shaking, where self-destabilizing delta failures frequently occur.

The sublacustrine landslide stratigraphy in these Alaskan lakes potentially represents a complete paleoseismic record of megathrust earthquakes, as long as a multi-lake approach is applied. Future work will include the study of long sediment cores in order to corroborate the combined record of the studied lakes with robust age models. This will provide insight in high-resolution temporal variations that can help assessing megathrust earthquake recurrence for the region. This information is crucial for understanding the seismic hazard of south-central Alaska, and particularly for the city of Anchorage, which is the population and infrastructure center of the state. A multi-lake approach also allows investigation of spatial variations of strong earthquakes, and it may reveal information about earthquake ground shaking from other seismic sources, such as intraslab earthquakes and crustal faults. Moreover, Skilak and Kenai lakes lie in a position between a more fully locked part of the megathrust beneath Prince William Sound, and a more creeping part of the megathrust offshore the Kenai Peninsula. Future results can be relevant for inferring information on variability between ruptures and interplate coupling.

## Acknowledgements

This research was funded by the Research Foundation Flanders (FWO-Vlaanderen). We want to acknowledge the Alaska Pacific University's Eklutna Glacier monitoring project for the financial support of the radionuclide dating. Jasper Moernaut acknowledges the support of the Swiss National Science Foundation (grant 133481). We also thank Adrian Bender for support during the seismic and coring surveys. We are grateful to Jean-Noël Proust, Jason Chaytor and an anonymous reviewer for
the constructive reviews, improving an earlier version of this paper. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the authors.

## References

Arnaud, F., Magand, O., Chapron, E., Bertrand, S., Boes, X., Charlet, F., Melieres, M., 2006.

Radionuclide dating ((210) Pb,(137) Cs,(241) Am) of recent lake sediments in a highly active geodynamic setting (Lakes Puyehue and Icalma-Chilean Lake District). Science of the Total Environment 366, 837-850.

Barclay, D., Wiles, G., Calkin, P., 2009. Holocene glacier fluctuations in Alaska. Quaternary Science Reviews 28, 2034-2048.

Bertrand, S., Charlet, F., Chapron, E., Fagel, N., De Batist, M., 2008. Reconstruction of the Holocene seismotectonic activity of the Southern Andes from seismites recorded in Lago Icalma, Chile, 39°S. Palaeogeography, Palaeoclimatology, Palaeoecology 259, 301-322.

Bouma, A.H., 1987. Megaturbidite - an acceptable term. Geo-Marine Letters 7, 63-67.

Brabets, T.P., 1993. Glacier runoff and sediment transport and deposition: Eklutna Lake Basin, Alaska, U.S. Geological Survey Water-Resources Investigations Report 92-4132, p. 52.

Bradley, D.C., Kusky, T.M., Haeussler, P.J., Goldfarb, R.J., Miller, M.L., Dumoulin, J.A., Nelson, S.W., Karl, S.M., 2003. Geologic signature of early Tertiary ridge subduction in Alaska, in: Sisson, V.B., Roeske, S.M., Pavlis, T.L. (Eds.), Geology of a transpressional orogen developed during reidge-trench interaction along the north Pacific margin: Geological Society of America Special Paper 371, pp. 19-49.

Burns, L.E., Pessel, G.H., Little, T.A., Pavlis, T.L., Newberry, R.J., Winkler, G.R., Decker, J.E., 1991.

Geology of the northern Chugach Mountains, southcentral Alaska, Alaska Division of Geological & Geophysical Surveys Professional Report 94, p. 63.
672 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., Sultan, N., Trincardi, F., Bryn, P., 2004. Slope failure dynamics and impacts from seafloor and shallow sub-seafloor geophysical data: case studies from the COSTA project. Marine Geology 213, 9-72.
673
674
675
676 Carver, G., Plafker, G., 2008. Paleoseismicity and Neotectonics of the Aleutian Subduction Zone—An Overview, in: Freymueller, J.T., Haeussler, P.J., Wesson, R.L., Ekström, G. (Eds.), Active Tectonics and Seismic Potential of Alaska. American Geophysical Union, Washington, D. C., pp. 43-63.
677
678
679 Chapron, E., Beck, C., Pourchet, M., Deconinck, J.F., 1999. 1822 earthquake-triggered homogenite in Lake Le Bourget (NW Alps). Terra Nova 11, 86-92.
680
681 Chen, C.T., Millero, F.J., 1977. Speed Of Sound In Seawater At High-Pressures. The Journal of the Acoustical Society of America 62, 1129-1135.
682
683 Christensen, D.H., Beck, S.L., 1994. The Rupture Process and Tectonic Implications of the Great 1964 Prince-William-Sound Earthquake. Pure and Applied Geophysics 142, 29-53.
684
685 Combellick, R.A., 1993. The penultimate great earthquake in south-central Alaska: Evidence from a buried forest near Girdwood, in: Solie, D.N., Tannian, F. (Eds.), Short notes on Alaskan geology 1993, Alaska Division of Geological and Geophysical Surveys Professional Report, 113, pp. 7-15.
686
687
688 DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophysical Journal International 101, 425-478.
689
690 Diedrich, K., Loso, M., 2012. Transient impacts of Little Ice Age glacier expansion on sedimentation processes at glacier-dammed Iceberg Lake, southcentral Alaska. Journal of Paleolimnology 48, 115-132.
691
692
693 Doughty, M., Eyles, N., Eyles, C., 2013. High-resolution seismic reflection profiling of neotectonic faults in Lake Timiskaming, Timiskaming Graben, Ontario-Quebec, Canada. Sedimentology 60, 983-1006.
694
695
696 Fanetti, D., Anselmetti, F.S., Chapron, E., Sturm, M., Vezzoli, L., 2008. Megaturbidite deposits in the Holocene basin fill of Lake Como (Southern Alps, Italy). Palaeogeography, Palaeoclimatology, Palaeoecology 259, 323-340.

699 Freymueller, J.T., Woodard, H., Cohen, S.C., Cross, R., Elliott, J., Larsen, C.F., Hreinsdóttir, S., Zweck, C., 2008. Active Deformation Processes in Alaska, Based on 15 Years of GPS Measurements, in: Freymueller, J.T., Haeussler, P.J., Wesson, R.L., Ekström, G. (Eds.), Active Tectonics and Seismic Potential of Alaska. American Geophysical Union, Washington, D.C., pp. 1-42.

703 Girardclos, S., Schmidt, O.T., Sturm, M., Ariztegui, D., Pugin, A., Anselmetti, F.S., 2007. The 1996 AD delta collapse and large turbidite in Lake Brienz. Marine Geology 241, 137-154.

705 Grantz, A., Plafker, G., Kachadoorian, R., 1964. Alaska's Good Friday earthquake, March 27, 1964, a preliminary geologic evaluation. U.S. Geological Survey Circular 491, 35 p.

707 Haeussler, P.J., Best, T.C., Waythomas, C.F., 2002. Paleoseismology at high latitudes: Seismic disturbance of upper Quaternary deposits along the Castle Mountain fault near Houston, Alaska. Geological Society of America Bulletin 114, 1296-1310.

710 Haeussler, P.J., Parsons, T., Finlayson, D.P., Hart, P., Chaytor, J.D., Ryan, H., Lee, H., Labay, K., Peterson, A., Liberty, L., 2014. New Imaging of Submarine Landslides from the 1964 Earthquake Near Whittier, Alaska, and a Comparison to Failures in Other Alaskan Fjords, in: Krastel, S., Behrmann, J.-H., Völker, D., Stipp, M., Berndt, C., Urgeles, R., Chaytor, J., Huhn, K., Strasser, M., Harbitz, C.B. (Eds.), Submarine Mass Movements and Their Consequences. Springer International Publishing, pp. 361-370.

716 Håkanson, L., Jansson, M., 2002. Principles of Lake Sedimentology. The Blackburn Press.

717 Hampton, M.A., Lee, H.J., Locat, J., 1996. Submarine landslides. Reviews of Geophysics 34, 33-59.

718 Holdahl, S., Sauber, J., 1994. Coseismic slip in the 1964 Prince William Sound earthquake: A new geodetic inversion. Pure and Applied Geophysics 142, 55-82.
720 Johnson, J.M., Satake, K., Holdahl, S.R., Sauber, J., 1996. The 1964 Prince William Sound earthquake: Joint inversion of tsunami and geodetic data. Journal of Geophysical Research-Solid Earth 101, 523-532.
721
722 Karlstrom, T.N.V., 1964. Quaternary geology of the Kenai Lowland and glacial history of the Cook Inlet region, Alaska. U.S. Geological Survey Professional Paper 443, p. 69.
723
724 Leonard, E.M., 1986. Varve studies at Hector Lake, Alberta, Canada, and the relationship between glacial activity and sedimentation. Quaternary Research 25, 199-214.
725
726 Leonard, E.M., 1997. The relationship between glacial activity and sediment production: Evidence from a 4450-year varve record of neoglacial sedimentation in Hector Lake, Alberta, Canada. Journal of Paleolimnology 17, 319-330.
727
728
729 Logan, M.H., 1967. Effect of the earthquake of March 27,1964, on the Eklutna Hydroelectric Project, Anchorage, Alaska, with a section on Television examination of earthquake damage to underground communication and electrical systems in Anchorage. U.S. Geological Survey Professional Paper 545-A, p. 30.
730
731
732
733 LP DAAC, 2009. ASTER Global Digital Elevation Model Version 2. NASA EOSDIS Land Processes DAAC, USGS Earth Resources Observation and Science (EROS) Center, Sioux Falls, South Dakota (https://lpdaac.usgs.gov).
734
735
736 Masson, D.G., Harbitz, C.B., Wynn, R.B., Pedersen, G., Lovholt, F., 2006. Submarine landslides: processes, triggers and hazard prediction. Philosophical transactions - Royal Society. Mathematical, Physical and Engineering Sciences 364, 2009-2039.
737
738
739 McCulloch, D.S., 1966. Slide-induced waves, seiching and ground fracturing caused by the earthquake of March 27, 1964 at Kenai Lake, Alaska. U.S. Geological Survey Professional Paper 543-A, p. 41.
740
741
742 Moernaut, J., De Batist, M., 2011. Frontal emplacement and mobility of sublacustrine landslides: Results from morphometric and seismostratigraphic analysis. Marine Geology 285, 29-45.
743
744 Moernaut, J., De Batist, M., Charlet, F., Heirman, K., Chapron, E., Pino, M., Brummer, R., Urrutia, R.,
745 2007. Giant earthquakes in South-Central Chile revealed by Holocene mass-wasting events in Lake
746 Puyehue. Sedimentary Geology 195, 239-256.

747 Moernaut, J., De Batist, M., Heirman, K., Van Daele, M., Pino, M., Brümmer, R., Urrutia, R., 2009.
748 Fluidization of burried mass-wasting deposits in lake sediments and its relevance for
749 paleoseismology: Results from a reflection seismic study of lakes Villarrica and Calafquén (South-
750 Central Chile). Sedimentary Geology 213, 121-135.

751 Moernaut, J., Van Daele, M., Heirman, K., Fontijn, K., Strasser, M., Pino, M., Urrutia, R., De Batist, M.,
752 2014. Lacustrine turbidites as a tool for quantitative earthquake reconstruction: New evidence for a
753 variable rupture mode in south central Chile. Journal of Geophysical Research-Solid Earth 119, 1607-
754 1633.

755 Monecke, K., Anselmetti, F.S., Becker, A., Sturm, M., Giardini, D., 2004. The record of historic
756 earthquakes in lake sediments of Central Switzerland. Tectonophysics 394, 21-40.

757 Morgenstern, N.R., 1967. Submarine slumping and initiation of turbidity currents, in: Richards, A.F.
758 (Ed.), Marine Geotechnique University Press, Urbana, Illinois, pp. 189-220.

759 Ólafsdóttir, K.B., Geirsdóttir, Á., Miller, G.H., Larsen, D.J., 2013. Evolution of NAO and AMO strength
760 and cyclicity derived from a 3-ka varve-thickness record from Iceland. Quaternary Science Reviews
761 69, 142-154.

762 Oldfield, F., Appleby, P.G., 1984. Empirical testing of Pb-210-dating models for lake-sediments, in:
763 Haworth, E.Y., Lund, J.G. (Eds.), Lake sediments and environmental history. Leicester University Press,
764 pp. 93-124.

765 Pennington, W., Tutin, T.G., Cambray, R.S., Fisher, E.M., 1973. Observations on Lake Sediments using
766 Fallout 137Cs as a Tracer. Nature 242, 324-326.

767 Perkins, J.A., Sims, J.D., 1983. Correlation of Alaskan varve thickness with climatic parameters, and
768 use in paleoclimatic reconstruction. Quaternary Research 20, 308-321.
769 Pinson, L.J.W., Vardy, M., Dix, J., Henstock, T., Bull, J., 2013. Deglacial history of glacial lake windermere, UK: implications for the central British and Irish Ice Sheet. JQS. Journal of quaternary science 28, 83-94.

772 Plafker, G., 1965. Tectonic Deformation Associated with the 1964 Alaska Earthquake. Science 148, 1675-1687.

774 Plafker, G., 1969. Tectonics of the March 27, 1964, Alaska earthquake. U.S. Geological Survey Professional Paper 543-I, p. 74.

776 Reger, R.D., Sturmann, A.G., Berg, E.E., Burns, P.A.C., 2007. A guide to the Late Quaternary history of northern and western Kenai Peninsula, Alaska. Alaska Division of Geological & Geophysical Surveys Guidebook 8, 112.

779 Robbins, J.A., Edgington, D.N., 1975. Determination of recent sedimentation rates in Lake Michigan using Pb-210 and Cs-137. Geochimica et Cosmochimica Acta 39, 285-304.

781 Ryan, H.F., Lee, H.J., Haeussler, P.J., Alexander, C.R., Kayen, R.E., 2010. Historic and Paleo-Submarine Landslide Deposits Imaged Beneath Port Valdez, Alaska: Implications for Tsunami Generation in a Glacial Fiord. Submarine Mass Movements and Their Consequences 28, 411-421.

784 Rymer, M.J., Sims, J.D., 1976. Preliminary survey of modern glaciolacustrine sediments for earthquake-induced deformational structures, south-central Alaska, U.S. Geological Survey Open-File Report 76-373, p. 32

787 Santini, S., Dragoni, A., Spada, G., 2003. Asperity distribution of the 1964 Great Alaska earthquake and its relation to subsequent seismicity in the region. Tectonophysics 374, 237-237.

789 Schmidt, S., De Deckker, P., 2015. Present-day sedimentation rates on the southern and southeastern Australian continental margins. Australian Journal of Earth Sciences 62, 143-150.

791 Schnellmann, M., Anselmetti, F., Giardini, D., McKenzie, J., 2006. 15,000 Years of mass-movement history in Lake Lucerne: Implications for seismic and tsunami hazards. Eclogae Geologicae Helvetiae 99, 409-428.
794 Schnellmann, M., Anselmetti, F.S., Giardini, D., McKenzie, J.A., Ward, S.N., 2002. Prehistoric earthquake history revealed by lacustrine slump deposits. Geology 30, 1131-1134.
795
796 Shennan, I., Bruhn, R., Barlow, N., Good, K., Hocking, E., 2014. Late Holocene great earthquakes in the eastern part of the Aleutian megathrust. Quaternary Science Reviews 84, 86-97.
797
798 Smith, S.B., Karlin, R.E., Kent, G.M., Seitz, G.G., Driscoll, N.W., 2013. Holocene subaqueous paleoseismology of Lake Tahoe. Geological Society of America Bulletin 125, 691-708.
799
800 Sonett, C.P., Williams, G.E., 1985. Solar periodicities expressed in varves from glacial Skilak Lake, Southern Alaska. Journal of Geophysical Research-Space Physics 90, 2019-2026.
801
802 Strasser, M., Hilbe, M., Anselmetti, F., 2011. Mapping basin-wide subaquatic slope failure susceptibility as a tool to assess regional seismic and tsunami hazards. Marine Geophysical Research 32, 331-347.
803
804
805 Strasser, M., Monecke, K., Schnellmann, M., Anselmetti, F.S., 2013. Lake sediments as natural seismographs: A compiled record of Late Quaternary earthquakes in Central Switzerland and its implication for Alpine deformation. Sedimentology 60, 319-341.
806
807
808 Strasser, M., Stegmann, S., Bussmann, F., Anselmetti, F.S., Rick, B., Kopf, A., 2007. Quantifying subaqueous slope stability during seismic shaking: Lake Lucerne as model for ocean margins. Marine Geology 240, 77-97.
809
810
811 Stupar, Y.V., Schäfer, J., García, M.G., Schmidt, S., Piovano, E., Blanc, G., Huneau, F., Le Coustumer, P., 2014. Historical mercury trends recorded in sediments from the Laguna del Plata, Córdoba, Argentina. Chemie der Erde - Geochemistry 74, 353-363.
812
813
814 Sturm, M., Matter, A., 1978. Turbidites and varves in Lake Brienz (Switzerland): deposition of clastic detritus by density currents, in: Matter, A., Tucker, M.E. (Eds.), Modern and Ancient Lake Sediments: International Association of Sedimentologists Special Publication 2, pp. 147-168.
815
816
817 Suito, H., Freymueller, J.T., 2009. A viscoelastic and afterslip postseismic deformation model for the 1964 Alaska earthquake. Journal of Geophysical Research-Solid Earth 114, 23.
818
819 Van Daele, M., Moernaut, J., Doom, L., Boes, E., Fontijn, K., Heirman, K., Vandoorne, W., Hebbeln, D.,
820 Pino, M., Urrutia, R., Brummer, R., De Batist, M., 2015. A comparison of the sedimentary records of
821 the 1960 and 2010 great Chilean earthquakes in 17 lakes: Implications for quantitative lacustrine
822 palaeoseismology. Sedimentology 62, 1466-1496.

823 Von Gunten, L., Grosjean, M., Beer, J., Grob, P., Morales, A., Urrutia, R., 2009. Age modeling of young
824 non-varved lake sediments: methods and limits. Examples from two lakes in Central Chile. Journal of
825 Paleolimnology 42, 401-412.

826 Waldmann, N., Anselmetti, F.S., Ariztegui, D., Austin, J.A., Jr., Pirouz, M., Moy, C.M., Dunbar, R., 2011.
827 Holocene mass-wasting events in Lago Fagnano, Tierra del Fuego (54 degrees S): implications for
828 paleoseismicity of the Magallanes-Fagnano transform fault. Basin Research 23, 171-190.
829 Waller, R.M., 1966, Effects of the March 1964 Alaska earthquake on the hydrology of south-central
830 Alaska: U.S. Geological Survey Professional Paper 544-A, 28 p., http://pubs.usgs.gov/pp/0544a/.

831 Wiemer, G., Moernaut, J., Stark, N., Kempf, P., De Batist, M., 2015. The role of sediment composition
832 and behavior under dynamic loading conditions on slope failure initiation: a study of a subaqueous
833 landslide in earthquake-prone South-Central Chile. International Journal of Earth Sciences 104, 1439-
834 1457.

835 Wilhelm, B., Nomade, J., Crouzet, C., Litty, C., Sabatier, P., Belle, S., Rolland, Y., Revel, M.,
836 Courboulex, F., Arnaud, F., Anselmetti, F.S., 2015a. Quantified sensitivity of small lake sediments to
837 record historic earthquakes: Implications for paleoseismology. Journal of Geophysical Research:
838 Earth Surface, n/a-n/a.

839 Wilhelm, B., Sabatier, P., Arnaud, F., 2015b. Is a regional flood signal reproducible from lake
840 sediments? Sedimentology 62, 1103-1117.

841 Zweck, C., Freymueller, J.T., Cohen, S.C., 2002. Three-dimensional elastic dislocation modeling of the
842 postseismic response to the 1964 Alaska earthquake. Journal of Geophysical Research: Solid Earth
843 107, ECV 1-1-ECV 1-11.
845 **Figures**

846

847 Figure 1: A. Tectonic setting of south-central Alaska with the location of the 1964 earthquake epicenter (yellow star) and rupture zone (red, e.g. Plafker, 1965; Freymueller et al., 2008) with respect to the Alaska-Aleutian megathrust (black line). Orange areas indicate high slip deficit after the 1964 earthquake ("locking") and blue colors indicate regions with afterslip ("creeping") (after Zweck et al., 2002). B. Regional effect of the 1964 earthquake in south-central Alaska with indication of areas of coseismic uplift (purple) and subsidence (green) in AD 1964 (after Plafker, 1969). C. Kenai Peninsula with Eklutna, Skilak and Kenai lakes. Background: ASTER Global Digital Elevation Model (LP DAAC, 2009; https://lpdaac.usgs.gov).

855

856 Figure 2: Bathymetric maps of A. Eklutna Lake, B. Skilak Lake and C. Kenai Lake with indication of seismic profiles, main river in- and outflows, core locations EK12-01 and SK12-03, other core locations (indicated by grey dots) and deltas (QD: quartz creek delta, PD: Porcupine delta, SD: Ship Creek delta, MD: Meadow Creek delta, LD: Lawing delta, RD: Rocky delta, LvD: Lakeview delta), superimposed on an ASTER Global Digital Elevation Model (Hill Shade view) (LP DAAC, 2009; https://lpdaac.usgs.gov) of the lake's region. Photographs of several deltas and alluvial fans are shown and indicated on the map with a number (1-6). Bathymetric contour lines are provided for Eklutna, Skilak and Kenai lakes every 5, 10 and 10 m, respectively.

864

865 Figure 3: Seismic profile (SW-NE) in Skilak Lake showing typical slope geometry features that point towards failure of slope sediments, i.e. a slide scarp and a landslide deposit located at the base of the slope. Landslide deposits are typically thicker at the base of the slope and thin out away from the slope and are typically acoustically transparent or chaotic.

869
Figure 4: Age model based on <sup>210</sup>Pb and <sup>137</sup>Cs concentrations in sediment from A. Eklutna Lake (core EK12-01) and B. Skilak Lake (core SK12-03). The graph shows isotope activity of <sup>210</sup>Pb and <sup>137</sup>Cs versus (corrected) depth. Extrapolations of the <sup>210</sup>Pb model are indicated with dotted lines. Locally thicker turbidites were considered as instantaneously deposited and removed for the construction of the model. The age model yields a background sedimentation rate of 0.43 cm yr<sup>-1</sup> and 0.25 cm yr<sup>-1</sup> for Eklutna and Skilak Lake, respectively.

876

Figure 5: Pinger profile (W-E) in the proximal sub-basin of Eklutna Lake showing landslide deposits and seismic event horizons. The top of a landslide deposit defines a seismic stratigraphic horizon, which is indicated by a solid line. The base of the landslide deposits represents an unconformity, which is indicated by a dashed line. In total 14 events can be identified, where Event Ek-1 represents the historic 1964 megathrust earthquake. The turbidite related to the 1964 earthquake is not visible on the seismic data (Rayleigh criterion). The in situ core length of EK12-05 is decreased by compaction of the sediments, during and after coring. Therefore the length of the core appears longer on the seismic data.

885

Figure 6: Pinger profile (SW-NE) in the distal sub-basin of Eklutna Lake showing landslide deposits, seismic event horizons (solid line) and unconformities (dashed line). Eight events can be identified, where Event Ek-A represents the historic 1964 megathrust earthquake. The turbidite related to the 1964 earthquake is not visible on the seismic data. The in situ core length of EK12-01 is decreased by compaction of the sediments, during and after coring. Therefore the length of the core appears longer on the seismic data.

892

Figure 7: Pinger profile (SSW-NNE) in Skilak Lake showing landslide deposits, megaturbidites and seismic event horizons (solid line). The base of the landslide deposits and megaturbidites represents an unconformity, which is indicated by a dashed line. Seven events can be identified, where Event Sk-
1 represents the historic 1964 megathrust earthquake. A transparent facies with ponding geometry is indicative of megaturbidites. The megaturbidite related to the 1964 earthquake is a graded deposit with coarse-grained base (~ 50 cm) in core SK12-01. The in situ core length of SK12-01 is decreased by compaction of the sediments, during and after coring. Therefore the length of the core appears longer on the seismic data.

901

Figure 8: Pinger profiles in Kenai Lake showing A. an east-west oriented transect with landslide deposits, seismic event horizons (solid line) and unconformities (dashed line) B. a north-south oriented transect with megaturbidites. The top of the megaturbidites is linked to a seismic horizon (solid line), while the base represents an unconformity (dashed line). In total seven events can be identified, where Event Ke-1 very likely represents the historic 1964 megathrust earthquake. The megaturbidite related to the 1964 earthquake is a graded deposit with coarse-grained base (~ 60 cm) in core KE12-05. The in situ core length of KE12-05 is decreased by compaction of the sediments, during and after coring. Therefore the length of the core appears longer on the seismic data.

910

Figure 9: Spatial distribution of (hemipelagic and tributary-sourced) landslide deposits produced by different events in Eklutna Lake. A. distal and B. proximal sub-basins are considered separately. The historic 1964 megathrust event (Ek-1/Ek-A) can be identified in both sub-basins based on correlation with short cores. For this event the bathymetric map is plotted on an ASTER Global Digital Elevation Model (Hill Shade view) (LP DAAC, 2009; https://lpdaac.usgs.gov). The tributary-sourced landslide deposits are the result of delta and alluvial fan failures. Landslide deposits that potentially consist of multiple individual deposits are considered as "suggested landslide deposits." Landslide deposits that could not be identified with high certainty are referred to as "uncertain deposits."

919

Figure 10: Spatial distribution of (hemipelagic and tributary-sourced) landslide deposits and megaturbidites produced by different events in Skilak Lake. For the historic 1964 megathrust event
922 the bathymetric map was plotted on an ASTER Global Digital Elevation Model (Hill Shade view) (LP
923 DAAC, 2009; https://lpdaac.usgs.gov). Identified slide scarps on seismic profiles are indicated on the
924 map. The tributary-sourced landslide deposits are the result of delta failures. Landslide deposits that
925 potentially consist of multiple individual deposits are considered as "suggested landslide deposits."
926 Landslide deposits that could not be identified with high certainty are referred to as "uncertain
deposits."
927

928

929 Figure 11: Spatial distribution of (hemipelagic and tributary-sourced) landslide deposits and
930 megaturbidites produced by different events in Kenai Lake. For the historic 1964 megathrust event
931 the bathymetric map was plotted on an ASTER Global Digital Elevation Model (Hill Shade view) (LP
932 DAAC, 2009; https://lpdaac.usgs.gov) with indication of deltas (QD: quartz creek delta, PD: Porcupine
delta, SD: Ship Creek delta, MD: Meadow Creek delta, LD: Lawing delta, RD: Rocky delta, LvD:
933 Lakeview delta). As the quality of the seismic data in the western north-south oriented segment was
934 not sufficient to identify deposits below the 1964 event, this segment is not shown for the
distribution of older landslides. The tributary-sourced landslide deposits are the result of delta and
936 alluvial fan failures. Landslide deposits that could not be identified with high certainty are indicated
937 as "uncertain deposits."
938

939

940 Figure 12: For Eklutna, Skilak and Kenai lakes: A. the number of successive landslide deposits at each
941 location in the lake; B. the slope gradient; C. the average post-1964 sedimentation rate. The number
942 of successive landslides is maximum when slopes have an angle of 5°-20° and when sedimentation
943 rates are high. White areas: cores on slopes are unavailable so sedimentation rates could not be
944 estimated. Hatched areas (Kenai Lake only): seismic data do not allow estimation of sedimentation
rates.
945

946

947 Tables
948

949

Table 1: Summary of landslide deposit and megaturbidite calculations (number and volume) for each event in the Eklutna Lake distal basin (Eklutna Distal), Eklutna Lake proximal basin (Eklutna Proximal), Skilak Lake and Kenai Lake. All volume values are rounded to one decimal place.

950

951

952

953

Table 2: Number of certain landslide deposits for each event and the confidence level of a seismic triggering mechanism. Confidence level of seismic triggering: moderate (1-2 landslide deposits), high (3-4 landslide deposits), very high (> 4 landslide deposits).

954

955

956

957

**Supplementary data**

958

959

Fs01: Uninterpreted pinger profile (W-E) in the proximal sub-basin of Eklutna Lake (Eklutna Proximal): line10.

960

961

962

Fs02: Uninterpreted pinger profile (SW-NE) in the distal sub-basin of Eklutna Lake (Eklutna Distal): line33.

963

964

965

Fs03: Uninterpreted pinger profile (SSW-NNE) in Skilak Lake: line03.

966

967

Fs04: Uninterpreted pinger profile (W-E) in Kenai Lake: line56.

968

969

Fs05: Uninterpreted pinger profile (S-N) in Kenai Lake: line69.

970

971
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>
A.

<figure><img src="image_5.png" /></figure>

C

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

B.

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /></figure>

slide scarp

N

1964 landslide deposit

SW

NE

25 m

5 m
A. Eklutna Lake (core EK12-01)

<figure><img src="image_16.png" /></figure>

B. Skilak Lake (core SK12-03)

<figure><img src="image_17.png" /></figure>
<figure><img src="image_18.png" /></figure>

EK12-05
<figure><img src="image_19.png" /></figure>

SW

NE

EK12-01

1964 Topographic Map
<figure><img src="image_20.png" /></figure>

SK12-01

1964 megaturbidite

SK12-01

<figure><img src="image_21.png" /></figure>

1964 megaturbidite
A. landslide deposits

<figure><img src="image_22.png" /></figure>

B. megaturbidites

<figure><img src="image_23.png" /></figure>
81° 20' 37.5' N 61° 22' 30.7' N 81° 20' 22.5' N

Event Ek-1/A: 1964 megathrust EQ

<figure><img src="image_24.png" /></figure>

149° 07' 30' W

149° 00' 00'

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>
Event Sk-1: 1964 megathrust EQ

<figure><img src="image_27.png" /><figcaption>Event Sk-2</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>Event Sk-3</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>Event Sk-4</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>Event Sk-5</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>Event Sk-6</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>Event Sk-7</figcaption></figure>

<figure><img src="image_33.png" /></figure>
Event Ke-1: 1964 megathrust EQ

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

Event Ke-2

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

Event Ke-3

<figure><img src="image_38.png" /></figure>

Event Ke-4

<figure><img src="image_39.png" /></figure>

Event Ke-5

<figure><img src="image_40.png" /></figure>

Event Ke-6

<figure><img src="image_41.png" /></figure>

Event Ke-7

<figure><img src="image_42.png" /></figure>
A. Number of landslide deposits

<figure><img src="image_43.png" /></figure>

B. Slope gradient

<figure><img src="image_44.png" /></figure>

C. Sedimentation rate

<figure><img src="image_45.png" /></figure>

EKLUTNA

0 14 0° 15° 0 cm yr⁻¹ 0.5 cm yr⁻¹

<figure><img src="image_46.png" /></figure>

DISTAL PROXIMAL

lower number higher number

<figure><img src="image_47.png" /></figure>

slope ~ 5°-15°

<figure><img src="image_48.png" /></figure>

lower sed rate higher sed rate

<figure><img src="image_49.png" /></figure>

SKILAK

1 7 0° 15° 0 cm yr⁻¹ 0.5 cm yr⁻¹

<figure><img src="image_50.png" /></figure>

no hemipelagic landslide deposits

<figure><img src="image_51.png" /></figure>

slopes > 20°

<figure><img src="image_52.png" /></figure>

<figure><img src="image_53.png" /></figure>

KENAI

1 7 0° 15° 0 cm yr⁻¹ 1 cm yr⁻¹
<table><thead><tr><th rowspan="2"></th><th rowspan="2"></th><th colspan="2">Hemipelagic landslide deposits</th><th colspan="2">Tributary landslide deposits</th><th rowspan="2">megatur bidites</th><th colspan="4">Total landslide deposits</th></tr><tr><th>number of landslide deposits</th><th>volume (m³)</th><th>number of landslide deposits</th><th>volume (m³)</th><th>volume (m³)</th><th>number of landslide deposits all (certain)</th><th>volume (m³)</th><th>volume per landslide (m³)</th><th>volume delta/ volume total</th></tr></thead><tbody><tr><td rowspan="7">Eklutna Distal</td><td>Ek-A</td><td>4</td><td>1.3 x 10<sup>6</sup></td><td>2</td><td>8.6 x 10<sup>5</sup></td><td>-</td><td>6 (6)</td><td>2.2 x 10<sup>6</sup></td><td>3.7 x 10<sup>5</sup></td><td>0.39</td></tr><tr><td>Ek-B</td><td>2</td><td>3.7 x 10<sup>4</sup></td><td>2</td><td>8.3 x 10<sup>4</sup></td><td>-</td><td>4 (4)</td><td>1.2 x 10<sup>5</sup></td><td>3.0 x 10<sup>4</sup></td><td>0.69</td></tr><tr><td>Ek-C</td><td>4</td><td>2.4 x 10<sup>5</sup></td><td>2</td><td>9.2 x 10<sup>4</sup></td><td>-</td><td>6 (4)</td><td>3.3 x 10<sup>5</sup></td><td>5.5 x 10<sup>4</sup></td><td>0.28</td></tr><tr><td>Ek-D</td><td>3</td><td>9.0 x 10<sup>4</sup></td><td>3</td><td>1.5 x 10<sup>5</sup></td><td>-</td><td>6 (5)</td><td>2.4 x 10<sup>5</sup></td><td>4.1 x 10<sup>4</sup></td><td>0.63</td></tr><tr><td>Ek-E</td><td>5</td><td>9.4 x 10<sup>4</sup></td><td>1</td><td>8.6 x 10<sup>4</sup></td><td>-</td><td>6 (5)</td><td>1.8 x 10<sup>5</sup></td><td>3.0 x 10<sup>4</sup></td><td>0.48</td></tr><tr><td>Ek-F</td><td>1</td><td>1.1 x 10<sup>5</sup></td><td>2</td><td>1.2 x 10<sup>5</sup></td><td>-</td><td>3 (3)</td><td>2.3 x 10<sup>5</sup></td><td>7.7 x 10<sup>4</sup></td><td>0.54</td></tr><tr><td>Ek-G</td><td>3</td><td>1.9 x 10<sup>5</sup></td><td>3</td><td>1.2 x 10<sup>5</sup></td><td>-</td><td>6 (4)</td><td>3.0 x 10<sup>5</sup></td><td>5.1 x 10<sup>4</sup></td><td>0.38</td></tr><tr><td>Ek-H</td><td>6</td><td>4.8 x 10<sup>5</sup></td><td>0</td><td>0.0</td><td>-</td><td>6 (4)</td><td>4.8 x 10<sup>5</sup></td><td>8.0 x 10<sup>4</sup></td><td>-</td></tr><tr><td rowspan="14">Eklutna Proximal</td><td>Ek-1</td><td>4</td><td>3.1 x 10<sup>6</sup></td><td>5</td><td>1.5 x 10<sup>6</sup></td><td>-</td><td>9 (9)</td><td>4.6 x 10<sup>6</sup></td><td>5.1 x 10<sup>5</sup></td><td>0.33</td></tr><tr><td>Ek-2</td><td>1</td><td>4.3 x 10<sup>4</sup></td><td>1</td><td>4.3 x 10<sup>4</sup></td><td>-</td><td>2 (2)</td><td>8.6 x 10<sup>4</sup></td><td>4.3 x 10<sup>4</sup></td><td>0.50</td></tr><tr><td>Ek-3</td><td>5</td><td>6.0 x 10<sup>5</sup></td><td>4</td><td>8.7 x 10<sup>5</sup></td><td>-</td><td>9 (5)</td><td>1.5 x 10<sup>6</sup></td><td>1.6 x 10<sup>5</sup></td><td>0.59</td></tr><tr><td>Ek-4</td><td>6</td><td>7.5 x 10<sup>5</sup></td><td>3</td><td>1.3 x 10<sup>5</sup></td><td>-</td><td>9 (6)</td><td>8.8 x 10<sup>5</sup></td><td>9.8 x 10<sup>4</sup></td><td>0.15</td></tr><tr><td>Ek-5</td><td>4</td><td>2.4 x 10<sup>5</sup></td><td>3</td><td>1.2 x 10<sup>5</sup></td><td>-</td><td>7 (5)</td><td>3.6 x 10<sup>5</sup></td><td>5.1 x 10<sup>4</sup></td><td>0.32</td></tr><tr><td>Ek-6</td><td>4</td><td>7.6 x 10<sup>5</sup></td><td>4</td><td>1.1 x 10<sup>6</sup></td><td>-</td><td>8 (8)</td><td>1.9 x 10<sup>6</sup></td><td>2.3 x 10<sup>5</sup></td><td>0.59</td></tr><tr><td>Ek-7</td><td>3</td><td>2.6 x 10<sup>5</sup></td><td>3</td><td>7.0 x 10<sup>5</sup></td><td>-</td><td>6 (6)</td><td>9.6 x 10<sup>5</sup></td><td>1.6 x 10<sup>5</sup></td><td>0.73</td></tr><tr><td>Ek-8</td><td>3</td><td>3.0 x 10<sup>5</sup></td><td>4</td><td>3.9 x 10<sup>5</sup></td><td>-</td><td>7 (6)</td><td>6.9 x 10<sup>5</sup></td><td>9.8 x 10<sup>4</sup></td><td>0.57</td></tr><tr><td>Ek-9</td><td>2</td><td>1.4 x 10<sup>5</sup></td><td>1</td><td>1.1 x 10<sup>4</sup></td><td>-</td><td>3 (3)</td><td>1.5 x 10<sup>5</sup></td><td>5.0 x 10<sup>4</sup></td><td>0.07</td></tr><tr><td>Ek-10</td><td>5</td><td>1.4 x 10<sup>6</sup></td><td>3</td><td>4.6 x 10<sup>5</sup></td><td>-</td><td>8 (8)</td><td>1.8 x 10<sup>6</sup></td><td>2.3 x 10<sup>5</sup></td><td>0.25</td></tr><tr><td>Ek-11</td><td>4</td><td>1.2 x 10<sup>6</sup></td><td>3</td><td>2.9 x 10<sup>5</sup></td><td>-</td><td>7 (7)</td><td>1.5 x 10<sup>6</sup></td><td>2.1 x 10<sup>5</sup></td><td>0.20</td></tr><tr><td>Ek-12</td><td>5</td><td>1.2 x 10<sup>5</sup></td><td>2</td><td>2.4 x 10<sup>4</sup></td><td>-</td><td>7 (2)</td><td>1.4 x 10<sup>5</sup></td><td>2.1 x 10<sup>4</sup></td><td>0.17</td></tr><tr><td>Ek-13</td><td>4</td><td>6.2 x 10<sup>5</sup></td><td>1</td><td>5.6 x 10<sup>4</sup></td><td>-</td><td>5 (4)</td><td>6.7 x 10<sup>5</sup></td><td>1.3 x 10<sup>5</sup></td><td>0.08</td></tr><tr><td>Ek-14</td><td>5</td><td>7.6 x 10<sup>5</sup></td><td>1</td><td>2.1 x 10<sup>4</sup></td><td>-</td><td>6 (5)</td><td>7.8 x 10<sup>5</sup></td><td>1.3 x 10<sup>5</sup></td><td>0.03</td></tr><tr><td rowspan="7">Skilak</td><td>Sk-1</td><td>21</td><td>8.4 x 10<sup>7</sup></td><td>2</td><td>1.5 x 10<sup>7</sup></td><td>2.7 x 10<sup>6</sup></td><td>23 (16)</td><td>9.9 x 10<sup>7</sup></td><td>4.3 x 10<sup>6</sup></td><td>0.15</td></tr><tr><td>Sk-2</td><td>19</td><td>1.4 x 10<sup>7</sup></td><td>1</td><td>2.3 x 10<sup>5</sup></td><td>3.8 x 10<sup>6</sup></td><td>20 (9)</td><td>1.4 x 10<sup>7</sup></td><td>7.2 x 10<sup>5</sup></td><td>0.02</td></tr><tr><td>Sk-3</td><td>13</td><td>1.2 x 10<sup>7</sup></td><td>1</td><td>2.5 x 10<sup>5</sup></td><td>-</td><td>14 (8)</td><td>1.2 x 10<sup>7</sup></td><td>8.8 x 10<sup>5</sup></td><td>0.02</td></tr><tr><td>Sk-4</td><td>14</td><td>3.3 x 10<sup>7</sup></td><td>1</td><td>1.2 x 10<sup>6</sup></td><td>4.8 x 10<sup>6</sup></td><td>15 (10)</td><td>3.4 x 10<sup>7</sup></td><td>2.3 x 10<sup>6</sup></td><td>0.03</td></tr><tr><td>Sk-5</td><td>22</td><td>2.6 x 10<sup>7</sup></td><td>1</td><td>2.8 x 10<sup>5</sup></td><td>2.6 x 10<sup>6</sup></td><td>23 (15)</td><td>2.6 x 10<sup>7</sup></td><td>1.2 x 10<sup>6</sup></td><td>0.01</td></tr><tr><td>Sk-6</td><td>13</td><td>6.6 x 10<sup>6</sup></td><td>1</td><td>2.2 x 10<sup>6</sup></td><td>-</td><td>14 (9)</td><td>8.8 x 10<sup>6</sup></td><td>6.3 x 10<sup>5</sup></td><td>0.25</td></tr><tr><td>Sk-7</td><td>17</td><td>6.4 x 10<sup>7</sup></td><td>1</td><td>7.7 x 10<sup>6</sup></td><td>2.2 x 10<sup>6</sup></td><td>18 (14)</td><td>7.2 x 10<sup>7</sup></td><td>4.0 x 10<sup>6</sup></td><td>0.11</td></tr><tr><td rowspan="7">Kenai</td><td>Ke-1</td><td>1</td><td>3.9 x 10<sup>6</sup></td><td>22</td><td>8.1 x 10<sup>7</sup></td><td>2.1 x 10<sup>6</sup></td><td>23 (19)</td><td>8.5 x 10<sup>7</sup></td><td>3.7 x 10<sup>6</sup></td><td>0.95</td></tr><tr><td>Ke-2</td><td>1</td><td>6.0 x 10<sup>5</sup></td><td>2</td><td>2.1 x 10<sup>5</sup></td><td>9.3 x 10<sup>5</sup></td><td>3 (2)</td><td>8.1 x 10<sup>5</sup></td><td>2.7 x 10<sup>5</sup></td><td>0.26</td></tr><tr><td>Ke-3</td><td>2</td><td>1.2 x 10<sup>6</sup></td><td>6</td><td>2.3 x 10<sup>6</sup></td><td>4.8 x 10<sup>5</sup></td><td>8 (8)</td><td>3.5 x 10<sup>6</sup></td><td>4.3 x 10<sup>5</sup></td><td>0.66</td></tr><tr><td>Ke-4</td><td>0</td><td>0.0</td><td>4</td><td>8.8 x 10<sup>5</sup></td><td>-</td><td>4 (1)</td><td>8.8 x 10<sup>5</sup></td><td>2.2 x 10<sup>5</sup></td><td>1.00</td></tr><tr><td>Ke-5</td><td>1</td><td>6.2 x 10<sup>5</sup></td><td>7</td><td>7.8 x 10<sup>6</sup></td><td>1.8 x 10<sup>6</sup></td><td>8 (7)</td><td>8.5 x 10<sup>6</sup></td><td>1.1 x 10<sup>6</sup></td><td>0.93</td></tr><tr><td>Ke-6</td><td>1</td><td>1.4 x 10<sup>5</sup></td><td>5</td><td>2.5 x 10<sup>6</sup></td><td>-</td><td>6 (4)</td><td>2.7 x 10<sup>6</sup></td><td>4.4 x 10<sup>5</sup></td><td>0.95</td></tr><tr><td>Ke-7</td><td>2</td><td>1.7 x 10<sup>6</sup></td><td>7</td><td>7.7 x 10<sup>6</sup></td><td>3.3 x 10<sup>6</sup></td><td>9 (9)</td><td>9.4 x 10<sup>6</sup></td><td>1.0 x 10<sup>6</sup></td><td>0.82</td></tr></tbody></table>
<table><thead><tr><th>Event</th><th>Number of certain landslide deposits</th><th>Historical earthquake</th><th>Confidence level of seismic triggering</th></tr></thead><tbody><tr><td>Ek-A</td><td>6</td><td rowspan="7">AD 1964</td><td>very high</td></tr><tr><td>Ek-B</td><td>4</td><td>high</td></tr><tr><td>Ek-C</td><td>4</td><td>high</td></tr><tr><td>Ek-D</td><td>5</td><td>Very high</td></tr><tr><td>Ek-E</td><td>5</td><td>Very high</td></tr><tr><td>Ek-F</td><td>3</td><td>high</td></tr><tr><td>Ek-G</td><td>4</td><td>high</td></tr><tr><td>Ek-H</td><td>4</td><td>high</td></tr><tr><td>Ek-1</td><td>9</td><td rowspan="14">AD 1964</td><td>very high</td></tr><tr><td>Ek-2</td><td>2</td><td>moderate</td></tr><tr><td>Ek-3</td><td>5</td><td>very high</td></tr><tr><td>Ek-4</td><td>6</td><td>very high</td></tr><tr><td>Ek-5</td><td>5</td><td>very high</td></tr><tr><td>Ek-6</td><td>8</td><td>very high</td></tr><tr><td>Ek-7</td><td>6</td><td>very high</td></tr><tr><td>Ek-8</td><td>6</td><td>very high</td></tr><tr><td>Ek-9</td><td>3</td><td>high</td></tr><tr><td>Ek-10</td><td>8</td><td>very high</td></tr><tr><td>Ek-11</td><td>7</td><td>very high</td></tr><tr><td>Ek-12</td><td>2</td><td>moderate</td></tr><tr><td>Ek-13</td><td>4</td><td>high</td></tr><tr><td>Ek-14</td><td>5</td><td>very high</td></tr><tr><td>SK-1</td><td>16</td><td rowspan="6">AD 1964</td><td>very high</td></tr><tr><td>SK-2</td><td>9</td><td>very high</td></tr><tr><td>SK-3</td><td>8</td><td>very high</td></tr><tr><td>SK-4</td><td>10</td><td>very high</td></tr><tr><td>SK-5</td><td>15</td><td>very high</td></tr><tr><td>SK-6</td><td>9</td><td>very high</td></tr><tr><td>SK-7</td><td>14</td><td>very high</td></tr><tr><td>Ke-1</td><td>19</td><td rowspan="7">AD 1964</td><td>very high</td></tr><tr><td>Ke-2</td><td>2</td><td>moderate</td></tr><tr><td>Ke-3</td><td>8</td><td>very high</td></tr><tr><td>Ke-4</td><td>1</td><td>moderate</td></tr><tr><td>Ke-5</td><td>7</td><td>very high</td></tr><tr><td>Ke-6</td><td>4</td><td>high</td></tr><tr><td>Ke-7</td><td>9</td><td>very high</td></tr></tbody></table>