Effect of Seismicity and Tectonic‐Glacial Interactions on Submarine Megaslides

Enhanced sedimentation at glacial margins can produce submarine megaslides (>10,000 km3). We report a single megaslide in the Surveyor Fan, Gulf of Alaska. Minimum extant size is ∼16,124 km2 in area and ∼9,080 km3 in volume. Slope failure occurred ∼1.2 Ma at the onset of the mid‐Pleistocene transition (MPT). With accretion along the Aleutian‐Alaska Trench, the original volume is conservatively ∼16,280 km3, with only a 140 km run‐out due to its blocky, high shear strength nature. We suggest the megaslide was triggered by a major sediment influx at the onset of the MPT, when glacial‐interglacial cycles shifted from 41 to 100 Kyr. The absence of repeat megaslides may reflect a changing balance between seismic strengthening and sediment flux, where later sedimentation driven by cross‐shelf ice streams results in thin, fluidized, non‐cohesive slides. Continued accretion of the Surveyor Fan and megaslide also reduces critical wedge taper, further inhibiting major failure.


Introduction
Mass Transport Deposits (MTDs) are ubiquitous on continental margins, the result of submarine sediment failure and redeposition (Masson et al., 2006).Many factors can individually, or together, contribute to slope failure, including earthquakes, which are one of the most effective processes triggering landslides (Masson et al., 2006;Mountjoy et al., 2018;Stigall & Dugan, 2010).
High accumulation rates, especially on glacial margins, appear to be the dominating pre-disposing factor in the occurrence of major submarine slope failure (Elverhøi et al., 2002;Lee, 2009;Sawyer et al., 2017;ten Brink et al., 2016).A majority of submarine slope failures have been shown to occur during or following glacial periods, due to the drastically increased sediment flux to the shelf edges during glacial maxima (Elverhøi et al., 2002;Hjelstuen et al., 2007;Lee, 2009;ten Brink et al., 2016).High accumulation rates, particularly in marine systems dominated by glacial clays (Elverhøi et al., 2002), can lead to subsurface overpressure and thereby slope instability (Dugan & Flemings, 2000;Lee et al., 2007;Stigall & Dugan, 2010).Several global studies reveal limited major slope failure on active margins compared to passive margins (Nelson et al., 2011;Sawyer & DeVore, 2015;Strozyk et al., 2010).
The southern Alaska-Aleutian margin includes a portion of the convergent North America-Pacific Plate boundary and subduction of the Yakutat microplate, an allochthonous oceanic plateau (Christeson et al., 2010), beneath North America (S.Gulick et al., 2007; S. P. S. Gulick et al., 2013;Pavlis et al., 2004;Worthington et al., 2012).Subduction and collision of the Yakutat microplate with North America has created the St. Elias Mountains, the highest coastal relief and most glaciated active orogen on Earth (Pavlis et al., 2004).Since the Miocene, glacial erosion on this active margin has contributed to the construction of the Surveyor Fan, the principal deepwater sediment body in the Gulf of Alaska (Reece et al., 2011).Major earthquakes occur frequently on the southern Alaskan-Aleutian margin, causing extensive surface deformation (e.g., Shennan et al., 2009).
The intensification of Northern Hemisphere glaciations (iNHGs) occurred at the Plio-Pleistocene transition (PPT, ∼2.8-2.6 Ma) (Lisecki & Raymo, 2005).The glacial-interglacial cycle since the mid-Pleistocene transition (MPT, 1.2-0.7 Ma) has evolved into 100 Kyr pacing.Prior to the MPT, the cycle was lower amplitude and more frequent at a 41 Kyr pacing (Clark et al., 2006;Lisecki & Raymo, 2005).In the Gulf of Alaska, tectonics and glacial climate are strongly coupled and recorded within the deep-sea Surveyor Fan, which consists of three sequences.Sequence I pre-dates the PPT.Sequence II sediments were deposited during the Early Pleistocene (2.8-1.2Ma) following the iNHG, and Sequence III sediments recorded mid-to Late Pleistocene (<1.2 Ma) deposition starting with the MPT (Jaeger et al., 2014).S. P. S. Gulick et al. (2015) showed that the sediment volume eroded from the St. Elias orogen to the Surveyor Fan increased by a factor of 2.75 since the MPT (Sequence III).
The Alaska-Aleutian subduction zone accretes Surveyor Fan sediments from Kodiak Island to Kayak Island (Figure 1, inset).Pre-stack depth migrated images along six crossings show the décollement that structurally

Geophysical Research Letters
10.1029/2024GL108374 divides the accreting from subducting sediments to be coincident with the Sequence I/II boundary (Frederik et al., 2020), which marks the onset of the iNHGs based on first occurrence of ice-rafted debris at Site U1417 (Jaeger et al., 2014).
The southern Alaska-Aleutian margin is prone to submarine landslides and tsunami hazards due to the seismically active plate boundary and extreme sedimentation rates from glacially enhanced mountain erosion.Contrary to other seismically active margins where seismic strengthening enhances slope stability (Sawyer & DeVore, 2015), the high-sedimentation margin offshore southern Alaska behaves like a passive margin from a shear strength perspective (Sawyer et al., 2017).The high sedimentation rates and fluid overpressure within the slope and Surveyor Fan potentially offset seismic strengthening that would otherwise occur.This is supported because shear strength follows an active margin profile outside of the fan, where slower background sedimentation rates occur.
Here we report the existence of a single, major submarine slope failure, the Surveyor MTD, on this tectonically active margin.We place the volume, geometry, timing, and setting along a tectonically active, glaciated margin in context with other known large MTDs globally (Figure 2, Table S1 in Supporting Information S1).Analysis of the Surveyor MTD provides insight into the relative influence of climate and tectonics on submarine slope stability by directly comparing the contributory influence of seismic activity versus high sediment accumulation rates.We further highlight negative feedbacks provided by critical wedge mechanics in a subduction setting, which potentially inhibit repetitive large submarine slides in selected margins.

Methods
This study utilizes 2-D seismic reflection data from multiple surveys in the Gulf of Alaska (Figure 1 S1 in Supporting Information S1 for sources).Surveyor MTD estimates from this study are shown both as extant and estimated assumed same width but additional run out accounting for 1.2 Ma of Pacific Plate subduction at 5 cm/yr (Doubrovine & Tarduno, 2008).
For conversion of time to depth and therefore thickness of the MTD, we applied a p-wave velocity of 3 km/s based on OBS velocity data (Christeson et al., 2010) acquired in nearby, equivalent depth Surveyor Fan sediments.Other studies use 2 km/s for slides at the seafloor (Mosher et al., 2010); however, the Surveyor MTD is buried 919 m below seafloor (mbsf) based on drilling data (Jaeger et al., 2014).We consider 3 km/s based on normal sediment velocities as a conservative estimate, as MTDs tend to be dewatered and more consolidated than fan sediments (Dugan, 2012;Sawyer et al., 2009), and thereby exhibit higher velocities than non-MTD sediment at similar depths.Area calculations were determined from the isopach map (interpolated between 2D seismic profiles) (Figure 1).Thicknesses determined at each 0.001 deg cell using 3 km/s velocity and summed over all locations within the isopach.For determining the minimum size of the MTD accreted to the margin, a Pacific Plate motion of 5 cm/yr and a conservative 0.5 s two-way-travel time thickness for the accreted portion were used.

Results
Deep water 2-D seismic reflection profiles (Figure 3) across the Aleutian Trench from Kayak Trough (Figure 1) reveal the typical layer cake stratigraphy of the Surveyor Fan (Reece et al., 2011), and the high-amplitude acoustic basement representing the top of the Pacific Plate.Between the Surveyor Fan strata and the underlying oceanic basement, there are two dominant units: one characterized by a chaotic seismic facies and another unit lacking internal reflectivity (Figure 3).The base and top of the chaotic unit exhibit high amplitude reflections, and within the chaotic facies there are discrete sections with highly deformed internal strata.These observations match wellestablished criteria for the identification of MTDs (Bull et al., 2009;Hampton et al., 1996).Therefore, we interpret this chaotic package to be an MTD and refer to it as the Surveyor MTD.
Within the boundaries of the Surveyor MTD, seismic profiles also exhibit sections of parallel, laterally continuous, and coherent reflectors.One such section exhibits curved parallel reflectors (Figure 3b), while reflectors in a second section are flat-lying (Figure 3c).The coherent reflectors are surrounded by the characteristic chaotic seismic signature of the MTD, and are within the high amplitude base and top reflectors of the MTD.We interpret the deformed sections with internal reflectivity as rafted blocks, consistent with similar seismic reflection studies of MTDs (Bull et al., 2009;Jackson, 2011).The blocks are likely intact stratigraphic sections mobilized from the source area that do not completely dissociate during failure or are entrained (ripped up) from the seafloor and carried along with the flow.The rafted blocks are up to 5-10 km wide, scattered throughout the deposit, and positioned as far as 50 km into the abyssal plain from the base of the modern slope.The curved reflectors of the first block (Figure 3b) could represent deformation associated with failure and transport.Perhaps this block was transported farther than a similar-size block with undeformed flat-lying reflectors (Figure 3c).Alternatively, the blocks may contain material with significant differences in shear strength or the curved strata may represent deformation during the accretion process prior to failure.
The rafted blocks appear to be vertically contiguous pieces, present in full vertical extent of the surrounding slide material, suggesting that the aggregate deposit thickness represents a single event rather than multiple layered events.An isopach map of MTD thickness does not appear to show multiple discrete lobes (Figure 1), but this is difficult to ascertain in the absence of denser data coverage.Based on available data, we suggest that the Surveyor MTD is a singular slope failure deposit.As such, the MTD is exceptionally thick and widespread, with a minimum area of ∼16,124 km 2 and a total volume of ∼9,080 km 3 (Figures 1 and 2).We are unable to determine the exact source location for the Surveyor MTD; this issue combined with the limitations of 2D seismic data coverage make it difficult to determine the exact 3D shape and position (Figure 1).Without insights on the source and flow direction, we are unable to determine the true slide runout distance, but instead measure the MTD length (Moscardelli & Wood, 2016) at ∼80 km from the base of the modern slope in the Aleutian Trench (Figure 2).Integrated Ocean Drilling Program Expedition 341 Site U1418 (Figure 1) drilled 7.79 m into the Surveyor MTD where it is labeled as lithologic unit IV (Jaeger et al., 2014).The drilling confirms the MTD interpretation, and dates the top boundary of the MTD, 919 mbsf (Jaeger et al., 2014), at ∼1.2 Ma through magnetic polarity stratigraphy (S.P. S. Gulick et al., 2015).This is a minimum age, as this was the first dateable horizon above the MTD.Due to the location of the Surveyor MTD atop the Pacific Plate and above the mapped décollement at the Aleutian subduction zone (Frederik et al., 2020), the MTD has been progressively accreted to the North American Plate.A plate tectonic reconstruction, restoring ∼1.2 myr of Pacific Plate subduction, with a rate of 5 cm/yr relative to North America (Doubrovine & Tarduno, 2008) shows the potential area of the original deposit at ∼25,724 km 2 , a volume of ∼16,280 km 3 , and a 140 km length (Figure 4).This is a conservative estimate and does not account for the likely thickening of the deposit toward the slope in the now accreted portion of the MTD.When compared to known large failures, this estimate places the Surveyor MTD among the largest known by volume, and comparable to many known MTDs in terms of runout distance (using length to estimate runout) (Figure 2).
The deposit ramps up laterally over adjacent strata in the east (Figure 3a).The ramped portion of the MTD is ∼300 ms (∼450 m) higher in the section than the base of the MTD to the southwest.We interpret this ramp-up as the edge of the deposit, where shear stress at the base of the slide became insufficient to scour the seafloor.This interpretation suggests that the bulk of the deposit scoured at least ∼450 m beneath the seafloor; therefore a large portion of the MTD is material incorporated from the scoured deposits of the Surveyor Fan.

Active Versus Passive Margins
Based on our estimate and comparison with past compilations (Moscardelli & Wood, 2016), the Surveyor MTD is the Fifth largest submarine slide by volume on record.No other MTDs imaged on seismic profiles (Figure 1 and Figure S1 in Supporting Information S1) approach the magnitude of the Surveyor MTD in the slope and deepwater regions of the Gulf of Alaska.This singular megaslide since the MPT stands in stark contrast to the correlative glacially influenced Barents Sea-Norwegian margin, home to at least 16 major submarine slides over the same time period, including the three Byørnøya Fan megaslides (Figure 2), which are among the largest known (Hjelstuen et al., 2007).In number of slides, the southern Alaska-Aleutian margin matches the global profile of limited slope failure on tectonically active margins, however sediment shear strength in the upper Surveyor Fan is more representative of a passive margin (Nelson et al., 2011;Sawyer & DeVore, 2015;Sawyer et al., 2017).

10.1029/2024GL108374
Additionally, the Surveyor MTD defies the established trend for volume versus runout distance.Generally, MTD runout increases with an increase in volume (Moscardelli & Wood, 2016), and most of the other large slides traveled significantly farther than Surveyor (Figure 2).The Surveyor MTD length is comparable to slides with volumes one to two orders of magnitude smaller and creates an atypical MTD geometry (Figure 2).Normal MTDs have a length to width ratio (L:W) greater than 2:1, but are often observed at 8:1 (Moscardelli & Wood, 2016).The original Surveyor MTD, prior to subduction, had a L:W close to 1:1, but could have been even slightly wider than long.
These unusual characteristics and blocky, thick style might imply a slide with a considerably higher shear strength.In laboratory experiments, strong pre-failure sediment leads to shorter runouts and a blocky deposit geometry (Sawyer et al., 2012).The Surveyor MTD includes kilometer-scale intact blocks, both internally deformed and undeformed, which may reflect relatively strong source sediments (Figure 2).Similar intact blocks have been observed within accretionary settings where a larger thickness of incoming material provides sufficient burial for lithification prior to accretion (e.g., S. P. S. Gulick et al., 2011).If correct, these blocks might represent early Pleistocene submarine fan material accreted to the margin prior to 1.2 Ma, then loaded by slope sediment flux prior to failure at ∼1.2 Ma.Alternatively, these could be older slope sections that consolidated/lithified within the slope cover of the accretionary wedge prior to failure.We note, however, that the thickness does not match slope cover thickness observations on accretionary margins which are typically <100 m (e.g., Bangs et al., 2009;S. P. S. Gulick et al., 2004).An exception is 100 s of m thickness present in rapidly deposited trough mouth fan settings (e.g., Montelli et al., 2017;Worthington et al., 2017), yet rapidly deposited glacigenic material may be less likely to fail in discrete kilometer-scale blocks (e.g., Elverhoi et al., 2002).
The exceptionally thick and blocky nature of the Surveyor MTD is in contrast to the style of failure expected from post-MPT high sedimentation rates and fluid overpressures which offset the potential strength gains from seismic strengthening (Sawyer et al., 2017).Prior to the MPT, however, sedimentation rates were lower (S.P. S. Gulick et al., 2015), thus the Surveyor MTD sediments may have developed enhanced shear strengths typical of active margin sediments with normal sedimentation rates (Sawyer & DeVore, 2015).When earthquake shaking is sufficiently strong to trigger submarine landslides, the post-failure behavior and mobility of the landslide will be strongly affected by the pre-failure strength properties.In particular, strong pre-failure sediments lead to blocky, thick, and short runout landslides similar to observed here, whereas those with limited or no seismic strengthening may more likely evolve into a liquefied debris flow.Laboratory experiments document this behavior (Sawyer et al., 2012), as do field studies of natural slides (Brothers et al., 2019;Greene et al., 2019;Lenz & Sawyer, 2022;Moore & Sawyer, 2016).

Sedimentary Flux, Glacial Intensification
To initiate failure in a seismically strengthened slope or one with intact accreted blocks, contributing factors must be of sufficient magnitude to overcome slope shear resistance.IODP Exp.341 drilling within the Surveyor Fan highlighted a major change in sedimentation rates recorded in the outer fan at Site U1417 (∼250 Km southwest of Site U1418), from ∼30 to 70 m/Myr during 5.2-2.8Ma to ∼90 m/Myr during 2.8-1.2Ma likely driven by intensified North Hemisphere glaciations with a 41 Kyr pacing.Sedimentation again increased to ∼140 m/Myr during 1.2-0.8Ma due to the onset of the MPT at 100 Kyr cycles (S.P. S. Gulick et al., 2015).Site U1418 overlying the Surveyor MTD records 600 m/Myr rates over the same time interval, with rates increasing after 0.8 Ma (Jaeger et al., 2014).The sedimentation at Site U1418 after 1.2 Ma was entirely glacigenic (Jaeger et al., 2014) with an inference that slope-to-fan glacigenic sediments are generated when ice streams reach the shelf edge which occurs only during glacial maxima.Thus, maximum slope sedimentation rates would be expected to be at the highest at these times, with the first such phase occurring around 1.2 Ma.Therefore, increased sediment flux at the start of the MPT is a potential driver for the Surveyor MTD to overcome seismic strengthening.
However, there is a conspicuous absence of large MTDs since the ∼1.2 Ma Surveyor MTD.One potential explanation is that negative feedbacks are possible due to critical wedge mechanics.Emplacement of the Surveyor MTD resulted in a large volume of material to be accreted at the toe of the accretionary wedge.If the décollement does not change dip then the outer wedge will be at a lower taper than prior to the slide.The inner wedge near the shelf edge, while initially steeper due to the slide, will then undergo significant sedimentation from the shelfcrossing ice streams.Thus a new competition from a critical wedge framework perspective occurs, in which the upper slope decreases in taper over time and internal deformation of the newly re-accreting portions of the Surveyor MTD increases the taper of the lower slope.The resulting feedback in an accretionary environment combined with continued earthquakes triggering small failures, could reduce the likelihood of a subsequent major slope failure such as the one that generated the Surveyor MTD.
An additional possible explanation for the lack of MTDs of similar magnitude is that since the MPT, shelfcrossing ice streams have delivered high rates (>10 m/Kyr documented; Montelli et al., 2017) of glacigenic sediment to the continental slopes in trough mouth fans (S.P. S. Gulick et al., 2015).While seismicity and associated earthquake forcings sufficient to trigger slope failure have not changed since the Surveyor MTD, the overall shear resistance of the slope sediments has dramatically decreased (Sawyer et al., 2017).Therefore, it is possible that the slope strata are unable to build up large sections before failing and instead fail in more frequent, thinner and more liquefied flows.
A similar competition between sedimentation rate and submarine slope failure frequency and style is observed farther south along the active strike-slip Queen Charlotte-Fairweather Fault margin.Geophysical and core data have revealed a prominent contrast regarding submarine slides and sediment physical properties along the QCF margin where distinct differences in sediment delivery systems occur along strike (Brothers et al., 2019;Greene et al., 2019).Sediments from the upper central Haida Gwaii slope have high densities, are sediment-starved, and have few slides or debris flows, suggesting that seismic strengthening is occurring here because sedimentation rate is low even though seismicity is high (Greene et al., 2019).In stark contrast, the northern sediment-dominated region contains broad depocenters of relatively weak (possibly overpressured based on fluid seeps) sediment where frequent and prominent headscarps mark submarine slides that ultimately transport material as relatively fluidized and non-cohesive styles of landslides (Greene et al., 2019).

Conclusions
We report on the existence of a megaslide contained within the Surveyor Fan in the Gulf of Alaska.The extant size has a minimum area of ∼16,124 km 2 and volume of ∼9,080 km 3 and occurred ∼1.2 Ma at the onset of the MPT and change in glacial extent and pacing.Taking into account tectonic accretion, the original volume of the slide is conservatively estimated at 16,280 km 3 , with only a 140 km runout due to its blocky, high shear strength nature.We suggest the pre-conditioning was enhanced sediment flux at the onset of the MPT likely then triggered Geophysical Research Letters 10.1029/2024GL108374 by an earthquake.The lack of subsequent failures of similar magnitude is hypothesized to reflect a change in the balance between seismic strengthening and sediment flux in which extremely high rates of slope sedimentation were driven by 100 Kyr glacial-interglacial cycles and direct delivery via shelf-crossing ice streams.Subsequent failures involve relatively thin, fluidized, non-cohesive styles of submarine slides.Further negative feedbacks may be linked to accelerated sediment flux to the slope combined with accretion of the Surveyor MTD reducing the critical wedge taper along the Aleutian-Alaska Trench, thus inhibiting oversteepening and major failure.

•
World's Fifth largest mapped megaslide documented beneath Surveyor Fan Gulf of Alaska • Timing of slope failure linked to onset of mid-Pleistocene glacial intensification • Absence of later failures due to changing balance of sediment flux/ seismic strengthening and negative feedbacks from critical wedge processes Supporting Information: Supporting Information may be found in the online version of this article.

Figure 1 .
Figure 1.(a) 3-D perspective bathymetric view of the southern Alaska margin showing the location of the Surveyor Mass Transport Deposit (MTD), line locations of seismic reflection profiles used in this study, and Integrated Ocean Drilling Program Site U1418.(b) An isopach map illustrating Surveyor MTD thickness and geometry.Contours are in 50 m intervals.Two-way travel time converted to thickness assuming 3,000 m/s sediment velocity.PWS, Prince William Sound; KT, Kayak Trough.
and Figure S1 in Supporting Information S1).The first survey was collected in conjunction with the NSF Continental Dynamics St. Elias Erosion/tectonics Project (STEEP) aboard R/V Marcus Langseth, in 2008.The remaining surveys are USGS surveys L378EG acquired in 1978 aboard the R/V Samuel Lee, and F689GA, acquired in 1989 aboard the M/V Farnella.Seismic acquisition and processing details for all surveys are summarized by Reece et al. (2013).

Figure 2 .
Figure 2. Comparison of volume versus lateral runout distance for major Mass Transport Deposits (MTDs) globally using compiled data (See TableS1in Supporting Information S1 for sources).Surveyor MTD estimates from this study are shown both as extant and estimated assumed same width but additional run out accounting for 1.2 Ma of Pacific Plate subduction at 5 cm/yr(Doubrovine & Tarduno, 2008).

Figure 3 .
Figure 3. Seismic reflection line 7 from 2008 St. Elias Erosion/tectonics Project (STEEP) survey showing the Surveyor Mass Transport Deposit (MTD).(a) A conservative interpretation for the MTD base is highlighted in yellow and used for volume estimates.1.2 Ma Surveyor Fan sequence boundary coinciding with top of MTD shown in green.(b, c) Enlarged images of MTD rafted blocks.See Figure 1 for line and drillsite location.Vertical axis is in two-way travel time.Vertical exaggeration is ∼10:1 at the seafloor.Additional Surveyor MTD seismic reflection images can be found in Supporting Information S1.

Figure 4 .
Figure 4. Map of the southern Alaska margin illustrating the present day and original (∼1.2 Ma) position of the Surveyor Mass Transport Deposit (MTD).The MTD has been accreting at the Aleutian Trench since its original deposition.