Slope failure repetition in active margin environments: Constraints from submarine landslides in the Hellenic fore arc, eastern Mediterranean

Abstract

[1] It has been shown that submarine landslides can occur less frequently at subduction zone fore arcs despite the general expectation of extensive slope failures from high neotectonic activity in active margin settings. The Hellenic subduction zone, Greece, represents an example where modern evidence for slope failure is scarce. Taking the deeper parts of the fore-arc basin into account, however, a sequence of massive landslide deposits is found at recurrence intervals of approximately 250 ± 70 ka. Given high seismicity in the fore-arc area, this rate of slope failure appears to be low. In order to improve our understanding on the relationship between low landslide recurrence rates and the frequency and settings of the required trigger mechanism, we here assess the mechanical behavior of the slope sediment cover during instability scenarios. Seismic profiles and geotechnical measurements from cores of midsize landslides found on the northeastern Cretan midslope are used to backanalyze slope destabilization in one-dimensional, infinite slope models for static conditions as well as for the case of seismic loading. Results reveal that today only critically steepened parts of the Cretan slope can fail from high loading stresses of peak ground acceleration (PGA) of 37%g, maybe up to ≥64%g. We further deduce that long-term tectonic processes are important preconditioning factors controlling an occasional development of critically inclined slope parts. Therefore, the impact of seismic triggers is strongly limited in time and space: For an initially stable slope, the critical seismic intensity to trigger failure is being reduced with increasing tectonically controlled steepening of the slope through time, until an earthquake is sufficient to trigger a large landslide. Before that, the slope rather gets more resistant, because smaller PGA may rather result in dynamic compaction (seismic strengthening). Our findings imply that low frequencies of landslides in the Hellenic fore arc are not in conflict with high seismicity in this region, because seismic loading is only sufficient to trigger major collapses of a generally shear-resistant "cohesive" slope (increasing with time due to seismic strengthening) if long-term tectonic movement provides a critical steepening. This may explain the relatively scarce occurrence of large submarine landslides in this and similar tectonically active environments.Citation: Strozyk, F., M. Strasser, A. Förster, A. Kopf, and K. Huhn (2010), Slope failure repetition in active margin environments: Constraints from submarine landslides in the Hellenic fore arc, eastern Mediterranean,

Seismicity and sedimentation rate effects on submarine slope stability

“…Direct evidence for the paucity of scars following the M 9.1 Sumatra (Indonesia) and the M 8.8 Maule (Chile) earthquakes reinforces this observation (Henstock et al, 2006;Völker et al, 2011). Drilling into the Hellenic forearc also encountered far fewer failure events than predicted by earthquake recurrence (Strozyk et al, 2010). The relative paucity of scars in those margins has been explained by lack of available sediments (Tappin et al, 2007), "dynamic compaction," or "seismic strengthening" by earthquakes (Lee et al, 2004;Strozyk et al, 2010).…”

“…To test the effect of intermargin morphotectonic differences on the correlations, we repeated the correlations in Figures 2A-2C, considering only margin areas and scar areas with gradients >3° (see Table DR2). A 3° gradient was used to differentiate between slopes that are likely to develop slope failures and those that would not (Strozyk et al, 2010), although slope failures occur also at lower gradients, especially in passive margins ( Table 1). Consideration of only the area steeper than 3° slightly improves the fit of the correlations, particularly for earthquake recurrence (Fig.…”

“…Drilling into the Hellenic forearc also encountered far fewer failure events than predicted by earthquake recurrence (Strozyk et al, 2010). The relative paucity of scars in those margins has been explained by lack of available sediments (Tappin et al, 2007), "dynamic compaction," or "seismic strengthening" by earthquakes (Lee et al, 2004;Strozyk et al, 2010).…”

Submarine Mass Movements and Their Consequences

“…Studies on the triggers of submarine slides are less well constrained; however, seismic acceleration due to earthquakes is often cited as a primary trigger (e.g., Biscontin and Pestana 2006;Kvalstad et al 2005a, b;Strasser et al 2006;Strozyk et al 2010). These studies define potential earthquake-related triggering mechanisms consistent with interpreted preconditioning (e.g., overpressure, weak layers), but also illustrate that more research is required to understand solid-fluid coupling and transmission of seismic energy through soft sediments during earthquakes.…”

Tsunamigenic potential of a Holocene submarine landslide along the North Anatolian Fault (North Aegean Sea, off Thasos Island): insights from numerical modeling

“…The slide evolved as a translational slide of well-bedded sediments, over a glide plane corresponding to a muddy This discrepancy between strong earthquakes and landslide frequencies has been highlighted in various tectono-sedimentary contexts (Völker et al, 2011;Strozyk et al, 2010;Pope et al, 2015Pope et al, , 2016. Various process may explain this discrepancy, 15 including the amount and the nature of sedimentary supply, local variations in physical properties of the sediments (Lafuerza et al, 2012) or the over-compaction of the sediments subsequent to the fluid release induced by the seismic wave shaking (Hampton et al, 1996;Strozyk et al, 2010). Some mass transport deposits, probably older than Holocene, can be guessed on sparker lines in the Sporadhes basin (Brooks and Ferentinos, 1980;Ferentinos et al, 1981) and on sub-bottom profiles crossing the Saros Gulf (McNeill et al, 2004).…”