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A transect of seafloor heat probe measurements on the Hikurangi Margin shows a significant increase of thermal gradients upslope of the updip limit of gas hydrate stability at the seafloor. We interpret these anomalously high thermal gradients as evidence for a fluid pulse leading to advective heat flux, while endothermic cooling from gas hydrate dissociation depresses temperatures in the hydrate stability field. Previous studies predict a seamount on the subducting Pacific Plate to cause significant overpressure beneath our study area, which may be the source of the fluid pulse. Double-bottom simulating reflections are present in our study area and likely caused by uplift based on gas hydrate phase boundary considerations, although we cannot exclude a thermogenic origin. We suggest that uplift may be associated with the leading edge of the subducting seamount. Our results provide further evidence for the transient nature of fluid expulsion in subduction zones.

[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,

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