Abstract Along continental margins with rapid sedimentation, overpressure may build up in porous and compressible sediments. Large-scale release of such overpressure has major implications for fluid migration and slope stability. Here, we study if the widespread crater-mound-shaped structures in the subsurface along the mid-Norwegian continental margin are caused by overpressure that accumulated within high-compressibility oozes sealed by low-permeability glacial muds. We interpret 56 000 km 2 of 3D and 150 000 km 2 of 2D-cubed seismic data in the Norwegian Sea, combining horizon picking, well ties and seismic geomorphological analyses of the crater-mound landforms. Along the mid-Norwegian margin, the base of the glacially influenced sediments abruptly deepens to form 28 craters with typical depths of c. 100 m, areal extents of up to 5130 km 2 and volumes of up to 820 km 3 . Mounds are observed in the vicinity of the craters at several stratigraphic levels above the craters. We present a new model for the formation of the craters and mounds where the mounds consist of remobilized oozes evacuated from the craters. In our model, repeated and overpressure-driven sediment failure is interpreted to cause the crater-mound structures, as opposed to erosive megaslides. Seismic geomorphological analyses suggest that ooze remobilization occurred as an abrupt energetic and extrusive process. The results also suggest that rapidly deposited, low-permeability and low-porosity glacial sediments seal overpressure that originated from fluids being expelled from the underlying high-permeability and high-compressibility biosiliceous oozes.

Abstract This paper presents the geohazard assessment for a proposed bridge across Bjørnafjorden in western Norway. The fjord is c. 5 km wide with a maximum depth of 550 m at the proposed bridge crossing. The main geohazards of concern are submarine slope instabilities. To identify locations of instability, their susceptibility to failure, and their potential runout distances, we performed the following analyses: (1) static and pseudo-static limit equilibrium analyses for the entire fjord crossing area; (2) 1D seismic slope stability sensitivity analyses for different slope angles and soil depths; (3) 2D static and pseudo-static finite element analyses for selected profiles; (4) back-analysis of a palaeolandslide; and (5) quasi-2D and quasi-3D landslide dynamic simulations calibrated using results from the back-analysis. The workflow progresses from simplified to more advanced analyses focusing on the most critical locations. The results show that the soils in many locations of the fjord are potentially unstable and could be the loci of landslides and debris flows. The evidence of numerous palaeosubmarine landslides identified on geophysical records reinforces this conclusion. However, the landslide triggers and timing are currently unknown. This paper demonstrates the need for comprehensive and multidisciplinary geohazard analyses for any infrastructure projects conducted in fjords.

Abstract Swath bathymetry data from the glacier-fed northern Svalbard margin reveal geomorphological details of a large submarine landslide, the Hinlope-Yermak Landslide. Multiple planar escarpments have several hundreds of meters of relief, with a maximum headwall height exceeding 1400 m at the mouth of the Hinlopen cross-shelf Trough. Within the slide-scar area, this landslide created a rugose seabed geomorphology, with little mass-transport deposition in the immediate vicinity of major escarpments. Beyond a pronounced constriction, occurrence of semitransparent acoustic units on seismic profiles indicates that mass-transport deposits are likely the accumulation of remolded and/or fluidized debris flows that are in places hundreds of meters thick. The surface expression of the masstransport deposits is hummocky with flow structures, arcuate pressure ridges, and rafted blocks. Smaller debris lobes close to landslide sidewalls are the result of secondary, marginal failures. At the outer rim of extensive mass-transport deposits, numerous rafted blocks rise from the semitransparent sediment unit, and tower hundreds of meters above the surrounding debris. Maximum remobilized volume from the slide-scar area, estimated from pre-landslide bathymetric reconstruction, is approximately 1350 km 3 . Headwall heights, the ratio of excavated volume and slide scar area, and the height of rafted blocks are large, compared to other landslides documented on siliciclastic margins. The position, thickness, and shape of the mass-transport deposits illustrate high mobility of sediments involved in submarine landsliding. Their dimensions require numerical modeling to understand landslide dynamics and potential to generate tsunamis. In simulations of sediment dynamics, large blocks are rafted by a loose debris flow, derived from disintegrating landslide material in the headwall area. The main failure process finishes after approximately 1 hour. The upper slide scar is probably not the source area for large rafted blocks. The Hinlopen-Yermak landslide most likely created a significant tsunami, considering that remobilized sediment volume, initial acceleration, maximum velocity, and possible retrogressive development govern landslide-generated tsunamis. Steep waves, implying dispersive and nonlinear effects, probably were more pronounced than for most other tsunamis induced by submarine landslides. These features exist because of the combination of high speed and substantial thickness of mass transport. Propagation and coastal impact of the tsunami is simulated by a weakly nonlinear and dispersive Boussinesq model. Close to the landslide area, simulations return sea-surface elevations exceeding 130 m, whereas sea-surface elevations along coasts of Svalbard and Greenland are on the order of tens of meters.

Abstract Four seeps and mud extrusion features at the lake floor were discovered in August 1999 in the gas hydrate area in Lake Baikal's South Basin. This paper describes these features in detail using side-scan sonar, detailed bathymetry, measurements of near-bottom water properties, selected seismic profiles and heat flow data calculated from the depth of the hydrate layer as well as obtained from in situ thermoprobe measurements. The interpretation of these data is integrated with published geochemical data from shallow cores. The seeps are identified as methane seeps and appear as mud cones (maximum 24 m high, 800 m in diameter) or low-relief craters (maximum 8 m deep, 500 m in diameter) at the lake floor. Mud cones (estimated to be approximately 50–100 ka old) appear to be older than the craters and have a different structural setting. Mud cones occur at the crest of rollover structures, in the footwall of a secondary normal fault, while the craters occur at fault splays. The seeps are found in an area of high heat flow where the base of the gas hydrate layer shallows rapidly towards the vent sites from about 400 m to about 160 m below the lake floor. At the site of the seep, a vertical seismic chimney disrupts the sedimentary stratification from the base of the hydrate layer to the lake floor. Integration of these results leads to the interpretation that focused destabilization of gas hydrate caused massive methane release and forced mud extrusion at the lake floor and that the gas seeps and mud diapirs in Lake Baikal do not have a deep origin. This is the first time that methane seeps and/or mud volcanoes associated with gas hydrate decomposition have been observed in a sub-lacustrine setting. The finding suggests that gas hydrate destabilization can create large pore fluid overpressures in the shallow subsurface (<500 m subsurface) and cause mud extrusion at the sediment surface.