Quantitative Seafloor Geomorphology and Offshore Geohazards

Abstract Seafloor geomorphology reflects the dominant geologic and tectonic processes on continental margins. Using the global, one-minute bathymetric grid (GEBCO), we use geostatistical techniques to quantify the scale of the dominant geomorphic features on several active and passive continental slopes to gain insight into the mechanisms that drive the erosive processes. Once the characteristic dimensional and spatial variability is determined, we can examine the regional physiography to surmise the processes that shape the bathymetry, and predict the occurrence of future mass-wasting events based on the past events that shaped the margin, be it sea-level lowstand canyon incision, large and (geologically) infrequent landslides, or frequent earthquake mass-wasting events. Introduction Our knowledge of offshore geomorphic processes increases as more and more high-quality data becomes available. For example, the 1929 Grand Banks (Newfoundland) passive margin earthquake generated a landslide which in turn produced fast-moving, erosive turbidity currents and a deadly tsunami [1,2]. While the 1998 Papua New Guinea earthquake was not sufficient in itself to generate a tsunami, the triggered landslide produced a wave that killed over 2,000 people [3]. Clearly, submarine landslides pose a significant threat to offshore installations such as cables, pipelines, rigs, etc., and also to coastal populations. Properly assessing the risk of this geohazard entails a comprehensive survey of the frequency and magnitude of past events. The goal of this study is to characterize the scale of dominant bathymetric features on active and passive continental margins at different latitudes. The erosive processes on these different margins (e.g. gullies, canyons, landslides, etc.) leave distinctive geomorphological signatures recognizable in bathymetry data [4,5]. The process that drives the erosion, be it earthquake-generated slope failures [6], climate-induced slope destabilization such as gas hydrate melting [7] or high latitude sediment loading at via glacial outwash [8], should occur with greater frequency in certain areas than others. We hope to build upon the studies of submarine landslide size distributions within given regions [4,9,10], by undertaking a systematic comparison of the scale of erosive features between tectonic regimes and latitude. Erosive processes on the seafloor should follow frequency-magnitude relationships related to driving mechanisms as they do on land (e.g. big and infrequent storms will cause lots of large landslides whereas small, frequent rainfall events will cause smaller landslides), therefore it stands to reason that the large submarine erosive features identified using this method (i.e. landslides) should be caused by infrequent events. Passive margins dominated by gullies and closely-spaced canyons generate semivariograms with relatively small sills and ranges. These erosive features suggest frequent (here, ‘frequent’ being anywhere from 1 per 100 years to 1 per 1000 years), small-scale erosive events, likely triggered by seasonal sedimentation events on the shelf, or perhaps earthquakes that trigger failure of sediment accumulated at the gullies' heads. On active margins, the erosive geomorphology largely depends on the nature of earthquakes (e.g. segmentation and ‘slow’ vs. ‘fast’ events). In contrast, margins with large and well-preserved landslides yield semivariograms with large sills and ranges, and are suggestive of a less frequent process, such as sea level change with interspersed intervals of relative quiescence.

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