Flank instability assessment in Dominica: Lesser Antilles Arc

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Rust, Derek; Teeuw, Richard; Solana, Carmen; Dewdney, Chris
Holding Date 03 September 2008

Dominica is a small (~750 km2) Caribbean island with nine volcanic peaks exceeding 1000 m, one of the highest concentration of active volcanoes in the world. Preliminary fieldwork and analyses of remote sensing, bathymetric and seismic data suggests that active coast-bounding dominantly dip-slip faulting is well developed, particularly along the very linear northern coast of the island. Here the faulting appears to truncate the northern slope of the Morne aux Diables edifice, producing well-developed breaks in the 100 m isobath in an area coinciding with epicentres of a late 2004 earthquake sequence that included a damaging M6.2 event. The northern volcanic flank uplifted along this fault, although largely unaffected by the earthquake, displays abundant evidence of instability in the recent geological past, including backtilted and rotated blocks at a range of scales and distinctive arcuate margins of slide masses that extend seawards from the shoreline. It may be that most instability occurs during the relatively short late-summer hurricane season when pore pressures are elevated, particularly when ground shaking events coincide. Despite the dense jungle cover it is possible to identify a number of potential sites for future instability, as well as to estimate the volume of material involved and the possible debuttressing effects on stability farther upslope. This analysis suggests that future instability could generate tsunami, including farther travelling tsunami of 2-4 m that would reach the low-lying and densely populated southern coast of Guadalupe, some 30-40 km away, in about 20 minutes. Such a scenario appears consistent with a recent review of tsunami activity in the Lesser Antilles, particularly Guadalupe. Twenty four tsunami are documented over the past 400 years, only 3 of which are attributed to a source beyond the Caribbean. Full evaluation of the instability potential of the fault-bounded northern flank of Dominica involves planned offshore bathymetric surveys employing portable and relatively low-cost echo-sounding equipment that can be mounted on a small fishing vessel and interfaced with specialised software to produce 3D graphical representation of the sea bed.