Finding, steep topography, collapse and slope failure caused by Sumatra Earthquake: Sea floor survey by Research Vessel gNATSUSHIMAh

Research Vessel gNATSUSHIMAh (1739 tons), which belongs to Japan Agency for Marine-Earth Science and Technology (president; Yasuhiro Kato), carried out the seafloor survey utilizing unmanned submersible gHYPER-DOLPHINh in and around the epicenter of Sumatra Earthquake. Steep relief of the seafloor, and evidences of collapse and slope failure caused by Sumatra Earthquake have been found by our seafloor observation.

1. A submarine active fault related to Sumatra Earthquake has not been found so far.

2. Following seafloor observations near the epicenter suggest taking place of collapse and slope failure, and traveling of collapsed sediment to deep water.

(1) Turbid water at downstream in a submarine canyon was found.The acoustic investigation detected a possible ragged relief.

(2) No turbid water at upstream in the submarine canyon was found (Photos1-1, 1-2). A number of collapsed relief, especially angular edged surface at where rupture took place was found (Photo 2). Abundant open fissures in the terrace were found. Fissures were not filled with sand, which could be transported by ripple current (Photo 3).

(3) The steep slopes at up and middle streams of canyon were covered with falling rocks and displaced sediments. No trace of organisms, and benthic creatures such as shrimp were found in the bottom surface.

The mentioned topographic details were observed throughout the survey area. Hence it follows that slope failures were cased widely, and a large mass of rock and sediment were transported to deep water in the seaward slope of outer ridge. This phenomenon must have changed drastically the state of sea surface.