Landslides Seen on Indian Ocean Seafloor Near Earthquake

SOUTHAMPTON, UK, February 11, 2005 (ENS) - The Royal Navy’s survey ship HMS Scott has collected unique images of the Indian Ocean seabed near the epicenter of the earthquake and tsunami that devastated the region on December 26, 2004. It is the first time that the seafloor has been observed so soon after an earthquake of this magnitude, which measured 9.0 on the Richter scale.

The scientific survey, led by the Ministry of Defence, is a collaboration including the UK Hydrographic Office, the British Geological Survey and Southampton Oceanography Centre. The scientists were onsite to start the surveying work on January 26, and sent back the first images on Wednesday. They expect to remain in the region for several weeks.

The Ministry of Defence says that the ship's first civilian task is being carried out in order to further the understanding of earthquakes and assist prediction of such events in the future. "It will be of considerable benefit to the Asia region as a whole and potentially give a global perspective," the ministry said.

The survey is taking bathymetric measurements of water depths and doing a geological assessment of the Asian earthquake epicentre and extended fracture zone.

ocean floor

This image from the HMS Scott shows the boundary between two plates, with arrows illustrating the direction of movement of the Indian Plate underneath the Burman Plate. White dashed line marks boundary between the two plates. Cold colors (purple, blue) indicate deeper water, hot colors (yellow, red) indicate shallower water. (Image courtesy Southampton Oceanography Centre)
HMS Scott, a deep water hydrographic survey vessel, uses swath bathymetry, a technique that produces high quality maps of the deep seafloor, to identify features that may be due to the recent earthquake or previous tectonic events.

Measurements taken at the site show depth of water in the vicinity of the epicentre varies between 200 to 5,000 meters (656 feet to a little more than three miles) which is well within HMS Scott’s capability using high-resolution multi-beam sonar.

Dr. Tim Henstock and Dr. Lisa McNeill of the Southampton Oceanography Centre were invited by the Royal Navy to join the survey ship for the investigation of the earthquake zone.

Dr. McNeill said, "This is the first major offshore earthquake since modern investigative techniques such as swath bathymetry have been available. It will provide the base map to further geological understanding of the region and will form the basis for planning possible future science missions.

"This will be the first time scientists have ever surveyed the seafloor so soon after an underwater earthquake – before erosion and other processes have changed the geology of the seafloor," he said.

ocean floor

This image shows an area of landsliding on the edge of the Indian Plate. Both slide scars and displaced slide blocks are visible. (Image courtesy Southampton Oceanography Centre)
The earthquake occurred some 40 kilometers (60 miles) below the seafloor where the Indian Plate is colliding with the Burman Plate. This collision has resulted in the Indian Plate being pushed underneath the Burman Plate, and it was a sudden movement of these two plates that led to the earthquake and tsunami.

The first results have yielded far more than the scientists had hoped for. Coloured contour maps of the seabed clearly show the boundary between the deep, flat Indian Plate, and the heavily deformed edge of the Burman Plate (boundary marked on images with a white dashed line). The collision has forced up spectacular large thrust ridges up to 1,500 meters (4,921 feet) high.

These unstable blocks have collapsed in places, producing large landslides several kilometers across that have scarred the seafloor, the images show.

These initial results will form the basis for further study in the area, as part of an international research effort.

Dr. McNeill said, "As a geologist I am really grateful for this opportunity to collect data across the earthquake zone. From this we hope to understand better the geological processes which produced the earthquake and ultimately help to determine future earthquake and tsunami hazards so that everyone can be aware and prepared."

The survey falls under the definition of marine scientific research under United Nations Convention on Law of the Sea (UNCLOS), the Defence Ministry said.