Scripps geophysicists develop technologies for monitoring underwater landslides.

By Chuck Colgan

When earthquakes strike along the coastline, it isn't just the shaking that can wreak havoc, but a one-two punch of tremors setting off undersea landslides that may produce the most devastation. That was the case in 1998 when a magnitude 7.1 quake off Papua New Guinea triggered an underwater slope collapse that pushed a 15-meter (50-foot) tsunami over the shoreline, destroying seven villages and killing more than 2,000 people.

Sites with the potential for underwater landslides exist all round the Pacific's Ring of Fire, where 90 percent of the world's earthquakes occur, but predicting where submarine slopes may slip is limited by a lack of monitoring capabilities. That's why several Scripps Institution of Oceanography geophysicists have come together to develop strategies and technologies for observing coastal sites where landslides are likely to happen. They've been joined in this effort by the energy producer BP America Inc. that put up $3 million for the research over the past three years. The project's goal has been to better understand seabed dynamics with an eye on improving design and safety of offshore facilities, from drilling platforms to pipelines.

"The partnership is part of a strategy at Scripps to work closely with the private sector in areas where there are mutual interests," said John Orcutt, a Scripps geophysicist and director of UC San Diego's Center for Earth Observations and Applications. "The seafloor technology being developed through this partnership with BP will be invaluable in future long-term state and federal observing systems in the oceans."

Scientists launch a high-resolution sonar instrument used to map underwater geological features.

The Scripps scientists in the project are advancing a wide variety of geological survey techniques, including electromagnetics, fiber optics, acoustics, ocean bottom seismographs, and autonomous underwater vehicles. Their objective is to be able to detect and measure very small seabed deformations with an arsenal of instruments to determine if specific geological features may be precursors to large-scale slope failures.

The initial studies focus on steep underwater slopes that dip down from the shoreline off Santa Barbara, Calif., where there is evidence of landslides at different times in the past. Along the northern flank of the Santa Barbara basin off Goleta there is a monstrous scar from a huge landslide of some 1.5 cubic kilometers (.35 cubic miles) of sediments that most likely collapsed some 2,500 years ago.

"The area has all of the ingredients that lead to slope failure–active cracks, a high level of earthquake activity, increased water pore pressure in the sediments, and tectonic deformation," said Neal Driscoll, a Scripps geologist. "We don't really know how these processes interact as we are rarely at such a site before events occur. We'll gain tremendous insight by placing instruments in this area and watching what happens."

The first step in the monitoring program was to conduct an extensive survey of the region to produce detailed seafloor maps. In 2004, Driscoll and his colleagues towed a high-resolution sonar instrument that he designed from behind Scripps research vessel Robert Gordon Sproul to acquire more than 1,000 kilometers (620 miles) of underway data. They also collected sediment cores to measure pore water pressure and for chemical analysis.

Fortunately for the scientists, yet potentially unfortunate for Santa Barbara residents, a small seafloor crack about 50 meters (165 feet) wide and 2 kilometers (1.25 miles) long exists on a steep slope in an area called the Gaviota slide where about 20 million cubic meters (26 million cubic yards) of material slid as a result of two offshore earthquakes in excess of 7 magnitude offshore Santa Barbara on Dec. 21, 1812. The scientists chose this slope as their study site in hope of answering the question of whether the crack is widening from slow creeping of the seafloor or if the material beneath it will move in a single slip, creating the threat of a tsunami. It is unclear from historical writings whether the 1812 earthquake and underwater landslide created a tsunami. While some maritime reports describe large waves that forced ships to sea, today scientists estimate at most a wave of about 1 meter (3.3 feet) would have been generated.

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