Research Highlight: Science on a Slippery Slope

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.”

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.

Three new technologies for monitoring seafloor hazards are being developed at Scripps to examine the Gaviota slope. Each has its own specific capabilities and research strategy based on the instrument’s spatial coverage and data-collection frequency so that the scientists can observe both sudden and long-term geological changes.

The first is a torpedo-shaped autonomous underwater vehicle (AUV) about 3.5 meters (12 feet) long and manufactured by Bluefin Robotics in Boston. Under the direction of geophysicist Gerald D’Spain, the AUV was outfitted with a state-of-the-art navigation system, an onboard pressure sensor, and a multibeam acoustic imaging sonar with a resolution of decimeters (several inches) when flying just above the seafloor.

The AUV is launched from a ship and then follows a preprogrammed underwater flight plan, making multiple passes over areas of interest to survey and map features. It worked well in initial tests conducted off San Diego, and has since been back to the engineering lab for further development. It will be deployed at the Gaviota slide several times over the next few years in a series of repeated surveys D’Spain describes as “mowing the same patch,” in an attempt to map any changes to the seafloor topography over periods of months and years. BP supplied funds for acquisition and integration of the navigation and sonar systems while the Office of Naval Research funded the vehicle and several of the onboard systems.

The second system is an array of high-precision acoustic transmitters and receivers that form a geodetic monitoring network similar to satellite-based measurement systems such as GPS used on land to detect surface deformation and earthquakes. The system was first developed in the 1980s by Scripps marine geophysicist Fred Spiess to study the movement of tectonic plates in the deep sea over months and years. It takes advantage of the fact that acoustic signals move through seawater quickly and with a consistency that allow their travel times to be used for precise positioning. The instruments are placed on the seafloor to straddle a feature of interest and by correlating signal transmission and reception times, the network can detect changes resulting from movements of the seafloor in distances of as little as 5 millimeters (0.2 inches) resulting from movements of the seafloor.

Scripps geophysicist David Chadwell has modified and adapted the acoustic network to work in a tighter configuration and to record on a daily basis for the Santa Barbara study. The system was set out at Gaviota last November from Scripps R/V Roger Revelle and continues to record its data, which will be recovered by ship later this year. If an earthquake occurs in the area, Chadwell plans to retrieve the data sooner.

Development of the third technology involves converting a standard electronic surveying tool, called an electronic distance meter, to work in the marine environment, which is proving to be a bit more difficult than anticipated. On land the meters emit light, usually infrared, to a distant reflector and measure the light’s return time to calculate distances several kilometers away with an accuracy of 1 to 2 millimeters (0.04-0.08 inches). Geophysicist Mark Zumberge had the idea that the light could be measured similarly by transmitting a beam into an optical fiber cable stretched between two anchors placed several hundred meters apart on the seafloor. If the two anchors are displaced with respect to one another by a geological process then the length of the fibers changes. The instrument records the change. For protection, the optical fibers are encased in hermetically sealed stainless steel tubing.

Zumberge took a prototype fiber-optic strain sensor, or FOSS, on R/V Roger Revelle during the November cruise and set it across the Gaviota fault. But when researchers laid it out at a depth of 400 meters (1,315 feet), the cable snapped, because Zumberge said, “we simply couldn’t keep the tension from becoming too great.” It was a setback, but he and colleagues are seeking further funding to continue their portion of the program.

With dozens of potential underwater landslide slopes along the West Coast from Seattle to San Diego that could trigger tsunamis in highly populated areas, there are plenty of reasons to continue the research. And for BP, there are very practical potential applications when it comes to offshore facilities.

“Even though we don’t yet have extensive trial results, at BP we’re already reaping the benefits in terms of new ideas and inspirations,” said BP senior advisor Hugh Banon said. “Collaborations with academic institutions that are at the forefront of ocean sciences are a key element in supporting our business. And Scripps is certainly among the very best.”