Trapped Water Could Be A Cause For Underwater Landslides, Tidal Waves

Dr. Peter B. Flemings, associate professor of geosciences and director of the research team, says, "Our analyses focused on the layered sediments and we found a potential for water trapped there under pressure to surge out and cause landslides or to seep out slowly. We have not calculated the probability of tidal waves. However, undersea landslides are known to cause tidal waves and we agree with recent reports from other researchers that there is potential for expulsive events in the continental slope along the East Coast. We offer a new, alternative explanation for the cause of expulsive events not only off New Jersey but around the world."

Flemings explains that the continental slope is a narrow region of steeply angled sea floor that connects the continental shelf, where the water is hundreds of feet deep, to the deep ocean floor where the water depths exceed many thousands of feet.

In their analyses, Flemings and graduate student Brandon Dugan showed that the slope off New Jersey may be only marginally stable as the result of the water trapped under high pressure in the layered sediments there. Even small shaking, from a mild earthquake, for example, could trigger release of the pressurized water and produce significant landslides. More importantly, the possibility exists that the water trapped under high pressure could trigger landslides independently, without an earthquake, and without warning.

Other researchers recently identified cracks in the continental slope off the Maryland, Virginia and North Carolina coast and cautioned that the rifts there could set off undersea landslides and subsequent tidal waves. In newspaper interviews, these researchers attributed the cracks to violent explosions of gas trapped under layers of sediment on the continental shelf. The Penn State researchers offer another possibility ñ water trapped under high pressure.

The researchers detailed their methods and results in a paper, "Overpressure and Fluid Flow in the New Jersey Continental Slope: Implications for Slope Failure and Cold Seeps," published July 14 in the journal Science. The authors are Brandon Dugan, a doctoral candidate in geosciences, and Flemings who is also director of both the Penn State GeoFluids Consortium and the Penn State Petroleum GeoSystems Initiative.

In their study, the researchers used a computer simulation they developed and the techniques and analyses commonly used to help the oil industry predict the location of zones where water is trapped under high pressure in undersea sediment layers. When crews conducting undersea oil drilling sink a well into one of these areas, the high pressure can cause "blow outs" that send water and sediment up to the sea floor or up to the drilling platform.

The researchers used data gathered by Flemings and other researchers during an expedition aboard the research ship D/V JOIDES Resolution in 1997. They were trying to study the history of sea level and climate change recorded in the layers of sediment in the continental shelf and slope. They bored holes in the shelf and slope and removed cores which showed the pattern of sedimentation caused by changing sea levels over millions of years.

Later, Flemings and Dugan used the sediment data and their computer simulation to estimate how the pressures evolved over the last million years. The simulation showed that water under high pressure in some of the lower layers of sediments could suddenly force its way out, laterally, through the slope face, creating undersea vents, cracks or landslides in the process. The same high-pressure zones that cause drilling problems for the oil industry could also unleash a landslide on the slope.

On the other hand, Dugan notes that the water trapped in the high pressure zones can also seep out slowly rather than exit forcefully. These seep fluids, he says, "may be rich in nutrients and provide energy for a variety of undersea life."

The Penn State researchers have not done calculations to predict when the high pressure zones could cause failures off New Jersey. Flemings says, "our contribution here is to recognize high fluid pressures in offshore New Jersey and present a quantitative model that describes how these fluid pressures could contribute to slope instability and fluid expulsion. We have not tried to predict the probability of a significant failure but recognize that further research is warranted."

Support for the Penn State study came from an Ocean Drilling Program grant, the National Science Foundation, and the Penn State GeoFluids Consortium, an association of companies in the oil industry. Dugan is supported by a Joint Oceanographic Institutions/U.S. Science Advisory Committee Ocean Drilling Fellowship.