Sliding on the Sea Floor
Goleta’s Offshore Landslide Probably Means Locally Generated Tsunami Will Come Someday
Monday, November 22, 2010
Two recent reports explore the possibility, or rather the probability, of another Goleta-born ocean floor landslide which could give rise to a tsunami.
Just outside Coal Oil Point is what’s known as the Goleta submarine landslide, sometimes called the Goleta landslide or simply the Goleta slide.
In 1981, using sound pulses to generate and analyze echoes, researchers first characterized the phenomenon as a steep-walled submarine canyon. At that point, interest focused on potential hazards in drilling for natural resources.
But since 1998, when a submarine landslide is suspected to have triggered the devastating Papua New Guinea tsunami, and then again in 2004, when an earthquake in the Indian Ocean likely generated a massive underwater landslide and consequent tsunami in Sumatra, interest has swelled in the subject of offshore slopes and locally generated large waves.
If an earthquake from a distant location were to trigger a propagating wave that would ultimately find its end at our beaches, our islands would probably dampen the effect. A locally generated event, however, is another story. Fault lines running along the Goleta slide itself, and a suspicious fissure that extends up coast to connect to the smaller Gaviota slide, could ring, sending a piece of steep slope moving down the greater than 40 degree incline to initiate quite a splash.
In fact, a similar scenario may have played out in December 1812, when several earthquakes rattled the area. Reports of sea waves and flooding indicated that rancheria-living locals retreated inland to avoid the water. Some think that a quake with a magnitude of approximately seven, within the Santa Barbara Channel, resulted in a sea level rise of six vertical feet.
In the last few years, the Monterey Bay Aquarium Research Institute, Scripps Institute of Oceanography, and others too numerous to name here, have used various techniques including remotely operated vehicles and sonar technology to provide detailed maps of the Goleta slide’s topographic slope, and the surrounding fissures.
The resulting images show a three-lobed disruption that looks as if a giant had scooped out part of the smooth sea floor. The area starts steeply swooping from a 300-foot depth to down below 1,800 feet. While a standard sea-floor slope is about 2 degrees, the top of the Goleta slide, sort of a cliff, starts at 40 to 45 degrees then transitions to 27 degrees, where it remains to the tip of what’s referred to as the toe. The complete complex, which does kind of look like footprints, is nine miles long and 6.5 miles wide. Drawn on a map of the Santa Barbara Channel, the grey blob just off of Coal Oil Point appears larger than San Miguel Island.
Most recently, researchers from the U.S. Geological Survey used various empirical data, such as data from previous geological dating studies, to estimate a probability of landslide occurrence. Their goal is to forecast these kinds of events. After crunching the numbers, one analysis suggested that there is a 30 to 40 percent probability of a local submarine landslide occurring within the next 60,000 years.
If that result doesn’t quell a sense of a looming disastrous sea-floor transformation, a few months ago, results from data that monitored sloped seafloor positions for minute changes—tiny, tiny, less than a half-an-inch changes—were reported. Assessing the crack that connects the large Goleta slide with the smaller Gaviota slide for a two-year period, the scientists conclude that “the slope is gravitationally stable absent large seismic triggers.”
Santa Barbara first became home to a seismograph following the large quake in 1925. Today the U.S. Geological Survey is looking for 35 SoCal residents to open their homes to digital seismographs. The Netquakes sensor, larger than a shoebox, uses the Internet to transmit information directly to the U.S. Geological Survey after an earthquake. There’s a list of preferred criteria for candidates, including residing near active faults or in densely populated areas or near major facilities—or maybe proximity to a submarine landslide.
So, until Goleta gives rise to another history-making submarine landslide, we can look forward to more sea floor-probing reports, to more equation-based forecasts, and to watching our Goleta-housed seismograms.