Underwater Robots Used to Detect Underwater Hidden Faults

A recent discovery by geologists from the Monterey Bay Aquarium Research Institute (MBARI) brings some relief to coastal residents, preparing them for seismic threats.

By using the latest technology available for underwater imaging, MBARI geologist Charlie Paull along with his colleagues at the U.S. Geological Survey have successfully detected underwater faults that are hidden beneath the ocean waves and sand, and are difficult to observe and study.

The undetected underwater faults are difficult to evaluate and geologists struggle to list the risks associated with them, because of which they cannot include them in the seismic hazard assessment.

"People have known that a significant portion of plate motion is going on offshore, but because there have been no techniques to get at this, it's something that is totally left out of the hazard assessments," Paull said in a press statement.

For this study, the team focused on the coast of Southern California, in a region called the California Borderland. The ultra-high-resolution image of the seafloor was collected with the help of MBARI's autonomous underwater vehicles (AUVs). With the aid of these images, they measured the rate at which two crust pieces slide past each other, known as 'slip rate'. These are located within a region of the Borderland called the San Diego Trough Fault Zone that runs from the Mexican border northward toward Catalina Island. This was the first slip rate that has ever been recorded for an offshore fault within California Borderland.

By using data from MBARI's seafloor-mapping AUV, they calculated the slip rate of the fault and measured the distance the fault had shifted. For this study, they focused on the ancient and inactive underwater channel and noticed a shift of 18 meters.

With high-resolution sub-bottom profiles from MBARI's AUV, they calculated the time the shift took to form, and also measured the thickness of the layers. Sediment samples were collected using MBARI's remotely operated vehicle (ROV) Doc Ricketts. The trapped animals in the sediment samples showed that the sediment had been accumulating for about 12,000 years since the channel had stopped eroding. Having covered a distance of 18 meters and formed some 12,000 years back, the average slip rate was calculated as 1.5 millimeters per year.

"I think we are going to see a lot more of this done in the future," Paull concluded.

The findings are reported in the December 2012 Bulletin of the Seismological Society of America.