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The mud carries debris, including rocks and boulders, as it gains speed.
The speed of the mud flow caught people off-guard. The power of nature can't be underestimated.
The U.S. Geological Survey says once a slide starts, the mud flowing down a hill can reach a velocity of 35-miles-per-hour.
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The chair of San Jose State's Department of Civil and Environmental Engineering described the force of the mud in a different way.
"If I threw a gallon jug of water at you," said Dr. Laura Sullivan-Green, "you're going to feel that impact pretty significantly because water's relatively heavy. Now imagine that by a factor of thousands hitting your home. This is a significant force."
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The mudslides started in the middle of the night, catching residents by surprise. However, Dr. Sullivan-Green said those living on hillsides or in canyons were vulnerable, even before the Thomas Fire.
"None of the soil near the surface that was affected by the fire and even the soil below that had any significant moisture after months of our dry season, so it's a double impact there," said Dr. Sullivan-Green.
Scientists say intense fires can also change the characteristics of the soil, giving it water repellent properties that promote runoff instead of saturation. There is also evidence to suggest that slide-prone areas tend to slide again in the future.
David Louie will have more on this story on ABC7 News at 4:00 and 6:00.