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Earthquake rocks, boulders unlikely to roll

Posted by on Mar 18th, 2010 and filed under News. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry


According to the U.S. Geological Survey website a 4.4 earthquake hit the greater Los Angeles area at about 4 a.m. on Tuesday; Glendale Parks & Recreation reported that no rocks were dislodged in Deukmejian Park.

According to officials, large rocks exposed by rainfall erosion have little chance of dislodging in an earthquake or light rain.

“A lot of stuff in Dunsmore Creek has been exposed, a lot of bed rocks,” said Russ Hauck of City of Glendale’s Parks & Recreation Department. “If we had a significant rain that was large enough to move those boulders, it would be large enough to move the K-rails.”

The city official spoke hopefully of the future, citing that, “February is historically the rainiest month…but it’s possible that we’ll have more rain this spring. But I don’t think it will be any heavier than what we’ve seen already. Hopefully we’re out of the woods.”

Employees of the U.S. Geological Survey argue that a flood or earthquake strong enough to carry heavy rocks and boulders would require a perfect storm of variables to be in play.

“There are a lot of different forces acting on steep hill flow. It takes the combination of different effects to get things moving,” said Research Geologist Susan Cannon of the U.S. Geological Survey.

But should there ever be a big enough rainstorm and earthquakes frequent enough to dislodge rocks into flood channels, “the rain, instead of creating a little flood, it can create a debris flow,” said Cannon. “Debris flows are much more destructive than floods.”

Recent rains in La Crescenta have resulted in debris flow that has inflicted property damage, though the magnitude of such flows has not been big enough to carry boulders yet.

“Since the Station Fire, we have many unstable rocks, several boulders the size of cars and a significant threat of slides,” said Steve Goldsworthy, vice president of the Crescenta Valley Town Council.

Soil erosion caused by decreased vegetation, fires and rain leave rocks exposed. That is why after the Station Fire had burned and we were preparing for the rains we had a number of scientists and geologists come in, said Hauck.

Those researchers prompted the installation of K-rails in the local area for the rainy season, which were put in place to divert water and debris flow.

“Our two primary concerns were the barn and the residences on Markridge [Avenue]. We put in a number of measures,” said Hauck. “We were hoping for the best and preparing for the worst. And it appears, at this point, we have managed to read the land properly”

Others say that although rocks dislodged by an earthquake are a great threat, residents in greater Los Angeles are not likely to see rocks dislodge due to seismic activity.

“There [are] only a handful of homes in any given place that are situated in a place where a boulder can be airborne and hit their home. That’s pretty rare,” said Randy Jibson, a Research Geologist of the U.S Geological Survey.

Rocks that are airborne can be far more devastating than rocks that roll down a steep slope. In order for an airborne rock or boulder to hit a home a house would have to be situated right below a cliff side, said Jibson.

However, if a rock is airborne, all it takes is a stone the size of a basketball to tear through a house said Jibson. “It’s all about momentum and momentum is all about mass times velocity.”

The likelihood that a rock will tumble toward a house as opposed to going airborne is locally more probable.

“There are many places where dislodged boulders can be rolling toward a house,” said Jibson. “I’ll be honest, in L.A. there are not many houses that are going to be hit by an airborne rock.”

Jibson advised concerned residents to “investigate the areas above the house and if there are loose boulders above your house you would have to hire a competent professional to assess the situation.”

Some commented on La Crescenta and the surrounding areas that are prone to landslides.

“The San Gabriel Mountains are some of the fastest growing and fastest eroding mountains in the country,” said Mike Lawler of the Crescenta Valley Historical Society. “The entire Crescenta Valley is formed from stuff that has fallen from the mountain.”

Earthquakes in the past led to the devastation of several brick buildings in the community, said Lawler, but added, “Our main threat right now is debris flow…It overshadows the earthquake right now.”

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