Liquefaction a real hazard in the Coachella Valley

Scientists say a major quake is long overdue along the southern stretch of the San Andreas fault which runs right through the Coachella Valley.

But many desert residents are unaware of liquefaction which can transform firm ground into a soupy mess during an earthquake, and the Coachella Valley is a high hazard zone for the phenomenon.

It happened during Japan’s 9.0 magnitude earthquake in 2011, during a 6.3 magnitude earthquake in coastal Christchurch, New Zealand the same year.

It also has occurred several times in California in 1857 during the Fort Tejon earthquake, in 1906 during the San Franciscoearthquake, in 1933 during the Long Beach earthquake, in 1971 during the San Fernando earthquake, in 1973 during the Point Mugu earthquake, in 1979 and 1981 during Imperial Valley earthquakes and in 1994 during the Northridge earthquake.

It also occurred in San Francisco during the 1989 Loma Prieta earthquake, a 6.9 seismic event that came without warning killing 63 people.

What many residents don’t realize however is liquefaction can also occur in the Coachella Valley where the sun almost always shines, there’s very little rain and the soil is almost always bone dry.

The answer lies just below the earth’s surface.

UC Riverside professor and seismologist David Oglesby said, “Unfortunately just because you’re living in a desert where the rain doesn’t fall very much doesn’t mean there’s a lot of water directly underneath the ground.”

Oglesby studies the San Andreas Fault as it travels through the Banning Pass to see if a major quake along the southern San Andreas might end it’s break there or travel further toward Los Angeles.

He and others also know the Coachella Valley lies atop a huge underground water reservoir which along with our fine sandy soil create ideal conditions for liquefaction.

Oglesby said that when strong shaking continues, “In a very short time you can go from perfectly solid soil to essentially mud.”

“That’s not to say the entire Coachella Valley would liquify,” Oglesby said, “but that just means there could be areas of liquefaction spread throughout the Coachella Valley.”

According to computer animations, the rupture from a major quake along the southern San Andreas fault at the Salton Sea could continue through the Coachella Valley and west toward Los Angeles.

Unlike the mountains around it, Oglesby said, the soft soils of the Coachella Valley would slosh around like water in a bathtub even after the main shockwaves passes. The longer the shaking lasts, the more the liquefaction hazard.

Riverside County Building and Safety’s Mike Lara said, “If you look at our maps typically the closer you get to the Salton Sea the higher the risk because the water table is higher.”

Lara saw liquefaction as he inspected San Francisco Marina District buildings after the Loma Prieta quake.

“You could see the sand actually,” Lara said, “what they called sand bubbles or sand boils that would come up or pop up under the sidewalks.”

Lara said liquefaction is most common in areas where there’s water within 50-feet of the surface.

It can damage pipelines, utilities, bridges and buildings with shallow foundations.

According to Riverside County maps, the entire Coachella Valley is at risk of liquefaction, much of it “high” to “very high” risk.

Buildings can crumble if their foundations crack because of liquid soil underneath, therefore liquefaction is a part of our local building code.

Some solutions, Lara said, are foundations with tightened steel rebar, harder concrete and highly compacted soil.

Those improvements will prevent foundations from breaking in a quake if the soil turns to liquid underneath.

“The intent of the building codes is to make sure a building stays intact during and after a seismic event as much as possible,” Lara said, “so the occupants can evacuate safely.”

But designing an “earthquake-safe” house doesn’t mean it will be habitable after a major quake.

State liquefaction hazard maps have been created for Los Angeles, Ventura and Orange counties, but there’s nothing for Riverside County. Funding isn’t expected anytime soon, according to state geologists.