Puente Hills Thrust Fault: Worse than The Big One?

Today's Los Angeles Times has a front-page article titled "Fault Could Spell Trouble" which has direct relevance to those who live in and near Carbon Canyon.

While much attention has been focused on The Big One, an 8.0 or greater earthquake expected to hit along the San Andreas Fault, the most recent local example coming in 1857, the article suggests that the lesser-known Puente Hills system could, with a 7.5 magnitude quake, do far more damage to the Los Angeles basin than a shaker on the San Andreas, which runs north of the San Gabriel Mountains.

The United States Geological Survey (USGS) has done preliminary analysis and found that Friday night's moderate 5.1 temblor emanated from the Puente Hills thrust fault system, the same one that, in 1987, erupted through the 5.9 Whittier Narrows quake that led to $350 million in damage (including the destruction of 100 houses and 1,000 apartments) and the deaths of eight persons.

USGS seismologist Lucy Jones, the go-to person for information on local quake activity, noted that a half-second of slipping along the fault led to the short ten second tremors.  If a 7.5 earthquake were to develop along the fault, the slippage would be for 20 second and the shaking would, of course, be far longer, even causing heavy objects like grand pianos to lift into the air and topple over.  The softer soil of much of the basin would contribute to the greater shaking effect of such a temblor.

Because the Puente Hills fault is horizontal, not vertical (like the Newport-Inglewood or the Hollywood faults) the shaking would be felt through a much wider area.  The USGS and the Southern California Earthquake Center estimate that a major quake on the fault could kill up to 18,000 people and lead to as much as $250 billion in damage, because of the heavy concentration of people in Los Angeles and its suburbs and the older buildings found along the path of the fault, especially in Los Angeles and Hollywood.  A Big One on the San Andreas might cause only one-tenth of the deaths because of that massive fault's distance from populated areas.

Strikingly, this fault system wasn't found until fifteen years ago, five years after the 6.7 Northridge quake occurred on another previously undiscovered fault.  Seismologists and others embarked on a study looking for other faults and learned of the Puente Hills, which happens to go right under downtown Los Angeles.

In fact, simulations show that a quake on the system could send massive amounts of generated energy west and south into heavily populated areas, in contrast to the 1994 Northridge shaker which moved to the north into lightly settled areas.

Friday's 5.1 main event and the 4.1 aftershock which hit Saturday at about 2 p.m. both came from the Puente Hills thrust fault system, as did the 5.5 Chino Hills quake, which was centered within Chino Hills State Park, just a short distance south of Carbon Canyon.

It bears remembering that each point higher on the Richter scale represents shaking 10 times greater.  So, a 7.5 would be 25 times stronger than the 5.1 experienced on Friday night.

The relatively minor damage, including the breaking of some water mains; damage to houses, apartments and businesses; and the rock slide along the still-closed Carbon Canyon Road, are just small preludes to the much greater devastation that would result.  With leaking or severed gas lines and broken power lines would come fire and the disruption in water, phone or other services would be significant. 

The bottom line is people have to be prepared to provide for themselves for up to several days before emergency and support services would be available.  A home earthquake kit and backpack kits for every car are fundamental to preparation for a major event.

To see the article, click here.