The Puente Hills Fault Threatens to Devastate Downtown LA

Most people assume that if LA were to be devastated by an earthquake it would be the long awaited “big one” caused by a slip on the San Andreas Fault but experts  now believe that the Puente Hills Thrust Fault rupturing could cause far more damage than the San Andreas. There are two reasons for this. Firstly most cracks local to LA are vertical whereas the Puente Hills Fault is horizontal. Secondly, the area it runs under is far more built up and highly populated than the Newport-Inglewood and Hollywood faults that are usually associated with quakes in the area.

From the LA Times:

Experts say a major, magnitude 7.5 earthquake on the fault could do more damage to the heart of Los Angeles than the dreaded Big One on the San Andreas fault, which is on the outskirts of metropolitan Southern California.

The size of Friday’s quake was considered moderate, but it packed a punch. Residents within 10 miles of the epicenter in La Habra reported toppled furniture, broken glass and fallen pictures. Several water mains broke, and a rock slide in Carbon Canyon caused a car to overturn, leaving those inside with minor injuries.

Officials said more than a dozen homes and apartments were red-tagged because of possible structural damage.

Preliminary checks by the U.S. Geological Survey after the quake show it erupted around the Puente Hills thrust fault system, said Caltech seismologist Egill Hauksson. Further research is underway.

In 1987, another “moderate” quake on that fault killed eight people and caused more than $350 million in damage. The magnitude 5.9 Whittier Narrows quake left old brick buildings in Whittier’s downtown area battered and also damaged some freeway bridges. More than 100 single-family homes and more than 1,000 apartment units were destroyed.

Friday night’s earthquake was caused by the underground fault slipping for half a second, said USGS seismologist Lucy Jones, prompting about 10 seconds of shaking at the surface.

But a 7.5 quake on the Puente Hills fault could cause the fault to slip for an entire 20 seconds — and the shaking could last far longer.

The Puente Hills fault could be especially hazardous over a larger area because of its shape. Other local faults, like the Newport-Inglewood and Hollywood, are a collection of vertical cracks, with the most intense shaking occurring near where the fault reaches the surface. The Puente Hills is a horizontal fault, with intense shaking likely to be felt over a much larger area, roughly 25 by 15 miles.

According to estimates by the USGS and Southern California Earthquake Center, a massive quake on the Puente Hills fault could kill from 3,000 to 18,000 people and cause up to $250 billion in damage. Under this worst-case scenario, people in as many as three-quarters of a million households would be left homeless.

One reason for the dire forecast is that both downtown L.A. and Hollywood are packed with old, vulnerable buildings, including those made of concrete, Jones said.

The ground under Los Angeles and the surrounding basins is soft and pliable which amplifies any shaking, allowing waves to travel across the basin unimpeded which causes a “wobbling” effect. Add that to a horizontal fault that could generate shaking that goes on for much longer than a “regular” quake of the same magnitude and you have a recipe for disaster.