Scientists believe the Puente Hills fault has a major quake roughly every 2,500 years but don’t know when the last one was. The San Andreas has quakes more frequently (both the Loma Prieta and 1906 San Francisco quakes were on this fault).
The Puente Hills fault was discovered relatively recently — in 1999. Five years earlier, the magnitude 6.7 Northridge earthquake hit on another “invisible” fault — completely underground — that scientists didn’t know about.
“When Northridge happened,” Jones said, “it was very sobering for us to think we could have that big of an earthquake that doesn’t come to the surface.”
So scientists launched a major study to discover more of these invisible faults. They strung thousands of sensors across the Los Angeles region and set off small explosions underground.
“From that, we saw this Northridge-like structure sitting under downtown L.A., which is horribly sobering,” Jones said.
Video simulations of a rupture on the Puente Hills fault system show how energy from a quake could erupt and be funneled toward L.A.’s densest neighborhoods, with the strongest waves rippling to the west and south across the Los Angeles Basin.
By contrast, the Northridge earthquake, which killed 57, channeled its strongest shaking north to the more sparsely populated Santa Susana Mountains.
Article Appeared @http://www.chicagotribune.com/news/nationworld/chi-earthquake-los-angeles-20140329,0,2520388,full.story