Lots of my relatives and friends in Southern California are reporting that they are feeling shaken up tonight. The
magnitude 5.1 earthquake (revised from 5.3) took place about 2 miles east of La Habra, which may place it on one of the strands of the Puente Hills blind thrust system, according the the
U.S. Geological Survey. My friends are reporting minor damage, mostly in the form of broken glass and fallen flat-screen televisions and monitors. The fault has been responsible for
a number of historical quakes, most notably the 5.9 Whittier Narrows quake of 1987, which killed 8 people and caused around 350 million dollars of damage. The 2008 Chino Hills quake 5.4 may have occurred on the same system.
The event on the Puente Hills blind thrust was oblique, with reverse (compressional) and right lateral displacement, as shown by the
focal mechanism from the quake (below). It is called a blind thrust because it doesn't have a clear surface expression which makes it difficult to assess the hazard level and the history of the fault. It is thought to be capable of generating a magnitude 7.2-7.5 magnitude event, which would have catastrophic consequences for the urban areas of Los Angeles and Orange counties. Such events are thought to have taken place
around four times in the last 11,000 years.
Earthquakes such as those tonight serve to remind all Californians that their home is earthquake country, and all who live in the region must take this into account. The Puente Hills fault is just one of numerous active faults in Southern California. The San Jacinto, Santa Susana, Cucamonga, Elsinore, Whittier Hills faults, and the granddaddy of them all the San Andreas are all capable of causing great mayhem. Education and preparation are the best defense against tragedy. Here is a good place to start:
http://www.data.scec.org/earthquake/preparedness.html.
