Western San Gabriel Mountains, CA

Stop #4a:  1971 Sylmar earthquake (blind thrust faults):  Epicenter

This marks the spot of the epicenter of the epicenter of the 1971 San Fernando Earthquake (also called the Sylmar earthquake).  Occurring at 6:01 am on February 9, 1971, the magnitude 6.6 earthquake was a wake-up call to Southern Californians.  The shallow focus earthquake ruptured a part of the San Fernando fault (thrust fault), whose fault plane extends south towards the San Fernando Valley (Wikipedia, 2018) (SCEDC, 2013). 

Damage was extensive, especially for structures built on the fault. That led to the Alquist-Priolo Special Studies Zone Act passed in 1972. The purpose of this act is to prohibit the location of most structures for human occupancy across the traces of active faults. Building codes were strengthened to minimize the impacts when faults rupture. The one problem is that some of these faults are what we call blind thrust faults (that is, we do not see a trace of the fault on the surface).

Note that there is now a golf course where the epicenter is. Ever wonder why there are so many golf courses around? The AP Zones surrounding an active fault limit what can and cannot be built in the area, and mostly focus on buildings in which humans will be occupying. However, so long as there are no buildings within the AP Zone, parks, golf courses, and the like are allowed. The owner of the land can make money, although perhaps not as much as building homes or office buildings would.

The Southern California region has thousands of faults and therefore a lot of AP Zones. What about buildings that existed before this 1972 law? They can still be there, although there are limitations of what can and cannot be done to the land within an AP Zone. For example, remodeling can be limited, or putting in a pool may not be allowed.