Western San Gabriel Mountains, CA

Stop #1:  The L.A. Aqueduct, the Cascades, & the Van Norman Dam

On November 4, 1913, the Los Angeles Aqueduct officially opened, and William Mulholland’s dream of providing thirsty Los Angelinos with a steady supply of fresh water was realized.  The Los Angeles Aqueduct stretches 233 miles from the Owens Valley just east of the Sierras to the San Fernando Valley, where it flows into the Van Norman Dam.

The photo on the left shows the Cascades, where water from the California Aqueduct flows over a series of concrete bumps to aerate the water before heading south into the Van Norman Dam. The wash leading to the Van Norman Dam can be seen in the lower center of the photo.

 

At 6:01 am on February 9. 1971, a magnitude 6.7 earthquake rocked the Southern California region.  While earthquakes were not uncommon, the region was largely unprepared for moderately sized earthquake such as this event.  The earthquake occurred on the San Fernando fault zone, a thrust fault.  The total amount of surface rupture was estimated to be approximately 19 km, and the resulting ground motions experienced was the most intense ever recorded by instruments at the time.  The San Fernando Valley to the south was heavily damaged due to numerous surface ruptures and ground failures.  Infrastructure such as medical facilities and bridges failed.  When all was said and done, the quake caused 65 deaths and $500 million in property damage.

Why was the damage so great?  Mostly it was due to regulations on where one could and couldn’t build.  While California had some pretty strict earthquakebuilding code at the time, there was no rules as to not building on a fault.   Legislation was passed in 1972 that prohibits “the siting of most structures for human occupancy across traces of active faults that constitute a potential hazard to structures from surface faulting or fault creep.”

The image above shows the Van Norman Dam; Yellow lines are major faults, and black lines are surface ruptures formed during the 1971 Sylmar EQ and the grey shaded areas are the Alquist-Priolo Fault Zones.

 
One of the structures that was damaged in the 1971 Sylmar earthquake were the Upper and Lower Van Norman Dam (also called the San Fernando Dam, see Figure 4, below).  The Lower Van Norman Dam came close to breeching – were it not for the quick thinking of one of the engineers, who decided to take a look at the dam after the earthquake and noticed the damage, it likely would have, and 80,000 residents would have had their homes inundated with flood water as a result.  The earthfill dam was 1,100 feet high, lined with concrete, and, when full, held 3.6 billion gallons of water.  Fortunately for all, the dam was only half full when the earthquake caused the embankment to fail. The top thirty feet of the embankment collapsed area slid into the dam, taking with it part of the concrete lining, lowering the embankment to within 5 to 6 feet of the water’s surface.