The Puente Hills Fault: The Los Angeles Area’s Biggest Monster
By Ali Sahabi
Tokyo had Godzilla. But, Los Angeles, for more than a century, has feared the notorious San Andreas Fault. However, we Angelenos have been overlooking a closer and even more deadly monster: the Puente Hills fault, which could kill more people and cause more damage in the Los Angeles Area than the San Andreas because it lies under vulnerable, older neighborhoods, and can produce heavy reverberations felt over a wide area.
A study by the University of Southern California found that the Puente Hills fault has the capacity to produce “the costliest disaster in U.S. history.” As many as 18,000 people could die, 735,000 could lose their homes, and up to 100,000 tons of debris may be generated. The total economic loss would be as high as $252 billion.[i] The United States Geological Survey (USGS) presented similar projections, noting that Puente Hills’ destructive power is five times that of the San Andreas. A 7.5 magnitude earthquake on the nearby Puente Hills fault would cause the same destruction as an 8.0 magnitude earthquake on the more distant San Andreas fault – with an 8.0 magnitude earthquake releasing 16 times the energy of a 7.5 magnitude earthquake.
The Giant Awakes?
A Puente Hills fault rupture happens once every 3,000 years, according to the USGS. The fault, first discovered in 1999, runs about 25 miles through the Los Angeles basin, from downtown Los Angeles, through southeast Los Angeles County creating a cross-stitch pattern under cities such as South Gate, Downey, Norwalk, and surrounding communities extending into Orange and San Bernardino counties.
The 6.0 magnitude October 1987 Whittier Narrows earthquake first led to the discovery of the Puente Hills fault. This was followed by quakes in Chino Hills (2008), Pico Rivera (2010), and La Habra (2014).
A team of scientists from Harvard, the University of Southern California, and the University of California at Los Angeles produced a report in 2017 showing accelerating slip rates along this fault. “This increase in rate implies that the magnitudes and/or the frequency of earthquakes on this fault segment have increased over time,” the report concluded. “This challenges the characteristic earthquake model and presents an evolving and potentially increasing seismic hazard to metropolitan Los Angeles[ii].”
Thrust Quake Dangers
The Puente Hills fault causes a thrust earthquake, which tends to rupture with one side of the fault pushing up and over the bottom side. It shares this quality with other nearby faults that sparked the Sylmar, Whittier Narrows and Northridge earthquakes. Thrust earthquakes create an accumulated strain that bends the earth’s crust in spots, where the fault has locked up. Seismologists believe that these areas are the spots most prone to future earthquakes. The force behind these types of earthquakes reflects the incredible amount of energy needed to move two tectonic plates toward each other at a rate of about one inch every three years.
It may sound easy to locate these trouble spots, but the reality is that many of these high-pressure areas are located deep underground and cannot be seen or detected – until they burst. Seismologists call these “blind thrust faults.”
These faults also present an added danger because they are located along the Los Angeles basin, where local soil conditions are prone to liquefaction – a phenomenon that causes the earth to move like Jell-O when shaken. When it occurs beneath vulnerable buildings and other structures, the results can be catastrophic. This is what happened during the 1994 Northridge earthquake on Balboa Boulevard in Granada Hills, where a gas main was ruptured by lateral spreading and caught fire, the Los Angeles Times reported.
Liquefaction also contributed to the devastation of the 1989 Loma Prieta earthquake – when soil and debris used to fill in a lagoon to build the Marina District subsided, fractured, and caused horizontal sliding of the ground surface. The Atlantic magazine, in 2019, describes the effects of liquefaction as horrifying: “Videos of its occurrence look like found-footage documentaries of the Second Coming, Buildings seem to simply slip away, the earth gives out, and the once-steady structures slide into the morass.”
Monster quakes are inevitable in California, but they do not have to be disasters. It is important to be prepared for their inevitable arrival. If your apartment building is in the Greater Los Angeles region, it may very well be at risk. Optimum Seismic offers complimentary consultations to help you know the risks you may face, and the steps you can take to save lives, protect your assets and your livelihood.
Ali Sahabi, a licensed General Engineering Contractor (GEC), is an expert in seismic resilience and sustainability. He is Chief Operating Officer of Optimum Seismic, Inc., which has completed more than 3,500 seismic retrofitting and renovation projects for multifamily residential, commercial, and industrial buildings throughout California.
[i] University of Southern California, “Loss Estimates for a Puente Hills Blind-Thrust Earthquake in Los Angeles, California,” Edward H. Field, et. al., 2005.
[ii] Geo Science World, “Accelerating slip rates on the Puente Hills blind thrust fault system beneath metropolitan Los Angeles, California, USA. Kristian Bergen et al, 2017.