Page 86 - AAGLA 2020-11
P. 86

 Member Update
The Los Angeles Area’s Biggest Monster
TBy Ali Sahabi, Optimum Seismic
okyo 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.1 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.
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 Angeles2.”
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.”

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