Is the Napa Quake a Precursor to the Big One?
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Was the 6.0-magnitude earthquake that recently struck Northern California an isolated seismic event, or the harbinger of a worse quake to come?
Jittery Californians and earthquake-watchers are asking themselves that question after Sunday's quake on the Hayward-Rodgers Creek Fault — the area’s largest in 25 years — struck the Napa Valley. The San Andreas Fault, a continental transform fault that runs some 810 miles through California, is overdue for an epic seismic event of near 8.0 magnitude. Such a quake is expected to create a national disaster and produce devastation to areas within 50-100 miles from the quake zone. The potential loss of life could be great if the “Big One” struck urban areas near the fault, such as San Francisco, Los Angeles or Palm Springs.
Despite the concerns, nobody really knows when a 7.0-8.0 magnitude is due as seismologists cannot predict the timing of earthquakes with any precision. However, there may be more cause for concern on the southern part of the fault than in the northern section.
According to the U.S. Geological Survey, catastrophic earthquakes strike along the northern, central, and southern sections of San Andreas every 150 years or so, citing the past history in the region spanning 1,400 years. The last epic shocks along the fault were on the central and northern areas of the fault, the 1857 and 1906 events. The 1857 quake, known as the Fort Tejon earthquake, had a magnitude of 7.9, and earthquake zone reached well into southern California. However, the epicenter of the quake was 85 miles north of Los Angeles. The 1906 earthquake, which had an epicenter in the northern fault area in the Bay Area, is estimated to have produced a magnitude of 7.8.
Thus, scientists say that Southern California may be overdue for another colossal temblor, but San Francisco is not as likely to see one soon, as only about a century has passed since its last epic quake. This is not to say that more damaging quakes are not possible for the San Francisco area. Quakes between 6.0 and 7.0 in magnitude can strike at any time, say seismologists.
One of those quakes happened in 1989. The Loma Prieta Earthquake, often dubbed “The World Series Quake” because it occured during the pre-game broadcast of Game 3 of the baseball championship between the San Francisco Giants and Oakland A’s. A magnitude 6.9 on the Richter scale, it caused 69 deaths, nearly 4,000 injuries, and more than $6 billion in property damage.
Southern California had a 6.7 magnitude quake in 1994 called the Northridge Earthquake. The original quake and its many aftershocks killed 57 people and caused $42 billion in property and infrastructure damage.
But despite their focus on the southern section of the fault, the “Big One” can possibly run far along the San Andreas, according to seismologists, creating damage spanning hundreds of miles.
Research conducted by the University of California eight years ago concluded that the fault is currently at sufficient stress level for the “Big One” to happen. The study also concluded that the risk of a large earthquake may be increasing more rapidly than researchers had previously thought. The study points to the more southern areas of the fault. It says that while the 1857 and 1906 quakes originated from the central and northern sections of the fault, the southern area of the fault hasn’t seen an equivalent rupture in more than 300 years.
“The information available suggests that the fault is ready for the next big earthquake but exactly when the triggering will happen and when the earthquake will occur we cannot tell,” said Yuri Fialko, a professor of geophysics at UC San Diego. “It could be tomorrow or it could be 10 years or more from now.”