Parts of OC Are in Earthquake ‘Doughnut Hole’

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Two earthquake faults that run along the edges of Orange County are part of a “doughnut hole” prediction theory that is attracting new interest from scientists studying the effects of April’s 7.2 Mexicali quake, according to a weekend story in the Los Angeles Times.

Developed in Japan in 1969 by seismologist Kiyoo Mogi, the theory studies the boundaries created by small quakes over a period of years to see if something bigger is developing in the center.

In the case of California, the Times reported, quakes to study as part of the Mogi doughnut hole theory are the Mexicali quake to the south, the 6.5 Eureka earthquake in January, and the 1989 Loma Prieta and 1994 Northridge temblors. The 5.4 quake earlier this month about 30 miles south of Palm Springs came, according to the Times, “several weeks after seismologists at the Jet Propulsion Laboratory and elsewhere warned that pressure was building in the San Jacinto fault zone, which is where the temblor occurred.”

The big question is whether the Mexicali quake has made a destructive temblor in the L.A. area more likely. Experts see strong evidence that there is more pressure now on the San Jacinto and nearby Elsinore fault networks to the east of Los Angeles.

The Elsinore fault zone is connected to the Whittier fault, which runs through densely populated sections of the L.A. area, including the San Gabriel Valley. As a result, there’s a concern that a quake on the Whittier fault might be more likely.

In Orange County, the Whittier fault passes through Yorba Linda, generally running southeast from the L.A. County line along the foothills to the 91 freeway. The Elsinore fault roughly parallels the Orange County border as it runs north from San Diego through Riverside County.

When the Elsinor fault reaches approximately the 91 Freeway in Riverside County, a branch of the fault heads northwest across the tip of Orange County.

The Times noted that “experts stress that the hypothesis is still unproven and not universally accepted. Skeptics say the concept could be applied to seemingly random earthquakes.”



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