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PROBABILITY: THE UNCERTAINTY FACTOR IN SCIENCE -- SCIENTISTS CLAIM 1,000-YEAR EARTHQUAKE LULL IN LOS ANGELES -- INDICATOR OF INCREASED RISK?

GREAT!  Days after posting my blog entry questioning the safety of some of the welded steel moment frame  (WSMF) high-rise buildings which experienced various degrees of failure in the Northridge Quake of 1994 and in which I noted that the famed seismologist Lucy Jones, of the United States Geological Survey, said that the Andreas Fault/San Jacinto fault area was 300 years overdue for a massive quake, we get another earthquake study affecting the Southern California region.

This time from the scientists at USC (University of Southern California) who now claim that there may be a link connecting the desert areas (which are experiencing more quakes) and the Los Angeles basin which is experiencing less quakes than would be predicted by the statistical analysis of historical records.

According to USC College Professor, James Dolan, "Earthquakes under the basin at historic low."  "The Los Angeles basin appears to be in a seismic 'lull'  characterized by relatively smaller and infrequent earthquakes, according to a study in the September issue of Geology.
By contrast, the Mojave Desert is in a seismically active period. Seismic activity alternates between the two regions, the study suggests. According to their findings, "the lull in the Los Angeles basin began 1,000 years ago.

“'The past 1,000 years has been relatively quiet,' Dolan said, referring to what he calls the 'urban fault network' under the Los Angeles metropolitan area."

Dolan said that even the Northridge earthquake of 1994, the costliest natural disaster in U.S. history at the time, was 'a drop in the bucket' compared to the massive jolts that would strike the basin during a period of high seismic activity."

Of course, the authors are quick to supply the caveats:

• The urban fault network does not include the more distant San Andreas fault. Though the San Andreas is storing energy at a slower than average rate, a major quake along the fault is always possible. About 10 San Andreas “big ones” have occurred during the current lull on the urban fault network.

The authors developed their theory from the discovery of several “clusters” of intense seismic activity in the geological record. It is not yet known if the clusters are statistically significant.

The authors studied the geological record going back 12,000 years. During that period, they found several clusters of seismic “bursts,” with the most recent lasting 4,000 years and ending about 1,000 years ago. The seismic clusters were separated by periods of relative calm lasting about 1,500 to 2,000 years.
Remarkably, the lulls in the Los Angeles region corresponded with seismic clusters in the Mojave Desert, as described in 2000 by Thomas Rockwell of San Diego State University and his colleagues. 

Which leads to Dolan's unproven phenomenological semi-empirical conclusion that:

“When we're having earthquakes in L.A., generally we don't have as many earthquakes in the Mojave,” and vice versa, Dolan said. 

The study in Geology proposes a mechanism by which periods of high seismic activity alternate between the urban fault network and the Mojave Desert.

Of course Dolan, as well as every major geology and seismology professional, will tell you  all major quake activity is conditioned on the movements and strain inherent in the massive San Andreas Fault system. Again, according to Dolan:

The two main cogs in the mechanism are the section of the San Andreas fault north of Los Angeles and the desert fault system known as the eastern California shear zone.
Rapid motion along one fault causes slower motion along the other, the authors suggest. During relatively rare periods when the San Andreas fault is moving slowly, the strain in the urban fault network drops accordingly, leading to a seismic lull in Los Angeles and to more seismic activity in the desert. 

“The San Andreas is always dominant. It's always the big brother,” Dolan said. “But at times the eastern California shear zone takes up its share of the load.”
During the current lull in Los Angeles, major earthquakes in the eastern California shear zone have included the magnitude 7.1 Hector Mine of 1999, the 7.3 Landers of 1992 and the 7.6 Owens Valley of 1872.
Each packed four to 20 times the energy of the Northridge quake.

While all three quakes occurred in sparsely populated areas, Palm Springs and other desert communities lie close to the eastern California shear zone and could be vulnerable.
“These are very large earthquakes,” Dolan said. 

If the authors' theory is confirmed, detecting the start and end of a lull will become extremely important. Predicting the end of the current lull is impossible at present, Dolan said.

“We do know that the Mojave part of the eastern California shear zone is still storing energy much more rapidly than usual (by a factor of about two), so I would tend to doubt that the recent 1994 (magnitude) 6.7 Northridge and 1971 (magnitude) 6.7 San Fernando earthquakes indicate that we are coming out of' the current lull,” he said.

So along comes Dr. Jones, claiming that the area near Palm Springs, as cited above, is approximately 300 years overdue for a major earthquake. Possibly as large as a projected Magnitude 7.8 striking in the Salton Sea between the two Coachella faults. And it is theoretically possible that this projected quake's energy may be able to ripple northward to trigger additional massive quakes along the entire southernmost San Andreas fault system.

Which, in my mind,  increases the urgency to find out if the Los Angeles Department of Building and Safety has adequately performed its duties relating to WSMF high-rise buildings.

What can YOU do?

About earthquakes -- nothing as they are a manifestation of the nature we so cavalierly claim that we can affect. But you can prepare yourself and your family to survive the rigors of a period when you will be unable to summon the appropriate authorities to protect your life and property during a natural event. For those who doubt this statement: see the accounts of Hurricane Katrina and the failure of official response mechanisms.

Do not obsess about the future as it is beyond your control. As for Dolan and the other dedicated earthquake researchers, do not fear their findings: at the worst, they are statistical anomalies and, at best, they are a type of early warning system for individual and community preparedness.

Demand that USC stop wasting money on its overblown football and basketball programs and spend it on earthquake research. [Editorial Note: GO BRUINS!]

As for all the predictive power of historical records and the application of statistics, this is a great illustration of the beauty of most mathematical studies: you can pick and choose the one you like most, right up until the time nature proves you catastrophically wrong. 

-- steve 

A reminder from OneCitizenSpeaking.com: a large improvement can result from a small change…

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane. -- Marcus Aurelius

References: 

Original Blog Entry - KNOWLEDGE IS RESPONSIBILITY: ARE LOS ANGELES OFFICIALS HIDING THE TRUTH ABOUT HI-RISE EARTHQUAKE VULNERABILITIES?

L.A. in 1,000-Year Earthquake Lull | USC Press Release


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