Quake conundrum
April 15 2019 01:14 AM
GRIM REMINDER: A red-tagged home near downtown Napa, California, on August 25, 2014, after a 6.0-magnitude earthquake rocked the area.

By Rong-Gong Lin II

California is in an earthquake drought.
It has been almost five years since the state experienced its last earthquake of magnitude 6 or stronger — in Napa. Southern California felt its last big quake on Easter Sunday 2010, and that shaker was actually centred across the border, causing the most damage in Mexicali.
Experts know this calm period will eventually end, with destructive results. They just don’t know when this well-documented geological pattern will shift.
“Earthquake rates are quite variable: We have a decade or two where we don’t have many earthquakes, and people expect that’s what California is always like,” said Elizabeth Cochran, seismologist with the US Geological Survey. Eventually, “we’re going to dramatically see a change in earthquake rates.”
Scientists have been warning California of seismic dangers, be it the Big One on the San Andreas fault or catastrophes that could come from lesser-known faults, such as Hayward or Newport-Inglewood.
One reason for the urgent alerts: Memories of a truly destructive quake in many urban areas have faded. And with that, some fear, the urgency of pushing seismic safety has also faded.
Experts say California ignores these realities at its own peril.
“Along the main plate boundary faults, we are in a deficit of earthquakes in the last 100 years,” said Tom Rockwell, a San Diego State paleoseismologist. “At some point, that’s going to change. We’re going to have some big earthquakes.”
Consider how much quieter California has been in regard to earthquakes of magnitude 6 or greater:
In the last 25 years, there have been 11 such temblors statewide. In the preceding generation, there were 32.
In the last quarter-century, there have been three such earthquakes that shook California’s 10 southernmost counties. In the prior generation, there were nine, including the Sylmar temblor of 1971 (magnitude 6.6) and Northridge of 1994 (magnitude 6.7).
The greater San Francisco Bay Area has been particularly quiet. Since the great 1906 earthquake destroyed much of San Francisco, there have been only three earthquakes of magnitude 6 or greater. But in the 75 years before that catastrophe, there were 14, according to geophysicist Ross Stein.
Earthquakes must happen at some point to relieve the immense tectonic forces that are pushing part of the state northwest toward Alaska and the rest southeast toward Mexico. On average, there’s about 12 feet of movement per century along the 730-mile San Andreas fault system between Point Arena in Mendocino County and the Mexican border, Rockwell said, but, except for a 90-mile section that creeps in San Benito and Monterey counties, there has been no significant movement along this primary boundary of faults between the Pacific and North American plates in a century.
There may be periods “where things get kind of all locked up and no earthquakes happen for a while. You store a lot of strain in the Earth’s crust,” said Tom Jordan, a University of Southern California professor of geophysics. “Once it gets going, it’s like a set of dominoes. You might get multiple events if you have enough strain energy stored in the crust, because it’s been a long time since an earthquake.”
When a time of elevated seismic activity will come is hard to say. Some thought the 1989 magnitude 6.9 Loma Prieta earthquake, which killed 63 people, would mark the end of decades of seismic quiescence in the Bay Area. Instead, the lack of extreme shaking has largely persisted, save for the Napa temblor of 2014.
Scientists have been particularly focused on the lack of major seismic activity on several California faults that as a group produce the most frequent earthquakes on the plate boundary — the San Andreas, San Jacinto and Hayward. They are the ones seen as most likely to cause trouble in our lifetime.
Yet they’ve been exceedingly quiet. A USGS analysis published in the journal Seismological Research Letters last week finds that, based on an analysis of sites where seismic data are known for the past millennium, the century between 1919 and 2018 is in all odds the only 100-year period in the past 1,000 years where there have been no earthquakes strong enough to break the ground on those three faults.
Ordinarily, there are roughly three to four of these earthquakes on these faults every 100 years.
“The next century is unlikely to be as quiet as this one. It’s hard to beat,” said USGS seismologist Glenn Biasi, the lead author of the study.
The 1800s, by comparison, was a far more active time for earthquakes. That century saw six large temblors on this trio of faults; between 1800 and 1918, there were eight. That’s an average of one major quake on those faults every 16 years.
Earthquake scientists have been buzzing for years about California’s hiatus in supersized earthquakes, thinking the chances of such a 100-year gap between ground-shattering seismic events to be improbable. “Did Someone Forget to Pay the Earthquake Bill?” was the title of a talk by UCLA geophysicist David Jackson at the Seismological Society of America conference in 2014.
Scientists focus on earthquakes that literally break the ground along the main plate boundary because they’re the ones that actually do the job of relieving centuries of tectonic strain. A famous example of the ground breaking was during the great 1906 earthquake that destroyed much of San Francisco; at Point Reyes in Marin County, a fence that intersected the fault was suddenly cut in two, separated on each side by the San Andreas by 18 feet.
Some quakes don’t do that job. The 1989 magnitude 6.9 Loma Prieta earthquake, while near the San Andreas fault, actually occurred on a neighbouring sub-parallel strand, according to USGS research geologist Kate Scharer, a co-author of the USGS study. Earthquakes of 1992 in Landers and 1999 in Hector Mine broke faults at the surface, but didn’t do it at the main plate boundary, Biasi said.
Only sections of the San Andreas, San Jacinto and Hayward faults that have 1,000 years’ worth of data on earthquakes were included in the analysis because the authors needed that data to look for periods of seismic activity and inactivity. Although the 1968 Borrego Mountain and other earthquakes at the extreme southern edge of the San Jacinto fault system have broken ground, those sections of the fault are not included in the study; there are no records available to determine if those areas produced many earthquakes over the last millennium, and as a result, are among California’s most worrisome.
There have been more mild seismic storms in recent memory.
The earthquake storm in the ’80s and ’90s is clearer in hindsight, said Caltech seismologist Egill Hauksson. It began with the Whittier Narrows temblor in 1987 (magnitude 5.9), which killed 8; followed by Pasadena in 1988 (4.9); Montebello in 1989 (4.4); Upland in 1990 (5.2); Sierra Madre in 1991 (5.8), which killed a woman; before ending with Northridge in 1994, which resulted in the deaths of at least 57 people.
In the 75 years before the great 1906 San Francisco earthquake, there were 14 earthquakes of magnitude 6 or greater in the Bay Area, according to geophysicist Ross Stein. Since 1906, there have been only three.
In the 75 years before the great 1906 San Francisco earthquake, there were 14 earthquakes of magnitude 6 or greater in the Bay Area, according to geophysicist Ross Stein. Since 1906, there have been only three. (Temblor.net geophysicist Ross Stein)
More severe periods of earthquake activity were seen in the 19th century. Mission San Juan Capistrano endured shaking and suffered damage from two earthquakes — both greater than magnitude 7 — within a span of just a dozen years.
The first came in 1800 from a quake on the San Jacinto fault east of Temecula. Then, in 1812, movement of the San Andreas and San Jacinto faults through present-day cities like San Bernardino, Rialto, Loma Linda, Yucaipa and Highland brought down the mission’s Great Stone Church, killing more than 40 people inside.
“The Inland Empire would be strongly impacted” if those quakes repeated today, Scharer said.
There are other examples of frequent major earthquakes. In the 1500s, for instance, three major San Andreas fault earthquakes hit along the Grapevine, according to Scharer, possibly all within 30 years. And over the last 1,000 years in the southernmost 100 miles of the San Andreas fault system, there have been three periods of increased seismic activity — between 900 and 1100, 1200 and 1400, and 1600 to 1800, according to Rockwell. In that section, it has been relatively quiet for the last 219 years.
“If this pattern continues, we could well start a new phase of activity,” Rockwell said.
Experts say it’s important that Californians are psychologically prepared for the prospect of a new era of earthquakes. 
                  —Los Angeles Times/TNS

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