Small Faults May Pose Big Threat
The earthquake risk posed by smaller, overlooked faults in the Los Angeles area may be more severe than many experts have long assumed, according to new research that suggests any one of them could cause a temblor like the 1994 Northridge earthquake.
At the same time, the existence of those smaller faults also means that the chances of a truly large local earthquake, like a prehistoric 7.6 temblor that researchers believe occurred on the Sierra Madre fault in Altadena, may be extremely small, the scientists reported Wednesday.
The new, more complex view of the area’s seismic hazard, published today in the journal Nature, is based on what experts called an innovative analysis of precise satellite data, geology studies and local seismic history. It arises from the effort to better understand the earthquake engine underlying Los Angeles. Scientists are trying to learn how the region’s tortured rocks balance the energy generated as massive tectonic plates push Palos Verdes toward Pasadena at about the same speed that fingernails grow.
In a comprehensive study of the metropolitan region, scientists at San Diego State University and three other research centers discovered that as much as half of the region’s potentially destructive seismic energy is slipping away into a series of small fractures, such as the Chino, Raymond, Verdugo and San Jose faults, that stripe regional hazard maps from east to west.
Researchers now suspect that any one of these smaller strike-slip faults could generate a damaging quake. The faults should be factored more prominently into official estimates of the area’s seismic hazard, they say. The local strike-slip faults are akin in type, but not in size, to the San Andreas fault.
The new view does not change the overall estimate of the area’s seismic hazard. No more or less energy is building up in the basin, experts explained. But it dramatically changes the character of the threat by spreading it more broadly, encompassing smaller fault zones located throughout the area.
“If it is true, it does suggest these strike-slip faults have a better chance of generating a damaging earthquake in our lifetimes than previously thought,” said Harvard University geologist John Shaw. “It matters quite a bit whether we should expect one huge earthquake or a lot of smaller events right under the metropolitan area. That makes a huge difference.”
Earthquake expert Scott Lindvall at William Lettis & Associates, a firm of earthquake geology consultants, said, “I think their research may be a watershed in how we think of the Southern California seismic picture. It is definitely a shift in the thinking.”
To determine how tectonic energy is absorbed and then cataclysmically released, many researchers in recent years have centered their attention on the danger posed by the region’s thrust faults. Those include the faults responsible for the Northridge and 1971 Sylmar-San Fernando quakes.
Those faults, which include the Elysian Park system, the Santa Monica fault and the Sierra Madre fault complex, tend to run from north to south and were thought to be the key mainsprings that stored the tectonic energy.
The new study, however, suggests that far less seismic stress is building up on those faults than previously believed.
Instead, the San Diego State group concluded that a large amount of the enormous energy building up as the metropolitan area is compressed from the north and the south may be released out the sides of the basin on fault lines running to the east and the west, much like wet clay squeezed in a clenched fist.
“Escape tectonics,” several geologists called the phenomenon.
“We now perceive a greater seismic hazard on those small faults than we thought before,” said San Diego State University geologist Christian Walls, who led the research team.
But “our overall seismic risk [in the metropolitan area] has not changed, because there is a finite amount of energy [pent up in the L.A. Basin] that can be released,” he said. “We just are shifting the emphasis from the thrust faults onto these strike-slip faults.”
Thomas Henyey, director of the Southern California Earthquake Center, however, praised the work but cautioned that it was too soon to conclude the area need only focus its worries on relatively small earthquakes, as the study suggests. “Whether it says anything about the distribution of big versus little earthquakes, I would say that is pure speculation at this point,” Henyey said.
“This is obviously a very interesting finding,” Henyey said. “It means maybe we have more work cut out for us. We will have to look more at some of these faults.”
Seismologist Lucile M. Jones at the U.S. Geological Survey in Pasadena said the study, while plausible, “is not the only solution to the problem.”
“The question of how the L.A. Basin fits together is a lot more complicated than people realize,” Jones said. “It is messy.”
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Escape Tectonics
Smaller strike-slip faults may be absorbing almost half of the seismic energy building up in the rocks that cradle metropolitan Los Angeles, new research suggests.
The energy builds up as the massive Pacific and North American tectonic plates squeeze the basin from the north and south, then escapes to the east and west along strike-slip fault lines--as shown by arrows in the map.
Scientists reassessing the quake hazard think the threat posed by these faults may have been underestimated.
Slip-Sliding Away
A strike-slip fault moves horizontally, with one side scraping by the other, unlike a thrust fault, in which the sides move up and down. The best known strike-slip fault is the San Andreas; the most prominent local thrust fault is the one that caused the 1994 Northridge quake.
Source: the journal Nature
