Baja earthquake shook up view of Southern California faults
Study of Easter 2010 temblor revealed previously unknown faults
Apr. 8, 2012 |
Written by
Keith Matheny
The Desert Sun
A new scientific study reveals that fault networks near the Salton Sea are even more complex than previously known — but what that may mean for earthquake potential in the region remains uncertain.
The U.S. Geological Survey and California Geological Survey released a study last month that found the Easter Day quake in northern Baja California on April 4, 2010 — the biggest quake to shake the Coachella Valley in recent years — triggered surface movement on many faults in the Imperial and Coachella valleys.
The magnitude-7.2 quake revealed faults southwest of the Salton Sea that were not previously known to scientists, and confirmed that other known faults were active, the new research shows.
It's not a finding that will quickly lead to the development of a reliable earthquake warning system, a primary goal of scientists. But it advances knowledge and will direct future research on one of the most intricate and studied fault zones on Earth, scientists said.
The Baja earthquake "has provided a geological treasure trove to our understanding of what is happening tectonically in this expansive region of northern Mexico and Southern California," said state geologist John Parrish in a statement about the study.
The 2010 Easter Day quake, dubbed the El Mayor-Cucapah quake by scientists, killed four people and injured more than 100 in Mexico and caused an estimated $440 million in damage in the Mexicali Valley of Baja California and $90 million in damage in the Imperial Valley.
The movements the quake caused on Southern California faults occurred at the surface and were very small, only centimeters. Similar fault movements were also observed after a magnitude-5.7 aftershock on June 14, 2010.
The discoveries show earth scientists that "the transfer of strain among the various faults in this region is not as simple as we thought before," said Jerry Treiman, a geologist with the California Geological Survey and co-author of the study.
Earthquakes like the El Mayor-Cucapah quake involve the sudden release of built-up energy from two of the Earth's tectonic plates moving against each other as one of the plates slips past the other. The quake was the largest in the northern Baja region in the past 120 years.Assisting in locating and mapping the surface movements on Southern California faults was NASA's UAVSAR, a sensitive radar system flown over the area at high altitudes on an unmanned aerial vehicle, or drone. Flyover data was compared with a previous examination of the area in October 2009 after the quakes, Treiman said.
Typical NASA radar images taken from satellites can show ground shifting down to centimeters, said Michael Rymer, U.S. Geological Survey's lead geologist on the research. With the drone system, however, "we were seeing down to millimeters of resolution," he said.
The images help crews in the field more accurately locate new faults in the remote Yuha desert west of El Centro near the U.S.-Mexico border, Rymer said.
Among the findings:
• The Ocotillo fault, a known but previously unnamed fault near the Imperial County town of the same name, was confirmed as an active fault by its 3.3 inches of movement.
• The Elsinore fault, one of the major northwest-trending faults in the area, showed movement from the quake extending 2 miles to the southeast beyond its previously known length.
Slight surface fault movement was discovered as close to the Coachella Valley as in the Mecca Hills on the San Andreas fault east of Thermal, Rymer said.
Research shows that historically, the southern San Andreas fault that runs along the Coachella Valley's northern edge has had a major earthquake every 150 years. But it's been at least 300 years since the last major temblor on that section of the fault.
But the 2010 event, a so-called triggered slip, occurred near the surface, from a few hundred meters to about 3 kilometers in depth — not far enough down to set off a major earthquake on the San Andreas, Rymer said.
"Those big earthquakes happen about 10 to 15 kilometers below the surface," he said. "These triggered slips do not affect that area, so they are not loading or unloading the faults."
The new faults have now been registered as Alquist-Priolo Earthquake Fault Zones, in accordance with a 1970s California law to prohibit most development across active faults, Treiman said.But many of the new faults are in remote areas of federal land managed by the U.S. Bureau of Land Management and are unlikely for development.
The study's findings raise questions for scientists to answer through further research, particularly regarding how strain is distributed across a wide area of faulting, Treiman said.
Rymer said the new faults could be sites for further seismic review, including excavation and examination of sediment layers to get a glimpse at their past eruptions.
"Now that we know about all of these new faults, it may be just the first step in understanding what the seismic hazard is down there," he said.http://www.mydesert.com/article/20120408/NEWS0805/204080309/Baja-earthquake-Easter-revealed-unknown-Southern-California-faults
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