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Monday, April 4, 2011

Geologist says Mexicali was lucky
last year's earthquake was not more devastating

By David Gaddis Smith

Although last year's Easter Sunday earthquake was devastating in many ways, geologist John Fletcher says, "Mexicali got really lucky with this earthquake."

quake talkThe 7.2-magnitude quake moved along small faults in remote areas "that have linked to form a newly discovered fault system," he told a standing-room only crowd at the Trans-Border Institute at the University of San Diego on Monday.

john fletcherFletcher (right) said that if the quake had moved along an established fault, "It could have been really devastating on the order of Haiti."

The 7.0 magnitude January 2010 earthquake in Haiti killed more than 200,000; depending on whose figures one follows, from two to four people died in the April 4, 2010 Mexicali quake, which is being referred to as the Mayor-Cucapah earthquake because it went through the Sierra Mayor and Sierra Cucapah ranges.

Some say, however, that as a result of the greater poverty and other socioeconomic problems brought on by the Mexicali quake, many others in Baja California have died prematurely.

The quake, whose epicenter was about 20 miles south of Mexicali, caused major structural damage in the Mexicali Valley and in Calexico in California's Imperial County. Damage in the Mexicali Valley was estimated at about $440 million.

Fletcher said the presence of more sophisticated equipment that was damaged on the U.S. side may have meant that the "epicenter of damage was north of the border."

Liquefaction caused major warping damage to Baja California farmland, which must be graded just so for the gravity-driven irrigation system to work properly. "Everything depends on inclination of the field. The leveling of fields got completely disrupted," Fletcher said. He said water-saturated sand below the surface may have "moved laterally into a different location and caused the surface to sink in some areas and rise in others." Canals were ruined; some wheat and hay fields became inundated with water.

Many fields must be precisely regraded in order to be able to produce again. That painstaking and costly grading work still continues.

Fletcher, a geologist at CICESE (Centro de Investigación Científica y Educación Superior— the Center for Scientific Study and Higher Learning) in Ensenada, said the new small fault system is "a much better route for really violent shaking to occur on than faults we knew about -- the Imperial and Cerro Prieto faults," as those faults are much closer to population centers. He said that when you consider that a 7.0 earthqake released the energy equivalent to 25 atomic bombs, Mexicali was fortunate.

He said the Imperial fault passes right along the margin of Mexicali, which he said has a million people. He said Mexicali would be helped if a future large quake occurs on the new fault system.

He said scientists are still puzzling about why the quake went the way that it did. He said scientists had not thought any of the faults the quake occurred on was capable of producing a 7.2-magnitude quake. "We have to pay more attention to small faults," he said.

earthquake displacementFletcher said the main surface rupture extends 140 kilometers, or about 85 miles. The said the seismogenic crust is about 12 kilometers, or more than seven miles, deep, and that two to five meters of separation occurred. The photo at left shows the over 6-foot-tall Fletcher in front of a Borrego fault rupture section that is taller than he is.

He said, "The epicenter and the primary rupture of this earthquake extend from near the northern tip of the Gulf of California all the way up to the international border.

"Most of the fractures lie in Mexico but a few of them kind of slipped across the border and these mayor cucapah earthquake mapfractures that came in from Mexico into the United States, ... (we) geologists have started to classify those as illegal fractures," he said to laughter. He added, "They didn't have permission to cross." (The map at left was a slide shown by Fletcher)

He said, "The Mayor-Cucapah earthquake started out as a north-striking fault that petered out and died really quickly and then spread out bilaterally in both directions, one across the Colorado River delta and another ... up into the the mountainous region of the Sierra Cucapah. We now think that the northernmost segment of the rupture actually propagated back towards its epicenter in a process called back rupturing. He said back rupturing, observed in computer simulations, has never previously "been observed in any natural earthquake."

He said it appears the "energy of the earthquake outran the surface rupture and the surface rupture came back." Fletcher said of the back rupturing, "It's a hypothesis right now, it's my borrego faultfavorite hypothesis."

He said the rupture followed the Laguna Salada fault, hit an intersection and turned up the Pescadores fault and then jumped over to the Borrego fault, whose new rupture can be seen in the Fletcher slide at left. Fletcher said a three-dimensional deformation below ground may have meant the quake could only follow a network of faults and no single fault.

new fumarole at pesdadores faultFletcher is a geologist with a sense of humor. He said when he hiked in to examine where the earthquake caused ruptures in the Pescadores area, he found a new fumarole (photo at right). "We started calling this 'New Faithful'; it's going to be the next new tourist attraction in Baja California," he said.

He said the fumarole is not far from the geothermal field at Cerro Prieto being used for power generation.

Fletcher said there was a continuous rupture all through the arid northern Cucapah mountains.

He said there was only one man who lives near the top 40 miles of the rupture. He showed an interview of the man, a tribal Cucapah elder, who said he heard a low humming sound and then told his wife there was going to be an earthquake. The man said he saw the eruption come through his property, saying the dust kicked up by the fault line as it moved along was like seeing the dust from a car going through the desert at high speed.

Fletcher said new technology has helped scientists at CalTech, the Jet Propulsion Laboratory, the U.S. Geological Survey, San Diego State University, UCSD, UCLA, UC Davis, CICESE and elsewhere better try to figure out what happened. Research came through satellite images, helicopter surveys and images taken from airplanes in addition to the old-fashioned boots on the ground, including the interview with the Cucapah elder.

"This earthquake has had data sets that have never been available for the study of any earthquake prior to this," Fletcher said. He called it "a turning point in understanding how faults work."

He said that luckily, LIDAR (light detecting and ranging) high resolution three-dimensional images of the earth's surface had been made in the area before the earthquake and also were taken afterward. He said the images can show whether a rock has moved since the previous survey.

He said scientists had taken LIDAR images of the San Andreas fault and "were just waiting for the San Andreas fault to slip in any one place."

But the first place the technique got to be used instead was for the Mayor-Cucapah quake. "We just beat them by very good luck," he said, adding, "Ganamos a los gringos" (We beat the gringos).

Fletcher is a gringo himself, who got his Ph.D. at the University of Utah in 1994. He decided the geology field was too crowded in the western United States and set his sights on working in Mexico, where he started learning Spanish from scratch at age 33.

He said a process called pixel matching provided by CalTech's Sebastien Leprince helped pinpoint the specific fault lines by mapping land in yellow, purple and other color pixels. "To my knowledge this is one of the best, if not the first, time that it's ever been useful in an earthquake," Fletcher said.

Fletcher said that while avalanches and ground failure buried some of route of quake, the technology helped scientists discover where to look for it. He said Ken Hudnut of the U.S. Geological Survey was able to provide a digitized yellow line that could be followed on Google Earth.

He said it was amazing that "somebody working out of their office was able to find some of the most picturesque parts of the rupture."

Scientists are still trying to figure out whether the reason the earthquake behaved as it did meant that some faults are old and slow, which he said would be good for humans, as there might not be another such earthquake for 10,000 years, or whether they were young faults and just getting started, which could mean greater devastation the next time. San Diego State University seismologist Tom Rockwell told the San Diego Union-Tribune he thought it was a once in a 10,000- to 20,000-year event.

Fletcher said the quake may have helped load faults across the border, saying it released significant earthquakes of 5.7 and 5.4 magnitudes within months.

He said a well-established theory of faulting indicates that faults should have an orientation of at least 60 degrees or be vertical in this kind of environment, but "what we are finding are fault planes that are very shallowly inclined." He said some inclination was as low as 22 degrees.

He said, "This might be one of the first earthquakes that have even been associated with a shallowly dipping fault plane in a major strike-slip environment."

Fletcher recommended a video showing plumes of earthquake-generated dust over mountains and quite a reaction to the quake by U.S. students that can be seen on the Internet. Fletcher noted that SDSU's Rockwell was able to notice a differential settling of dust clouds over the mountains in the video, something that appears to have helped scientists further figure out what may happened with the earthquake. The video also can be seen on the website of UCSD's Yuri Fialko.

Trans-Border Institute Director David Shirk said the quake set back several months a program with the Autonomous University of Baja California law school for training lawyers and public defenders because of building damage. Photos of damaged UABC buildings

Shirk also pointed out that many feel the story of the earthquake has been undercovered in the media.

Update, July 11: Agricultural payouts to farmers affected by earthquake end.

Update, Feb. 11, 2012: CICESE helps create specialized 3D map of quake, published in Science. The article is entitled "Near-Field deformation from the El Mayor Cucapáh earthquake revealed by differential Lidar." Among the authors is John Fletcher of CICESE (Centro de Investigación Científica y Educación Superior— the Center for Scientific Study and Higher Learning) in Ensenada. Story, Frontera (PDF).
Science magazine abstract.

Update, March 29, 2012: New earthquake-resistance standards for buildings in Baja California.

Update, Nov. 20, 2012: U.S. to buy some of Mexico's share of Colorado River water in exchange for money Mexico will use to repair earthquake damage to Mexicali irrigation canals

Update, April 3, 2013: State legislators say many of those affected by earthquake have been abandoned by the government, urge action. Story, El Mexicano (PDF).

Update, April 4, 2013: Agriculture Ministry says it has spent 355.5 million pesos ($29 million) to rehabilitate 54,440 hectares (21 square miles) of land damaged by the quake, and state education system says 293 damaged schools have been repaired. Stories, Frontera (PDF).

Update, April 5: Governor José Guadalupe Osuna Millán reinaugurates State Public Library in Mexicali, which was badly damaged in the Easter 2010 earthquake. Story, El Mexicano (PDF). Jump.

Update, Feb. 27, 2014 — Dissatisfied farmers threaten boycott of Agrobaja conference: El Mexicano reported that the Frente Campesino de la Zona Cero (The Ground Zero Farmers Front) is threatening to boycott the March 7-9 Agrobaja conference in Mexicali because it is dissatisfied with the accounting of how 60 million pesos ($4.5 million) has been spent to help agriculture recover from the Easter 2010 earthquake. The group seeks greater clarification of how the money has been spent. The group said 10,000 hectares have been rendered useless and 36,000 are experiencing greater salinity and are in danger of being lost for agricultural use.

To see some good photos of what the earthquake did
Other photos taken by SDSU's Tom Rockwell
A scientific look at the earthquake
KPBS' one-year anniversary story
Union-Tribune one-year anniversary story

Earthquake photos are from John Fletcher.