|The epicenter of Sunday's quake was located roughly 100 miles east of the Straits of Gibraltar. However its focus lay 400 miles below the ancient city of Granada (Click to view larger image.)|
Most of us seismologists think we know it all. Just look at this year's large earthquakes: Within a few hours after the devastating Haiti Earthquake in January, the fault on which it occurred was identified (see blog January 13, 2010); after the giant temblor in Chile in late February, it took only minutes to issue a tsunami warning (see blog March 1, 2010); and within 24 hours, scientists found ruptures in the Earth's surface caused by the strong quake which shook northern Baja California and the southern part of our state on April 4th (see blog April 9, 2010). This blog is full of entries describing all the things we know about earthquakes - but here is a real puzzler from Europe, which nobody has been able to explain so far.
Last Sunday night, shortly after midnight, the Earth rumbled under the Alhambra, the old Moorish citadel overlooking the ancient Spanish town of Granada. Seismic sensors all over Europe reacted to this 6.3 magnitude quake, but few people felt it and no damage was reported from anywhere. The computer programs in the data centers of the European seismic networks and at USGS's Earthquake Information Center in Golden, Colorado, quickly and automatically calculated the exact location of the quake: Its hypocenter lay just 8 miles southeast of Granada's city center - but at a depth of almost 400 miles.
|This map shows all earthquakes east of the Strait of Gibraltar, which have been recorded in the last 20 years. Their colors depict the depth of the hypocenters. The depest temblors recorded before Sunday's quake (red star) was the green cluster with depths not exceeding 100 miles.|
When seismologists got to their offices on Monday morning, they scratched their heads: "400 miles? Something must be wrong!" The deepest quakes in Europe occur under the Tyrrhenian Sea just north of the Italian island of Sicily. There, the African plate subducts under the European plate, similar to the movement of the Gorda plate beneath North America north of California's Cape Mendocino. But even in Italy, the quakes' foci do not reach deeper than 300 miles. In addition, there is no known subduction zone within 1000 miles of Granada - and all deep earthquakes known to researchers occur within the confines of these zones. Hence the question arose: Did some measurement go awry which led to locating the Granada quake wrongly to such super depth?
The answer is that the computations were all correct and consistent. All data centers came to the same conclusion. There was no doubt that the temblor occurred 400 miles below Granada. However, when researchers had time to search the archives, they found, that this quake wasn't so unusual after all. Since seismic recordings began, three very similar earthquakes have occurred in exactly the same location, in 1990, in 1973 and the strongest one (M=7.0) on March 29, 1954. European scientists even wrote three scientific papers describing these extremely deep events, but they left the question unanswered, why and how they occured. Sunday's quake only adds to the mystery. (hra056)
|Figure 1: Crossing the fault over the newly created "staircase" in Mexico's Highway 2 (from Southern California Seismic Network's webpage) (Click to view larger image.)|
In the last five days, Earth scientists have been swarming all over the northern part of Mexico's Baja California searching for geologic clues to the largest earthquake to strike in California and its immediate vicinity in almost 20 years. The Sierra El Mayor quake, as Sunday's 7.2 temblor has been officially named, occurred in a remote desert landscape about 30 miles south of the Mexican border town of Mexicali (see blog April 5, 2010). What have these scientists found so far?
|Figure 2: A map of the aftershocks, after E. Hauksson (Caltech) (Click to view larger image.)|
The quake's origin lay along the Laguna Salada Fault system, named after a very large, almost 40 mile long dry lakebed which lies immediately west of the fault. According to every study read by the blogger, this is a godforsaken place, more akin to Death Valley than any other location in Mexico's Sonoran Desert. The average precipitation is only one fifth of an inch per year; in the summer the mercury can rise to more than 120 degrees F and the wind can blow at hurricane strength, whipping up sand and salt particles into choking dust storms. This "Salty Lake," the translation of Laguna Salada from Spanish, is bound to the south by the Sierra El Mayor mountain range. Sunday's quake was named after these mountains, because it started adjacent to them at the southern end of the Laguna Salada fault at a depth of 11 miles. From its hypocenter, the fault system ruptured for at least 55 miles to the northwest.
Geologists from the CICESE Research Institute in Ensenada (Mexico) found many places east of Laguna Salada where the earthquake ruptured the Earth's crust all the way to the surface. One of the field geologists, John Fletcher, followed these ruptures for 17 miles along a stretch parallel to the Borrego Fault, one of the subfaults of the Laguna Salada system. The maximum displacement, or offset, at these breaks was more than 8 feet - an example of the extremely strong forces which zip open the Earth along this stretch of desert.
The most prominent surface faulting is seen on Mexico's Highway 2, the major "autopista" that connects Tijuana with Ciudad Juarez, the notorious city across the border from El Paso, Texas. At the north end of the Laguna Salada, where the fault crosses the highway, the earthquake has turned the roadway into a staircase. 12 distinct breaks were observed over a section of road stretching less than 15 yards, with each step about 5 inches in height (see Figure 1). The total displacement on this stretch of road alone was about 50 inches.
In the meantime, there is no sign that aftershocks are abating (see map, Figure 2). At least eight have had magnitudes of more than 5. The most recent of these occurred on Thursday morning. It had a magnitude of 5.3. (hra055)
When the Earth rumbled and high rises swayed on Sunday afternoon all over Southern California and parts of Arizona, most people were sure that the "Big One" had hit. This major temblor is expected to rip through the southern end of the San Andreas Fault and race northwest along the fault line causing damage and devastation on its way (see blog November, 10 2008). But when Sunday's swaying motion stopped and seismologists in Southern California Seismic Network's data center in Pasadena checked their records, it very quickly became clear that the "Big One" is still to come. Instead, the shaking was caused by a magnitude 7.2 quake south of the border, with its epicenter near the village of Guadelupe Victoria in the Mexican state of Baja California, about 35 miles south of Mexicali and 100 miles east of Tijuana.
Guadelupe Victoria lies quite a ways south of the southern tip of the San Andreas Fault (see blog November 4, 2008), roughly halfway between the Salton Sea and the northern end of the Gulf of California. From a geologist's point of view, this area is rather unique, because it marks the transition zone between two completely different tectonic regimes. All of California, from Cape Mendocino in the north to Cabo San Lucas at the southern tip of Baja California is dominated by the boundary between the Pacific and the North American Plates. North of the US-Mexican border, this boundary is represented by the San Andreas Fault system, along which the two plates slide past each other with a speed of about 2 inches per year.
South of the border, the picture is completely different. When looking at a map of northwestern Mexico, did you ever wonder about the fingerlike peninsula Baja California? How did this almost 800 mile long, extremely narrow stretch of land come into being? The cause lies under the seafloor of the Gulf of California, which separates Baja from the Mexican mainland. In this area the East Pacific Rise (EPR) rules the boundary between the two plates. Like the Mid Atlantic Ridge or the Red Sea Rift, the Earth's crust splits apart along the EPR. This split causes Baja - located on the Pacific Plate - to move westward, slowly drifting away from the Mexican mainland at less than 2 inches per year.
This movement is also causing the Gulf of California to slowly open up and to extend its shores further and further to the north. The epicenter of Sunday's quake lay exactly in the region where the Gulf is creeping north. This is a slow but steady movement, which has been going on for at least 8 million years. Strong earthquakes like Sunday's temblor have been observed in this area at least four times in the last hundred years, each one cracking the Earth's crust a little bit more. Down there in the salty marshlands where the Colorado River drains into the ocean, the land is slowly ripping apart, like opening a zipper in slow motion, one tooth at a time. (hra054)
|Figure 1: A theodelite.|
Earthquakes do not only generate seismic waves, which can be as devastating like those which recently hit Southern Chile and Haiti. They also cause a lasting movement of the Earth's crust. In fact, it is this very movement which - over millenia - defines the rate at which the tectonic plates drift over the Earth's mantle.
This permanent displacement was first noted a little more than one hundred years ago right on our doorstep. When scientists under the leadership of UC Berkeley's Andrew Lawson fanned out to investigate the effects of the Great San Francisco Earthquake of 1906, one of them, Harry F. Reid from Johns Hopkins University in Baltimore, took his theodelite along. In those days, these instruments (see Figure 1) were used by surveyors to measure the angles between fixed points in the field exactly, be they corner monuments of homesteading parcels or the locations of railroad tracks. Before the 1906 earthquake several such survey lines had been laid across the San Andreas Fault, which was then known among geologists as the "Rift Zone." When Reid revisited these points after the quake and took measurements with his scope, he noted that invariably points on opposite sides of the fault had moved to the right, in some cases by up to 18 feet.
In today's age of satellite navigation, theodelites are an item of the past, having been replaced by highly sensitive, geodetic GPS receivers. These instruments measure movement with much greater precision than your ordinary GPS instrument in a car. They also transmit the data in real-time to research labs, like the Berkeley Seismological Laboratory. In the Bay Area alone, researchers from half a dozen institutions jointly operate a network of 67 such GPS receivers and continuously measure the movement caused by the slip of the Pacific and North American Plates. (See blog October 17, 2008.)
A similar, international cooperation has existed for years in South America under the acronym CAP - Central and Southern Andes GPS Project. This group, led by Mike Bevis from Ohio State University, operates several dozen GPS sites in many countries in Latin American, from Bogota in the North to Punta Arenas in the South. When these researchers compared the GPS data from before and after the Great Chilean Earthquake of February 27th, 2010, they noticed a huge movement. The giant temblor had moved the city of Concepcion in the heart of the epicentral region by more than 10 feet to the west. Chile's capital Santiago moved more than 10 inches, and even in Buenos Aires, more than one thousand miles from the earthquake's focus, the Earth's crust had moved by 1.5 inches towards the Pacific. (hra053)
|Figure 2: Preliminary Coseismic Displacement Field M 8.8 Maule Earthquake, Chile, Feb 27 2010 (courtesy of CAP).|
|This map shows all earthquakes in southern Chile over the last 100 years with magnitudes larger than 5.5. Until Saturday, the area around Concepcion had much fewer quakes than the rest of coastal Chile. The yellow star depicts the epicenter of the most recent 8.8 temblor. (Map courtesy of USGS)|
The huge earthquake, which on Saturday devastated large areas of Chile, was the largest temblor anywhere on Earth in more than five years. With a magnitude of 8.8, it was about 30 percent smaller than the great Sumatra earthquake of December 28, 2004, but also almost one thousand times more energy than the last destructive temblor in our area, the Loma Prieta earthquake of October 1989.
Like the Haiti earthquake of January 12, 2010, this great Chilean earthquake was not unexpected. Here is why: The entire Pacific coast of South America is prone to very large earthquakes, because it defines the collision zone of the westward moving South American plate and the eastbound Nazca Plate. These plates move toward each other at a rate of almost 3.2 inches per year. The South American plate is continental and carries all of Latin America south of the Isthmus of Panama. The Nazca-Plate underlies most of the southeastern Pacific Ocean and, like all oceanic plates, it is much denser than a continental plate. Due to this density difference, the Nazca-Plate is pushed under by the South American Plate by the collision and dives into the Earth's mantle. In the region of the epicenter of Saturday's quake, the angle under which the Nazca Plate subducts beneath South America is about 27 degrees.
As the oceanic plates dives, it tries to take the leading edge of South America with it, thereby bending it downwards. As continental crustal rocks are neither plastic nor elastic, but rather brittle, the bending causes an enormous strain in the rocks of the westernmost South American plate. Finally the bending continues so far that the strain exceeds the strength of the rock. It then breaks and the bending edge of the plate snaps back. This happens extremely quickly and results in huge earthquake - as we saw on Saturday.
However, not all areas affected by the subduction of the Nazca-Plate snap back at the same time. During the last century, regions along most of Chile's 2000 mile long coastline experienced such a snap back in the form of a huge earthquake. Seismologists counted at least four quakes with magnitudes larger than 8 and countless temblors between 7.0 and 7.9. In fact, the largest temblor ever measured occurred just 200 miles south of Saturday's epicenter, when on May 22, 1960, the Earth shook in a magnitude 9.5 earthquake.
The only region along Chile's coast which has been an exception, and had not experienced a major quake in the last hundred years, was the coastal area of Concepcion. And this is exactly, where Saturday's "terremoto" hit with its devastating effects. (hra052)