|Figure 1: This relief map of the area around New Madrid shows the low-lying areas of the Mississippi Valley in blue and purple. The higher elevations are depicted in green, yellow and brown. The white dots are the epicenters of recent earthquakes. The Reelfoot Fault is shown as a small red line. (Source: Center for Earthquake Research Information, Memphis, Tenn.)|
The earthquake sequence that struck the central Mississippi Valley two hundred years ago (see most recent blog entry) was remarkable. It occurred far from any tectonic plate boundary, the regions normally marked by high levels of seismic activity. Although temblors in these areas are much less frequent than in California or Alaska, such intra-plate earthquakes are not that unusual. The 5.8 quake that shook Virginia and the capital in August was a reminder that such quakes do happen. But what is it that causes the Earth to tremble thousands of miles from any plate boundary?
Put in simple terms, we can compare the stable interior of a continent with a pane of glass. Virtually every place in the contiguous United States east of the Rocky Mountains is part of this solid platform. Even though the Earth's crust in this area is quite solid when compared to California, it is not totally free of mechanical stress. On the one hand, the continental interior is battered by the actions at the plate boundary in the West. It is also still recovering from the huge load of the ice sheet of the last ice age. During that era, the northern part of the continent was completely blanketed with layers of ice several kilometers thick. It melted only 12,000 years ago, relieving the crust of its load. Another factor is the age of the crust far away from any active tectonic boundary. While the rocks along plate boundaries are usually young, continents can be several hundred million years old. Their age makes them extremely rigid, in contrast to the more ductile crust at the continental fringes.
And this is where the comparison with the pane of glass comes in: When battered at the edges, the mechanical stresses in the center of the pane can build up. Once under stress, a slight puncture at one point is enough to make the entire pane shatter. The area around New Madrid is such a puncture point. There the Reelfoot Fault, an ancient rupture line, crosses the Mississippi Valley (red line in Figure 1). Because of the accumulated stresses and the rigidity of the continental crust, slight movements along this fault can lead to larger ruptures, such as those of two hundred years ago.
|Figure 2: Earthquake hazard map of the United States. The large oval in the eastern half of the continent shows the hazard associated with the New Madrid Seismic Zone. Its center is purple, depicting a hazard similar to the one we have to live with here in California. (Source: USGS)|
Paleoseismic investigations in this area have shown that the earthquake sequence of 1811-1812 was not unique. Similar strong shaking occurred at least three times during prehistoric times - probably around 1450 and 900 A.D. and 2350 B.C. Because other strong temblors cannot be ruled out in the future, scientists from the USGS have determined that the seismic hazard in the area around New Madrid is as high as that along the San Andreas Fault or the Pacific coast of Alaska (see Figure 2). (hra070)
December 16, 2011
|Figure 1: Image from the archives the State Historical Society of Missouri, courtesy of the National Information Service for Earthquake Engineering, EERC, University of California, Berkeley.|
When you drive North on Kentucky State Highway 94 away from the Tennessee Border but close to the Mississippi, you may notice one of those ubiquitous solid bronze Historical Highway Markers (see figure 2). The inscription talks about the "greatest earthquake recorded in North America," which was centered in this area. What, major earthquakes in Kentucky? Aren't the really big ones supposed to happen only at the West Coast or in Alaska? Indeed the area where today's states Kentucky, Tennessee and Missouri meet was shaken by a major temblor exactly two hundred years ago today.
The earthquake struck at about 2:15 in the morning. One eyewitness, Eliza Bryan, described the unusual events: "The screams of the affrighted inhabitants running to and fro, not knowing where to go, or what to do - the cries of the fowls and beasts of every species - the cracking of trees falling, and the roaring of the Mississippi - the current of which was retrograde for a few minutes, owing as is supposed, to an irruption in its bed -- formed a scene truly horrible." Two more strong earthquakes struck the area within the next two months, one on January 23 and one on February 7, 1812.
Two hundred years ago, the Mississippi River valley was the frontier to the American West. Only eight years before the earthquake struck, the US had bought from the French the Louisiana Purchase, a huge swath of land west of the mighty river. New Madrid, a town across the river from today's historical marker, had been settled in 1789, and its 3000 residents were proud to live in the oldest city west of the river. The earthquakes, however, did not only destroy most of the ramshackle houses and log cabins in this frontier town. As the marker indicates, the tremors were also felt almost everywhere the white man had settled in America, from Boston and Maine all the way to New Orleans. The shaking was so severe that chimneys toppled in Cinncinati, Ohio, and church bells started ringing on their own in South Carolina.
Taking into account the huge area affected by the seismic waves of these three shakers, seismologists first rated the quakes with magnitudes of 8 or greater, clearly topping the magnitude 7.8 quake that shook San Francisco in 1906. When the historical marker was erected in 1964, the superlative of "greatest earthquake" mirrored the state of knowledge at the time. Now, however, more thorough investigations have led to a significant downgrade of the "New Madrid Earthquake Series." At most, the quakes had magnitudes of 7, maybe even only 6.5.
|Figure 2: The New Madrid historical marker.|
As in the case of the most recent quakes in Oklahoma, the area of seismic shaking was deceiving to the early students of the New Madrid quakes (see blog 9 Nov 11). Read more about the causes of the mid-continent earthquakes in the next blog.
When high-rises in Mexico City swayed for more than a minute on Saturday night, people feared for their lives. Many inhabitants of the Mexican capital remembered the nightmarish scenario 26 years ago, when similar movements led to one of the worst natural disasters our neighbors to the south ever suffered. On 19 September 1985 an earthquake, with a focus more than 200 miles away, wreaked havoc on many buildings in the capital city (see blog 19 September 2008). But the swaying on Saturday subsided without causing any significant damage.
Nevertheless, the latest 6.5 earthquake was a reminder that Mexico City is vulnerable to seismic waves approaching from country's distant Pacific coast. The earthquakes there are caused by the subduction of the Cocos Plate, one of the dozen or so tectonic plates which make up the outer layer of the Earth, under the North American Plate. The Cocos Plate approaches from the southwest with a speed of just over 2 inches a year. Upon colliding with the North American Plate, it bends downward and begins to sink into the Earth's hot mantle. The mechanical stresses associated with the bending and the temperature change cause the subducting Cocos Plate to break, hence the temblors.
The Cocos Plate diverges from the Pacific Plate along the East Pacific Rise. Its subduction dominates the tectonic regime along most of the western coast of Central America, from Panama all the way to the Mexican resort town of Puerto Vallarta. In the past century alone, dozens of earthquakes occurred along this subduction zone, among them the 8.1 temblor which cost thousands of lives and produced severe damage in Mexico City. The locations of each of the strong quakes which occurred along Mexico's Pacific coastline are shown as grey areas on the map, with the extremely destructive 1985 quake marked in green.
The area just to the southeast of the green region is conspicuously empty. For more than 150 years, no significant quake with a magnitude greater than 7 has occurred in the immediate vicinity of the tourist town of Acapulco in the Mexican state of Guerrero. The absence of large earthquakes in one region along a tectonic front is called a seismic gap. Such gaps are tectonic time bombs waiting to go off in a major earthquake. At most other locations along the Cocos subduction zone the tectonic stresses have been release by earthquakes over the last century. However, during that time, nothing happened in the Guerrero gap. There the stresses caused by subduction have built up. As a result, seismologists expect a really big earthquake in this region. It would not only flatten Acapulco but, like the 1985 temblor, would cause significant damage in Mexico City.
Although Saturday's quake (red star) occurred exactly in this gap, it did not release much tectonic energy. With a magnitude of 6.5 it was more than 75 times weaker than the 1985 temblor. That means that the wait for big one which will hit Acapulco is still on. (hra068)
Last Saturday's magnitude 5.6 earthquake in Oklahoma not only shook the Boone Pickens Stadium at Oklahoma State, where the home team, the Cowboys, had just beaten the Kansas State Wildcats 52 to 45. This largest quake in the Sooner State's history shook a huge area. Almost 60,000 people tuned into the "Did you feel it?" website from the US Geological Survey and reported how they felt the shaking. Computers automatically plotted each of these reports on a map, where the color depicts the intensity of the shaking felt by the person reporting. Blue represents a very light rumble, yellow stands for strong shaking and dark red and brown means that you would be lucky to survive.
The map of responses for the Oklahoma shaker covers an enormous area, ranging west to east from the Texas panhandle to Memphis, Tennessee, and north to south from Omaha, Nebraska, and Des Moines, Iowa, to Austin, Texas (see left panel in our figure). In total, the M 5.6 earthquake was felt in an area of more than 300,000 square miles, roughly three times the size of California.
Now compare that with the shake area of the largest Northern California Earthquake in the last 22 years, the October 31, 2007, temblor on the Calaveras Fault near Alum Rock northeast of San Jose. With a magnitude of 5.4, this quake was just a little smaller than Oklahoma's quake. Although it was felt as far away as Redding in the north, Carson City, Nevada, in the east and Bakersfield in the southeast, the area of more intensely felt shaking was concentrated around the greater Bay Area (see right panel in our figure). The region where shaking was felt consistently covers only about 50,000 square miles. Even if you double that number because California is a coastal state and the shaking cannot be felt on water, the shake area of the Alum Rock quake is only about a third of shake region of the Oklahoma quake - even though the two quakes were comparable in magnitude.
What causes this discrepancy? The answer is that the tectonic regime in the contiguous United States east of the Rocky Mountains differs substantially from the situation west of the mountain range. The Earth's crust here in the West is torn and jumbled by the tectonic collision of the Pacific and the North American plates. The boundary is not confined only to the San Andreas Fault, but influences the entire region from the Pacific Coast all the way to the Rockies. East of the range, however, most of the Earth's crust is old and geologically very stable. It has not seen any plate collisions for hundreds of millions of years. Seismic waves can travel through such stable regions with much less interference than in areas that have recently been torn up by tectonic forces. When an earthquake hits east the Rockies, the Midwest and East ring like a bell, to be "heard" for hundreds of miles. Here in California and the West, however, the seismic waves are more like a "thud." They are scattered, absorbed, and attenuated by the battered crust, thereby losing their punch in a much shorter distance. (hra067)
Late Saturday night, Oklahoma, a state more known for its destructive tornadoes than for earthquakes, was shaken by the strongest temblor in its history. An earthquake with a magnitude of 5.6 struck near the town of Prague in Lincoln County, about 40 miles east of Oklahoma City. It was preceded by a magnitude 4.7 foreshock, which occurred about twenty hours before the main event at almost the same epicenter.
The main shock caused damage to at least five homes, mostly when chimneys caved in. One man was reportedly injured when he tripped and hit his head while attempting to flee his home near Prague. U.S. Highway 62 buckled in at least two places during the quake, causing a sinkhole east of Meeker, but road crews repaired the damage overnight. The quake also damaged a 40-foot spire at St. Gregory University in Shawnee and ruptured a water pipe in Chandler.
These two quakes are, so far, the culmination of a very unusual series of temblors, which has been shaking Oklahoma since 2009. Last year alone, the Oklahoma Geological Survey recorded 1047 earthquakes in the Sooner State, more than a hundred of them were felt, mostly in the greater vicinity of the State's capital, Oklahoma City. While such numbers are normal for California and other earthquake prone regions, they are certainly unusual for a state in what geologists consider to be the stable part of the Nation.
|Since 2009 the number of earthquakes in Oklahoma has risen dramatically. This figure shows the annual number of earthquakes in Oklahoma listed in the catalog of the Advanced National Seismic System. The local network of the Oklahoma Geological Survey has registered even more temblors.|
Even more puzzling to seismologists is the fact that these quakes are not associated with the only known earthquake fault in Oklahoma, the Meers Fault. It runs over a length of approximately 30 miles through Comanche and Kiowa counties in the southwestern part of the state. Geological studies showed, that this fault was active for the last time about 1300 years ago. It may then have produced an earthquake with a magnitude between 6.5 and 7.0.
Until Saturday's temblor struck the biggest recorded earthquake in Oklahoma occured in 1952, when the town of El Reno, just west of Oklahoma City was shaken by a magnitude 5.5 event. Before Oklahoma became a state in 1907, a quake of similar magnitude struck in northeastern Indian Territory in 1882. The exact location of that tremor is unknown.
Saturday's late-night quake was slightly lower in magnitude than the one that rattled the East Coast on August 23. That 5.8 magnitude earthquake was centered in Virginia and felt from Georgia to Canada. No major damage was reported then, although cracks appeared in the Washington Monument. The National Cathedral suffered costly damage to elaborately sculpted stonework, and a number of federal buildings were evacuated. (hra066)