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)
|Seismic hazard map of Eastern Turkey (Source: USGS/NEIC)|
Looking at the world's seismic hazard map, the area around Lake Van in the border region between Turkey, Iran and Iraq is marked in a suspiciously dark brown color: It indicates that this area in far eastern Anatolia has one of the highest earthquake hazards in the world. Sunday's quake and the destruction it caused in the city of Van and the neighboring villages was therefore no surprise to seismologists. In Ercis alone, the town of 75,000 inhabitants closest to the epicenter, more than 80 multi-story buildings collapsed, trapping dozens, if not hundreds of people in the rubble.
Turkey is one of the most seismically active countries in the world. The northward push of the Arabian Plate into the Euasian Plate with a speed of approximately one inch per year is the ultimate cause of the tectonic activity. However, this collision is not as clean and well defined as the plate boundary in our Californian backyard. While here, a rather narrow San Andreas Fault zone marks the boundary between the Pacific and the North American plates, the collision zone in Eastern Turkey and the neighboring areas looks more like a complex tectonic jumble. Over several hundred square miles of high mountain ranges and dozens of separate faults dominate the region.
Before Sunday's quake, the area had already seen many destructive temblors. A magnitude 7.3 earthquake shook the region on November 24, 1976. Its epicenter was less than 40 miles from the most recent quake. Many villages in Turkey and neighboring Iran were destroyed, and more than 5000 people died. On May 22, 1971 a moderate earthquake with a magnitude of 6.3 shook a region west of the current zone of destruction. Then more than 90 percent of all houses in the city of Bingol collapsed and several thousand people died.
Although the region around Van in East Anatolia has a large seismic risk, even greater hazards lurk in the Earth's crust further to the West. The North Anatolian Fault zone, a tectonic regime similar to our San Andreas Fault, underlies some of the largest cities in Turkey, including Istanbul, the gateway between Europe and Asia. The devastating Izmit earthquake of 1999 with a magnitude of 7.6 broke a section of this fault. More than 17,000 people were killed, 50,000 were injured and 500,000 were left homeless. The most destructive quake in Turkey in the last hundred years also occurred along the North Anatolian Fault. A magnitude 7.8 earthquake struck the city of Erzincan in 1939, killing an estimated 33,000 people. (hra065)
|Community Internet Intensity Map for October 20 M4.0 Berkeley earthquake Click to view larger image.|
It was early Thursday afternoon, at 2:41 pm to be exact, when the Earth shook under Berkeley. Nothing was damaged, but the jolt was widely felt. At this time of the year, people here in the East Bay are always somewhat on the edge. Almost exactly 22 years ago, on October 17, 1989, the Loma Prieta quake hit and dozens of people died during the collapse of the double-decker Cypress Structure carrying freeway 880. Two years later, on October 19, 1991, the Oakland Hills firestorm claimed many more lives, and several thousand houses burned to oblivion. No wonder then, that long time residents of the area took cover when the latest temblor hit the area, even though it was mild compared to other temblors. Thursday's quake had a magnitude of 4.0 and was located on the Northern Hayward Fault more than 6 miles below the Clark Kerr Campus of UC Berkeley. It was felt over a large area in Northern California, from Santa Rosa in the North, to Sacramento in the East and all the way to Gilroy to the South. Within three hours of the quake, more than 15,000 people had reported their observations. If you felt the quake but have not yet reported, it is not too late. You can do so on the "Did you feel it?" website. Your report is important, because from the the collective observations, seismologists draw conclusions about the different ways the ground in the Bay Area shakes in response to seismic waves.
The quake was the strongest temblor to be felt in the Bay Area since March 1, when a quake with a magnitude of 4.5 struck the area near the Geysers hydrothermal area in Sonoma and Lake counties.
The quake under Berkeley behaved exactly the way seismologists expected. It was a strike slip motion along the Hayward Fault. The area west of the fault, that is, the lowlands of Berkeley, Oakland and Albany moved a few tenths of an inch to the North with respect to the Berkeley Hills. This northward movement is caused by the sliding of the Pacific Plate against the North American Plate. The Hayward Fault is one of the three major earthquake faults in the Bay Area which convey this plate movement. The others are the San Andreas Fault further to the West and the Calaveras Fault in the East. Even with a modest earthquake like Thursday's temblor, aftershocks can be expected. By 5 pm, two aftershocks, one of magnitude 1.8, the other a 2.2, had already occurred. Shortly after 8 pm, a third aftershock with a magnitude of 3.8 again rattled the nerves of the people in the East Bay. (hra064)
Every textbook about Earth Science tells you that the giant lithospheric plates move past each other at a velocity of a few inches per year. The most common metaphor is that their speed compares directly to the rate at which a fingernail grows. That sounds gentle, harmless, and most of all managable. Unfortunately, these numbers are utterly misleading, because they are very long term averages measured over hundreds or even thousands of years. When you look at plate movement in the short term, you recognize a completely different picture. Most of the time, nothing happens at all. The plates are tightly locked against each other, snapping perhaps in a few small earthquakes here or there. This tectonic inactivity gives us, the residents of earthquake zones, a false sense of security. Because suddenly, within a fraction of a second and without any warning, the geologic interlocking may fail and then all hell breaks loose. The mechanical energy, caused by the constant push of the churning viscous mantle below and stored in the plates over hundreds of years, is released. In a few seconds the plates jump past each other by dozens of yards. In many respects, the plate movement is like the fate of a drag racing car. Most of time, the machine sits around idly, but once in a while, its engine is fired up and the dragster races a few hundred yards at lightning speed.
This is what happened last Friday off the east coast of Honshu. When the westward moving Pacific Plate finally unlocked itself from the Eurasian Plate, the result was the giant earthquake. Over the long term the Pacific and Eurasian Plates drift past each at an average rate of about 3.25 inches per year. But on Friday, they raced passed each other by dozens of yards in about two minutes.
Several researchers have already made models of how the interlocking between the plates actually failed, the quake's so called "rupture process". Gavin Hayes from the USGS office in Golden, CO, used the recordings of 60 seismic stations from all over the globe to compute the rupture. According to (his calculations) the rupture area underground was almost 200 miles long and 125 miles wide. Imagine, the entire state of West Virginia slipping up to 20 yards eastwards in less than two minutes.
A group of scientists from the Geoforschungszentrum in Potsdam, Germany, used the recordings of almost 500 sensitive GPS-stations in Japan to model the rupture. Their calculations yielded a 250 mile long zone with a offset of almost 28 yards (see Figure 2). Researchers from Harvard University used several hundred earthquake stations on the US mainland for their calculations and came to similar conclusions. They estimate the rupture to be 240 miles long by 150 miles across. They even simulated how the rupture spread over this area (see their animation).
In the meantime, the massive temblor has been given a name, and the magnitude of the "Great Tohoku Earthquake" has officially been upgraded to 9.0. (hra063)