The massive earthquake off Indonesia surprised scientists: Usually this type of jolt isn't this powerful. The biggest earthquakes tend to occur in subduction zones where one plate of the Earth's crust dives under another. This grind produced the 2004 magnitude-9.1 Indian Ocean disaster and the magnitude-9 Japan quake last year.
Yesterday's magnitude-8.6 occurred along a strike-slip fault line similar to California's San Andreas Fault. Scientists say it's rare for strike-slip quakes, in which blocks of rocks slide horizontally past each other, to be this large.
"It's clearly a bit of an odd duck," said seismologist Susan Hough of the US Geological Survey in Pasadena, Calif.
As one of the world's most seismically active places, Indonesia is located on the Pacific "Ring of Fire," an arc of volcanos and fault lines encircling the Pacific Basin.
Pressure builds up in the rocks over time and is eventually released in an earthquake. Wednesday's quake was followed by a magnitude-8.2 aftershock. Both were strike-slip quakes.
"A week ago, we wouldn't have thought we could have a strike-slip earthquake of this size. This is very, very large," said Kevin Furlong, a professor of geosciences at Penn State University.
So large, in fact, that the main shock went into the history books. Record-keeping by the USGS National Earthquake Information Center ranks Wednesday's shaker as the 11th largest since 1900. It's probably the largest strike-slip event though there's debate about whether a similar-sized Tibet quake in 1950 was the same kind.
A preliminary analysis indicates one side of the fault lurched 70 feet past the other a major reason for the quake's size. By contrast, during the 1906 magnitude-7.8 San Francisco earthquake along the San Andreas perhaps the best known strike-slip event the ground shifted 15 feet. The Sumatra coast has been rattled by three strong strike-slip quakes since 2004, but yesterday's was the largest.
Earthquake-hit Aceh rushes for hills
BANDA ACEH: Thousands of panic-stricken people packed the streets of Banda Aceh as they streamed out of the city in cars and motorcycles seeking higher ground moments after a massive earthquake measuring 8.6 struck about 434 km southwest of the city on Wednesday.
A series of aftershocks, one of them measuring an equally serious 8.2, rattled people further as they packed the streets hoping to find a way out of the city to the hills. There were no immediate reports of casualties in Aceh.
Immediately after the temblor struck, a high tsunami alert was issued along the Indian Ocean rim and fears of a wall of waves coming down on the coastal areas looked real. People fled buildings. Terrified residents screamed "God is great" as they jumped into cars and motorcycles, clogging streets as they fled.Initial estimates had put the magnitude of the quake at 8.9, but later this was scaled down to 8.6.
Quake officials said they feared a tsunami had been generated and that was racing for the Aceh coast. For close to two hours, the city was on the edge. The ground had shaken for four minutes and one resident recalled. "This isn't the strongest earthquake I have felt, but it simply refused to end." Thousands feared a repeat of the deadly 2004 disaster, a similar quake that had devastated the province and killed upwards of 200,000 people.
As tsunami fears compounded the quake shock, electricity was cut in Aceh and traffic jams kept getting worse. "Sirens blared and Koran recitals began in mosques everywhere," Reuters reported. There were reports of people jumping from windows in a desperate attempt to escape. Patients poured out on to the streets with drips still attached to their arms.
But when the fearsome wall of water failed to hit Aceh for close to two hours, panic seemed to be subsiding. Later, an expert described the quake at Aceh as a strike-slip quake, not a thrust quake. "In a slip quake, the earth moves horizontally and doesn't displace large volumes of water," he said explaining why a tsunami hadn't struck. But, two hours later, a massive aftershock - with a similarly huge magnitude of 8.2 - struck only 110 miles further out to sea, unleashing fresh fears. A new tsunami alert was hoisted.
Indonesian president Susilo Bambang Yudhoyono moved to calm nerves, saying that there appeared to be no serious casualties and local residents had been taken to safety. He added that "as of this time there is no threat of tsunami" - despite an international warning being issued. "The situation is under control so far. It is a very different situation from 2004 when Aceh was faced with a deadly tsunami," he said reassuringly.
A series of aftershocks, one of them measuring an equally serious 8.2, rattled people further as they packed the streets hoping to find a way out of the city to the hills. There were no immediate reports of casualties in Aceh.
Immediately after the temblor struck, a high tsunami alert was issued along the Indian Ocean rim and fears of a wall of waves coming down on the coastal areas looked real. People fled buildings. Terrified residents screamed "God is great" as they jumped into cars and motorcycles, clogging streets as they fled.Initial estimates had put the magnitude of the quake at 8.9, but later this was scaled down to 8.6.
Quake officials said they feared a tsunami had been generated and that was racing for the Aceh coast. For close to two hours, the city was on the edge. The ground had shaken for four minutes and one resident recalled. "This isn't the strongest earthquake I have felt, but it simply refused to end." Thousands feared a repeat of the deadly 2004 disaster, a similar quake that had devastated the province and killed upwards of 200,000 people.
As tsunami fears compounded the quake shock, electricity was cut in Aceh and traffic jams kept getting worse. "Sirens blared and Koran recitals began in mosques everywhere," Reuters reported. There were reports of people jumping from windows in a desperate attempt to escape. Patients poured out on to the streets with drips still attached to their arms.
But when the fearsome wall of water failed to hit Aceh for close to two hours, panic seemed to be subsiding. Later, an expert described the quake at Aceh as a strike-slip quake, not a thrust quake. "In a slip quake, the earth moves horizontally and doesn't displace large volumes of water," he said explaining why a tsunami hadn't struck. But, two hours later, a massive aftershock - with a similarly huge magnitude of 8.2 - struck only 110 miles further out to sea, unleashing fresh fears. A new tsunami alert was hoisted.
Indonesian president Susilo Bambang Yudhoyono moved to calm nerves, saying that there appeared to be no serious casualties and local residents had been taken to safety. He added that "as of this time there is no threat of tsunami" - despite an international warning being issued. "The situation is under control so far. It is a very different situation from 2004 when Aceh was faced with a deadly tsunami," he said reassuringly.
The Indonesian island of Sumatra is located in a highly seismic area of the world. Other than the subduction zone, where the Indian plate dives under the Burmese plate, the Indian Ocean has a diffused seismicity zone that lies to the west of the subduction zone.
ReplyDeleteClick here, for a PDF version of the graphic presentation on tsunami.
There have been seven major earthquakes between 6 and 7.5 magnitude in this region in the last 25 years. The 8.6 magnitude quake of April 11, 2012, and the aftershock of 8.2 magnitude should now be added to this list. There have so far been 22 aftershocks, and except for one aftershock of 8.2 magnitude, the rest have a magnitude between 5 and 6.
The fault along which these major quakes have occurred in the last 25 years has an east-west direction. The quakes occurring in this fault are of strike-slip nature. The east-west trending zone extends from 80 degree East longitude to around 93 degree East longitude (where the April 11 quake occurred). Hence the length of the zone (about 13 degrees) is around 1,300 km.
The April 11 quake was not in the subduction zone but to the west of it. “It is an intra-plate earthquake,” said R. K. Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad. An earthquake is called as intra-plate when it occurs within the plate and not at the edges of two plates. Since it is an intra-plate earthquake, no subduction takes place, and the movement is along the fault where one edge of the plate grinds past the other horizontally causing a strike-slip fault.
“The April 11 earthquake was about 100 km west of the subduction zone,” he said. “It is within the Indo-Australian plate.” According to him, the movement of this plate in this region of diffused zone of seismicity is governed by mid-ocean ridges in the Indian Ocean.
There are two ridges In the Indian Ocean – southeast Indian Ocean ridge and the South Indian Ocean ridge. “These two ridges govern the plate movement in this part of the Indian Ocean, where the April 11 earthquake took place,” Dr. Chadha said. In this part, the plate movement is at a rate of 52 mm per year in a NNE direction.
“There is a belief that the Indo-Australian plate is breaking along the diffused zone of seismicity,” he said. “So this may become the new boundary between the Indian plate and the Australian plate.”
“The April 11 earthquake appears to be near the subduction zone,” he said. “Looks to be near the junction where the strike-slip fault meets the subduction zone, causing a triple junction.”
Explaining about the occurrence of large magnitude earthquakes in the last few years, he said some kind of clustering of quakes with magnitude greater than 8.5 has been happening since the December 2004 Sumatra quake.
In the 100 years of earthquake data, the 15-year period between 1950 and 1965 witnessed a clustering of large quakes of magnitude greater than 8.5. “A similar clustering of large-magnitude earthquakes is now being observed since the December 2004 Sumatra earthquake,” he said. “Starting from December 2004, there have been about six quakes of magnitude greater than 8.5 globally. It is following the same pattern of 1950-1965.” The chances of another high-magnitude quake occurring at some place in the world cannot be ruled out if this clustering is indeed happening. “Earthquakes occur randomly, but we are still able to see some kind of pattern now emerging,” the NGRI scientist said.