Science: Apr 12, 2012

The Indonesian island of Sumatra is located in a seismic area of the world. Other than the subduction zone, where the Indian plate Dives under the Burmese plate, the Indian Ocean has diffused seismicity zone which is situated to the west of the subduction zone.

Click here for a PDF version of the graphical representation of a tsunami.

There were seven major earthquakes between 6 and 7.5 magnitude in this region in the last 25 years. The 8.6 magnitude earthquake on April 11, 2012, and the aftershock of 8.2 magnitude should now be added to this list. There are up to now has been 22 aftershocks, and in addition to a replica of a 8.2 magnitude, the rest have a magnitude between 5 and 6.

The fault in which these major quakes occurred in the last 25 years have East-West direction. The quakes occurring in the fault is a strike-slip nature. To the east-west trending zone extends from 80 degrees East longitude at about 93 degrees East longitude (where the April 11 earthquake occurred). Therefore the length of the area (about 13 degrees) is about 1,300 km.

The April 11 earthquake was not in the subduction zone, but to the west of it. "There are intra-plate earthquake," said RK Chadha, Chief Scientist at the National Geophysical Research Institute (NGRI), Hyderabad. An earthquake is called intra-plate when it happens within the plate and not on the edges of two plates. Because it is intra-plate earthquake, no subduction takes place, and the movement is along the fault where one side of the plate grinds past the other horizontally causing a strike-slip fault.

"On April 11 an 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 the diffused zone of seismicity is dominated by mid-ocean ridges in the Indian Ocean.

There are two ridges in the Indian Ocean - Southeast Indian Ocean ridge and the North Atlantic ridge. "These two ridges dominate the plate movement in that part of the Indian Ocean, where the April 11 earthquake occurred," Dr. Chadha said. In this part, the plate movement is at a rate of 52 mm per year in a NNE direction.

"It's faith, that the Indo-Australian plate is broken in the diffused zone of seismicity," he said. "Then it can become the new border between the Indian plate and the Australian plate."

"The April 11 earthquake appears to be near the subduction zone," he said. "Looks to be close to the junction where the strike-slip fault meets the subduction zone, causing a triple junction."

Explaining the emergence of large magnitude earthquakes in the past few years, he said, a kind of clustering of quakes with magnitude greater than 8.5 have been happening since the December 2004 Sumatra earthquake.

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 watching from the December 2004 Sumatra earthquake," he said. "Beginning December 2004, is approximately six quakes of magnitude greater than 8.5 globally. It follows the same pattern of 1950-1965." The chances of another high-magnitude earthquake occurring somewhere in the world can not be ruled out, if this clustering is indeed happening. "Earthquakes occur randomly, but we still could see some kind of pattern is now emerging," the NGRI scientist said.

Science

Thursday, April 12, 2012

Why April 11 quake was caused by a strike-slip fault

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