The U.S. Geological Survey (USGS) initially estimated Wednesday's
earthquake off the west coast of Northern Sumatra, at 8.9 magnitude. But
it was subsequently lowered to 8.7 and then to 8.6. Similarly, the
focus of the quake was first thought to be 33 km from the surface, but
was later changed to 22.9 km.
The signals from a high-magnitude quake flood the nearby earthquake
recording station, leading to an initial estimation of high magnitude.
Click here, for a PDF version of the graphic.
Explaining why tremors were felt in several Indian cities, R.K. Chadha,
Chief Scientist at the National Geophysical Research Institute (NGRI),
Hyderabad, said: “Tremors are felt at faraway locations due to surface
waves produced by an earthquake. Surface waves cause a lateral movement
of the particles in the earth's medium. The earth behaves like an
elastic medium when seismic waves are travelling.”
Unlike the December 26, 2004 quake caused by a thrust fault, Wednesday's
quake was caused by a strike-slip fault. The fault had moved in a north
northwest-south southeast direction. In the case of a strike-slip
fault, the fractured crust slides past each other laterally.
“The movement along the fault should be in the order of a few metres,”
said Dr. Chadha. “Only a detailed modelling using data from 40-50
stations can reveal the actual amount of displacement.”
The reason why the 8.6-magnitude quake did not cause killer tsunami
waves was the nature of the faulting. “To generate giant tsunami waves,
there should be great vertical displacement of the water column,” Dr.
Chadha said. “This happens only in the case of a thrust fault [the
December 2004 quake] or a dip-slip fault. Strike-slip fault will not
generate tsunami waves.”
According to him, though the quake was a strike-slip fault, there should
have been a small amount of oblique movement along the fault. This is
the reason why Wednesday's quake caused small tsunami waves.
Even the 7.2-magnitude quake of January 10, 2012, at a depth of 20.5 km
from the surface off the west coast of northern Sumatra, was due to
strike-slip faulting.
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