Dec. 26 quake measured 9.3 on Richter scale

Dec. 26 quake measured 9.3 on Richter scale

Richard Ingham, Agence France-Presse/Paris

In a reassessment of the Dec. 26 earthquake that unleashed the
Indian Ocean killer tsunami, scientists say the temblor measured
9.3 on the Richter scale -- more than twice as powerful as
originally estimated and the second biggest quake ever recorded.

The quake split the ocean floor northward from Sumatra along
1,200 kilometers, twice as long as previously thought, according
to their research, which appears on Thursday in Nature, the
weekly British science journal.

The event released so much strain along this particular part
of the fault that in theory there should be no quake of similar
magnitude, or a similar tsunami, there for another 400 years,
said geologists Seth Stein and Emile Okal of Northwestern
University, Illinois.

But farther south, it is a different picture.

The scientists -- who wrote before last Monday's quake, which
also struck western Sumatra -- warned with uncanny prescience
that "a great earthquake" with the potential to generate a large
tsunami remained a threat south of the Dec. 26 site.

The Dec. 26 quake occurred off northwestern Sumatra, at the
nexus where the Indian plate of the Earth's crust is sliding
under a tongue-shaped sliver called the Burma microplate.

It was initially thought to be 9.0 on the Richter scale. But
an evaluation of very low frequency data from seismograms shows
that the quake was in fact 9.3 magnitude.

As the Richter scale is logarithmic, the difference between
9.3 and 9.0 is 2.5 times, the study said.

"Conventional methods used to assess earthquake size
dramatically underestimated it," the study said. "(...) This
makes the Indonesian earthquake the second largest ever to be
instrumentally recorded."

Only one measured quake has been bigger: a 9.5 event that
struck Chile in 1960.

However, that quake caused far less damage when compared with
Dec. 26, largely because the grinding plates in East Asia meet at
an unusually oblique angle, causing the energy to propagate
northwards along a weakened fault.

Research led by Chinese seismologist Ni Sidao of the
University of Science and Technology at Hefei, Anhui province,
sheds dramatic light on what happened on the floor of the eastern
Indian Ocean.

Their computer model, also published in Nature, suggests that
the quake delivered a high-frequency shock that lasted a stunning
500 seconds, compared with 340 seconds for the Chile event in
1960.

From Indonesia to just south of Myanmar, the ocean ruptured at
2.5 kilometers per second in an arc measuring 1,200 kms.

The rip occurred along the so-called Sunda megathrust -- the
great tectonic frontier along which the Australian and Indian
plates begin their descent beneath Southeast Asia.

Stein and Okal also estimate a rupture of 1,200 kms , and say
that this figure would explain why Sri Lanka and southern India
were so badly hit by the tsunami.

The reason: the biggest waves that struck their shores came
not from the quake site off Sumatra, to the southeast, but from
the thrust of the ocean floor to the east.

"Tsunami amplitudes are largest when perpendicular to the
fault," Stein and Okal note.

Seismologists had said that the energy transmitted by the Dec.
26 added to tension in this region and fueled the likelihood of
an imminent, very big shock. There was no tsunami in the March 28
quake, probably because the shock was much smaller and occurred
beneath the ocean floor.