Dec. 26 quake measured 9.3 on Richter scale<\/p>\n
Richard Ingham, Agence France-Presse\/Paris<\/p>\n
In a reassessment of the Dec. 26 earthquake that unleashed the
\nIndian Ocean killer tsunami, scientists say the temblor measured
\n9.3 on the Richter scale -- more than twice as powerful as
\noriginally estimated and the second biggest quake ever recorded.<\/p>\n
The quake split the ocean floor northward from Sumatra along
\n1,200 kilometers, twice as long as previously thought, according
\nto their research, which appears on Thursday in Nature, the
\nweekly British science journal.<\/p>\n
The event released so much strain along this particular part
\nof the fault that in theory there should be no quake of similar
\nmagnitude, or a similar tsunami, there for another 400 years,
\nsaid geologists Seth Stein and Emile Okal of Northwestern
\nUniversity, Illinois.<\/p>\n
But farther south, it is a different picture.<\/p>\n
The scientists -- who wrote before last Monday's quake, which
\nalso struck western Sumatra -- warned with uncanny prescience
\nthat \"a great earthquake\" with the potential to generate a large
\ntsunami remained a threat south of the Dec. 26 site.<\/p>\n
The Dec. 26 quake occurred off northwestern Sumatra, at the
\nnexus where the Indian plate of the Earth's crust is sliding
\nunder a tongue-shaped sliver called the Burma microplate.<\/p>\n
It was initially thought to be 9.0 on the Richter scale. But
\nan evaluation of very low frequency data from seismograms shows
\nthat the quake was in fact 9.3 magnitude.<\/p>\n
As the Richter scale is logarithmic, the difference between
\n9.3 and 9.0 is 2.5 times, the study said.<\/p>\n
\"Conventional methods used to assess earthquake size
\ndramatically underestimated it,\" the study said. \"(...) This
\nmakes the Indonesian earthquake the second largest ever to be
\ninstrumentally recorded.\"<\/p>\n
Only one measured quake has been bigger: a 9.5 event that
\nstruck Chile in 1960.<\/p>\n
However, that quake caused far less damage when compared with
\nDec. 26, largely because the grinding plates in East Asia meet at
\nan unusually oblique angle, causing the energy to propagate
\nnorthwards along a weakened fault.<\/p>\n
Research led by Chinese seismologist Ni Sidao of the
\nUniversity of Science and Technology at Hefei, Anhui province,
\nsheds dramatic light on what happened on the floor of the eastern
\nIndian Ocean.<\/p>\n
Their computer model, also published in Nature, suggests that
\nthe quake delivered a high-frequency shock that lasted a stunning
\n500 seconds, compared with 340 seconds for the Chile event in
\n1960.<\/p>\n
From Indonesia to just south of Myanmar, the ocean ruptured at
\n2.5 kilometers per second in an arc measuring 1,200 kms.<\/p>\n
The rip occurred along the so-called Sunda megathrust -- the
\ngreat tectonic frontier along which the Australian and Indian
\nplates begin their descent beneath Southeast Asia.<\/p>\n
Stein and Okal also estimate a rupture of 1,200 kms , and say
\nthat this figure would explain why Sri Lanka and southern India
\nwere so badly hit by the tsunami.<\/p>\n
The reason: the biggest waves that struck their shores came
\nnot from the quake site off Sumatra, to the southeast, but from
\nthe thrust of the ocean floor to the east.<\/p>\n
\"Tsunami amplitudes are largest when perpendicular to the
\nfault,\" Stein and Okal note.<\/p>\n
Seismologists had said that the energy transmitted by the Dec.
\n26 added to tension in this region and fueled the likelihood of
\nan imminent, very big shock. There was no tsunami in the March 28
\nquake, probably because the shock was much smaller and occurred
\nbeneath the ocean floor.<\/p>",
"url": "https:\/\/jawawa.id\/newsitem\/dec-26-quake-measured-93-on-richter-scale-1447893297",
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