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TOPIC: Sumatra-Andaman earthquake


L

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Great Indian Ocean earthquake
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Great Indian Ocean earthquake of 2004 set off tremors in San Andreas fault
In the last few years there has been a growing number of documented cases in which large earthquakes set off unfelt tremors in earthquake faults hundreds, sometimes even thousands, of miles away.
New research shows that the great Indian Ocean earthquake that struck off the Indonesian island of Sumatra on the day after Christmas in 2004 set off such tremors nearly 9,000 miles away in the San Andreas fault at Parkfield, California.

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L

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Sumatra-Andaman earthquake
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Ceremonies have been taking place to mark the third anniversary of the devastating Asian Tsunami.
More than 200,000 people in 13 countries died in the 26 December 2004 disaster, which was triggered by an undersea earthquake.
At least 128,000 people died in Indonesia alone.
About 1,000 people attended an open-air prayer ceremony in a village outside Calang town in Indonesia's worst-affected province, Aceh.

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Title: Active deformation across the Sumatran forearc over the December 2004 Mw9.2 rupture
Authors: Donald Fisher, David Mosher, James A. Austin Jr, Sean P.S. Gulick, Timothy Masterlark, and Kathryn Moran

A 220-km-long, single-channel seismic reflection profile crosses the northern Sumatra margin and presumed rupture zone of the December 2004 Mw9.2 tsunamigenic earthquake and images active deformation across the forearc. At the largest wavelength (tens of kilometers), the forearc surface is defined by a steep, 55-km-wide outer slope, a 110-km-wide upper slope forming a broad depression between two forearc highs, and a 25-km-wide steep inner slope between the landward high and forearc basin. Superimposed on these prism-wide variations are anticlinal ridges spaced 13 km apart; the inner and outer slopes are characterized by landward and seaward fold vergence, respectively. Between anticlines, growth strata deposited in slope basins are folded at ~23 km wavelengths. These small folds deform the seafloor and increase in amplitude with depth, verging toward anticlinal hinges. We suggest that long-wavelength variations are consistent with variations in strength across the forearc. The 13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior. Growth strata geometries indicate ongoing deformation within individual basins. Our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 Mw9.2 event, peeling up shallower and less competent trench fill, deforming the toe and the upper slope of the forearc, and producing seabottom uplift responsible for the tsunami.

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Bay of Bengal tsunamis
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The northern end of the Bay of Bengal could be at risk of giant earthquakes and tsunamis in the coming decades, an Australian study concludes.
Such events have been thought unlikely there, in contrast to the area further south where the 2004 tsunami began.
But the new work, published in the journal Nature, has found "compelling evidence" for tsunami-triggering earthquake activity to the north.
Geologists have said this warning should be taken "very seriously".
The area is densely populated, and more than a million of people could potentially be at risk.

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L

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Myanmar region tsunamis
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The Sumatra earthquake of December 2004 caught seismologists by surprise. It just wasn't the sort of place they expected a long fault to break all at once. Now researchers are realising that the same fault poses a threat all around the Bay of Bengal, from Myanmar to Bangladesh and India.

"You could lose millions of people up there in one earthquake" - Kerry Sieh seismologist of the California Institute of Technology in Pasadena.

The unsettling realisation comes after an analysis of recently published geological and geophysical data, historical accounts from the region, and modelling of tsunamis. In the 6 September issue of Nature, seismologist Phil Cummins of Geoscience Australia in Canberra argues that the Indian tectonic plate is not harmlessly sliding by the adjacent plate along a fault far inland in Myanmar, as previously assumed. Instead, the Indian plate is diving beneath the other plate well offshore of Myanmar, buried beneath the thick sediments of the Bay of Bengal. Any large quake there could generate a tsunami.
Such a quake would not be without precedent. Historical reports suggest a major quake shook the Myanmar region in 1762, and Cummins's analysis backs this up. He finds that the way land rose in the south and sank in the north supports a rupture of 700 kilometres of the fault, which may have generated a magnitude-8.8 earthquake. In a model, the quake generates a large tsunami along the nearby Myanmar coast. If a similar rupture were to recur, the quake and tsunami combined would threaten Chittagong and Dhaka, Bangladesh, and Kolkata, India.

Given the dense populations there, "it seems likely that the number of lives at risk may be over a million".

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Ancient tsunami
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Two major tsunamis or storms had hit Kirinda 2,200 years and 1,600 years ago, a team of scientists have discovered through dating layers of sand deposits from the estuary in Kirinda.
It was a tsunami study of a different kind that the CCF team comprising Dr. Abeyratne and Research Scientist (Geology), Pathmakumara Jayasingha and Ashok Kumara of the Post-Graduate Institute of Archaeology, were carrying out.

The objectives of the study were two fold. The first was to locate deposits from the most-recent 2004 tsunami and the second was to find deposits if available of any other tsunamis or large storms which may have occurred in and around Kirinda in ancient times - Dr. Abeyratne pointing out that historical records suggest at least one catastrophic tsunami did take place in the area.

And the technique used by the CCF team is thermo-luminescence (TL) dating. According to Dr. Abeyratne, whose speciality is dating of the archaeology type, a TL dating laboratory was established at the CCF way back in 1985.

This one and only dating lab in Sri Lanka even now, was set up to scientifically date archaeological objects such as pottery, bricks etc.

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RE: Sumatra-Andaman earthquake
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Scientists at the University of Texas have used a pair of satellites to measure the seismic deformations produced in the Earth during and after the huge Sumatra-Andaman earthquake of December 2004, the one whose associated tsunami killed hundreds of thousands around the Indian Ocean coastline.
The Gravity Recovery and Climate Change Experiment (GRACE) consists of two Earth-orbiting satellites. The satellite's relative spacing, monitored continuously, can be altered by the shifting gravitational subtleties triggered by the movement of massive objects beneath. This can mean big changes in land water and lakes, sea level changes, polar ice sheet melting, or sea floor changes caused by earthquakes.

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The 2004 Sumatra-Andaman earthquake and resulting tsunami are now infamous for the damage they caused, but at the time many scientists believed this area was unlikely to create a quake of such magnitude. In the March 23 issue of the journal Science, a geophysicist from Rensselaer Polytechnic Institute urges the public and policy makers to consider all subduction-type tectonic boundaries to be locked, loaded, and dangerous.

Seismologists have long tried to determine which subduction boundaries are more likely than others to break. Yet, the great earthquake of 2004 ruptured a segment that was thought to be among the least likely to go - Robert McCaffrey, professor of earth and environmental sciences at Rensselaer.

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Sediment folding may have added to the exceptionally large tsunami that struck Sumatra on Dec. 26, 2004, according to an international team of geologists.

"Tsunami models consider the rebound of the plate during the earthquake, but do not include permanent deformation, like folding, of the upper plate" - Donald M. Fisher, professor of geosciences at Penn State.

Tsunamis propagate only when earthquakes occur under water and have an up and down component to their motion. Earthquakes where tectonic plate boundaries slide side by side, do not cause tsunamis. Subduction zone earthquakes, those areas where one plate moves beneath another, are prime candidates for tsunami generation, but if the two plates slide smoothly across each other, water is displaced very slowly.

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BSSA Special Issue on the 2004 Sumatra Earthquake and Indian Ocean Tsunami
The Bulletin of the Seismological Society of America (BSSA) has published a special issue on the 26 December 2004 magnitude 9.0 Sumatra-Andaman Islands earthquake and the resulting tsunami in January 2007. The special issue contains 23 articles. These events demand our attention because of the devastating effects, which included the most casualties from a tsunami in recorded history. In addition, it is the largest earthquake in the age of global digital seismic networks, and the resulting tsunami was recorded nearly worldwide on tide gauges in the Indian, Pacific, and Atlantic Oceans. Thus, the data sets produced by this earthquake will provide unique insights into Earth processes and how these create natural disasters.
This BSSA special issue will focus on results of investigations into all seismological, geodetic, other geophysical, geological, and engineering aspects of the 2004 Sumatra earthquake and the ensuing tsunami. All authors working in this area are encouraged to submit research papers for consideration as part of this special issue.
The special issue guest editors include Susan Bilken, New Mexico Tech, Kenji Satake, National Institute of Advanced Industrial Science and Technology and Kerry Sieh, Caltech.

Source: Seismological Society of America

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