* Astronomy

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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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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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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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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