HR: 10:20h
AN: T32B-01    [Abstracts]
TI: Subduction and accretion across the Ganges-Brahmaputra Delta: is it seismogenic?
AU: Akhter, S H
EM: shakhter@univdhaka.edu
AF: University of Dhaka, Dept of Geology, Dhaka, 1000 Bangladesh
AU: Seeber, L
EM: nano@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964 United States
AU: * Steckler, M
EM: steckler@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964 United States
AU: Armbruster, J G
EM: armb@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964 United States
AU: Kogan, M
EM: Kogan@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964 United States
AU: Small, C
EM: cs184@columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964 United States
AU: Ho, E C
EM: eh2005@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964 United States
AB: Huge earthquakes generally contribute a major portion of the strain at subduction zones, but are rare, and may thus be overlooked in hazard analysis. The India-Sunda boundary is increasingly oblique to convergence from northern Sumatra to the eastern Himalayan Syntaxis and had not experienced a well-documented great subduction earthquake prior to the Mw9.3 1,300km rupture of 26 Dec 2004. The 500km long Arakan segment north of the 2004 rupture may have ruptured in 1762, when a regional earthquake caused changes in coastal elevation and ejection of sand and water from anticlinal ridges. Further north, we found some evidence for a 1548 event which may have ruptured the 300km long Tripura segment from Chittagong to the Shillong Plateau. In this segment, the subduction zone encounters the huge clastic wedge of the Ganges-Brahmaputra delta, which is prograding onto oceanic or transitional crust. This wedge is being accreted into a wide active foldbelt that extends beyond its topographic expression into the subsurface of the delta as far as the main estuary (Megna). This 50 km-wide blind area of foldbelt shows pedological evidence (the Tippera Surface) of Holocene uplift and is higher than the active part of the delta to the west by 3-4m, an amount that may be expected in a one or more large subduction ruptures. Interseismic tectonic deformation is evident in both drainage changes and GPS data. An eastward shift of the major channel of the Ganges into the Bay of Bengal from the Hooghly (at Calcutta) to the present Meghna Estuary since the 1600s may reflect subsidence and tilting of the delta. Preliminary results from a 6-station continuous GPS network in Bangladesh show rapid regional subsidence, that may reflect a combination of compaction and lithospheric flexure in response to tectonic and sediment load, and north-south shortening, consistent with the Shillong Plateau moving south relative to India. E-W motions are inconsistent with aseismic accretion along the Tripura segment and the network's resolution in this direction is expected to improve soon. In light of the 1762 and 2004 examples, the lack of a well-known historical precedent, high pore-fluid pressure and oblique convergence at the Tripura segment do not rule out large subduction earthquakes across the delta, which need to be included in Hazard estimates. Their effects are difficult to predict given the unusual conditions of a >20 km foredeep and rapid sedimentation, but they are likely to be catastrophic for the 200M people living in this area.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: Fall Meeting 2005