The Mw 7.5 2009 Coco earthquake, north Andaman region |
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Authors: | P Mahesh Amit Bansal B Kundu J K Catherine and V K Gahalaut |
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Institution: | (1) Lamont-Doherty Earth Observatory of Columbia University, 1000, Palisades, NY 10964-8000, USA;(2) Department of Geology, Wright Geological Laboratory, Rutgers University, 610 Taylor Road, Busch Campus, Piscataway, NJ 08854, USA |
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Abstract: | The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred
within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004
Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with
predominant normal motion on approximately north-south to northeast-southwest oriented plane is very similar to the 27 June
2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate
of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south
of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent
with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake
too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting
90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing
postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively
low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal
arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could
trigger only a few aftershocks in its immediate vicinity. |
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