P.: Partial and complete rupture of the Indo-Andaman plate boundary 1847–2004

We review seismicity along the Nicobar/Andaman plate boundary prior to the Mw=9 earthquake of 26 December 2004, with particular attention to reverse slip in the central and northern parts of the rupture zone 600-1300 km north from the epicenter. Slip is partitioned between convergence and strike-slip motion, which in the northern Andamans is assisted by back-arc spreading. Subduction zone earthquakes prior to the rupture occurred largely to the east, and at deeper depths than the area ruptured in the shallow 2004 megathrust. Large thrust earthquakes in 1847 (Mw>7.5), 1881 (Mw=7.9) and 1941 (Mw=7.7) appear to have occurred on intermediate regions of the down-dip boundary, areas that have been surrounded and probably incorporated into the 2004 rupture. Preliminary reports of 1-4 m of subsidence of the Nicobar islands and 1-2 m uplift of western shorelines of the Andaman islands are consistent with a down-dip fault width of 150-180 km, and a slip of 7-23 m. Based on preliminary reports from the Port Blair tide gage, slip in the Andaman islands, 800 km north of the epicenter, appears to have started no sooner than 36 minutes after the mainshock, some 30 minutes after the primary mainshock rupture is inferred to have arrived from the epicenter, but consistent with large aftershocks occurring in this region 85 minutes after the mainshock, and suggestive of slow slip. The delayed slip was not accompanied by shaking except that from aftershocks. GPS measurements in the Andaman islands prior to the earthquake indicate a plate convergence rate of 14 mm/year suggesting that great earthquakes with similar slip to the 2004 event cannot occur more frequently than once every 1000 years. A shorter recurrence interval of 400 years is calculated for the epicentral region where convergence rates are higher. The apparent indifference of the 2004 earthquake to the lowered slip deficits caused by previous major earthquakes, and its release of significant seismic moment without evidence for comparable shaking, has implications for the analysis of historical earthquakes in other plate boundaries