It is commonly thought that the longer the time since last earthquake, the larger the next earthquake's slip will be. But this logical predictor of earthquake size, unsuccessful for large earthquakes on a strike-slip fault, fails also with the giant 1960 Chile earthquake of magnitude 9.5 (ref. 3). Although the time since the preceding earthquake spanned 123 years (refs 4, 5), the estimated slip in 1960, which occurred on a fault between the Nazca and South American tectonic plates, equalled 250-350 years' worth of the plate motion. Thus the average interval between such giant earthquakes on this fault should span several centuries. Here we present evidence that such long intervals were indeed typical of the last two millennia. We use buried soils and sand layers as records of tectonic subsidence and tsunami inundation at an estuary midway along the 1960 rupture. In these records, the 1960 earthquake ended a recurrence interval that had begun almost four centuries before, with an earthquake documented by Spanish conquistadors in 1575. Two later earthquakes, in 1737 and 1837, produced little if any subsidence or tsunami at the estuary and they therefore probably left the fault partly loaded with accumulated plate motion that the 1960 earthquake then expended.

The 1897 Shillong (Assam), northeast India, earthquake is considered to be one of the largest in the modern history. Although Oldham's [1899] classic memoir on this event opened new vistas in observational seismology, many questions on its style of faulting remain unresolved. Most previous studies considered this as a detachment earthquake that occurred on a gently north dipping fault, extending from the Himalayan front. A recent model proposed an alternate geometry governed by high‐angle faults to the north and south of the plateau, and it suggested that the 1897 earthquake occurred on a south dipping reverse fault, coinciding with the northern plateau margin. In this paper, we explore the available database, together with the coseismic observations from the region, to further understand the nature of faulting. The geophysical and geological data examined in this paper conform to a south dipping structure, but its location is inferred to be in the Brahmaputra basin, further north of the present plateau front. Our analyses of paleoseismic data suggest a 1200‐year interval between the 1897 event and its predecessor, and we identify the northern boundary fault as a major seismic source. The Shillong Plateau bounded by major faults behaves as an independent tectonic entity, with its own style of faulting, seismic productivity, and hazard potential, distinct from the Himalayan thrust front, a point that provides fresh insight into the regional geodynamics.

The 1819 earthquake in Kutch, northwestern India, is one of the most significant events to have occurred in a plate-interior setting. Despite being the second largest among the stable continental region (SCR) earthquakes, this event has not been analyzed within the context of present-day understanding of earthquake seismology. Coseismic changes related to this earthquake include massive ground deformation in a wide low-lying tidal-flat area. Although detailed historic accounts of this earthquake exist, many questions regarding the mode of deformation and the seismic history of the region remain unresolved. We explored the region nearly 180 years after the earthquake, and the information gathered adds to our understanding of this event and provides a fresh perspective on this unique intraplate seismogenic zone. A 90-km-long tract of elevated land with a peak height of 4.3 m is the most visible surface expression of this earthquake. We surveyed and analyzed the morphological features of this scarp and also carried out exploratory trenching in this region. The scarp morphology is suggestive of a growing fold related to a buried north-dipping thrust rather than a discrete fault that could have resulted from a surface rupture. The extensive liquefaction field associated with the earthquake offered an ideal setting to explore the paleoearthquake history. Age data of liquefaction features suggest that a previous event of comparable size must have occurred 800-1000 years ago. Seismic activity appears to be related to the reactivation of an ancient rift in a stress regime that is dominated by nearly north-south compression.

The Indian Ocean earthquake of 26 December 2004 led to significant ground deformation in the Andaman and Nicobar region, accounting for ϳ800 km of the rupture. Part of this article deals with coseismic changes along these islands, observable from coastal morphology, biological indicators, and Global Positioning System (GPS) data. Our studies indicate that the islands south of 10Њ N latitude coseismically subsided by 1-1.5 m, both on their eastern and western margins, whereas those to the north showed a mixed response. The western margin of the Middle Andaman emerged by Ͼ1 m, and the eastern margin submerged by the same amount. In the North Andaman, both western and eastern margins emerged by Ͼ1 m. We also assess the pattern of long-term deformation (uplift/subsidence) and attempt to reconstruct earthquake/tsunami history, with the available data. Geological evidence for past submergence includes dead mangrove vegetation dating to 740 ‫ע‬ 100 yr B.P., near Port Blair and peat layers at 2-4 m and 10-15 m depths observed in core samples from nearby locations. Preliminary paleoseismological/tsunami evidence from the Andaman and Nicobar region and from the east coast of India, suggest at least one predecessor for the 2004 earthquake 900-1000 years ago. The history of earthquakes, although incomplete at this stage, seems to imply that the 2004-type earthquakes are infrequent and follow variable intervals.

The Himalaya has experienced three great earthquakes during the last century-1934 Nepal-Bihar,

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More than six years after the great (M w 9.2) Sumatra-Andaman earthquake, postevent processes responsible for relaxation of the coseismic stress change remain controversial. Modeling of Andaman Islands Global Positioning System (GPS) displacements indicated early near-field motions were dominated by slip down-dip of the rupture, but various researchers ascribe elements of relaxation to dominantly poroelastic, dominantly viscoelastic and dominantly fault slip processes, depending primarily on their measurement sampling and modeling tools used. After subtracting a pre-2004 interseismic velocity, significant transient motion during the 2008.5-2010.5 epoch confirms that postseismic relaxation processes continue in Andaman. Modeling three-component velocities as viscoelastic flow yields a weighted root-mean-square (WRMS) misfit that always exceeds the WRMS 26.3 mm/yr of the measured signal. The bestfitting models are those that yield negligible deformation, indicating the model parameters have no real physical meaning. GPS velocities are well-fit (WRMS 4.0 mm/yr) by combining a viscoelastic flow model that best-fits the horizontal velocities with ~50 cm/yr thrust slip downdip of the coseismic rupture. Both deep slip and flow respond to stress changes, and each can significantly change stress in the realm of the other, so it is reasonable to expect that both transient deep slip and viscoelastic flow will influence surface deformation long after a great earthquake.