Northeast Himalaya and its adjoining region India, has been delineated into nineteen seismogenic sources on the basis of certain seismological and criteria for the estimation of repeat times of earthquakes, to apply a regional time- and magnitude-predictable model for all these sources to study the future seismic hazard. For this, published earthquake data since 1906 to 2008 from different available earthquake catalogues and books have been used. Considering the inter-event time between successive mainshocks, the following two predictive relations were computed: logTt = 0.01 Mmin + 0.22 Mp - 0.05 log m0 + 0.98 and Mf = 0.89 Mmin - 0.26 Mp + 0.29 log m0 - 5. Multiple correlation coefficient and standard deviation have been calculated as 0.50 and 0.26, respectively for the first relation and 0.75 and 0.41, respectively for the second relation. The positive dependence of Tt on Mp indicates the validity of time predictable model on the area considered in this study.on the basis of these relations and using the magnitude and time of occurrence of the last mainshocks in each seismogenic source, time dependent conditional probabilities of the next mainshocks and their occurrence during the next 10, 20 and 30 years as well as the magnitude of the expected main shocks are forecast.

The frequently occurring strong earthquakes in the Himalayan region signify the seismic vulnerability in the region. The continued northward movement of Indian plate is generating large amount of stress at the plate boundary which is being released in form of large and great earthquakes (M≥7). Absence of such great events in the Himalayan front for last six decades and in some segments for last two centuries envisages the region as a high potential zone for future seismic hazard. In this paper we studied the larger events in the central Himalayan region.Key words: Central Himalaya; Large earthquakes; Seismic hazardsThe Himalayan Physics Vol.1, No.1, May, 2010Page: 14-17Uploaded Date: 28 July, 2011

Northeast India and its vicinity is one of the seismically most active regions in the world, where a few large and several moderate earthquakes have occurred in the past. In this study the region of northeast India has been considered for an earthquake generation model using earthquake data as reported by earthquake catalogues National Geophysical Data Centre, National Earthquake Information Centre, United States Geological Survey and from book prepared by Gupta et al. (1986) for the period 1906–2008. The events having a surface wave magnitude of Ms≥5.5 were considered for statistical analysis. In this region, nineteen seismogenic sources were identified by the observation of clustering of earthquakes. It is observed that the time interval between the two consecutive mainshocks depends upon the preceding mainshock magnitude (Mp) and not on the following mainshock (Mf). This result corroborates the validity of time-predictable model in northeast India and its adjoining regions. A linear relation between the logarithm of repeat time (T) of two consecutive events and the magnitude of the preceding mainshock is established in the form LogT = cMp+a, where "c" is a positive slope of line and "a" is function of minimum magnitude of the earthquake considered. The values of the parameters "c" and "a" are estimated to be 0.21 and 0.35 in northeast India and its adjoining regions. The less value of c than the average implies that the earthquake occurrence in this region is different from those of plate boundaries. The result derived can be used for long term seismic hazard estimation in the delineated seismogenic regions.

A long-range correlation between earthquakes is indicated by some phenomena precursory to strong earthquakes. Most of the major earthquakes show prior seismic activity that in hindsight seems anomalous. The features include changes in regional activity rate and changes in the pattern of small earthquakes, including alignments on unmapped linear features near the (future) main shock. It has long been suggested that large earthquakes are preceded by observable variations in regional seismicity. Studies on seismic precursors preceding large to great earthquakes with M ≥ 7.5 were carried out in the northeast India region bounded by the area 20°–32°N and 88°–100°E using the earthquake database from 1853 to 1988. It is observed that all earthquakes of M ≥ 7.5, including the two great earthquakes of 1897 and 1950, were preceded by abnormally low anomalous seismicity phases some 11–27 years prior to their occurrence. On the other hand, precursory time periods ranged from 440 to 1,768 days for main shocks with M 5.6–6.5 for the period from 1963 to 1988. Furthermore, the 6 August, 1988 main shock of M 7.5 in the Arakan Yoma fold belt was preceded by well-defined patterns of anomalous seismicity that occurred during 1963–1964, about 25.2 years prior to its occurrence. The pattern of anomalous seismicity in the form of earthquake swarms preceding major earthquakes in the northeast India region can be regarded as one of the potential seismic precursors. Database constraints have been the main barrier to searching for this precursor preceding smaller earthquakes, which otherwise might have provided additional information on its existence. The entire exercise indicates that anomalous seismicity preceding major shocks is a common seismic pattern for the northeast India region, and can be employed for long-range earthquake prediction when better quality seismological data sets covering a wide range of magnitudes are available. Anomalous seismic activity is distinguished by a much higher annual frequency of earthquake occurrence than in the preceding normal and the following gap episodes.

Precursory swarms associated with major earthquakes in the Western Nepal Himalaya and its adjoining region (bounded by 28.0°–31.0°N and 79.5°–82.2°E) have been studied using seismicity data from 1963 to 2006. The delineation of preparation zones for future seismic disturbances is carried out using the temporal and the spatial distribution of earthquakes, considering the events with cutoff magnitude m b ≥ 4.3 in four anomalous episodes: normal/background (N); anomalous/swarm (A); precursory gap (G) and main shock sequence (M), respectively. Five cases of anomalous seismicity have been identified, including two cases for which quiescence episodes still continue. Three moderate earthquakes of 1980 (m b 6.1, Bajhang), 1984 (m b 5.6, Bajura) and 1999 (m b 6.6, Chamoli) in Western Nepal and its adjoining Indian region were preceded by well-defined patterns of anomalous seismicity/precursory swarm. Two additional cases of anomalous seismicity patterns were observed: (1) 1999–2006, and (2) 2003–2006. In these two cases no main shock has yet occurred. However, the seismicity from 1999 onwards has fluctuated from low to high to low, as in the precursory sequences for previous earthquakes. The occurrence of the swarm sequence followed by a quiescence phase, which is still continuing, is an indication of a precursory seismicity gap in the region. From the predictive equations developed for the Himalayan frontal arc, it is estimated that an earthquake of M 6.5 ± 0.5 may occur at any time up to 2011 in an area bounded by 29.3°–30.5°N and 81.2°–81.9°E, in the focal depth range 10–30 km.

The study of seismic activity at some stage in 1963 to 2006 in the Western Nepal Himalaya and its adjoining regions (28–31°N and 79–82.3°E), reveal that seismicity is non-uniform in space and time. The analyses of fault-plane solutions of twenty-four earthquakes inferred that the Western part of Nepal Himalayan frontal arc is in compressed state in which seismic activity is dominated by thrust faulting. Based on orientation of P-axes, compressive stress directed north-south to northeast-southwest approximately perpendicular to the prevailing stress along the major trend of the Himalaya. Thrust faulting coupled with shallow dip of nodal planes reflects that the Indian continental lithosphere is under-thrusting at a shallow angle. This information suggests crustal shortening in north-south direction in which earthquakes are generated due to northward compression. In the adjoining Tibet parts earthquake activity is due to normal faulting with east-west extension. These might be due to the presence of a relatively strong Main Himalayan Thrust, the plate boundary fault below the Himalayas, would have favored the occurrence of thrusting. While, a weak Main Himalayan Thrust below Tibet along with initiation of the Main Central Thrust can explain South Tibetan Detachment (geodynamic process) and associated stress field in Western Nepal Himalaya and its adjoining regions.