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— The Indo-Burma (Myanmar) subduction boundary is highly oblique to the direction of relative velocity of the Indian tectonic plate with respect to the Eurasian plate. The area includes features of active subduction zones such as a Wadati-Benioff zone of earthquakes, a magmatic arc, thrust and fold belts. It also has features of oblique subduction such as: an arc-parallel strike-slip fault (Sagaing Fault) that takes up a large fraction of the northward component of motion and a buttress (the Mishmi block) that resists the motion of the fore-arc sliver. In this paper, I have examined the seismicity, slip vectors and principal axes of the focal mechanisms of the earthquakes to look for features of active subduction zones and for evidence of slip partitioning as observed in other subduction zones. The data set consists of Harvard CMT solutions of 89 earthquakes (1977–1999 with 4.8≦̸Mw≦̸7.2 and depths between 3–140 km). Most of these events are shallow and intermediate depth events occurring within the Indian plate subducting eastward beneath the Indo-Burman ranges. Some shallow events within the fore-arc region have arc-parallel Paxes, reflecting buttressing of the fore-arc sliver at its leading edge. Some of the shallowest events have nearly E-W oriented P axes which might account for recent folding and thrusting. Examination of earthquake slip vectors in this region shows that the slip vector azimuths of earthquakes in the region between 20°–26°N are rotated towards the trench normal, which is an indication of partial partitioning of the oblique convergence. It is seen that all aspects of seismicity, including the paucity of shallow underthrusting earthquakes and the orientation of P axes, are consistent with oblique convergence. The conclusions of this paper are consistent with recent geological studies and interpretations such as the coexistence of eastward subduction, volcanic activity and transcurrent movement through mid-Miocene to Quaternary period. [ABSTRACT FROM AUTHOR]

In this paper, ground motion during the Independence Day earthquake of August 15, 1950 (Mw 8.6, B en-M enahem et al., 1974) in the northeastern part of India is estimated by seismological approaches. A hybrid simulation technique which combines the low frequency ground motion simulated from an analytical source mechanism model with the stochastically simulated high-frequency components is used for obtaining the acceleration time histories. A series of ground motion simulations are carried out to estimate the peak ground acceleration (PGA) and spectral accelerations at important cities and towns in the epicentral region. One sample PGA distribution in the epicentral region encompassing the epicenter is also obtained. It is found that PGA in the epicentral region has exceeded 1 g during this earthquake. The estimated PGA’s are validated to the extent possible using the MMI values. The simulated acceleration time histories can be used for the assessment of important engineering structures in northeastern India. [ABSTRACT FROM AUTHOR]

- The geothermal structure beneath a seismoactive region of central India has been determined by using two independent geophysical parameters, i.e.,P n velocity and Curie point depth, which are temperature dependent.P n velocity has been determined by reanalyzing deep seismic sounding (DSS) records obtained for this region. Surface heat flow has been computed using theP n velocity data from this region and found to be 55 ± 5 mW m-2. Curie point depths have been derived from the magnetic satellite data in this region, and reveal an identical value of heat flow of around 56 ± 4 mW m-2. We estimated temperatures of 600-700°C at a Moho depth of 39 km by using surface heat flow ranging from 50 to 60 mW m-2 and a simple 1-D geothermal model. The present study suggests the presence of a brittle-ductile transition (high strength) zone ranging in depth from 20 to 35 km, where the temperatures range from 350 to 550°C. We also suggest that the nucleation of earthquakes in this region took place in a relatively high strength zone. This is supported by the fact that the focal depths of all the recent earthquakes (from 1978 to 2000 of magnitude = 3.5) lie in this zone of high strength. [ABSTRACT FROM AUTHOR]