Geological process

Himalayan fault lines

Print edition : May 29, 2015

The destroyed village of Barpak, at the epicentre of the quake, seen from a helicopter during a relief and rescue mission in Nepal, on May 3. Photo: DANIEL BEREHULAK/NYT

Villagers sort through rubble in search of their belongings in Barpak on May 6. Photo: DANIEL BEREHULAK/NYT

Figure 1: Colour-coded shakemap depicting the severity of shaking. The table shows the Mercalli intensity categories along with the corresponding qualitative strength of shaking, peak-ground acceleration as percentage of g (the acceleration due to gravity) and the peak velocity of ground motion in cm/s from the observed impact on structures above ground (Credit: USGS).

Figure 2: The northward drift of India from 71 million years ago to present time. Note the simultaneous counter-clockwise rotation of India. Collision of the Indian continent with Eurasia occurred at about 55 million years ago (Credit: Wikipedia sourced from www.usgs.org).

Figure 3 (left): Epicentres of greater than M4.0 earthquakes within the India-Eurasia collision zone from 1990 to date. The inset yellow border defines the boundary of Nepal (Credit: Incorporated Research Institutions for Seismology (IRIS) at www.iris.edu).Figure 6 (right): The aftershock distribution outlines the rupture zone of the main shock. The rupture during the main shock initiated beneath the epicentre and propagated towards southeast (Credit: IRIS www.iris.edu).

Table 1: The estimated population affected by the different levels of shaking intensity on ground (Credit: USGS).

Figure 4: Picture showing the main tectonic features of the Himalayan geology (Credit: Geological Society of America Special Papers, 2007, 419, pages 135-151).

Figure 5: Dip-slip faults occur in inclined fracture interface between two rock blocks. It is called ‘thrust fault’ when the interface inclination angle is gentle and the block above the fault plane overrides the block below the fault plane. When it is steep (above 45°) it is called ‘reverse fault’.

Figure 7: Map showing the April 25 Nepal earthquake among the large and great earthquakes along the Himalayan arc. The epicentre of the present quake is located in the central seismic gap closer to the 1934 Nepal-Bihar (M8.1) earthquake (Credit: Rajendran, C.P. et al., 2015, Journal of Geophysical Research, DOI: 10.1002/2014JB011015).

Graph 1: Plot of number of aftershocks with magnitude 4.0 or more till May 1. Most of the aftershocks were in the magnitude range 4-5 and also the number dropped significantly after the first 36 hours (Credit: Nepal Seismological Centre, www.seismonepal.gov.np).

The geology of the Himalayas, determined largely by the ongoing collision of the Indian tectonic plate with the Eurasian plate, makes the region particularly vulnerable to earthquakes. There is a great need for better disaster preparedness as seismologists, through analysis of available data, expect a ‘great’ quake to hit the region in the near future.
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