Localised Strain and Partial Melts in Shear Zones

According to geophysical surveys using seismic (sound wave) reflection and gravity methods, the crustal thickness beneath the Himalayas is approximately 70 km, which is twice the thickness of a standard continental lithosphere (for example, the Indian Peninsula). In other words, the Himalayas are higher because they sit on an over-thickened continental crust formed by compression, thrusting, and folding. According to Himalayan Geology, the landscape is particularly prone to landslides. The mountain range formed due to the collision of the Indian and Eurasian plates. The Indian plate’s northward movement stresses the rocks, making them brittle, weak, and susceptible to landslides and earthquakes.

Research on seismic and landslide hazards in the Himalayas by scientists:

Most research on seismic and landslide hazards in the Himalayas emphasises geophysical and geomorphological character traits. In many cases, however, the underlying cause of these natural disasters may be deep in the subsurface, with geological implications such as rock types, rheology, strain localisation, etc. As a result, acknowledging the geodynamic scenario of a particular portion of the Himalayas is just as important as understanding its seismic and geological characteristics in the aftermath of natural disasters.

Scientists from the Wadia Institute of Himalayan Geology, Dehradun, a fully independent institute under the Department of Science and Technology (DST), Govt. of India, suggest that, unlike other parts of the western Himalaya, partial melting of the crust in Kumaun is affected by induction of an environment that is conducive to sheetlike, planar or curvilinear zone constituted of rocks that are more strained than rocks adjacent to the zone (major shear zones), rather than a continuous zone of mid crust The research on seismic and landslide hazards in the Himalayas Also indicate that brittle deformation of these shear zones/thrust planes may still regulate exhumation and seismicity in this Himalayan region.

Because the Kumaon Himalaya is located in the central seismic gap, it has the potential to host a large Himalayan earthquake. Based on local seismic activity data recorded at 18 seismological stations, the seismicity and seismotectonics of the Kumaon Himalaya and the surrounding region have been investigated. The seismically active zone of the Kumaon Himalayas eastern segment deviates from the regular cycle of seismicity in the Himalayan Seismic Belt of the NW Himalaya. The Chiplakot Crystalline Belt (CCB) southern edge of the Vaikrita Thrust is a hotspot for seismic activity in this region.

Investigation made after the Research on seismic and landslide hazards in the Himalayas.

To investigate the kinematics of the region, the Focal Mechanism Solutions of 41 earthquakes were computed using the waveform inversion technique, as well as the solutions of 12 earthquakes procured from the Worldwide Centroid Moment Tensor solution catalogue were used. The investigation uncovered a complex rift pattern in the Inner Lesser Himalaya. The stress inversion results show a widely distributed stress pattern and a low frictional coefficient, which are credited as one of the major causes of clustered seismicity observed in the CCB. A careful examination of the fault orientations shows the presence of a hinterland dipping Lesser Himalayan Duplex over the Main Himalayan Thrust ramp structure.

This duplex structure considers the CCB’s high compressive stress and strain rate. The presence of a fluid-rich zone, strain localisation, and massive stress build-up due to holding in the ramp framework on the Main Himalayan Thrust beneath the CCB account for the high accumulation of shallow-focus earthquakes in the CCB.

What is strain localisation?

When a ductile material is subjected to a uniform loading process that induces extreme strain, the deformation may localise into narrow and planar bands, frequently positioned in standard lattice patterns in Himalayan Geology. This concept is quite common and occurs at many scales, ranging from the kilometric scale in the earth’s crust to the nanoscale in metallic glass.

Conclusion 

The research on seismic and landslide hazards in the Himalayas found short and differentiated pulses of magmatism, indicating strain localisation and deflections along discrete shear zones. Later in the brittle regime, these shear zones controlled exhumation and the emergence of social shear zones or plunges that did help amplify and excavate the Himalayan core. Characterisation of these thrust zones using Himalayan geology proxies aids in denoting seismicity and even landslides, as strongly strained and crushed rocks displayed in thrust or fault zones are more prone to slope failure.