The autonomous Wadia Institute of Himalayan Geology (WIHG), a part of India’s Department of Science & Technology (DST), has revealed new insights on seismic activity in Arunachal Himalaya. The study focuses on the exploration of the elastic properties of rocks and seismicity in the region, which is a part of the Seismic Zone V and thus, highly susceptible to earthquakes.
The Study Undertaken by WIHG
To understand the rock elasticity and seismicity in Arunachal Himalaya, WIHG installed 11 broadband seismic stations along the Lohit River Valley. These stations were connected through the Global Positioning System. To collect the data, the study deployed both teleseismic (related to earthquakes that occur over 1000km from the measurement site) and local earthquake data. The seismic information was gathered via seismometers, sensitive instruments that can detect ground movements caused by earthquakes, volcanic eruptions and explosions.
Latest Findings
The findings of the study pointed out that moderate earthquakes are being generated at two different crustal depths in the area. Earthquakes of lower magnitude were concentrated at a depth of 1-15 km, while those slightly higher than 4.0 magnitude mostly originated from the 25-35 km depth. The intermediate depth showed no seismic activity and aligned with the zone of fluid or partial melts.
High Poisson’s Ratio
The study also revealed an extremely high Poisson’s ratio in the upper regions of the Lohit Valley. This indicates the prevalence of fluid or partial melt at crustal depths. The Poisson’s ratio is a measure that describes how a material reacts under strain; a high ratio suggests that the material undergoes significant elastic deformation even when exposed to minor levels of strain.
Underthrusting of the Indian Plate
The Himalayas were formed due to the collision between the Indian and Eurasian plates around 50-60 million years ago. The continuous underthrusting of the Indian plate beneath the Eurasian plate leads to progressively increasing stress in the Himalayas. This ongoing process is responsible for the modification of drainage patterns and landforms in the region. It also increases seismic hazards in the Himalayan mountain belt and the adjoining areas.
The Tuting-Tidding Suture Zone
The Tuting-Tidding Suture Zone (TTSZ) forms a significant part of the Eastern Himalaya. Here, the Himalayas take a sharp southward bend, connecting with the Indo-Burma Range. Recent development plans for roads and hydropower projects in this zone has underscored the need to understand the pattern of seismicity in the area.
Crustal Thickness
The crustal thickness in the region under study varies noticeably. It measures about 46.7 km under the Brahmaputra Valley and approximately 55 km in the elevated regions of Arunachal. This difference results in a marginal uplift of the contact, marking the boundary known as the Moho discontinuity. The Moho discontinuity is identifiable by the change in velocity of seismological waves as they pass through different densities of rock.