Discerning structure and seismic hazards in the Sikkim Himalayas
By Mita Uthaman, Arun Singh, Chandrani Singh, Arun Dubey, and Gaurav Kumar
India is on the move and has been for a while. For the past 120 million years, since India and Africa began to split apart (leaving Madagascar in between), the tectonic plate on which India sits has been moving northward. This relentless movement, along with the region’s complex geological structure, eventually led to the formation of the towering Himalayas.
The Himalayas dramatically affect many Earth system processes across the region, from monsoons and drought to water flows and sediment transport in great rivers like the Ganges and the Brahmaputra. The Himalayan region, home to hundreds of millions of people, is also the site of numerous earthquakes great and small. Studying earthquakes in this region is key to understanding not only the potential for future earthquakes but also the complex subsurface structures where these earthquakes originate.
Sandwiched between Bhutan, Nepal, and Tibet, the Sikkim Himalayas in northeastern India are one of the most seismically hazardous regions on Earth; the likelihood, timing, and effects of future great earthquakes here are uncertain, adding urgency to efforts to study the area. Research to illuminate the region’s structure and seismogenesis (earthquake initiation) has been hindered, though, by the temporary and limited distribution of seismic station deployments. Starting in September 2019, our research team from the Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, installed 27 broadband seismic stations in Sikkim. These stations have been running continuously since and will be operational for at least 3 years, helping address the vital need for more seismic observations in this region.
A Brief History of the Himalayas
The origins of today’s seismic activity in the Himalayas started when the supercontinent Pangea began breaking up about 200 million years ago. The Indian Plate drifted northward at a rate of 10–15 centimeters per year, eventually colliding with Asia. After this initial collision, the Indian Plate continued its northward movement, albeit more slowly at 4–5 centimeters per year [Treloar and Coward, 1991], steadily pushing against the Asian continent to form the Himalayas.
This region is one of the best examples of an active continent-continent collision zone. As a result of the high-impact collision, the Indian Plate began subducting under the Eurasian Plate. Continued northward movement of the Indian Plate has caused a series of intense compression, folding, and faulting events [Valdiya, 1984]—demarcated at the surface by the Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Main Frontal Thrust (MFT)—as well as a number of transcurrent (strike-slip) faults at right angles to these larger compressional zones (Figure 1).
At depth, the thrust zones are narrow regions of ductile shear—where rock formations shear past each other and deform rather than breaking into faults—that run roughly parallel to each other and to the Himalayan mountain belt for much of its length. Kilometers below the surface, these thrust zones all intersect and merge into a single northward dipping thrust, the Main Himalayan Thrust (MHT) [Hauck et al., 1998]. The MHT is a décollement surface that serves as the main plane along which the Indian and Eurasian Plates grind past each other and is the source of major Himalayan earthquakes.
The presence of multiple, interacting fault zones makes this belt, including the Sikkim Himalayas, a structurally complex region. Adding to this complexity, the MCT bends and meanders through Sikkim, exposing several thrust sheets. The exposed thrust sheets, which are folded and piled up over each other, suggest that the crust in Sikkim is more likely to fold compared to the rest of the Himalayas (Figure 2).
Temporal and spatial limitations in data collected by previous seismic networks in this region have made it difficult to resolve critical structural issues, such as the depth and geometry of the décollement surface where seismicity is localized. The combination of active seismicity and poorly resolved subsurface features—especially the décollement surface—makes the Sikkim Himalayas a compelling region for seismological research.
Where Earthquakes Begin
Historic earthquakes testify to the active seismicity of the regions in and around Sikkim. Over the past 50 years, these regions have experienced strong earthquakes (magnitude 6.0–6.9; Bhutan, 1964; Nepal, 1965; Sikkim, 1980; Imphal, 2016), major earthquakes (magnitude 7.0–7.9; Nepal, 1988; Sikkim-Nepal, 2011; Gorkha 2015), and great earthquakes (magnitude 8.0 or greater; Bihar-Nepal, 1934; Assam, 1950).
Most of the seismicity in the Himalayas results from shallow-angle thrust faults associated with the subduction of the Indian Plate beneath Tibet along the MHT. The MHT is notable for its strong anisotropy (directionality), caused by the shear on the décollement surface [Schulte-Pelkum et al., 2005], which may contribute to unexpected redistribution of energy generated during slip. High-magnitude, deep earthquakes with thrust focal mechanisms are attributed to this décollement surface. For example, the 2015 Gorkha, Nepal, earthquake (magnitude 7.8) seems to have happened when accumulated stress caused a locked portion of the MHT to mobilize [Avouac et al., 2015].
The origin depths of these earthquakes, ranging from the upper crust down to the mantle, increase to the north in the Himalayas [Feldl and Bilham, 2006]. Events originating in the lower crust and upper mantle have been reported, for example, beneath eastern Nepal and the southern Tibetan Plateau [Monsalve et al., 2006].
Earthquakes in the Sikkim Himalayas have both shallow and deep origins. Deeper earthquakes are localized in the north of Sikkim and are associated with thrusting along the MHT [De and Kayal, 2003]. Most of the shallow-origin earthquakes involve strike-slip fault motions rather than thrusting and are attributed to the multiple transcurrent faults that pass through Sikkim between the MCT and MBT [Hazarika et al., 2010]. Past research showed that these earthquakes originate in the upper or middle crust, with a few occurring at uncertain lower crust depths. However, pinning down the exact locations of these earthquake origins proved difficult because it involved using data from seismic stations installed along a linear profile oriented in the north–south direction, which provided poor azimuthal coverage [Hazarika et al., 2010].
Slip on smaller faults during the shallow earthquakes on the Himalayan front contributes to slip on the MHT. Studying these earthquakes is thus crucial in delineating the geometry of the MHT and locating locked portions of the fault where stress and strain accumulate during periods between earthquakes [Monsalve et al., 2006].
A New Seismic Sensor Network
Early in 2019, we conducted an exhaustive survey to find suitable sites to install the broadband instruments in the new seismic network. Most of the seismic stations are installed on private land, and landowners were compensated to watch and guard the stations. A few stations are located in highly remote areas, siting that required specific permissions from the Science and Technology Department of Sikkim State.
Reaching sites for the remote installations also required challenging treks over difficult terrain. For example, it took at least 10 days of hiking to install station SK23 located in Dzongri. The station is installed on Forest Department land, just a few kilometers away from Dzongri Peak. The trek route would occasionally get narrow enough for just one person to pass through at a time, which proved to be challenging amid the heavy rains, especially when carrying sensitive instrumentation weighing about 100 kilograms along with the mountaineering gear.
All the stations in the new network were installed either underground or on concrete piers, depending on site conditions. Sites were selected on the basis of their accessibility and to provide good azimuthal distribution. Each station is equipped with a three-component broadband seismic sensor, a 24-bit digitizer, two batteries to power the sensor and digitizer, two solar panels to continuously charge the batteries, and two charge controllers to regulate the charge dispensed to the equipment.
The dense coverage of the broadband stations will ensure recording of seismic events as small as magnitude 1.0. This enhanced data set will enable us to obtain high-resolution results and fill the data gaps encountered in previous studies of this region. The data obtained so far are of excellent quality and are already yielding preliminary results.
The structural complexity of the region often presents ambiguities that make interpreting seismic data more challenging and interesting. For example, the delineation of MHT still remains unresolved despite past studies. We hope to resolve some of these ambiguities by supplementing data from the new network with open-source data sets obtained from other networks like the Himalayan Nepal Tibet Seismic Experiment (HIMNT), the Nepal-Himalaya-Tibet Seismic Transect (HiCLIMB) network, the Bhutan Seismic Network (BHUTAN), and International Deep Profiling of Tibet and the Himalaya (INDEPTH II).
Fundamental Science and Societal Benefits
Our project is conducting continuous monitoring of earthquakes in the region using the new dense network of seismic stations. These stations are being deployed for a longer duration than the relatively short-term deployments of seismic networks in the past, which have typically been put into operation in phases, each lasting for a couple of years.
Data sets generated in this project will be crucial to answer fundamental tectonic questions and will provide the highest-resolution views yet of Earth’s structure amid the Sikkim Himalayas. The high-resolution data density we obtain from our network will be used in illuminating 3D velocity structure and mantle flow patterns, which will advance globally accepted geodynamic models for the Himalayan collision zone.
We anticipate that our study will document crustal and possibly upper mantle earthquakes regionally during the 3-year instrument deployment and that it will provide new understanding of the genesis and fundamental mechanisms of earthquakes in the Himalayas.
We also aim to initiate programs in the schools of Sikkim to raise awareness among students about earthquakes and the associated risks. We plan to demonstrate the functioning of the seismic stations to students in nearby schools (once the schools reopen after the COVID-19 pandemic) and to show them how the data will be beneficial in assessing the seismic hazard potential of the region. Altogether, these scientific and outreach efforts should lead to more realistic assessments of seismic hazards and risks in Sikkim and should improve public safety when future earthquakes strike.