Abstract
ALL EARTH 2023, VOL. 35, NO. 1, 2191917 https://doi.org/10.1080/27669645.2023.2191917 Seismic hazard estimation and medium-term earthquake precursor analysis of North East India: an assessment on large earthquake scenario a a,b Anshuman Phukan and Debasis D Mohanty a b Geoscience and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat, Assam, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India ABSTRACT ARTICLE HISTORY Received 28 December 2022 The North-Eastern region of India has the distinction of being one of the most seismically active Accepted 13 March 2023 and tectonically complex regions in the world. The region has periodically jolted by a number of large earthquakes. The temporal and spatial frequency-magnitude distribution (FMD) of KEYWORDS earthquakes in three major selected regions of North-East India, namely Shillong–Mikir Hills, North East India; b-value; Arunachal Himalaya, and Assam Foredeep region, are examined in this study. Temporal Seismic Hazard; Large variation of ‘b’ shows a significant declination prior to a large earthquake, which strongly Eathquake; Earthquake advocates for a medium-term (months-year) earthquake precursor. Similarly, spatial distribu- Precursor o o tion map is prepared by estimating b-value at every 0:01 � 0:01 grid using the nearest 150 events, which is vital in understanding the stress regime of a specific region. The present study critically demonstrates the higher b-value regions associated with minimum seismicity and vice-versa, especially focusing on large earthquake scenario. High anomalies of ‘b’ were found in the regions of Shillong–Mikir hills and Arunachal Himalaya, whereas the Assam-Foredeep region, free from seismic activity for a long period, shows a low. It may be due to the accumulation of stress energy within the region for a long time and could be an alarming sign for future large events. 1. Introduction events in near future from the available earthquake catalogue. However, the quality of the catalogue is The North Eastern region (NER) of India exists in the crucial while determining the seismic hazards. Based zone V of seismic zonation map (BIS, 2004) of India, on the capacity and coverage of seismic recording holds the distinction of being one of the most complex stations, earthquake events may vary throughout dif- tectonic regimes in the world (Kayal, 2001, 2008; ferent time periods. At that time, where major focus Nandy, 2001). Over the past few decades, the area stays with the magnitude of completeness (M ) of the has been the site of numerous big earthquakes catalogue, which is always changing. It often gets (M � 5:0). The persistent seismicity as well as the smaller over time as instruments and processing tech- diverse geological features of the region have always niques become more effective at detecting events. piqued the interest of geoscientists, prompting them The statistical distribution of sizes of group of earth- to investigate the region’s tectonics thoroughly. quakes is very complex and has a power law distribu- Earthquakes are one of the natural hazards that tion (Ishimoto & Iida, 1939). In view of these power law, have always been important to human beings due to the tectonic implications of seismically prone zones the serious threats they pose. Forecasting the major can be well investigated by studying the b-value of upcoming events has always been a common interest seismicity. The b-value obtained from Gutenberg- in the research field. Although prediction of the Richter frequency magnitude relation (Gutenberg & exact day and time of a future earthquake is not pos- Richter, 1944) characterises the earthquakes over the sible at this time, one can characterise earthquake- observed range of magnitudes. The b-value which is prone areas to reduce the devastating effect on related to the differential stress distribution of earth human life. Understanding the statistical distribution crust can be used for seismic hazard assessment study of earthquake events can be used to calculate the in a region. Depending on the tectonic setting and probability of the occurrence of large events. The seismicity the value ‘b’ changes accordingly (C. Singh Gutenberg-Richter relation (Gutenberg & Richter, & Chadha, 2010; C. Singh et al., 2008; C. Singh, 2014; 1944), is one of the most popular approaches that Pacheco et al., 1992; Wiemer & Wyss, 1997), however can be used to assess the probability of significant for a short time window it reaches to 1.0 for a particular CONTACT Debasis D Mohanty devlinkan06@yahoo.com Geoscience and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat, India © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. ALL EARTH 47 region. The mapping of b-value on a local to regional high accumulated shear stress in a region whereas scale in various tectonic regimes results in a value a high value indicates some major tectonic events between 0.4 and 2.0, (Wiemer, 2000). Study of b-value that have already occurred in the past or the release both in time and space can be used as medium-term of shear stress (Chan et al., 2012; Prasad & Singh, 2015). (months-year) precursor for major future earthquakes This inverse relation of b-value with the accumulated (Nuannin and Kulhánek, 2012; Nuannin, 2005). Several stress in and around epicentral region of major shocks researchers studied spatial variation of b-value (C. can be used in estimating the possible future large Singh et al., 2008, 2009; C. Singh, 2014; Wiemer & earthquake. Katsumata, 1999) in seismically active areas, which The main goal of the present study is to conduct reflects locally the effective stress (Scholz, 1968). a thorough study on the evaluation of b-value with There are many factors that are responsible for the respect to both space and time for the selected three deviation of b-value from the normal value of 1.0. regions of North-East India, (Figure 1) in order to assess Generally high b-value is observed when material den- the high probability or expectation of large earth- sity or crack density increases (Mogi, 1962) and signifi - quakes in the NER. This study also aims to understand cant low values are observed in the areas of increased the present hazard scenario and the futuristic assess- shear stress (Scholz, 1968) or effective stress (Wyss, ment of earthquake probabilities for a better social 1973). In simple words, b-value can be used as benefit of the general public with a constructive urba- a stress metre, which can be used to measure the nisation of this region. The North-Eastern region of accumulated stress in a tectonically active zones (C. India tends to be a region of complex tectonics and Singh, 2014; Wiemer & Wyss, 1997). In the context of seismically active zone from time to time. Many large shear stress, a low b-value indicates that there is very earthquakes (M � 6) jolted the whole region in past Figure 1. Seismicity map of study area, representing three major regions (solid boundaries) of study; Shillong–Mikir Hills, Arunachal Himalaya and Assam Foredeep, where the probabilistic hazard analysis is made. The major geological structures (faults & folds) are represented by black dotted lines with abbreviations represents as. MBT: Main Boundary Thrust, MCT: Main Central thrust, MFT: Main Frontal Thrust, ITSZ: Indus-Tsangpo Suture Zone, KF: Kopili Fault, DF: Dudhnoi Fault, DAUKI.F: Dauki Fault, BF: Brahmaputra Fault, CMF: Churachandpur Mao Fault, DF: Dapsi Thrust, Du. F, Dudhnoi Fault, OF: Oldham Fault, BS: Barapani Shear Zone, LT: Lohit Thrust, MT, Mishmi Thrust, and WT: Walong Thrust. Circular sub-regions selected within the Shillong–Mikir Plateau (S 1, S 2) and Arunachal Himalaya (S 3 and S 4) for temporal distribution analysis of b-value are shown by red region region region region circles. The dashed line (black colour) represents the E-W cross-section (AB) drawn in the Arunachal Himalaya region for the micro- seismic clusters’ interaction analysis. 48 A. PHUKAN AND D. D. MOHANTY and there is a high chance of occurrence of major Cachar earthquake (M ~7.5) and the 1943 Assam events in near future. This active seismicity of the Earthquake (M ~7.2) (Baro & Kumar, 2015). The north- North-East India and the high probability of future ern and southern boundary of the Shillong plateau is earthquakes always being the major concern in the demarcated by E-W trending Brahmaputra fault and seismological research. In our present study, we have E-W trending Dauki fault respectively (Angelier & divided the whole region into three major entities as Baruah, 2009). Dauki fault also divides the plateau Shillong–Mikir Plateau, Arunachal Himalaya and from the Bengal basin (Dasgupta & Nandy, 1982). The Assam-Foredeep and try to estimate the b-value for E-W trending Dauki fault had witnessed some of the the respective sub-regions to understand the present prominent earthquakes in the past including 1923 and future hazard scenario. For the estimation of Meghalaya earthquake (M ~7.1) (Bilham & England, b-value data collected from 1900 to 2022, including 2001). The plateau also has two significant thrusts, some major historical events are used in this study Barapani shear zone and the Dapsi thrust, which is (Figure 1). the western extension of the Dauki fault, runs through the plateau in a NW-SE orientation and is 90 ~ 100 km long (Kayal, 1991). The NW-SE trending Kopili fault 2. Geodynamics of NER divides the entire massif into two parts Shillong Plateau and Mikir hills (Mohanty & Singh, 2021; The North-Eastern region (NER) of India is one of the Mohanty, 2022). The Kopili fault is 300–400 km long most tectonically active zones in the world, and is and 50 km wide fault and it may commence in Bomdila represented by India-Asia collision to the north and and stretch through the Assam valley across the east Indo-Burmese subduction to the east (Angelier & margin of the Mikir plateau to the Naga Hills (Abdel- Baruah, 2009; Bilham & England, 2001; Bora et al., Gawad, 1972; Baruah et al., 1997; Nandy, 1980). The 2014). The region has experienced numerous big depth variation of the past seismic events along the earthquakes ðM> 5:0Þ in the past few decades. The Kopili fault indicates most of the events have a depth persistent seismicity in the region from time to time range less than 70 km. The N-S trending Dhubri fault, as well as diverse geological features has a direct which separates the plateau from the Indian subconti- impact on the region’s complex tectonics structure. nent, lies to the west of the plateau. Other faults in and The region’s significant geological features include around the plateau include the Chedrang fault, the the Shillong–Mikir Plateau, Brahmaputra valley, Indo- Samin fault, the Dudhnoi fault. The considerable seis- Burmese Subduction zone, and Eastern Himalayan mic activity in and around the Shillong plateau com- Syntaxis (Mohanty & Mondal, 2019, 2020, 2021; plicates the geodynamic setting of the region. Mohanty, 2023; Mondal & Mohanty, 2021). The easternmost part of the Himalaya, between The Shillong Plateau, which is a part of the Indian longitude 91.30’E and 96E occupied by Arunachal Shield (Evans, 1964), is notable for being a seismically Himalaya. The eastern Himalayan syntaxis is also active zone in the India. Numerous lineaments can be a part of it (Yin et al., 2006). This part of the found in the Plateau and its surrounding areas as Himalaya is located in the easternmost side of the a result of complex tectonic forces from the Indo- Bhutan. Many pioneer researchers had done exten- Burmese subduction zone and the Himalayan collision sive studies in the region to study the geology and zone (Kayal, 2001). The Shillong Plateau–Mikir hills tectonic structure of the region. The Himalaya’s extends from the Himalayas in the north to exhumation and growth are a continual process Bangladesh in the south. About 60 million years ago, caused mostly by reverse faults in which the rocks when the Indian and Eurasian plates began to collide on the bottom surface of a fault plane move beneath in a north-south orientation, the Shillong Plateau was relatively static materials on the top surface, formed. The Shillong Plateau is an ancient cratonic a process known as under-thrusting of the Indian block in North-Eastern India, comprising roughly plate beneath its Eurasian plate. This process is fun- 47,614 km (Mohanty & Mondal, 2020; Mohanty et al., damentally responsible for creating a significant seis- 2021; Sarma, 2014), with regional heights of up to 2.0 mic hazard in the Himalayan Mountain belt and km (Najman et al., 2016). The Shillong Plateau is com- surrounding areas by continuously changing the posed of Precambrian gneissic rocks in the south, east, drainage patterns and landforms. The Himalaya con- and west, with Proterozoic and Cenozoic sediments nects with Indo-Burmese range taking a sharp south- partially overlain (Sarma, 2014). There are numerous ward bend and the zone is known as Tuting-Tidding faults which encircle the Shillong plateau, and the Suture Zone (TTSZ) (Acharyya & Sengupta, 1998; Holt plateau itself has numerous faults, namely Dhubri et al., 1991). A study carried out by Wadia Institute of fault at the west and the Kopili fault at the east Himalayan Geology, Govt. of India suggest that most (Evans, 1964). The most prominent one is the Kopili of the low magnitude earthquakes are concentrated fault which runs through the Shillong plains to the at depth 1–15 km and higher magnitude earthquakes Mikir hills. This fault zone was the site of the majority ðM> 4Þ concentrated in the depth 25–35 km in the of the large earthquakes in the past, including the ALL EARTH 49 easternmost part of India. The fluid/partial melt zone the Assam earthquake of 1943 (M ~7.2) are among the is located at middle depth and is free from seismic most powerful earthquakes ever recorded. activity. The crustal thickness in this area varies from The Great Assam earthquake (M ~8.7) was the most 46.7 km beneath the Brahmaputra Valley to about 55 powerful earthquake to strike Assam in its history. The km in the higher elevations of Arunachal (A. Singh earthquake hit the Shillong plateau and its neighbour- th et al., 2017; Mohanty et al., 2016), with a marginal ing areas on 12 June, 1897 (as reported by Bilham & uplift of the contact that defines the boundary England, 2001). Many aftershocks were reported until th between crust and the mantle technically called the July 25 1897. The damages caused by the earthquake Moho discontinuity. This, in turn, reveals the under- were severe enough to cause fissure in the ground, thrusting mechanism of Indian plate in the Tuting- cracks in construction, landslides and flooding in var- Tidding Suture Zone. Extremely high Poisson’s ratio ious sites of the Assam valley as well. The course was also obtained in the higher parts of the Lohit pattern of river Brahmaputra also changed significantly Valley, indicating the presence of fluid or partial melt due to the 1897 Assam earthquake. This earthquake at crustal depths. This detailed assessment of seismi- shook various parts of the Indian subcontinent, caus- city in this region will be helpful for planning any ing major damage to structures in Kolkata and large-scale construction in this region in the future. Myanmar (Baro & Kumar, 2015). Another strong earth- The Assam-Foredeep is a small valley that stretches quake (M ~7.5) hit the entire North-eastern region on from ENE-WSE and is bordered to the north and east by 10 January, 1869. The rupture zone of the earthquake two young mountain ranges. The seismicity of the was located within the Kopili fault. Severe damages Assam-Foredeep region is quiet low for a long time. were caused by this earthquake with influences ran- There is an area called ‘Assam gap’ in the upper Assam ging from Dibrugarh in the north to Manipur in the valley (Khattri et al., 1983) of North-East India that has east, Patna in the west, and Kolkata in the south. This experienced essentially no seismic activity for the past earthquake caused a number of ground fractures and 60 years (between Kopili fault and Mishmi sand vents in various regions. o o thrust,92 E 96 E). After such a long time, the region Shillong plateau has the distinction of being the of Assam gap was jolted by an earthquake of magni- most seismically active region in Assam’s history. The tude, M ~6.2 on 28 April 2021, followed by a series of plateau had previously undergone two major earth- aftershocks. The epicentre of this earthquake is located quakes in the past of magnitudes 7.1 and 7.2. On on the Kopili fault. This earthquake draws an immedi- 9 September,1923, a 7.1 magnitude earthquake struck ate attention following a sudden burst of accumulated the southern edge of the plateau. The epicentre of the stress that has been building up for a long time, and it earthquake is located on the Dauki fault. This earth- may also be a precursor to larger earthquakes in the quake had an impact on parts of Meghalaya, Assam, near future (Mohanty et al., 2021). and West Bengal. The region was once again shocked by an earthquake of magnitude 7.2 on 23 October, 1943, causing fractures in the ground, tree falling, 3. Historical seismic events of North-East and structural damage. The stress pattern of Kopili India fault could be the source of this earthquake, however very less information is available related to these earth- India’s North-Eastern region and the surrounding areas quakes. The North-eastern region of India had experi- are located in a tectonically unstable and seismically nd enced another historic event (M ~7.1) on 2 July, active area. The North-eastern region of India falls in 1930. According to Kayal (2008); main source of this the zone V of Seismic Hazard Zoning Map (BIS, 2004) earthquake was the Dhubri fault. Dhubri town which is which is more prone to earthquakes; being a highly near to the Dhubri fault was greatly affected with an populated area it draws the attention of the many intensity of IX on the Rossi-Forel scale (Olympa and researchers to understand the mechanism of seismic Kumar, 2015). A number of aftershocks followed by the events, active faults, seismogenic sources and their main shock are reported (Gee, 1934) till 5 July, 1930. effects. Using seismological data captured by the per- manent and temporary analog networks between 1980 and 1990, the region has been well investigated 4. Data and methodology (e, g, Khattri et al., 1983). The region is presently well equipped with various broadband seismological In the present study we have selected the study area instruments, making it possible to fairly understand between longitudes 90–98° and latitudes 24–30°, its seismotectonics. The Shillong Plateau and the (Figure 1). The event data that used in this study areas around it have historically experienced were collected from International Seismological a number of strong earthquakes. The great Assam Centre (ISC), United States Geological Survey (USGS), Earthquake of 1897 (M ~8.7), the Cachar earthquake National Centre for Seismology (NCS) and CSIR- North- of 1869 (M ~7.5), the Dhubri earthquake of 1930 (M East Institute of Science and Technology (CSIR-NEIST) w s ~7.1), the Meghalaya earthquake of 1923 (M ~7.1), and catalogue from the year 1900 to 2022. With the s 50 A. PHUKAN AND D. D. MOHANTY available data we try to estimate the b-value for the into consideration to avoid the asymmetry in scaling whole region and also studied the variation of ‘b’ with laws to analyse the seismicity and therefore b-values. space and time. One essential parameter for mapping The declustering of the catalogue is important because the spatial b-value is the magnitude of completeness it removes the effects of dependent events (after- (M ). The magnitude of completeness (M ) is calculated shocks) (Chan and Chandler, 2001). However, there is c c using the maximum curvature method from the no direct procedure to identify the aftershocks, the Gutenberg-Richter relation (Wiemer, 2000). The mag- mostly applied method is Reasenberg decluster algo- nitude of completeness (M ) is an important parameter rithm (Reasenberg, 1985) for aftershocks identification which is related to the quality of the data or the or removal. In this method an interaction zone is cre- catalogue. M can be defined as the smallest magni- ated around each earthquake in the catalogue. tude for which all events in a selected space-time Earthquakes that have occurred after a main shock volume are detected. It varies both spatially and tem- and falling into the same interaction zone are classified porally. Generally, M decreases with time as the num- as aftershocks and are considered to be dependent ber of seismic monitoring stations increases and events. Several parameters have to be chosen for this method improves continuously. In the present study procedure. The original database contains a total of events with magnitude (M � M ) were selected for 7250 events which is then limited to 6228 events after mapping the b-value (Figure 2). the declustering. This declustered database is then The statistical distribution of size for a group of used for the estimation of b-value for the study region. o o earthquakes is complicated and has a power law dis- The entire study area is divided into a 0:01 � 0:01 tribution. Several research on the frequency–magni- grid cells to map the variation of b-value using the tude relationship of earthquakes as a function of declustered database consisting of 6228 events time, space, and depth have been conducted, estab- (Figures 1, 2 and Figure S1). For each node lishing an empirical relationship between the occur- a minimum number of events (N ) was assigned min rence-frequency and magnitude of earthquakes in and the grid cell consisting of less than N were min a specific region within a finite time-window removed. In the present study we have estimated (Gutenberg & Richter, 1944; Ishimoto & Iida, 1939). b-value for the whole study area as well as for three The simplest earthquake frequency–magnitude rela- main regions of interest namely Arunachal Himalaya, tionship provided by Gutenberg and Richter (1944) Shillong–Mikir Plateau and Assam-Foredeep. The which is as follows, obtained results of spatial b-value for these three regions give a better picture for interpreting the log N ¼ a bM (1) hazard scenario of the study area. Where ‘a’ and ‘b’ are constants and N is the number of earthquakes in the group with magnitudes greater 5. Results and discussions than M. The constant ‘a’ relates to the length of the window under examination as well as its volume, and The frequency-magnitude distribution (FMD) of the earthquakes in the North-Eastern region of India is ‘b’ measures the ratio of strong to weak earthquakes analysed in the current study both in a temporal and and is thought to be related to the tectonic regime of the region in question. There are two possible meth- spatial scale. The findings of analysis show that b-value significantly declines before the occurrence of big ods available to estimate the b-value, least square events, suggesting that the changes in the ‘b’ may be methods (LS) and maximum likelihood methods (ML). The maximum likelihood method (ML) is frequently employed as a medium-term (month-to-year) earth- cited as a more accurate estimation of b-value than quake precursor. During the studied period, no large earthquakes were observed in areas resulting in a high the least-squares method (LS) (Aki, 1965; Hirata, 1989; b-value. Mapping the b-value provides information Utsu, 1965). In the present study we have used the maximum likelihood method for mapping the b-value about the stress regime of a specific region. Changes in the b-value with time following large earthquakes using the software package ZMAP (Wiemer, 2001). also suggest that b(t) can be used as a short-term (day- Accordingly, b-value can be defined as, to-month) earthquake precursor for the aftershocks log e b ¼ (2) sequence. M M Where M is the average magnitude and M is the thresh- 5.1. b-value mapping old magnitude used in the analysis, and log e = 0.4343. An overall b-value map is prepared using the avail- At first the entire database consisting of 7250 num- able database collected from various seismological ber of events were declustered using Reasenberg agencies as mentioned above. The database was decluster algorithm (Reasenberg, 1985). The events declustered first and the map was prepared using with magnitudes scaled in M - scale are only taken the events, M � M . The obtained result (Figure 3a) b c ALL EARTH 51 Figure 2. Frequency-magnitude Distribution (FMD) of earthquakes database for the regions (a) Shillong–Mikir Plateau, (b) Arunachal Himalaya, and (c) Assam Foredeep. The straight line (red colour) is the best fit line by maximum likelihood analysis of events. The threshold magnitude, M , is shown by the triangle. The average b-value is shown with the standard deviation. shows a significant b-value anomalies in the entire (Table 1), which are associated with the areas of study area. Epicenters of large earthquakes low b-value, (low b-value areas are shown by blue (M � 6) are shown by yellow star marks in the colour in the map (Figure 3a). On the other hand, map. There are a number of large earthquakes the higher b-value regions are associated with no (M � 6), found in various parts of the study area large earthquakes (Figure 3a). b 52 A. PHUKAN AND D. D. MOHANTY Figure 3. a. The spatial distribution of b-value for the entire study area; b. The resolution map, the area with high resolution is shown by blue color. Epicenters of large earthquakes (M � 6) are shown by yellow star marks. 5.2. Resolution map 2015). In our study we have taken N (minimum min number of events) equal to 20. In the present The geographical resolution is an important para- study the area under investigation shows meter in the study of b-value map. Resolution map a resolution of 20–100 km for the epicentral region, is directly linked to the density of the earthquakes except for 1904 (M ~7.6) earthquake which is associated with each grid nodes where the resolu- slightly towards on higher sides (~40–150 km) of tion of the map is inversely proportional to the radii (Figure 3b). The region located to the north of radii of the circle containing N events min the latitude 28° and west of longitude 94° shows (Nuannin & Kulhánek, 2012; Prasad & Singh, a resolution of about 120–200 km, due to the low ALL EARTH 53 Table 1. List of large earthquakes (M ≥ 6) including some historical events and their epicenters (Lat, Lon) in the study area are presented below. Day Month Year Longitude Latitude Magnitude Depth (km) 28 04 2021 92.45 26.78 6.2 34.0 17 11 2017 95.06 29.83 6.1 6.40 09 01 2013 94.77 25.32 6.0 95.1 21 09 2009 91.44 27.38 6.0 17.4 01 06 2005 94.60 28.90 6.0 25.7 07 06 2000 97.14 26.81 6.3 24.2 06 08 1988 95.16 25.06 6.9 98.1 12 08 1976 97.07 26.68 6.4 27.0 12 08 1976 97.14 26.67 6.4 26.6 29 07 1970 95.32 25.95 6.6 73.5 15 09 1967 91.87 27.33 6.0 13.0 21 10 1964 93.83 28.08 6.0 20.2 15 08 1950 96.50 28.50 8.7 – 29 07 1947 93.70 28.80 7.7 – 23 10 1943 93.00 26.00 7.2 – 21 01 1941 92.50 26.50 6.5 – 02 07 1930 90.20 25.80 7.1 – 09 09 1923 91.00 25.50 7.1 – 30 08 1904 97.02 25.51 7.6 15.0 12 06 1897 91.00 25.00 8.7 – 10 01 1869 93.00 25.00 7.5 – seismicity of the area. The resolution of the map in the past. The occurrence time of large earthquakes decreases as we move away from the epicentre of (M � 5) and the major drop in b-value are shown by the large earthquakes (Prasad & Singh, 2015), arrow marks (Figure 4). It can be observed from the which can be seen from the prepared map figure that there is a noticeable amount of decrease in (Figure 3b). b-value during such large earthquakes, after which the value gets increased significantly. For Shillong–Mikir Plateau, a slight declination of 5.3. Temporal variation of b-value the curve observed during the year 1900, which is due to the occurrence of many earthquakes (M � 5) The average b-values for three main regions of interest, during that nearby period. In the next half a significant the Shillong–Mikir Plateau, the Arunachal Himalaya, drop of b-value observed during the period 2006– and the Assam-Foredeep were estimated for two 2007. It may be due to the occurrence of a large earth- time periods, the first period includes all events that quake of magnitude 5.5 in the year 2006 followed by occurred until 1999 and the second period includes a cluster of earthquakes (M � 4:5) during that time events that occurred after 2000 (01.01.2000) till period. The region once more experienced some sig- the year 2022. The corresponding average b-value are nificant earthquakes (M � 5) in the years 2012, 2013, listed in Table 2. For Assam-Foredeep, the average 2015 and 2016, which might be the cause of the rapid b-value is presented only for the time period 1900– variation of ‘b’ between the years 2012 and 2017. The 2022 period due to the low seismicity in this region. Shillong–Mikir Plateau region had experienced a large Hence, a comparative study for the two time periods earthquake (M ~6.2) after a long gap in 2021. It can be could not be possible because of the less seismic seen from the figure (Figure 4b) that the curve events in this time period. decreases after 2018 which indicates building up of From Table 2, it can be seen that the b-value high stress in this region. A very significant variation decreases significantly for the second period for all of of b-value from (~0.6–1.4 units) can be observed in the the three regions. It may be due to the increasing in region. The earthquakes (M � 5:5) which occurred in seismicity in the region of interests. The Figure 4 shows the region Shillong–Mikir Plateau are listed in Table 3. the temporal variation of b-value (b(t)) for the three The temporal variation of b-value for Arunachal regions. However, there are few numbers of major Himalaya also shows a significant anomaly (~0.6–1.5 earthquakes (M � 6) occurred in these three regions Table 2. ‘b-value for the consecutive two periods; first period (1900–1999) and second period (2000–2022). REGION TIME PERIOD B-VALUE M SHILLONG–MIKIR PLATEAU 1900–1999 1:63� 0:3 4.8 2000–2022 0:554� 0:01 2.7 ARUNACHAL HIMALAYA 1900–1999 1:45� 0:1 4.7 2000–2022 0:928� 0:04 3.7 ASSAM FOREDEEP 1900–2022 1:05� 0:2 3.9 54 A. PHUKAN AND D. D. MOHANTY Figure 4. Temporal variation of b-value for three regions; Shillong–Mikir Plateau, Arunachal Himalaya and Assam-Foredeep. Left side panel of the figure (a and c) shows the distributions for b-value for the period 1900–1999 and the right-side panel with figures (b and d) shows the distribution for the period 2000–2022 for the Shillong and Arunachal region, respectively. Due to low seismicity in the Assam-Foredeep region, temporal distribution is evaluated only for the period 1900–2022, without any divisions in time periods. The major drop in b-values are shown by arrow marks (red colour). units), as the region had undergone some major earth- (M � 5:5) in the year 1985 (Figure 4c). During the quakes in the past. The list of earthquakes (M � 5:5) next half when the region was once more jolted by are given in Table 4. An observable drop in the b-value a major event (M 6) in 2005, an observable drop of was found in the region of Arunachal Himalaya for b-value was noticed. Between the time periods 2012 both time periods (Figure 4c & 4d). In the first half and 2019, the region had undergone many earth- (Figure 4c) the first drop of the value ‘b’ was found quakes (M � 5) causing an undulation in the b-value between the time period 1982 to 1983; which may be which can be seen in the figure (Figure 4d). The due to the occurrence of a series of earthquakes Arunachal Himalaya region, which is located in the (M � 5) during that period of time. This decline of ‘b’ eastern part of the Himalaya is seismically very active. continued, and attained a minimum value when the The accumulated stress from crustal shortening fre- region was again hit by some large earthquakes quently releases huge amounts of energy, resulting in ALL EARTH 55 Table 3. List of large earthquakes (M ≥ 5.5) in the region of Shillong-Mikir Plateau. As temporal variation of ‘b-value' changes significantly due to large earthquakes, the scenario of these large events are represented below. Day Month Year Longitude Latitude Magnitude Depth (km) 12 06 1897 91.00 25.00 8.7 – 10 01 1869 93.00 25.00 7.5 – 23 10 1943 93.85 26.63 7.4 15.0 02 07 1930 90.20 25.80 7.1 – 09 09 1923 91.00 25.50 7.1 – 21 01 1941 92.50 26.50 6.5 – 28 04 2021 92.45 26.78 6.2 34.0 01 09 1964 92.20 26.96 5.7 22.0 28 06 2015 90.31 26.63 5.6 24.4 12 09 2018 90.17 26.35 5.5 14.1 06 11 2013 93.59 26.47 5.5 51.5 23 02 2006 91.72 26.97 5.5 15.9 17 07 1971 93.12 26.45 5.5 41.1 Table 4. List of large earthquakes (M ≥ 5.5) in the Arunachal Himalaya region. Day Month Year Longitude Latitude Magnitude Depth (Km) 15 08 1950 96.50 28.50 8.7 – 29 07 1947 93.70 28.80 7.7 – 15 09 1967 91.87 27.33 6.0 13.0 21 10 1964 93.83 28.08 6.0 20.2 01 06 2005 94.60 28.90 6.0 25.7 23 04 2019 94.52 28.44 5.9 19.0 14 03 1967 94.21 28.43 5.8 18.8 19 07 2019 92.77 27.63 5.7 13.6 07 01 1985 91.97 27.10 5.7 14.9 01 09 1964 92.20 26.96 5.7 22.0 07 01 1985 91.98 27.15 5.6 11.8 26 09 1966 92.55 27.45 5.6 16.3 01 08 1985 95.18 29.17 5.5 24.0 26 09 1998 92.82 27.58 5.5 18.6 19 02 1970 93.91 27.43 5.5 12.8 List of large earthquakes (M ≥ 5.5) in the region Arunachal Himalaya. As temporal variation of ‘b-value’ varies depending on large earthquakes, list of large earthquakes is given. moderate to large earthquakes in this region. The cor- earthquake occurrence in this region with respect to responding figures which show the declination of a spatial-temporal domain. b-value prior to such large events are presented in To make the analysis more precise, b-value map for figures 4c and 4d. the selected three regions of interest is prepared sepa- The variation of b-value is very less for the region, rately. Largest events (M � 6) occurred during the Assam-Foredeep. No significant earthquakes study period, are marked with star (Figure 1). For the (M � 5:5) can be seen in the region during the study three regions: Shillong–Mikir Plateau, Arunachal period. There is a very minimal amount of b-value Himalaya and Assam-Foredeep, the largest events anomaly (~0.5–0.7 units) and a very small amount of occurred are listed in Tables 3 and 4. However, there drop in b-value can be seen in this region (Figure 4e), is no large event (M � 5:5) can be seen in the region which may be due to comparatively less seismic activ- Assam-Foredeep. From the Figure 5, it can be seen that ities in this region since several years. all large events occur in the low b-value areas (b < 1; dark blue/blue colour), ranging from~0.5–1. It is impor- tant to note that the average b-value calculated for each case are different (Figure 5), where the same 5.4. Spatial variations in seismicity and hazard colour index is used to prepare the spatial distribution estimation map for an easier interpretation purpose. The spatial The overall b-value map is prepared using the available distribution for the corresponding three regions is esti- database (Figure 3a), which shows a variation ranging mated for two time periods, as is the case with the from~0.4 to 2.4 units. The region North-East India had temporal distribution. Events that took place prior to experienced many large earthquakes (M � 6) from 1999 are included in the first period, while the events time to time, which can be observed from the that occurred after 2000 and up to 2022 are included in Figure 1. Hence, the b-value estimates the stress pat- the second period. The time period is divided only for the Shillong–Mikir plateau and Arunachal Himalaya tern and the hazard analysis on the basis of the 56 A. PHUKAN AND D. D. MOHANTY Figure 5. Spatial variation of b-values for the three regions; Shillong–Mikir Plateau, Arunachal Himalaya and Assam-Foredeep. The distributions for b-value for the periods 1900–1999 are shown in the left side panels of the figure (Figures 5a and 5c), whereas the distributions for the periods 2000–2022 are shown in the right-side panels of the figure (Figs. 5b and 5d), respectively, for the Shillong–Mikir Plateau and Arunachal Himalaya regions. The temporal distribution for Assam Foredeep region is evaluated only for the period 1900–2022, with no divisions in time period, because of minimal seismic activities in the region. region, whereas due to lower seismicity, the spatial 95.5° and 98° and latitudes 28–29.5°. In the second distribution of b-value for Assam-Foredeep region is period, the region had witnessed a large earthquake calculated only for the time period 1900–2022. (M ~6) in the year of 2005. The area near the epicentral In Shillong–Mikir Plateau, the major changes in region shows a low b value (Figure 5d), which indicates b-value observed in the two period roughly in between the accumulation of high stress in this region. No large longitude 90–92.5°. The b-value map for second period earthquakes in the area of high b-value can be seen in preceding the earthquake (M ~6.2, 2021) shows a low this region, (Figures 5c and 5d). value (Figure 5b), indicating the increase of accumula- No significant variation of b-value can be observed tion of stress in the region. Also, during the second for the region Assam-Foredeep. No such large events period the region had experienced many large events (M � 6) can be found in the region during the study (M � 5:5) (Table 3), which can be observed in the low period. The seismicity of the region is quiet low for b-value areas, (blue colour) in figures as precondition a long time as compared to other two regions. The of stress patterns before large earthquakes. spatial distribution map of b-value shows that the major- The region Arunachal Himalaya also shows an ity of the region’s areas are associated with low b-value observable anomaly in the b-value between longitudes ranging roughly from 0.5 to 1.0 (Figure 5e), which might ALL EARTH 57 be an indication of large-scale accumulation of stress in earthquakes that occurred in the region is represented this region for quite a long time period. The spatial map here. Analysis of earthquakes that occurred prior to for b-value, representing the lowest b-values (dark blue 1980 is impossible due to the discontinuous nature of colour), suggests a possibility of future larger events/ data. The analysis is performed only for the Shillong– earthquakes in this particular region associated with Mikir plateau and the Arunachal Himalaya region, a large accumulation of stress energy. because of their high seismicity compared to the Assam-Foredeep region. For these two regions, we have selected some circular sub-regions (Table 5 & 5.5. Larger earthquake scenario on temporal Figure 1) with a specific radius (120 km) from distribution of b and hazard estimation a central coordinate. While selecting the central coor- dinate and the sub-regions it was taken into considera- To strictly observe the variations in seismicity in the tion that, maximum number of earthquake data enters NER, specific research for the b-value concerning large Table 5. Temporal changes of b-value in four sub-regions of interest. Number of Earthquakes used for the Changes in ‘b-value’ Sl No. Regions Central Coordinate computations (Range) 01 Sub-region 1 (S 1, Shillong–Mikir 91.00°E and 26.00°N 535 ~0.016–0.7 units region Plateau) 02 Sub-region 2 93.00°E and 26.10°N 1103 ~0.21–0.65 units (S 2, Shillong– Mikir Plateau) region 03 Sub-region 3 92.52°E and 27.80°N 546 ~0.3–0.51 units (S 3, Arunachal Himalaya) region 04 Sub-region 4 95.50°E and 29.00°N 298 ~0.07–0.48 units (S 4, Arunachal Himalaya) region Details of the selected four circular sub-regions and their respective central coordinates along with the number of earthquakes used to perform the computation of ‘b-value’ and their respective results are presented Figure 6. Scenario of temporal variations of large events, where the analysis is performed around four coordinates within the study area and then determined the temporal variations of b-value for the events within a radius of ~120 km range. Figures (a) & (b) show the temporal distribution for Shillong–Mikir Plateau region (S 1 and S 2) and figures (c) & (d) are for Arunachal region region Himalaya region (S 3 and S 4), respectively. Large events (M � 5) are shown by star marks (yellow colour). The respective region region peak values of ‘b’ are marked by inverted triangles and the areas associated with major deviation of b-value are shown by red circle. The corresponding changes in b-values (δb) of clusters are marked by arrows. 58 A. PHUKAN AND D. D. MOHANTY into the computation of the b-value. Table 5 contains and 1991. The results reveal a steep decline in the information on the earthquake database, including the value ‘b’ starting in the year 1982 when the area was central coordinate and the observed results, where hit by three large earthquakes (M ~5.2, 5, and 5.3) only large earthquakes (M � 5) were taken into con- during the period 1983 and 1984. It reached a low sideration (Figure 6). Corresponding temporal distribu- value again (~0.95 units) when a cluster of earthquakes tion maps of the calculation (b-value variation) are occurred between the period 1986 and 1987. This shown in Figure 6, and the results are summarised in decline of ‘b’ continued until it reached another mini- Table 5. Interestingly, the b-value dropped in the range mum value (~0.9 units) when a cluster of earthquakes of 0.016–0.7 units (Table 5) prior to the major events, occurred in between 1989 – 1991 (Figure 6b). After the corresponding to a significant increase in tension/ occurrence of these events, the value ‘b’ rose exponen- stress levels for the occurrence of large events. So, by tially, reaching a maximum value of 1.34 units, since measuring the temporal variations in the b-value, we there were no such significant events observed in the can lead to characterise the hazard analysis or advent area during this period. This embarks the significant of major seismicity of a particular region of interest. loss in the stress patterns after these series of earth- The observations manifest a drastic drop in the b-value quakes prior to 1996, which have released a major at the time of occurrence of large events. stress accumulated in this region responsible for To observe the temporal variation of b-value for these large events. After reaching its peak value, it the earthquakes occurring within the Shillong–Mikir started to fall once more until two significant events plateau region, we have chosen two coordinates as with magnitudes of 5 and 5.3 shocked the area in 1998 the central location of two circular sub-regions as; and 1999 corresponding to a drop of ‘b’ (Δb) by 0.21 S 1 (91°E, 26°N) and S 2 (93°E, 26.10°N) unit. Afterwards, again an increase till the year 2003 region region (Figure 1). The corresponding results were sum- has been seen, reaching a new peak of 1.26 units, marised in Table 5. The observed results for S following which there is a gradual decline of ‘b’ is region 1 show a drastic drop in the b-value prior to major observed that approached a minimum value (~0.61 eventsðM � 5Þ. The first drop of ‘b’ is observed unit) in the year 2012, indicating an increase in tectonic between the periods 1986 to 1993 (Δb ~ 0.016 stress in the region. This led to again two large earth- unit) when two large earthquakes (M ~5 and M quakes of magnitudes 5.2 and 5.4 shocked the region b b ~5.1) occurred in the year 1992 (Figure 6a). After in 2012, followed by a significant event (M ~5) in 2009, the occurrence of these events, there is a gap of corresponds to a significant drop in ‘b’ (Δb ~ 0.65 about 13 years, during which the region had not units). gone through any major events and the corre- To analyse the changes in b-value for the Arunachal sponding b-value rose to a maximum value of 1.4 Himalaya region within the time window considered, units during 2002–2003. This suggests a less tension two sub-regions were selected (S 3 & S 4) region region scenario/period which has lasted for about 13 years (Figure 1) whose central point lies at the coordinates after the two major earthquakes of 1992, as major- 92.52°E, 27.80°N and 95.50°E, 29.00°N. The temporal ity of the stress was already released in these distribution map of b-value variation for the S 3 region events. After reaching the peak, it began to (Figure 6c) shows three major deviations of ‘b’ (Δb) decrease once more (Δb ~ 0.25 unit) until 2006 ranging from 0.3 to 0.51 units. During the periods (from 2003), when S 1 was again shocked by region 1985–1991, the first deviation is observed (Figure 6c) another large earthquake of magnitude 5.5. The in the year 1985 where a cluster of four earthquakes obtained results show a fluctuation of ‘b’ after with magnitudes ranging from M ~5 to 5.7 occurred, 2010, which underlines the region’s instability. b During the period 2005 to 2006 the region had causing the value of ‘b’ to fall from its peak value of 1.4 undergone two major events (M ~5.5 and M ~5) units. Following two more significant events (M ~5 b b which leads to a major drop of b-value (Δb) roughly and 5.1) in the years 1987 and 1989, this declination around 0.7 units. continued and reached a minimum value of 1.1 units The S 2 (Figure 1), which is located very near the region (Δb ~ 0.3 units). In the years, 1993 to 1998, a series of Kopili fault zone, has a notable amount of b-value earthquakes with magnitudes of M ~5 to M ~5.5 b b anomalies. The Kopili fault is one of the most seismi- jolted the area once again. ‘Figure 6c’ shows how the cally active lineaments in the North-Eastern region. The value ‘b’ rose dramatically after 2013 and peaked at obtained results for S 2 show two significant region 1.22 units in 1993. When the region was again hit by deviations of ‘b’ from 1983 to 1991 and 2003 to 2012 a series of three earthquakes between the years 2000 (Δb ~ 0.6 and 0.65 units, respectively), supporting the and 2001, it corresponds to a declination in b-value tectonically active nature of the Kopili fault zone. The once more, reaching a low value of 0.82 units in 1999 region experienced a series of earthquakes with mag- nitudes ranging from 5 to 5.4 between the period 1983 (M ~ 5.1, 5.2 &5.3). After these events, there was a 10- ALL EARTH 59 year gap during which no more significant events were which the author used a micro-seismic cluster coupling seen, and the related b-value grows gradually until procedure to analyse the 2017 Bodrum earthquake (M ~6.6). The study identified several relocated events the year 2010, suggesting a low-stress regime and w within the study area nearby SW corner of Turkey, undergoing the accumulation period of stress. After which were then statistically analysed in both spatial the peak of 2010 (1.12 units), there was a sharp decline and temporal forum to establish a symptomatic rela- of ‘b’ that significantly manifests the 2012 event (M tion with the major Bodrum event of 2017. The term ~5.2), reaching a minimum value of 0.61 units (Δb ~ ‘relocated events’ refers to events that are repeated in 0.51) in 2013. each cluster and have the same seismic parameters The second sub-region of Arunachal Himalaya (same hypo-central depths or focal mechanism solu- (S 4), likewise exhibits abrupt fluctuations in region tions or similar magnitudes). The temporal and spatial b-value (Figure 6d). The area has occasionally been distribution of these micro-seismic clusters were eval- the epicentre of numerous large earthquakes. A series uated following the 26 November 2012 Bozburun of nine earthquakes with a magnitude of 5 to 5.5 struck mainshock (Mw 4.8) and the aftershocks thereafter in the area during the years 1982 and 1988, triggering Bozburun, Ula, and Bodrum areas of the gulf (the a significant drop of ‘b’ (Δb) up to 0.32 units. This 16 May 2013 Ula aftershock (Mw 4.6), the declination was maintained following two clusters of 25 March 2014 Bozburun aftershock (M ~4.0), and earthquakes that occurred in the years 1992 and 1993, the 1 May 2014 Bodrum aftershock (M ~4.0)). The with magnitudes ranging from 5 to 5.6. It then study considered the 2012 Bozburun earthquake (M dropped to a minimum value of 0.6 units (Δb ~ 0.48 ~4.8) to be the first main shock and identified five units) until another significant event occurred in 1996 clusters of events in the study area and their interac- (M ~5). Following the occurrence of these events, the tion was symptomatically established to evaluate the b-value had a dramatic increase, peaking at 0.84 units 2017 main shock of M ~. 6.6 in Bodrum. The analytical in 2000. It again began to fall gradually during a series w model is closely related to Toker (2021) interaction of earthquakes that occurred between 2002 and 2007, coupling model, known as the ‘chain reaction model’, with magnitudes ranging from 5 to 6 and attaining which is applicable to multiple-shock events as well as a minimum value of 0.6 units (Δb ~ 0.24 units). When a series of five earthquakes with magnitudes ranging event clusters. However, there is a detectable connec- from 5 to 5.2 rocked the area in 2013, further declina- tion over large distances, implying that the interaction tion ‘b’ (Δb) from that point caused it to fall even more, extends even further and for a longer period. by 0.07 units. This abruptly manifests the significant We have attempted to analyse the interactions loss in strain patterns after a series of major earth- between micro-earthquake clusters in the Arunachal quakes. A large part of stress, which was stored for Himalayan region for the first time. An E-W cross- a gap of around 10 years was released in a series of section of the Arunachal Himalaya region (Figure 1) events, not with a single stroke. After the serial advent is established and depth distribution of the events are of these cluster of earthquakes, the regime tries to gain mapped along this particular profile. The depth sec- the gradual stress pattern again, and the b-values keep tion reveals that the seismicity of the area extends up gradually increasing again as suggested by the figure to a depth of 80 km. In a time, frame of 2014–2020, (Figure 6d). we have clearly noticed three major micro-seismic clusters whose epicentral locations and depth distri- 5.6. Systematic discretisation of small earthquake bution are very close to one another (Figure 7). To clusters and coupling to 2019 Arunachal large evaluate the interactions of these clusters, we have earthquake (M ~5.7) closely analysed the clusters related to the event with In the present study, we try to establish a symptomatic M ~4.8 (2015) and statistically established the relation discretisation of small earthquake clusters and relate of the clusters associated with this event towards the their coupling with a major earthquake event in the occurrence of a large event with M ~5.7, 2019 north eastern region. A recent study carried out in the through a symptomatic coupling of these cluster of SW corner of Turkey (Toker, 2021) has revealed events in this particular time frame. The first main a significant coupling of clusters of events, which pro- shock in 2015 (M 4.8) was associated with three gressively contribute towards the outburst of a larger large pre-shocks (M ~4, 4.2 and 4.4; purple colour event. Our present study stressed upon this new ideol- dots) and many minor shocks in the same cluster ogy and fortunately mapped that the large M ~5.7 (Figure 7, represented by shadow regions). The inci- (2019) earthquake in the Arunachal Himalaya is dence times of these pre-shock and main-shock series a consequence of a series of events, related to sympto- are fairly close. The 2015 (M ~4.8) event had after- matic clustering of events since 2015. The current shocks that lasted for years, and led large shocks findings are comparable to those in Toker (2021) in 60 A. PHUKAN AND D. D. MOHANTY Figure 7. An E-W Event-depth section over the Arunachal Himalaya region, across the profile AB (Figure 1). The red dot corresponds to the epicentre of 2015 (M ~ 4.8) event (C the main event of a cluster) in the west. The yellow dot corresponds b M, to the epicentre of the 2019 (M 5.7) event in the east, which is the treated as the major event as a consequence of the cluster of previous events. The micro-seismic clusters under consideration are shown by rectangular shadow and the corresponding foreshocks (C ) and aftershocks (C , C , C ) are shown by purple and green colour, respectively. Notice three aftershock F A1 A2 A3 sequences that have moved from E-W direction, which took place in different time frames: C (during 2015), C (during 2016), A1 A2 and C (from 2017 to 2019). A3 (4 � M < 4:8) to occur quite frequently in the nearby This periodic build-up of stress energy led to a rapid area. These aftershocks had an E-W alignment with an rise in stress in this region, which is also active due to upward migration which can be seen from the depth the collisional tectonic scenario, resulted in a sudden section (Figure 7), which lasted for almost three years. burst of energy that in the form of the 2019 event These aftershocks have three unique sequences, (M 5.7). This experiment is a success example of the which are further classified as C , C , and C symptomatic interaction of micro-seismic clusters, A1 A2 A3 based on their occurrence times. The aftershock leading to a rapid growth in the stress regime in sequence C is related with the 2015 main shock this Arunachal Himalayan region and successfully A1 (M ~4.8), which occurred in 2015. Following the explains the earthquake scenario associated with occurrence of these events, another series of events 2019, M ~5.7 event. (C ) is reported within few months which continued A2 throughout 2016, with the epicentres migrated 6. NE hazard estimations and tectonic towards the east of the 2015 major shock with implications a few kilometres away. Continuing to this serial occur- The temporal and spatial variation of b-value were rence of events, a tight cluster of earthquakes (C ) A3 analysed for the North-Eastern region of India was observed near the epicentre of the 2019 event (between longitudes 95.5° and 98° and latitudes 28°– (M ~5.7) in the time span of 2017–2019. It is signifi - 29.5°). The prime objective of the present study is to cant to note that the aftershock series has a periodic make a comparison of seismic b-value among the three migration to the east, as seen in Figure 7. These main regions of Interest; Shillong–Mikir Plateau, micro-cluster migrations indicate the transfer of stress Arunachal Himalaya and Assam-Foredeep. The entire energy in an E-W direction, which then accumulated database was divided into two periods for a robust in the epicentral region of the 2019 M ~5.7 event. analysis. The results from these two periods show ALL EARTH 61 observable changes in b-values for the three regions earthquake focal mechanism solutions indicates two prior to and after large earthquakes. broad dominating stress directions in the Arunachal Major changes of the b-value were observed in the Himalaya, ranging from NNW-SSE in the western and region of Shillong–Mikir Plateau. Shillong plateau has central regions to N-S in the eastern part (Angelier & a distinctive pop-up tectonics block of Northeast India Baruah, 2009). The obtained results of b-value from the due to its uplift dynamics, which are restricted by current study also suggest the huge accumulation of deep-rooted thrust faults (Bilham & England, 2001; tectonic stress beneath the region. Interestingly, for Hossain et al., 2021; Mohanty & Singh, 2021, 2021). the last 6 years the strain pattern is gradually attaining This distinct and complex plateau structure was most the increment without any major fluctuations and may probably formed by a combination of N-S-directed be a close signal for a major seismicity, as evidenced by stress from the Himalayan collisional zone and temporal patterns of b-value in this region in the recent E-W-directed compressional force from the Indo- and further past periods. The Indo-Eurasian collisional Burma subduction zone (Rao & Kumar, 1997). tectonics with a significant crustal shortening is attrib- According to Bilham and England (2001), there is uted towards this large scale stress accumulation in a blind reverse fault (Oldham fault) about 110 km a temporal scale and may be a snoozing alarm for long that dips steeply away from the Himalayas near future for a large event. beneath the Shillong plateau. This blind reverse fault The region ‘Assam gap’ in the upper Assam valley was believed to be the source of the 1897 great earth- (Khattri et al., 1983) of North-East India has experi- quake, which caused pop-up deformation of around enced essentially no seismic activity for the past 60 11 m of the plateau (Mohanty & Singh, 2021). The years. The seismic activities in the region are quiet low. southern part of the plateau is defined by the Dauki The unavailability of the earthquake data during the fault, which is responsible for a portion of the short- study period limits the stress analysis or the estimation ening between the Himalayas and India (Bilham & of b-value for the region in the present study. However, England, 2001; Singh, Eken, et al., 2016). The plateau’s the obtained results from the spatial distribution of uplift began in the mid-Cretaceous and reached b-value with the available data show that the majority a maximum during the Mid-Miocene epoch, which of the region’s areas are associated with low b-value was followed by the India-Eurasia collision (Mitra (~0.5–1.0 units, blue colour). From this study, it can be et al., 2005). Its current elevation is around 1 kilometre inferred that the Assam Foredeep region might have (Bilham & England, 2001) and it exhibits a small posi- accumulated a sufficient amount of stress for a long tive bouguer gravity anomaly (Das Gupta & Biswas, time, which may lead towards an intense and larger 2000). The complex dynamics of the region makes earthquake event in a near future. Again, jawed in the plateau seismically active and historically it has between collisional tectonics towards north, enforced gone through many devastating and great earth- stress from Indo-Burmese Range from east, northward quakes. The average estimated b-value for the region push from Bengal basin and overall absolute plate show a very low value (~0.4). The temporal variation in motion related strain guided by asthenosphere drag b is also in agreement in almost no such variations in of Indian plate towards NE direction, really makes this the patterns in recent past. There is a high possibility of geological entity a most complex tectonic domain in large-scale tectonic stress building up within the northeast Indian subcontinent. Hence, a complex region, posing a high risk of future large earthquakes. strain pattern development, assisted by a near 60 There are also noticeable b-value anomalies for the years gap of major seismicity as suggested by lower Arunachal Himalaya region. A significant drop of b-value scenario, is indeed an alarming condition and b-value was observed (~0.52 unit, (Table 2)) during a serious seismic issue of the present day, leads to the second time period (2000–2022). The Arunachal a need of the hour research in a broader scale. Himalaya encompasses the eastern Himalayan syntaxis and is located between longitudes 91° 30 E and 96° 00 7. Conclusions E. This northeast region of the Indian plate is sur- rounded by major thrust zones (MBT – MCT to the The frequency-magnitude analysis and systematic var- north, Lohit – Mishmi thrusts to the northeast, Naga, iations in b-values on a temporal and spatial scale allow Disang and Eastern Boundary thrusts to the east, front us to decipher and understand the present seismic thrusts of the Arakan Yoma Belt to the southeast) (C. hazard scenario of the whole NER of the Indian sub- Singh et al., 2015; Singh, Eken, et al., 2016). In compar- continent. These analyses allow us to understand the ison to the central and western Himalayas, the eastern present stress regime, creation of resolution scale map Himalayan orogeny has undergone significant crustal and by logical reasoning the continuous variations in shortening (Yin, 2006). Because of the rapid rate of seismicity, herewith b-values, since the very past in crustal shortening and tectonic deformation, the entire a continuous scale. Further, the systematic spatial dis- region attains a more activity in seismicity (C. Singh tributions of frequency-magnitude relations, allow us et al., 2015). The inversion of stress tensors from to clearly understand the hazard potential regions in 62 A. PHUKAN AND D. D. MOHANTY the NER. Taking into consideration of the very complex India from teleseismic receiver function analysis. (2014). Journal of Asian Earth Sciences, 90(1), 14. ISSN 1367-9120. tectonics and the critical geodynamic processes https://doi.org/10.1016/j.jseaes.2014.04.005 beneath the NER, along with the present scenario of Chan, L., & Chandler, A. (2001). Spatial bias in b-value of the stress patterns, this region is all set for large seismicity frequency– magnitude relation for the Hong Kong region. in near future. The need of the present hour is to Journal of Asian Earth Sciences, 20(1), 73–81. https://doi. analyse the complex geophysical processes with high org/10.1016/S1367-9120(01)00025-6 resolution models to estimate the proper stress pat- Chan, C. -H., Wu, Y. -M., Tseng, T. -L., Lin, T. -L., & Chen, C. -C. (2012). Spatial and temporal evolution of b-values before terns and levels. A further study on characterisation of large earthquakes in Taiwan. Tectonophysics, 532/535, 215 the most seismic potential zones, for a better prepa- 222. https://doi.org/10.1016/j.tecto.2012.02.004 redness in the near future is a real need of the hour. Das Gupta, A. B., & Biswas, A. K. (2000). Microseismicity and tectonics in northeast India (1st edn ed.). Geol Soc. of India. Dasgupta, S., & Nandy, D. (1982). Seismicity and tectonics of Acknowledgements Meghalaya Plateau, Northeastern India. In VII Symposium on earthquake engineering (I) . University of Roorkee, 10–12 DDM is thankful to the Director, CSIR-NEIST, G Narahari Nov 1982. Sastry, for the permission to publish this manuscript. This Evans, P. (1964). The tectonic framework of Assam. Journal of study is fully supported by a Mathematical Research Impact the Geological Society of India, 5, 80–96. Centric Support grant (MATRICS), vide Project grant no. MTR/ Gee, E. R. (1934). The Dhubri earthquake of the 3rd July 1930. 2022/000343 and FBR Project grant from CSIR, India vide Memoirs of the Geological Survey of India, 65(1), 1–106. Project no. MLP-0005. Gutenberg, B., & Richter, C. F. (1944). Frequency of earth- quakes in California, Bull. 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Journal
All Earth
– Taylor & Francis
Published: Dec 31, 2023
Keywords: North East India; b-value; Seismic Hazard; Large Eathquake; Earthquake Precursor
