Accepted: August 10, 2021 | Published Online: August 12, 2021

Citation: Siangshai DM, Walia D, Saxena A, Lyngdoh AC (2021) Assessing the Seismicity of Shillong Plateau Using 'b value'. Int J Earth Sci Geophys 7:051.

Copyright: © 2021 Siangshai DM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

The Shillong Plateau is a detached North-eastern portion of the Indian plate and is part of the Eastern Himalayan Syntaxial zone. It is one of the most seismically active plateaus in the world and falls under zone V of the Indian seismic hazard zonation map, the highest seismic hazard zone. During the period of study (2013-2017) numerous local earthquakes of varied magnitude were recorded by the installed broadband seismic stations. The present study is focused on the calculation of the 'b value' using the Gutenberg-Richter relation, which is the rate of fall in the frequency of earthquake occurrences with their increasing magnitude. The 'b-value' is estimated by the maximum likelihood method at five different layers of the lithosphere considering the geoelectric crustal configuration up to the depth of 50 km. The calculated 'b values' at different layers are estimated to be 0.78 (0 < depth ≤ 10), 0.83 (10 < depth ≤ 20), 0.74 (20 < depth ≤ 30), 0.66 (30 < depth ≤ 40), 0.71 (40 < depth ≤ 50), with an average of 0.77 (0 < depth ≤ 50). The result indicates that the overall b value of the Shillong Plateau has a moderate value and the variation of the 'b value' at a different range of depth layers is due to the converging varying stresses due to strength, stress orientations, and physical properties of the layers.

Keywords

Shillong Plateau, Gutenberg-Richter relation, b value, Maximum likelihood method

Introduction

The Gutenberg-Richter relation describes the relationship between the frequencies of occurrence of earthquakes with increasing magnitudes. The relation is given as

$${\log }_{10}N=a-bM$$

'N' denotes the cumulative number of earthquakes having a magnitude equal to or larger than 'M'. The constant 'a' measures the level of seismic activity and the constant 'b' also known as the 'b value' is the slope of a straight line, which determines the rate of fall of frequency of occurrence of events with increasing magnitude. Based on the tectonic setup of the region the b value normally varies from 0.45 to 1.50 [1] and is commonly close to 1.0 in seismically active regions.

The 'b value' is an important parameter in seismic studies as it indicates the stress regime of a region [2]. The 'b value' of the region where stress is continually released by an earthquake will be high; on the other hand, the b value will be low when stress is getting accumulated [3,4]. A high 'b value' indicates large numbers of earthquakes are of lower magnitude and a low 'b value' indicates large numbers of earthquakes are of higher magnitude [5,6]. The 'b value' varies on the strength and composition of the material constituting the rock mass, stress orientations, and physical properties of the layers at different depth range. The region with low strength and large heterogeneous rock mass has a high 'b value' and the region with asperity and homogeneity has a low 'b value' [3,7-9]. In this paper, the 'b-value' is estimated in the Shillong Plateau at five different layers of the lithosphere considering the geoelectric crustal configuration up to the depth of 50 km [10].

Study Area

The Shillong Plateau is considered to be one of the most active seismic regions in the world. It is a detached portion of the Indian plate and falls under zone V [11] the highest earthquake hazard zone. The plateau experienced a great earthquake on 12^th June 1897 with a magnitude of 8.7 [12]. The Great Shillong earthquake has caused huge property damage and killed 1542 people [13]. Our study area falls under 25° N to 26° N latitude and 90° E to 92.5° E longitude and covers almost the entire state of Meghalaya. The recent strong earthquake on 28th April 2021 of magnitude 6.4 with epicentre at 26.690° N and 92.360° E, around 132 km N of Shillong, and focal depth of ~17 km is indicative of the fact that the region is active and requires consistent efforts to understand the seismotectonic setup of the region. Geologically, the Shillong Plateau is mainly comprised of the Precambrian Gneissic Complex and younger Cretaceous Shillong Group of sedimentary rocks sequence unconformably overlying the Precambrian Gneissic Complex and Tertiary sediments towards its southern margin [14-16]. The Shillong Plateau acts as a shallow crustal block in which the lithosphere beneath it subducts along the Dauki and Brahmaputra thrust [10]. The geoelectric cross-section along with the earthquake hypocentres in the Shillong Plateau and Lower Brahmaputra valley is shown in Figure 1.

Data Analysis and Methodology

Earthquakes waveform data are obtained from three broadband seismic stations set up at Cherrapunji, Kongwang, and Lurniang on the Shillong Plateau and data from Indian Meteorological Department (IMD), Shillong from March 2013 to December 2017 (Figure 2). The geographical coordinate of the seismic stations is given in Table 1.

The earthquake epicentres are located using the location program, HYPO71 [17], and the crustal velocity model of Bhattacharya, et al. [18].

The 'b value' is estimated by the Maximum likelihood method [19-21] given as

$$b=\frac{{\log }_{10}e}{M-M_o}$$

Where 'M' is the mean magnitude of the set of events and M_o is the minimum or threshold magnitude.

The 'b value' is calculated at depths ranging from 0 to 50 km. The earthquake maximum depth of 50 km is chosen considering the geoelectric crustal configuration [7]. The depth (km) is classified into 5 zones; 0 < depth ≤ 10, 10 < depth ≤ 20, 20 < depth ≤ 30, 30 < depth ≤ 40, 40 < depth ≤ 50. Each depth layers consist of a minimum of 14 events above the threshold magnitude and up to a maximum of 180 events.

The standard deviation of b-value (δb) is given as [19,22]

$$\delta b=2.3b^2\sqrt{\frac{{\displaystyle \sum _{{}_i}^n{\left(M_i-M\right)}^2}}{n\left(n-1\right)}}$$

Where 'Mi' is the magnitude of the ith event, 'M' is the average magnitude for a set of events and 'n' denotes the total number of events in a set.

Results and Discussion

The seismicity map showing the earthquake epicentres on the Shillong Plateau for the years 2013 to 2017 is presented in Figure 3. The earthquake epicentres are distributed in almost every part of the plateau, but the highest density of the epicentral distribution is in the central part of the plateau, trending in the NW-SE direction. The hypocentres of the earthquakes are confined within a depth of 50 km. The earthquakes of the Shillong Plateau are plate boundary earthquakes with a maximum hypocentral depth of 50 km [10,23,24]. The E-W depth distribution of the events is shown in Figure 4, the highest seismic activity (a-value) is observed at a depth range of 10 to 20 km. The seismicity of the Shillong Plateau is influenced by the tectonic driving forces from the Himalayan Arc to the north and the Burmese Arc to the east [10,25]. About 1.5-3.5 mm/yr. of the present-day N-S convergence in the Eastern Himalaya is accommodated in Shillong Plateau [26].

During the study period (2013 to 2017), the results show that the Shillong Plateau has a moderate 'b value' (0.77) as derived in Figure 5 which is lower than the 'b value' (1.29) calculated during the period 1983 to 1986 [24]. The cumulative frequency magnitude distribution of Shillong plateau for the study period (2013 to 2017) at depth less than 50 km is shown in Figure 5. According to Wyss [27] the 'b value' of a region changes in time and space. The decrease in 'b value' from 1.29 to 0.77 indicates that the tectonic stress is getting accumulated in the recent past after 1986. The standard deviation of the 'b value' varies from 0.01 to 0.02. The lower limit magnitude is taken to be 2.5 and a maximum limit of 5.1. The 'b value' at different depth layers is shown in Table 2. The b value obtained using the Maximum likelihood method is comparable with the least square fit. Lower 'b value' in the lower layers compared to the upper layers indicates low strength of the tertiary and Cretaceous sediments as compared to Precambrian Gneissic Complex. The variation of the 'b value' at a different range of depth layers is due to the converging varying stresses due to strength, stress orientations, and physical properties of the layers. The lower 'b value' at greater depth is a result of higher effective stress on the deeper earth's layer [27,28].

Conclusions

The Shillong Plateau shows high seismicity during the recent past (2013-2017). The epicentres are scattered on almost every part of the plateau but the highest density is found in the central part of the plateau. This indicates that the central part of the Shillong Plateau is more active. A detailed microseismic zonation study is required for earthquake risk reduction as urbanisation and infrastructural setup are rapidly growing in this area. The minimum 'b value' 0.66 is from the layer of depth around 35 km. A lower 'b value' indicates the accumulation of tectonic stress in the Shillong Plateau (Table 2). The results show that stress is being built up in the Shillong plateau and considering the shallow seismicity confined to depths within 50 km and the plateau's potential to generate earthquakes such as the Magnitude 8.7 earthquake experienced on the 12^th of June 1897, the vulnerability and risk in the Shillong Plateau grow exceedingly high. The average 'b value' of this region is 0.77 (Figure 5). This indicates a decreasing trend as compared to 1986 which is a matter of concern that needs a concerted effort to study the stress accumulation and earthquake activity in the Shillong plateau.

Acknowledgments

The authors would like to thank the Department of Environmental Studies and Department of Physics, North-Eastern Hill University, Shillong for supporting the research work. The authors also acknowledge IMD, Shillong for providing broadband seismic data.

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