The seismicity parameters and their spatial-temporal distribution are investigated in nine seismogenic source zone by using the updated homogeneous earthquake catalogue from the period of 1976–2020 for Northeast India. These seismicity parameters are providing the information to identify the high seismically active zone in the near prospect of this study region. Thus, the spatial distribution of b-value has been estimated by using the Maximum-likelihood method which varies from 0.6 to 1.52 in this study region as depicted in Figure. 2. High b-value is exhibited by the distribution of low stress which indicates the creeping of the faults in the southern part of the Indo-Burman range (Mizo folds and Chin Hills) and also in the Sikkim region due to the occurrence of low magnitude earthquakes along the fault in recent past. However, the low b-value is present in most of the Indo-Burman range and the surrounding region of the Sagaing fault in the study region. These low b-values are associated with the high magnitude of earthquakes occurrence, high crustal stress build-up in the locking segments of the fault, and heterogeneity of material (Gautam et al., 2018, Pandey et al., 2019). Furthermore, the low b-value is primarily due to the large earthquake occurrences in the region as suggested by Nuannin et al., (2005). The moderate b-value is found in the rest of the study region. From Figure. 3, it has been found that most of the Indo-Burman range and other areas exhibited a less standard b-value (i.e., 0.01), which ensures less uncertainty in the estimation of b-value in this study region. The result of the b-value in the present study is well matching with the previously obtained by several authors such as Yadav et al., 2009; Thingbaijam et al., 2009; Pandey et al., 2017; Pandey et al., 2019; Nayak and Sitaram, 2019 for Northeast India. There are several reasons such as earthquake events time period, source, and methodology which associate the difference in the estimation of b-value (Nayak and Sitaram, 2019).
The spatial distribution of parameter “a-value” which describes the level of seismicity is computed like the parameter “b-value” for Northeast India. The estimated a-value varies from 3.13 to 8.73 for this study region as shown in Figure. 4. From this Figure, high a-value is observed in the southern part of the Indo-Burman range in the range of 6.5 to 8.73. The high hazard level in terms of seismicity has been observed by estimation of a-value due to the occurrence of large earthquakes in the study region. Most of the study regions such as the northern part of the Indo-Burman range, the Main Boundary Thrust, and the vicinity of the Sagaing fault is showing a low a-value in the range of 3.13 to 4.0 as depicted in Figure. 4. Also, some areas such as in and around the Sikkim area and Chin Hills, Naga Disang Thrust) shows the moderate a-values which varies from 4.0 to 6.5. Previously, a-value has been estimated for the Indo-Burman range and observed that a-value lies in the range of 2.05 to 4.06 (Gautam et al., 2018). Kolathayar et al., (2012) have been estimated the a-value for the entire Himalayan region and observed that the low a-value is an exhibit in the eastern Himalaya. Moreover, the low and high a-value has been calculated by several researchers such as (Nayak and Sitaram, 2019; Thingbaijam et al., 2008; Yadav et al., 2009; Pandey et al., 2019) for this study region and its good matches with the present study.
The magnitude of completeness (MC) has been estimated for Northeast India and it is varying from 4.0 to 4.8 as shown in Figure. 5. The low MC value has been observed in the range of 4.0 to 4.2 in the area of northern and southern Indo-Burman range, Main Boundary Thrust, and also in and around the Sikkim (Figure. 5). The southern part of the Indo-Burman range and Chin Hills area represent the high MC value of the range 4.7–4.8 in this study region. In addition, the moderate MC value is obtained which varies from 4.3–4.5 in between the area of the southern part of the Indo-Burman range and volcanic line and Sikkim area. Gautam et al., (2018) have been estimated the MC value which varies from 2.0 to 3.5 in the Indo-Burman range to shows the spatial-temporal distribution of seismicity parameter for Northeast India. It has been observed that low MC values are present in the range 2.0 to 2.7 in Naga and Disang Thrust and the northern part of Sagaing fault of the study region. Also, high MC values are observed by Gautam et al., (2018) in the area of the southern part of the Indo-Burman range, Chin Hills, and Mizo fold which well-matches with the present study. The spatial distribution of MC value has been calculated by Nayak and Sitaram, (2019) for the entire Himalayan region and observed that the low MC value is present in Northeast India.
Further, the frequency-magnitude distribution plot has been computed by using ZMAP software for Northeast India as shown in Figure. 6. From Figure. 6, the seismicity parameters values have been obtained such as a-value, b-value, and MC value are 7.37, 0.93(± 0.013), and 4.6 respectively. Also, the seismicity parameters for nine seismogenic source zone have been estimated in frequency-magnitude distribution as listed in Table-2. Recently, the seismicity parameter values have been estimated by several authors such as Nayak and Sitaram, (2019); Thingbaijam et al., (2008); Pandey et al., (2019) for this study region. Yadav et al., (2009) has been computed the value of a, b, and MC as 7.94, 0.93(± 0.023), and 4.6 respectively for the same study region, which shows a good correlation with the present result of this study region. Also, seismicity parameters value has been estimated for this study region and it was found to be the value of a, b, and MC as 7.50, 0.95(± 0.023), and 4.6 respectively (Pandey et al., 2017).
Finally, it has been concluded that the obtained values (discussed above) from the estimated seismicity parameters for this study region indicating that the Northeast region is currently high seismically active region, and some areas are reflected as most hazardous. Further, the seismicity parameters values are estimated based on the frequency-magnitude distribution for each zone as listed in Table-2. Resulting from this, low and high b-values are present in Zone-2 and Zone-7 along with MC values such as 3.5 and 4.8 respectively. Also, the low and high seismicity activity rate values are present in Zone-2 and Zone-1 respectively.
Table-2 List of seismicity parameters values is estimated by FMD for nine seismogenic source zone of Northeast India.
S. No.
|
Seismic Zones
|
No. of Earthquakes
|
Mmax
|
MC
|
a-value
|
b-value
|
1.
|
Zone 1
|
664
|
6.9
|
4.7
|
7.31
|
1.05(± 0.06)
|
2.
|
Zone 2
|
746
|
6.1
|
3.5
|
4.21
|
0.45(± 0.01)
|
3.
|
Zone 3
|
286
|
6.3
|
4.6
|
5.90
|
0.84(± 0.05)
|
4.
|
Zone 4
|
522
|
6.1
|
4.6
|
6.78
|
0.95(± 0.05)
|
5.
|
Zone 5
|
621
|
5.9
|
3.4
|
4.24
|
0.49(± 0.02)
|
6.
|
Zone 6
|
947
|
6.1
|
3.2
|
4.27
|
0.48(± 0.01)
|
7.
|
Zone 7
|
1459
|
6.9
|
4.8
|
8.12
|
1.17(± 0.03)
|
8.
|
Zone 8
|
1492
|
7.2
|
4.8
|
6.42
|
0.85(± 0.03)
|
9.
|
Zone 9
|
575
|
6.9
|
4.6
|
6.66
|
0.93(± 0.43)
|
The return period or recurrence interval (R) and annual probability of occurrence of the different magnitude of the earthquake are calculated for each zone and it dramatically varies from one to another zone in the study region. The return period and annual probability of future earthquakes for 40 yrs have been shown in Figure. 7 and Figure. 8. Also, the return period of different magnitude (M = 4.0, M = 5.0 and M = 6.0) respectively, for 40 yrs has been estimated as listed in Table-2. From Table-2, it has been observed that low return period values are present in Zone-7, Zone-7, and Zone-8 while high return period values are present in Zone-3, Zone-5, and Zone-3. However, Zone-5 has never experienced an earthquake of M = 6.0 or above magnitude in the 40 yrs past. Thus, a low value of return period for different magnitude (M = 4.0 and M = 5.0), indicating the Zone-7 is the high seismically active zone of Northeast India. Recently, the return period has been estimated for Northeast India by Nayak and Sitaram, (2019) and observed that a high return period was present for a magnitude of M = 7.0 in this study region. Also, suggested that this study region has never experienced a large earthquake above M = 8.0 in the past 50 year. Further, annual probability percentage of different magnitude (M = 4.0, M = 5.0 and M = 6.0) has been computed for 40 yrs by using equ (2) as shown in Table-2. Resulting from this, a low probability of occurrence of future earthquakes for different magnitudes (M = 4.0, M = 5.0, and M = 6.0) has been observed in the Zone-7, Zone-7, and Zone-8. However, Zone-7, Zone-5, and Zone-6 are exhibiting a high probability of occurrence of future earthquakes for different magnitudes (M = 4.0, M = 5.0, and M = 6.0) in Northeast India. The probability of occurrence of moderate to a large earthquake in 40 yrs time period for the rest of the zones is 50% or less than 50%. This analysis is indicating the probability of occurrence of low to high magnitude earthquake in Zone-7 i.e., the southern part of the Indo-Burman range and the vicinity of this area. Nayak and Sitaram, (2019) have been observed that this study region has been experienced a different magnitude of earthquake M = 6.0 and M = 7.0 in both 50 yrs and 100 yrs. The probability of earthquake occurrence is high for magnitude (M = 6.0 and M = 7.0), but for magnitude, M = 8.0, the probability of earthquake occurrence is less than 50%. Also, suggested that probability of occurrence of small to moderate magnitude earthquake in this study region.
Table-3 Illustrate the Return period or Recurrence interval and Annual probability for 40 yrs in each seismogenic source zone of Northeast India.
S. No.
|
Seismic Zone
|
Return Period (Yrs)
|
Annual Probability (in 40 Yrs)
|
M = 4.0
|
M = 5.0
|
M = 6.0
|
M = 4.0
|
M = 5.0
|
M = 6.0
|
1.
|
Zone 1
|
0.115
|
0.263
|
5.00
|
833.3
|
380.2
|
20
|
2.
|
Zone 2
|
0.158
|
0.357
|
8.00
|
625
|
280.11
|
12.5
|
3.
|
Zone 3
|
0.298
|
0.512
|
13.33
|
333
|
195.31
|
7.50
|
4.
|
Zone 4
|
0.175
|
0.396
|
5.71
|
555.6
|
252.5
|
17.51
|
5.
|
Zone 5
|
0.231
|
0.526
|
-
|
434.8
|
190.11
|
-
|
6.
|
Zone 6
|
0.121
|
0.500
|
40.0
|
833.3
|
200
|
2.5
|
7.
|
Zone 7
|
0.059
|
0.098
|
5.00
|
166
|
1020.4
|
20
|
8.
|
Zone 8
|
0.064
|
0.131
|
2.00
|
1562.5
|
763.3
|
50
|
9.
|
Zone 9
|
0.145
|
0.290
|
3.33
|
689.7
|
344.8
|
30.0
|