Seismicity Parameters Analysis In Space-Time Distribution For Northeast India.

A complete homogeneous earthquake catalogue is prepared to estimate seismicity parameters and their spatial-temporal variation in nine seismogenic source zones for Northeast India. The value of seismicity parameters like a-value, b-value, and M C value i.e., 7.37, 0.93(± 0.013), and 4.6, respectively have been estimated from the frequency-magnitude distribution. Moreover, the maximum-likelihood method has been utilized to map the spatial variation of the above parameters. The spatial variation of low b-values is dominant in the Indo-Burman Range, the Main Boundary Thrust, and the vicinity of the Sagaing fault. High seismic activity rate has been obtained in the Indo-Burman range as observed from spatial variation of a-value parameter. Furthermore, the return periods and the annual probability of an earthquake have been calculated for each zone. The results of this seismicity parameter provide useful information about the hazard level of a particular zone and further helps in preparing the hazard map of Northeast India.


Introduction
Natural hazards such as earthquake, tsunami, and ood are a serious threat to the people living. The earthquake is considered as one of them a destructive natural phenomenon in the world due to which several damages to man-made buildings, loss of human life, and environmental affect. The occurrence of moderate to large magnitude earthquakes makes this study region as high seismically active of the Indian sub-continent. In the past 120 years, several low to high magnitude earthquakes (M W ≥ 3.0 to above 7.0) have occurred in Northeast India. But this region has been experienced by two devastating seismicity and seismotectonic analysis. Thus, the seismicity parameters study can help to reveal some aspects to minimize the seismic activity rate and vulnerability in any complex study region (Jafari, 2013).
The seismicity parameters investigation has been done in the past by several researchers such as ; Nayak and Sitaram, 2019; Gautam et al., 2018) in and around Northeast India.
These studies were based on different techniques by using the instrumental or both historical and instrumental earthquake catalogue (Nayak and Sitaram, 2019). Also, the seismicity parameters analysis in terms of spatial-temporal distribution has been done by several authors such as Shankar and Sharma, (2012); El-Quliti, (2016) for the different tectonic settings of the world in the recent past.
has been adopted to prepare the seismotectonic map ( Figure.   Based on the active tectonic structures, focal mechanism of earthquakes, and seismicity level this study region has been categorized into four seismogenic source zones, i.e., the Eastern Syntaxis (

Earthquake Catalogue
The comprehensive and complete homogeneous earthquake catalogue is an essential input to quantify the seismicity parameters for any tectonic setting. A homogeneous earthquake catalogue (M W ≥ 3.0) has been prepared covering the period from 1st January, 1976 to 31st June, 2020 for the study region. The earthquake catalogue is compiled from various agencies such as the International Seismological Centre The earthquake catalogue should be entirely independent of foreshocks and aftershocks for accurate assessment of seismicity parameters. The homogeneous earthquake catalogue is encompassing the foreshocks, mainshocks, and aftershocks from various sources and it may be repeated which have to be ltered . The earthquake catalogue has been performed in the ZMAP software (Weimer, 2001) by utilizing a decluster technique developed by Uhrhammer, (1986) to lter the dependent events (i.e., foreshocks, aftershocks) and repeated earthquake events from the mainshocks. Also, the magnitude of completeness which is de ned as the minimum magnitude at which 100% of the earthquake has been detected in space-time volume i.e., the seismic network provides the complete detection of an event without missing a single event has been examined for this study region (Rydelek and Sacks, 1989; Schorlemmer and Woessner, 2008; Woessner and Wiemer, 2005).

Methodology b-value-
The basic seismological parameter is the b-value which ensemble of earthquakes in the frequencymagnitude relation for any tectonic setting (Singh, 2016). A well-document simplest relationship has been established by Gutenberg and Richter, (1944) (1) where N is the number of earthquakes in a group having a magnitude larger than M. The parameter a and b are positive real constant and these coe cients change values are depending on the various tectonic feature of each earthquake source zone. The parameter "a" is described as seismicity activity which depends on the size of the area, observation period length, largest seismic magnitude, and the stress level of the area (Allen, 1986). The parameter "b" is the slope of the log-linear relation and it is known as the bvalue (Srivastava, 2014). Normally, the b-value is 1.0 on a global scale, but the range of b-value is lying in between 0.5 to 1.5 which depends on the tectonic settings of the seismically active region (Singh, 2016;Pacheco et al., 1992;Wiemer and Wyss, 1997;Singh et al., 2008;Singh and Chadha, 2010). The b-value can be used for the study of seismicity, seismotectonic, and seismic hazard assessment for any geological setting. The b-value can be computed by using two methods such as least-square t method and the Maximum-likelihood method for any tectonic setting. In the least square t method, the log values of the cumulative number of earthquakes (N) are plotted with a magnitude sequence. The b-value is estimated from the slope of the least square t line, the log-linear relation between N and M. However, the Maximum-likelihood method (MLM) (Aki, 1965) is the most robust and accepted method utilized to estimate the b-value for any study region. Thus, the Maximum-likelihood method based on theoretical consideration has been utilized to compute the b-value in 0.5°×0.5° grid interval for this study region. The b-value parameter estimates from the given formula as: b = log 10 (e) / m mean -(Mc-( Δm/2)) equ (2) where m mean is the mean magnitude of the sample, Mc is the magnitude of completeness and Δm is the magnitude bin size.

Return Period and Annual Probability-
The return period and annual probability of M W = 3.0 earthquake in nine seismogenic source zones are estimated by utilizing the value of seismicity parameters such as "a" and "b". Return period or Recurrence interval is the average interval of time within which an earthquake of speci ed magnitude is probable to be equaled or surpassed at least once (El-Quliti, 2016). Percentage probability is estimated by dividing 1 by the Return period or Recurrence interval and multiplying by 100 (El-Quliti, 2016). Thus, the return period and annual probability Percentage has been estimated by equation 1 and 2 respectively as given below: Return Period (T R ) = (n+1)/m equ (3) Annual Probability Percentage (P(M)) = 100.m/(n+1) equ (4) Where m is event ranking and is the number of events in the period of record.

Result And Discussion
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.  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.  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 M C 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 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 M C 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 re ected as most hazardous. Further, the seismicity parameters values are estimated based on the frequency-magnitude distribution for each zone as listed in Table-2  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- 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

Conclusion
The seismicity parameter analysis is helpful with regular updated seismological datasets from a seismological point of view. Seismicity parameters have been computed by using the available maximum information of seismology for this study region. Such kind of examination is support to understand the earthquake generation process of any tectonic setting. The reliability of results depends upon the input used like methodology information for the investigation of seismicity parameters for any study region. In the present article, different approaches have been utilized to estimate the seismicity parameters with the seismotectonic framework of the study region. This study region has been categorized into nine seismogenic source zone based on geological features, seismicity, and orientation of the faults in which the seismicity parameter has been estimated. The seismicity parameters such as b-value (0.6-1.52) with standard b-value (0.01-0.38) and a-value (3.13-8.73) are estimated by using the maximum likelihood method which indicating Northeast India is the most hazardous region. In this analysis, the b-value analysis providing information about the crustal stress level of the study region. Based on the b-value result, it has been observed that most areas of the Indo-Burman Range show high crustal stress and material heterogeneity due to the subduction of the Indian plate beneath the Burmese plate. Resulting, the probability of the occurrence of future large magnitude earthquakes is in Northeast India. It has been observed that Zone-7, 9 are low and Zone-8 is the high seismic active zone in this study region. Finally, it has been concluded that all zones are indicating the different types of hazard levels of the study region. Also, the return period and annual probability of occurrence of the earthquake have been estimated for 40 yrs by using the obtained value of seismicity parameters. The estimated seismicity level from one zone to another zone is very informatively and useful from both theoretical and practical points of view in this study. Resulting from this analysis, it has been noticed the seismicity level is high in most areas of Northeast India which indicating the vulnerability to hazard in the near prospect. The resultant hazard parameter, return periods, and the probability of occurrence may be utilized as quantitative measures of seismicity for any study region. In addition, this type of analysis will be helpful to prepare the probabilistic seismic hazard map in Northeast India.
Declarations 37. Woessner, J. and S. Wiemer, Assessing the quality of earthquake catalogues: Estimating the magnitude of completeness and its uncertainty, Bull. Seismol. Soc. Am., 95 (2) Figure 1 Seismotectonic map of Northeast India with decluster events MW ≥ 3.0 covering the period from 1st January, 1976 to 30th June, 2020. Yellow lines indicated the seismogenic source zones based on seismicity, focal mechanism, and tectonic activity of earthquake. Nine seismogenic source zones are denoted by 1 to 9. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
The spatial distribution of "b-value" from the declustered catalogue by using the Maximum-likelihood method from the period of 1st January, 1976 to 30th June, 2020 for Northeast India.