**2. Methodology**

For estimating the seismic hazard at places of critical infrastructures, such as high-rise building, dams, bridges, and subsea tunnels, are of the utmost importance in the field of earthquake engineering, as damage to such structures results in severe economic loss and threats to the environment. Seismic hazard can be quantified by adopting two globally accepted techniques: the deterministic seismic hazard analysis (DSHA) and the probabilistic seismic hazard analysis (PSHA). Both approaches can be defined in a four-step process, and their initial steps are identical [1]. In the deterministic approach, the ground-shaking hazard at any point is evaluated based on the controlling source, which is expected to produce the maximum hazard at that point among all the potential sources [2]. On the other hand, in the probabilistic approach, the inherent uncertainties in the forecast of earthquake size, site, and earth motion parameters are explicitly combined to obtain the hazard for a given probability of exceedance in a particular time period [3]. Despite their benefits and drawbacks, these two methods can harmonize each other and give additional insights into the question of seismic hazard [4]. Moreover, a deterministic framework accounts for the hazard from each source independently; thus, it is more capable in the regions, where seismic activity is low to moderate, particularly, where limited earthquake data is available [5].

#### **2.1 Deterministic seismic hazard analysis (DSHA)**

For identification of linear seismic sources, a seismotectonic atlas developed by the Survey of India [6] has been taken as the base for the present study. The seismotectonic maps are prepared for each district headquarters of north Chhattisgarh, keeping the headquarter at the center of the circle, with a radius of 300 km, based on the latitude and longitude as shown in **Figure 1**. The faults having fault length (Li) ≥ 25 km coming in a 300 km radius were identified, numbered, and

*A Seismic Hazard Assessment of North Chhattisgarh (India) DOI: http://dx.doi.org/10.5772/intechopen.109490*

#### **Figure 1.**

*Seismotectonic map of district headquarters of north Chhattisgarh: (a) Ambikapur, (b) Baikunthpur [Koria], (c) Korba, and (d) Jashpurnagar.*

measured the length of the faults. The minimum map distances of identified faults were measured from the center of the circle for the study area.

The total number of linear sources as 33, 36, 41, and 27 were identified for major districts Ambikapur, Baikunthpur [Koria], Korba, and Jashpurnagar, respectively. In next step, the source-to-site distance or minimum map distance for the above linear sources for study area was measured using seismotectonic maps as shown in **Figure 1** and tabulated in **Appendix Tables A1–A4**.

For estimation of seismic parameters, the linear least-square fit method developed by Stepp [7] has been applied over the past earthquake data collected from various catalog and research agencies (USGS website and NICE) for all four-district headquarters of north Chhattisgarh. In order to assess the magnitude of completeness, time-magnitude plots of the final catalog have been generated for different time periods as shown in **Figure 2**.

The seismic activity of a region is characterized by the Gutenberg-Richter [8], recurrence relation as given below:

$$\log 10 \text{ (N)} = \text{a-b Mw} \tag{1}$$

where N is the number of earthquakes greater than or equal to magnitude m, a denotes the seismicity rate computed by the logarithm of the average number of earthquakes of magnitude Mmin or Mmax, and b value characterizes the proportion of large earthquakes relative to small earthquakes [9] as shown in **Figure 3**. For tectonic earthquakes, the b value is to be confined within the range 0.7018 < b < 0.8429 as tabulated in **Table 1**.

The "b" value assesses the frequency of the occurrence of earthquakes of different sizes. The maximum magnitude is the highest potential of accumulated strain energy to be released in the region or in a seismic source [10]. The maximum probable earthquake is defined as the upper limit of earthquake magnitude for a given entire region and is synonymous with the magnitude of the largest possible earthquake in that region [11]. It assumes a sharp cut-off magnitude at a maximum magnitude, so by definition, no earthquake is to be expected with a magnitude exceeding Mmax [12]. In the present study, Mmax is estimated by using two methods. The first method is of Wells and Coppersmith [13] and the second method is of Gupta [14]. In Wells and Coppersmith method, a relation between Mw and surface rupture length (SRL) was developed using reliable source parameters, and this is further applicable to all types of faults, shallow earthquakes, and interplate or intraplate earthquakes.

$$\text{Log}\,(\text{SRL}) = \mathbf{0.57M}\_{\text{W}} - \text{2.33} \tag{2}$$

The above equation was used to estimate the maximum magnitude (Mmax) for all sources of the study area. Gupta's method in which maximum magnitude (Mmax) was estimated from Eq. (3) as given below:

$$\mathbf{M}\_{\text{max}} = \mathbf{M}\_{\text{obs}+} \mathbf{0.5} \tag{3}$$

Mmax = Maximum magnitude

Mobs = Observed magnitude

After comparing the outcome of the above two methods, maximum magnitude (Mmax) values for seismic sources for the district headquarters of north Chhattisgarh are tabulated in Appendix A. The recurrence relation developed in the present study is

*A Seismic Hazard Assessment of North Chhattisgarh (India) DOI: http://dx.doi.org/10.5772/intechopen.109490*

**Figure 2.**

*Earthquake data completeness analysis for district headquarters of north Chhattisgarh: (a) Ambikapur, (b) Baikunthpur [Koria], (c) Korba, and (d) Jashpurnagar.*

not for the particular seismic source but for the entire study region. It is required to differentiate the activity rates among the seismic sources and to develop the frequency magnitude relationship for an individual fault. The truncated exponential recurrence

**Figure 3.** *Frequency-magnitude relationship for district headquarters of north Chhattisgarh: (a) Ambikapur, (b) Baikunthpur [Koria], (c) Korba, and (d) Jashpurnagar.*


**Table 1.**

*Regional recurrence relationship and "b" values for district headquarters of north Chhattisgarh.*

relationship is commonly used in practice: to estimate the most likely earthquake magnitude and the most likely source-site distance, for the calculation of PGA values using the ground motion records [15]. This process of disaggregation requires the mean annual rate of exceedance (λm), expressed as a function of magnitude. A MATLAB computer program has been developed to solve Eq. (4) and the graphs were plotted as shown in **Figure 4**.

$$\lambda\_m = w\_i \ast \nu \ast \frac{\exp[-\beta(m - m\_0) - \exp[-\beta(m\_{\max} - m\_0)]]}{1 - \exp[-\beta(m\_{\max} - m\_0)]} \tag{4}$$

where *υ* ¼ expð Þ *α* � *β* ∗ *m*<sup>0</sup> α = 2.303\*a, β = 2.303\*b, and wi is the weight factor for a particular source.

An attenuation relationship includes the source geometry, earthquake magnitude, source-to-site distance, and site conditions. Regional geology plays an important role in the selection of an appropriate relationship in any seismic hazard study. In the recent past, devastating events have occurred in Peninsular India (PI), which is a warning about the possibility of such earthquakes in future [16]. In the present research, the study area comes under the region of PI. So, the attenuation relationship developed by Iyengar and Raghu Kanth [17] has been used. The GMPE proposed by Iyengear and Raghu Kanth is given below:

$$\ln \text{ Y = C1 + C2 (M - 6) + C3(M - 6)^2 - \ln(R) - C4(R) + \ln(\varepsilon)} \tag{5}$$

where Y, M, and R refer to PGA(g), moment magnitude, and hypo-central distance, respectively.

Peninsular India: C1 = 1.6858; C2 = 0.9241; C3 = �0.0760; C4 = 0.0057;

σ (ln ε) = standard deviation of error = 0 [50 Percentile, for DSHA ε = 0]

σ (ln ε) = standard deviation of error = 0.4648 [84 Percentile]

M100 = magnitude of earthquake [100 years Recurrence—period calculated by using **Figure 4**] R = hypo-central distance = √ (D<sup>2</sup> +F<sup>2</sup> ) D = minimum map distance to the sources, and F = focal depth = 10 km.

The peak ground acceleration at bedrock level for all the sources of the study area has been estimated for a return period of 100 years, using the attenuation relationship of Iyengear and Raghu Kanth [17]. The estimated peak ground acceleration values at bedrock level of seismic sources of the district headquarters of north Chhattisgarh are tabulated in **Appendix Tables B1**–**B4**. It is observed that in the attenuation relationship the highest estimated PGA (g) value is found for fault no. F8 for district headquarter Baikunthpur [Koria] and tabulated in **Table 2**.

*A Seismic Hazard Assessment of North Chhattisgarh (India) DOI: http://dx.doi.org/10.5772/intechopen.109490*

**Figure 4.** *Disaggregation of seismic sources near district headquarters: (a) Ambikapur, (b) Baikunthpur [Koria], (c) Korba, and (d) Jashpurnagar.*

The seismic zonation map of any country acts as a guide to the seismic status of the regions and their susceptibility to earthquakes. India has been divided into five zones with respect to the severity of the earthquake. Zone V is the seismically most active zone, where an earthquake of magnitude 8 or more could occur.


#### **Table 2.**

*Maximum PGA values for seismic sources for district headquarters of north Chhattisgarh.*

Recent strong motion observations around the world have revolutionized the thinking on the aspect of the design of engineering structures, placing emphasis on the characteristics of the structures. BIS 1893 (Part 1): 2016 (sixth revision) [18], prepared a seismic zone map of India as shown in **Figure 5** with zone factors that are tabulated in **Table 3**.

The outcome of the present study with the recommended value for zone II is 0.1g as per IS 1893 (Part 1): 2016 (sixth revision). The maximum PGA (g) value for 50 percentile for a return period 100 years for Baikunthpur [Koria] is more and is 0.14554 g for Iyengear and Raghu Kanth's model.

#### **Figure 5.**

*Indian seismic zone map as per BIS 189 (Part 1): 2016 [Map taken from BIS 1893 (Part 1): 2016, sixth revision].*

#### *Natural Hazards – New Insights*


**Table 3.**

*Seismic zone factor, Z, IS 1893 (Part 1): 2016 (Clause 6.4.2).*

#### **Figure 6.** *Chhattisgarh district map.*

As **Figure 6** (i) shows the district headquarters map of Chhattisgarh and the study area is highlighted by a rectangle. Using the outcome of the DSHA approach and IS 1893 (Part 1): 2016 (sixth revision) recommendation the seismic zone map for north Chhattisgarh has been developed and as shown in **Figure 7a** and **b**.

### **2.2 Probabilistic seismic hazard analysis (PSHA)**

Thus, seismic hazard estimation has been performed considering the classical Cornell [19] approach. This approach comprises considering all potential seismic sources and their activity rate. Many researchers have considered this approach to carry out the hazard analysis of different regions of India [20–27]. The seismic hazard curves can be used to evaluate the probability of ground motion exceedance for a specified return period. Seismic hazard curves can be obtained for a particular seismic source and are combined to express aggregate hazards at a particular site. The probability of exceedance can be written as:

*A Seismic Hazard Assessment of North Chhattisgarh (India) DOI: http://dx.doi.org/10.5772/intechopen.109490*

#### **Figure 7.**

*Seismic zone map of north Chhattisgarh. (a) PGA (g) values for 50 percentile. (b) PGA (g) values for 84 percentile.*

$$\lambda\_{\mathcal{Y}^\*} = \sum\_{i=1}^{N\_\iota} \nu\_i \int \left[ P[Y > \mathcal{Y} \* | m, r] f\_{Mi}(m) f\_{Ri}(r) dmdr \right] \tag{6}$$

The final PSHA equation is given by

$$\lambda\_{\mathcal{V}^{\ast}} = \sum\_{i=1}^{N\_{\mathcal{S}}} \sum\_{j=1}^{N\_{\mathcal{M}}} \sum\_{k=1}^{N\_{\mathcal{R}}} \nu\_i \left[ \int P\left[Y > \mathcal{y} \* |m\_j, r\_k\right] P\left[M = m\_j\right] P[R = r\_k] \right] \tag{7}$$

where P[Y > y\* | m, r] is obtained from the predictive relationship and the probability density functions for magnitude and distance, respectively. The mean annual rate of exceedance λy\* is computed at a site for different specified ground motion values y\* in a life period of time. A computer program has been developed in MATLAB, which is used to draw the seismic hazard curves for the major district headquarters of north Chhattisgarh. The curves are depicted in **Figure 8**. Using Iyengear and Raghu Kanth's [17] attenuation relationship, the peak ground accelerations at bedrock level for 2% and 10% probability of exceedance in 50 years have been computed [28]. The aggregate value of PGA from this relationship has been worked out for each district headquarter (**Figure 9**). For 2% and 10% probability of exceedance in 50 years the PGA values for district headquarters Korba, Jashpurnagar, Ambikapur, and Baikunthpur [Koria] ranges from 0.016 to 0.086 g and from 0.008 to 0.036 g, respectively. The outcome highlighted that the seismically activity district headquarters are Ambikapur and Baikunthpur [Koria]. Thus, for PGA values 0.05 g, 0.10 g, and 0.015 g at bedrock level, the return periods have been estimated for Ambikapur and Baikunthpur [Koria] and have been tabulated in **Table 4**.

### **3. Conclusions**

After carrying out the deterministic and probabilistic seismic hazard analysis for major district headquarters of north Chhattisgarh, considering the local site effects, the following conclusions were drawn and discussed below:

*A Seismic Hazard Assessment of North Chhattisgarh (India) DOI: http://dx.doi.org/10.5772/intechopen.109490*

#### **Figure 8.**

*Seismic hazard curves for district headquarters of north Chhattisgarh: (a) Ambikapur, (b) Baikunthpur [Koria], (c) Korba, and (d) Jashpurnagar.*

#### **Figure 9.**

*Seismic hazard curves for district headquarters of north Chhattisgarh.*


#### **Table 4.**

*Return periods for various PGA (g) values for district headquarters of north Chhattisgarh.*

The frequency magnitude relationship was established from the research, for the study area, after carrying out the completeness analysis as per Stepp. Completeness of the data was observed for the north Chhattisgarh region and a seismic hazard parameter of "b value" estimated was found to vary from 0.7018 to
