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Seismicity Analysis of Chenab Valley Seismic Zone, NW Himalaya: the Seismogenic of Kirthai-I Hydro Electric Project

Sana, Hamid.Department of Geology & Geophysics, Indian Institute of Technology Kharagpur,

West Bengal, IndiaMukhtar, Gulzar Ahmad.

Geotechnical Laboratory, Engineering Geology Wing, Jammu & Kashmir State Pow-er Development Corporation, Bemina, Srinagar

Nath, Sankar Kumar.Department of Geology & Geophysics, Indian Institute of Technology Kharagpur,

West Bengal, India

Abstract

We present the seismicity analysis of Chenab Valley Seismic Zone, the seismogenic source of Kirthai-I HEP. Chenab Valley is a tectonic pull-apart basin, situated to the southeast of Kashmir nappe and north-west of Chamba nappe in the northwestern Himalaya. Geologically, the Palaeozoic-Mesozoic sequences of the Kashmir and Chamba nappes are separated by this deeply eroded basin and metamorphic rocks at the base of the sequence are exposed. Tectonically, this basin is traversed by the Main Central Thrust, Kishtwar Fault and Main Boundary Thrust. The seismicity clusters around these faults speak about the tectonic activity of this basin. Seismically, this seismic zone is known for the 22 June 1945 Kathua earth-quake of Mw 6.3. The earthquake catalogue used in the analysis is an extensive one from IMD (1945-2012) with a magnitude of completeness (Mc) of 3.0 (±0.19). The Gutenberg-Richter parameters, a-value and b-value assessed for the Chenab Valley Seismic Zone are 4.30 and 0.50 (±0.03) respectively. The mmax estimation of the seismogenic source zone as well as the major faults was carried out. The mmax for the Chenab Valley Seismic Zone is Mw 6.8 (±0.58), whereas mmax for the Main Boundary Thrust, Main Central Thrust, and the Kishtwar Fault is Mw 7.4, 7.3 and 6.8 respectively. These seismicity parameters are used in the site specific seismic hazard assessment of the Kirthai-I Hydro Electric Project.

1. Introduction:

The Kirthai-I Hydroelectric Project is located on the Chenab River in the Kishtwar Dis-trict of Jammu and Kashmir. The geographical coordinates of this project are 33º15’33” and 76º10’10”. Detailed investigations for this project are being carried out by the Jam-mu and Kashmir State Power Development Corporation (JKSPDC). Geologically, this hydroelectric project (HEP) lies in the Kishtwar Basin of NW Himalaya. This basin is a tectonic window, which is bound by the Main Crystalline Thrust (MCT) and the Kisht-war Fault (KF). This active tectonic setup is manifested by the earthquake activity in the area. The 22 June 1945 Kathua earthquake of Mw 6.3 (Ambrasseyes and Douglas, 2004) and the recent 1 May 2013 Kishtwar earthquake of Mw 5.8 (www.imd.gov.in) are two prominent earthquakes that rocked the Chenab valley. Broadly speaking, this HEP lies in the Seismic Zone-IV of the Indian Standard Criteria for Earthquake Resis-tant Design of Structures (BIS: 1893, 2002), which corresponds to an intensity of VIII on the MSK64 earthquake intensity scale (Fig. 1)

The seismotectonic setup of the area makes it imperative to carry out the detailed site specific seismic hazard analysis of the dam site. In order to accomplish seismic hazard assessment, the seismogenic source zoning of the area of influence is necessary. In this work, we present seismicity analysis of one of the seismogenic source zones of Kirthai-I HEP, the Chenab Valley Seismic Source Zone (CVSZ). The CVSZ is defined

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primarily by the Kishtwar window and the surrounding seismic cluster. The seismicity analysis involves calculation of Gutenburg-Richter parameters (a-value and b-value), magnitude of completeness (Mc) and the maximum credible earthquake (mmax). These parameters are used as input for the detailed site specific seismic hazard assessment of a particular site of interest. The earthquake catalogue used in the present study is pro-vided by Indian Meteorological Department (IMD) spanning from 1945 to 2012 (www.imd.gov.in). Figure 2 shows the distribution of the seismicity in CVSZ according to focal depth.

Figure 1: a) Map showing lithotectonic units of Himalaya, after Stephenson et al. (2001). b) Tectonic map of NW Himalaya modified after Singh (2010) and c) Geotec-

tonic map of Kishtwar window and the surroundings modified after Singh (2010).

2. Geology and Tectonics around the Project Area:

The Palaeozoic-Mesozoic sequences of the Kashmir and Chamba nappes are separated by the deeply eroded Chenab valley, exposing the metamorphic rocks at the base of the sequence (Fig. 1b). Kirthia-I HEP is situated in the Kishtwar window (Fuchs, 1975), in proximity of the MCT. The tectonic contact between the phyllites and schists of the lesser Himalaya and a sequence of garnet-biotite schist and marble of amphibolite grade of the Higher Himalayan Crystallines (HHC) towards the eastern end of the win-dow near Atholi, is described as the Main Central Thrust. However, on the western end of the window, the tectonic contact between the quartzite of the Lesser Himalaya and the biotite schist of the Higher Himalayan Crystallines is a high angle dipping fault, adjacent to Kishtwar town. This tectonic contact has been referred as the Kishtwar Fault (Singh, 2010). Overall, the non-cylindrical geometry of Kishtwar Basin suggest that it is right-lateral pull-apart tectonic basin (Winter, 1994). The project is located at the foothills of the Paddar area exposing high grade metamorphic rocks in the Upper Chenab stretch of Jammu and Kashmir. The Paddar area exposes a high grade suite of meta-sediments and metamorphites belonging to the Higher Himalayan Crystalline

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of Proterozoic age. The litho-units identified here in the ascending order are garnetif-erous mica gneiss; hornblende gneiss; garnetiferous gneiss with marble and graphite; calc-silicate gneiss; garnetiferous hornblende gneiss; and marble. In addition, gneissose granite, amphibolite, actinolite-tremolite schist, pegmatite and quartz veins are found frequently. In the Kirthai-I HEP area quartz-mica schists dominate. The general trend of the rocks here is NNW-SSE with an average dip of 35° due NE (Fig. 1c).

3. Methodology:

The Gutenberg-Richter parameters (a-value and b-value) are defined by the classic magnitude –frequency relationship:

where ‘N’ is the cumulative number of earthquakes with magnitude ≥M (Gutenberg and Richter 1944). There are various methods to estimate the b-value; we have used Maximum-Likelihood estimation method which is given as:

where, meanM is the average magnitude, is the magnitude of completeness and m∆ is the magnitude bin size (Aki 1965; Utsu 1999). is introduced to get rid of the

non-linearity at lower magnitudes, above this magnitude the catalogue is considered complete for seismicity analysis (Weimer and Wyss 2000).The maximum credible earthquake (mmax) is the highest magnitude the earthquake ex-pected to occur in an area with known seismotectonic framework. Various approaches are used to assess the mmax, in the present case we have used fault attributes (Wells and Coppersmith, 1994) and seismogenic source zone approach (Kijko, 2004). As described by Kijko (2004), the estimator of, after considering the uncertainty of the b-value, is performed as an iterative solution of the following equation:

Where, is the maximum observed magnitude, is the b-value,

, , is the Incomplete Gam-

ma function and and are expressed in terms of the mean and variance of .

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Figure 2 a) Seismicity map of Chenab Valley Seismic Zone (CVSZ), z represents depth b) Histogram showing distribution of Seismicity according to focal depth.

4. Seismicity Analysis of Chenab Valley Seismic Source Zone:

The results of seismicity analysis for the Chenab Valley Seismogenic Zone (CVSZ) are presented in table 1 and mmax of the faults in this zone in table 2. The IMD catalogue used in the present study covers a period from 1945 to 2012 (Fig. 2). The magnitude of completeness (Mc) of this catalogue is calculated to be 3.0 (±0.19). The earthquake catalogue is considered complete above this magnitude for seismicity analysis. The seismicity parameters, a-value and b-value are 4.30 and 0.50 (±0.03) respectively for CVSZ (Fig. 3). The lower b-value (<1.0) emphasize the fact that the area is tectonically active (Wiemer and Wyss 1997). The seismogenic zone based mmax for CVSZ is esti-mated to be Mw 6.8 (±0.58). The fault parameters’ based approach estimates the mmax for the MBT, MCT and Kishtwar fault as Mw 7.4, Mw 7.3 and Mw 6.8 respectively.

Table 1Showing results of seismicity analysis of Chenab Valley Seismic Source Zone

a-value b-value Mc mmax mobs

4.30 0.50±0.03 3.0 ±0.19 6.8 ±0.58 6.3

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Table 2Showing mmax of all the major faults in the Chenab Valley Seismic Source Zone

Fault mmax

MBT 7.4MCT 7.3

Kishtwar 6.8

Figure 3 Frequency-magnitude plot of Chenab Valley Seismic Source Zone.

5. Results:

This study presents the seismicity analysis of one of the seismogenic zones, the Chenab Valley Seismic Source Zone, of Kirthai-I HEP. The earthquake catalogue of Indian Me-teorological Department (1945 to 2012) was used to carry out the analysis. The seismic-ity analysis involves evaluation of Magnitude of Completeness (Mc), Gutenberg-Rich-ter parameters (a-value and b-value) and Maximum Credible earthquake (mmax). The results of the analysis reveal that Mc, a-value and b-value are 3.0 ((±0.19), 4.3 and 0.50 (±0.03) respectively. Hence, the catalogue is complete above the determined Mc of 3.0 ((±0.19) for seismic hazard analysis. Lower b-value is indicative of the active tectonic setup of the area. The mmax estimated for CVSZ is Mw 6.8 (±0.58) and that for MBT, MCT and Kishtwar fault is Mw 7.4, Mw 7.3 and Mw 6.8 respectively. These estimations have important bearings on the design of the engineering structures of the Kirthai-I HEP.

References:

1. Ambraseys, N. and Douglas, J. (2004). Magnitude calibration of north Indian earthquakes. Geophysical Journal International, 159: 165-206.

2. Indian Meteorological Department, Seismology Division, New Delhi. www.imd.gov.in

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6. Singh, K. (2010). Tectonic evolution of Kishtwar Window with respect to the Main Central Thrust, northwest Himalaya, India. Journal of Asian Earth Sciences, 39: 125–135.

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