A Revisit to Seismic Hazard at Uttarakhand

A Revisit to Seismic Hazard at Uttarakhand

Monalisha Nayak (Department of Civil Engineering, Indian Institute of Science, Bangalore, India), T. G. Sitharam (Department of Civil Engineering, Indian Institute of Science, Bangalore, India) and Sreevalasa Kolathayar (Department of Civil Engineering, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India)
Copyright: © 2015 |Pages: 18
DOI: 10.4018/IJGEE.2015070104
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Abstract

This paper presents the seismic hazard of the state of Uttarakhand in India, located at the foothills of the seismically active Himalayan mountain ranges. In the present study, an updated catalog of earthquakes has been prepared for Uttarakhand which was homogenized into a unified moment magnitude scale after declustering of the catalog to remove aftershocks and foreshocks. Various source zones were identified in the study area to account for local variability in seismicity characteristics. The seismicity parameters were estimated for each of these source zones, which are necessary inputs into seismic hazard estimation of a region. The seismic hazard evaluation of the region based on a state-of-the art PSHA study was performed using the classical Cornell–McGuire approach with different source models and attenuation relations. The most recent knowledge of seismic activity in the region was used to evaluate the hazard, incorporating uncertainty associated with different modeling parameters as well as spatial and temporal uncertainties. The PSHA was performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models. The hazard maps were produced for horizontal ground motion at the bedrock level and an attempt was done to bring the hazard at surface level using appropriate amplification factors. The maximum PHA value at bedrock level for 10% Probability of exceedance (PE) in 50 years is 0.34g and same for 2% PE in 50 years is 0.54g.
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Seismicity Of The Study Area

Tectonics of the Area

The study area, Uttarakhand, is a state located in northern part of India in a latitude of 28-320N and longitude of 77-810E (Fig. 1). The seismicity of the area is governed by seismicity of Himalaya. Himalaya is situated near the Indo-Eurasian plate boundary and is formed due to continental-continental collision in Cenozoic era. Currently Indian plate is moving at a rate of about 5cm/year (Kumar et al. 2007). Due to collision of plates a complex network of thrusts, faults and suture is formed in this region. The seismicity of the plate boundary region is mainly influenced by different prominent Himalayan tectonic thrusts named main boundary thrust (MBT), main central thrust (MCT) and main frontal thrust (MFT). Uttarakhand almost lies in the central part of Himalaya and possess significant segment of MBT, MCT and MFT. Again a large part of central Himalaya was identified as central seismic gap (Bilham and Szeliga, 2008). Seismic gaps supposed to have high potential for future earthquake in any region. Besides these Himalayan thrusts, several faults are found in and around Uttarakhand. Among those Karakoram fault is important one that runs more than 1000km from central Pamir to north of Uttarakhand Himalaya. Other than these huge thrusts and faults, other seismically important geological tectonic features of the study area are Jwalamukhi thrust (JMT), Drang Thrust(DT), Ramgharh thrust (RT), North Almora Thrust (NAT), South Almora Thrust (SAT), Great Boundary Fault (GBF), Alaknanda Fault (AF), Mahendragarh Dehradun Fault (MDF), Moradabad Fault (MF), Manali Fault (MF), etc. Along with these important thrust and faults a large number of smaller thrusts, faults and lineaments are also present in and around Uttarakhand.

Figure 1.

Location of study area

The seismic activity in Uttarakhand is related to all the regional tectonic features like thrust and faults. Many earthquakes of moderate to large magnitude have occurred in this area in the past. In addition to 1991-Uttarkashi earthquake of magnitude Mw 6.8 (USGS) and 1999-Chamoli earthquake of magnitude Ms 6.6 (USGS), many earthquakes have occurred in nearby locations out of Uttarakhand along the Himalayan thrusts and faults. Among these the great Kangra earthquake of 1905 of magnitude 8 having epicenter in the vicinity of MBT, the Kashmir earthquake of magnitude 7.6 are large ones and the 1945 Chamba earthquake, 1975 Kinnaur earthquake and 1986 Dharmasala earthquake are of moderate magnitude earthquakes. Most of the earthquakes in this region are shallow crustal earthquakes with focal depth less than 60km from surface.

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