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The statistically point process model known as epidemic-type aftershock sequence (ETAS) model is employed for systematically investigating the seismic quiescence or seismic anomalies around the focal regions of large/strong earthquakes for NW Himalaya. For this propose, the model predicted the expected occurrence rates of earthquakes by estimating the model parameters from the earthquake occurrences times using maximum likelihood method, has been used. Then the exhibited relative quiescence due to decreasing occurrence rates from the modeled ones can be identified by inspecting the abnormally downward deviated plot from the extended cumulative curve of the Residual Point Process (RPP) events. Examination of such RPP events in the whole time interval exhibits significant 1.5 years and 2.0 years of relative seismic quiescence before the strong 1991 Uttarkashi (MW 6.8) and 1999 Chamoli (MW 6.6) earthquakes, respectively. Considering the optimally oriented planes of Uttarkashi earthquake, the Coulomb stress changes (ΔCFS) have been investigated to check the rate of seismicity around the focal region of Chamoli earthquake. It has been found that ΔCFS of Uttarkashi earthquake exhibits stress shadow in or near the source zone of Chamoli earthquake and eventually decreases seismicity rates due to seismic quiescence in the source zone. On the other hand, the detected quiescence and activation relative to the predicted seismicity rate are consistent with the obtained Coulomb stress to depict the associated anomalies being sensitive enough to detect a slight stress change in the study region. Henceforth, the increased or decreased seismic activity due to seismic activation or quiescence is found to be consistent with the patterns of the Coulomb's stress changes calculated on the ruptured fault planes of Uttarkashi earthquake. Hence, this ETAS model based on statistical technique can thus be incorporated with other sensitive geophysical instruments for identifying seismically quiet period not only in the seismic gaps, but also in its neighborhoods along the Himalayan range for mitigating seismic hazards due to impending great earthquakes. [ABSTRACT FROM AUTHOR]

Site effect estimation using recorded ground motion is an effective approach to assess the seismic hazard of a region. Keeping the same thing in mind, an endeavor is being made to study the local site effects at different locations in the North East region of India through analysis of recorded strong ground motion data provided by Indian strong motion network. The data recorded at 25 sites from 37 earthquakes with a magnitude range M L 4.0–6.9 have been utilized. The estimated predominant frequencies ( f p e a k ) using horizontal to the vertical spectral ratio (HVSR) is well observed for various sites placed in various geological formations like the Precambrian, Tertiary and Quaternary Consequently, the pseudo velocity response (PSV) for 5% critical damping is estimated and compared with the regional geological formations, especially in the Brahmaputra Valley region. It is further noticed that the HVSR, as well as PSV, show a noticeable correlation according to the geological set up of the region for most of the sites, giving a clear idea about the site effect evaluation. Analyzed strong motion data also show the effect of non-linearity, which is an important parameter as it explains the inelastic behavior of the soil causing a reduced or non-linear amplification. Since some of the sites used in present work do not correlate with geology, the sites used for HVSR estimation are used to classify them according to the classical approach employing f p e a k . All the sites are classified in four different classes namely A, B, C, and D based on shear wave velocity in the upper 30 m. The outcome of the present study is being shown in the form of contour maps for f p e a k and site amplification for various types of buildings in order to assess the seismic hazard and risk mitigation. [ABSTRACT FROM AUTHOR]

The historical 1 September 1803 Garhwal–Kumaon Himalaya earthquake (Mw 8.1) was a solitary Himalayan earthquake reported felt widely in the Indo-Gangetic Plain with severe shaking observed from Punjab to Bengal. There was no common consensus among 9 different researchers on the source location and size of the Pre-instrumented 1 September 1803 earthquake. In this study, we attempted to resolve the issues based on source, path, and site characterization by estimating peak ground accelerations (PGAs) and correlation between estimated and reported intensities based on the concept of strong motion seismology. In order to arrive at a plausible epicenter location of the pre-instrumented 1803 earthquake, we conducted a detailed postmortem, for the first time of its all of 9-different source locations propounded by different researchers and examined those locations based on the nature and extent of damage pattern; extent of earthquake shaking; intensity at source rock; source characterization; path characterization; and estimates of peak ground acceleration (PGA) at different earthquake hypocenters. In order to achieve a better correlation, the hypocentre is considered at 11 locations at MHT from Higher Himalaya to Sub Himalaya. The surface PGA and MMI are estimated at all locations where the intensity was reported for the 1803 earthquake by considering about 100 scenario earthquakes. Out of nine locations proposed by previous researchers, the five locations fall in the highest and one in the higher seismic attenuation zone that negates the nature and extent of earthquake shaking up to the distance of more than 1200 km at Kolkata then Calcutta where the intensity was reported to be IV. The other locations propsed in least atteuation zone were of less magnitude. We, therefore infer that the 1803 earthquake occurred at the depth of 20 km located in the least seismic attenuation zone in the NW Himalaya with a location of 31.2oN, 78.7°E (130 km north of the HFT, 50 km north of the 1991 Uttarkashi earthquake) with a rupture dimension of 190 × 81 Sq. km with a slip of 3.0 m that corresponds to a magnitude of Mw 8.1. We also inferred that the mainshock rupture was directed toward the south along the Main Himalayan Thrust and the rupture found stopped at 50 km north of the HFT below the Ton Thrust in the Lesser Himalaya. This study also confirms that the rupture of the 1803 Garhwal–Kumaon earthquake has not reached up to the HFT. The 1991 Uttarkashi and 1999 Chamoli earthquakes are inferred to be located at the southern end of the 1803 Garhwal Kumaon earthquake rupture area. A slip deficiency of 1.3 m has been computed with reference to the source zones of the 1803 Garhwal–Kumaon, 1991 Uttarkashi, and 1999 Chamoli earthquakes, suggesting a maximum magnitude potential of Mw 7.2 The Tehri Dam is located approximately 20 km south of the 1991 Uttarkashi earthquake source zone and is believed to lie outside the rupture zone of the 1803 Garhwal–Kumaon earthquake. • For the 1803 Garhwal-Kumaon earthquake, eight magnitudes (Mw7.3 to 8.1) and seven locations were proposed. • Seismic Intensities have been computed based on strong motion simulation and site effects for all proposed magnitudes and locations. • Earthquakes (Mw 7.3 to 8.1) are also simulated considering hypocenter from Higher to sub-Himalaya along MHT. • The location and magnitude of the historical 1803 Garhwal-Kumaon earthquake are reassessed. • The rupture of the 1803 Garhwal-Kumaon earthquake was found not to reach up to HFT. [ABSTRACT FROM AUTHOR]

In the present work, acceleration response spectra are determined from earthquakes which have occurred in the NE region and the effect of local geology on its shape is studied. One hundred and ninety-five strong ground motion time histories from 45 earthquakes which have occurred in the NE region having a magnitude range of 3.5 ≤ Mw ≤ 6.9 and a distance range of 20-600 kms are used. It is observed that the shape of the normalized acceleration response spectra is influenced by the local site conditions and regional geology. The influence of magnitude and distance on the spectra is also studied. The present study is carried out for three categories of rocks: Pre-Cambrian, Tertiary and Quaternary. It is inferred that the acceleration response spectra in the current Indian code designed for the entire country are applicable for NE region as it is within the spectral limits prescribed in Indian code. The ground motion is amplified at higher frequencies for stations located on hard rock, while for stations located on alluvium sites, it is amplified at lower frequencies. The sites located on hard rock show lowest values of spectral acceleration than the sites located on alluvium sites. The results obtained in the present study are compared with the similar results obtained in the stable continent region like Gujarat. It is found that the dominating period of response spectrum of similar rock types is found to be at higher side for NE region as compared to Gujarat region. This may be attributed towards the tectonic complexity of the NE region than the stable continent region like Gujarat. [ABSTRACT FROM AUTHOR]

Earthquakes are deadliest among all the natural disasters. The areas that have experienced great/large earthquakes in the past may experience big event in future. In this study, we have simulated Kangra earthquake (1905, Mw 7.8) and a hypothetical great earthquake (Mw 8.5) in the north-west Himalaya using Empirical Green's Function (EGF) technique. Recordings of Dharamsala earthquake (1986, Mw 5.4) are used as Green function with a heterogeneous source model and an asperity. It has been observed that the towns of Kangra and Dharamsala can expect ground accelerations in excess of 1 g in case of a Mw 8.5 earthquake and could have experienced an acceleration close to 1 g during 1905 Kangra earthquake. The entire study region can expect acceleration in excess of 100 cm/s in case of Mw 7.8 and 200 cm/s in case of Mw 8.5. The sites located near the rupture initiation point can expect accelerations in excess of 1 g for the magnitudes simulated. For validation, the estimates of the PGA for Mw 7.8 simulation are compared with isoseismal studies carried out in the same region after the Kangra earthquake of 1905 by converting PGA values to intensities. It was found that the results are comparable. The target earthquakes (Mw 7.8 and Mw 8.5) are simulated at depth of 20 km and 30 km to examine the effect of PGA for different depths. The PGA values obtained in the present analysis gave us an idea about the level of accelerations experienced in the area during 1905 Kangra earthquake. Future construction in the area can be regulated, and built environ can be strengthened using PGA values obtained in the present analysis. [ABSTRACT FROM AUTHOR]

In the present study ground motions for a Mw 8.5 scenario earthquake are estimated at 13 sites in Kumaun-Garhwal region using the empirical Green’s function technique. The recordings of 1991 Uttarkashi earthquake of Mw 6.8 at these sites are used as an element earthquake. A heterogeneous source model consisting of two asperities is considered for simulating the ground motions. The entire central seismic gap (CSG) can expect acceleration in excess of 100 cm/s 2 with NW portion in excess of 400 cm/s 2 and SE between 100 and 200 cm/s 2. The central portion can expect peak ground acceleration (PGA) between 200 and 400 cm/s 2. It has been observed from simulation of strong ground motion that sites located near the rupture initiation point can expect accelerations in excess of 1 g. In the present analysis, Bhatwari and Uttarkashi can expect ground accelerations in excess of 1 g. The estimates of the PGA are compared with earlier studies in the same region using different methodologies and it was found that the results are comparable. This has put constrains on the expected PGAs in this region. The obtained PGA values can be used in identifying the vulnerable areas in the central Himalaya, thereby facilitating the planning, design and construction of new structures and strengthening of the existing structures in the region. [ABSTRACT FROM AUTHOR]

The attenuation characteristics based on coda waves of two areas-Jamnagar and Junagarh of Saurashtra, Gujarat (India)-have been investigated in the present study. The frequency dependent relationships have been developed for both the areas using single back scattering model. The broadband waveforms of the vertical components of 33 earthquakes (Mw 1.5-3.5) recorded at six stations of the Jamnagar area, and broadband waveforms of 68 earthquakes (Mw 1.6-5) recorded at five stations of the Junagarh area have been used for the analysis. The estimated relations for the Junagarh area are: Q = (158 ± 5)f (lapse time : 20 s), Q = (170 ± 4.4)f (lapse time : 30 s) and Q = (229 ± 6.6)f (lapse time : 40 s) and for the Jamnagar area are: Q = (178 ± 3)f (lapse time : 20 s), Q = (224 ± 6)f (lapse time : 30 s) and Q = (282 ± 7)f (lapse time : 40 s). These are the first estimates for the areas under consideration. The Junagarh area appears to be more attenuative as compared to the Jamnagar area. The increase in Q values with lapse time found here for both the areas show the depth dependence of Q as longer lapse time windows will sample larger area. The rate of decay of attenuation ( Q) with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ. A comparison of the coda-Q estimated for the Saurashtra region with those of the nearby Kachchh region shows that the Saurashtra region is less heterogeneous. The obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the Saurashtra region of Gujarat where no such relations were available earlier. These relations are also important for the simulation of earthquake strong ground motions in the region. [ABSTRACT FROM AUTHOR]

The attenuation properties of the crust in the Chamoli region of Himalaya have been examined by estimating the frequency-dependent relationships of quality factors for P waves (Qα) and for S waves (Qβ) in the frequency range 1.5–24 Hz. The extended coda normalization method has been applied on the waveforms of 25 aftershocks of the 1999 Chamoli earthquake (M 6.4) recorded at five stations. The average value of Qα is found to be varied from 68 at 1.5 Hz to 588 at 24 Hz while it varies from 126 at 1.5 Hz to 868 at 24 Hz for Qβ. The estimated frequency-dependent relations for quality factors are Qα = (44 ± 1)f(0.82±.04) and Qβ = (87 ± 3)f(0.71±.03). The rate of increase of Q(f) for P and S waves in the Chamoli region is comparable with the other regions of the world. The ratio Qβ/Qα is greater than one in the region which along with the frequency dependence of quality factors indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of attenuation relation for S wave estimated here (Qβ = 87f0.71) with that of coda waves (Qc = 30f1.21) obtained by M andal et al. (2001) for the same region shows that Qc > Qβ for higher frequencies (>8 Hz) in the region. This indicates a possible high frequency coda enrichment which suggests that the scattering attenuation significantly influences the attenuation of S waves at frequencies >8 Hz. This observation may be further investigated using multiple scattering models. The attenuation relations for quality factors obtained here may be used for the estimation of source parameters and near-source simulation of earthquake ground motion of the earthquakes, which in turn are required for the assessment of seismic hazard in the region. [ABSTRACT FROM AUTHOR]

The attenuation properties of the crust in the Kachchh region, Gujarat, India, have been investigated. For this purpose, 49 local earthquakes having focal depths in the 3-38 km range have been used. The quality factors Qα (the quality factor using P waves) and Qβ (the quality factor using S waves) have been estimated using the coda normalization method. The quality factor of coda waves (Qc) has been estimated using the single backscattering model. Multiple scattering models have been used to estimate Qi (the intrinsic attenuation parameter) and Qs (scattering attenuation parameter) for the region. The values of Qα, Qβ, Qc, Qi, and Qs show a dependence on frequency in the range of 1.5-24 Hz for the Kachchh region. The average frequency-dependent relationships (Q = Q0fn) estimated for the region are Qα = (77 ± 2)f(0:87±0:03), Qβ = (100 ± 4)f(0:86±0:04), and Qc = (148 ± 3)f(1:01±0:02). The estimate of Qc is found to be higher than Qβ in this region. This analysis supports the Zeng et al. (1991) model that predicts that the effects of intrinsic and scattering attenuation combine in a manner that Qc should be more than Qβ. The estimated values of Qs and Qi vary from 529 and 183 at 1.5 Hz to 3053 and 2668 at 24 Hz, respectively. We find that the Qc estimates lie between the estimates of Qi and Qs but are closer to Qi at lower frequencies. This is in agreement with the theoretical as well as laboratory measurements. A comparison between Qi and Qs shows that intrinsic absorption is predominant over scattering. [ABSTRACT FROM AUTHOR]

We simulate the strong ground motion for an expected major earthquake (Mw 7.0) beneath the Bhutan Himalayan region with an empirical Green's function method using waveforms recorded from the 2009 Bhutan mainshock (Mw 6.1) and its largest aftershock (Mw 5.1). Fault orientation and location of the simulated event extend the length of the fault plane determined from the 2009 Bhutan mainshock and aftershock along its strike. The simulated PGA values are compared with the results derived from a ground motion prediction equation (GMPE) for the Himalayan region and it is found that different levels of accelerations are associated with different rupture initiation points on the fault plane. It is observed that the NE Indian region is capable of generating peak ground acceleration (PGA) in exceedance to 121 cm/s2 for simulated earthquake (Mw 7.0). The maximum impacts of shaking will be on the sites located near the rupture initiation points that are poised to generate higher values of ground acceleration. To validate our simulation, we also estimate the extent of rupture directivity of the simulated earthquake with respect to four initiation points indicating that higher value of PGA and shaking duration exist either to South or to South-West azimuths from the imitated locations, which are corroborated with respective geology of the sites. This study suggests that areas of maximum ground shaking would occur in the vicinity of the source initiation where possibility of relatively stronger earthquake hazards does exist, which in turn requires attention for adoption of earthquake risk mitigation plans in view of impacts of trans-boundary earthquakes in the region. • Different acceleration levels are associated with different rupture initiation points beneath Bhutan extended to NE-India. • Maximum impacts of shaking is on sites located in S or SW of rupture initiation which is confirmed by rupture directivity. • Cities in NE India are capable to generate PGA equivalent to a maximum of 121 cm/s2 in case of simulated earthquake. • Proactive steps should be regulated to mitigate the trans-boundary disastrous earthquakes in NE-India. [ABSTRACT FROM AUTHOR]

Maximum magnitude (MM) earthquake in the Kopili fault zone of North-East India has been assessed using different approaches, which are primarily dependent on various parameters such as fault geometry, slip rate, geodetic moment rate, and convergence rate. The analyses reveal that the source zone has accumulated strain energy, during the last 72 years since 1943, enough to produce a strong earthquake of magnitude ≥ 7. On supplementing with the historical data, we conclude M w 7.3 as the maximum potential earthquake for the Kopili source zone. Such large earthquake, on its occurrence, may cause widespread significant ground shakings and damage to infrastructures in the study region. We, therefore, also simulated strong ground motion, in the form of peak ground acceleration (PGA), for the M w 7.3 potential earthquake using Empirical Green’s Function (EGF) approach for ten different sites. In the analysis, an earthquake of magnitude M w 6.5, which has been simulated using a recorded M w 5.3 earthquake, is used as Green’s Function. The two-step approach is adopted in the simulation process, as the required criteria, i.e., moment ratio of < 1000 between the target potential event (M w 7.3) and the element event (M w 5.3) could not be met. We found that the cities like Tezpur, Masamari, Tumuki, Dhekiajuli, Nagaon, Bomdila, Udalguri, Seppa, Hajoi, Behali, Guwahati, and Itanagar that are located ∼ 60–130 km from the source zone may experience very strong to moderate ground shaking with PGA ranging between 0.36–0.14 g. However, the cities located in the distance range of ∼ 130 – 300 km from the source, namely Jorhat, Ziro, Mokokchung, Dhubri, and Kokrajhar are expected to have low ground shaking with PGA < 0.14 g. The study therefore provides valuable insights to the likely seismic hazard scenario in north-east India. [ABSTRACT FROM AUTHOR]