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Site and path effects are always required to isolate the source term from a recorded seismograph for the estimation of source characteristics. In this paper, the site and path effects are assessed in terms of site amplification and anelastic attenuation (quality factor), respectively, to obtain an isolated source term for the estimation of earthquake source parameters in the Kinnaur Himalaya. These terms are then applied to calculate the source parameters of 75 local earthquakes in the range 1.5 ≤ moment magnitude (Mw) ≤ 3.6 recorded in the Kinnaur region. The scaling relations of earthquake source parameters are also established for this region. The site amplification curves and S-wave quality factor Qs(f) are determined and further utilized to correct the Fourier spectrum of earthquake records. The obtained source spectrum corrected for these two terms is compared with the theoretical source spectrum based on the Brune (J Geophys Res 75:4997–5009, 1970) source model. The root-mean-square error (RMSE) between the observed and theoretical spectrum is formulated by implementing iterative forward modeling. The obtained range of source parameters of 75 events reveals ranges of 2.73 × 1011–3.44 × 1014 N-m for seismic moment, 0.03–13 bar for stress drop, 0.3–0.9 km for source radius, and 5.64 × 1002–1.19 × 1008 J for radiated energy. [ABSTRACT FROM AUTHOR]

This paper presents the first uniformly processed comprehensive database on strong motion acceleration records for the extensive regions of western Himalaya, northeast India, and the alluvial plains juxtaposing the Himalaya. This includes 146 three components of old analog records corrected for the instrument response and baseline distortions and 471 three components of recent digital records corrected for baseline errors. The paper first provides a background of the evolution of strong motion data in India and the seismotectonics of the areas of recording, then describes the details of the recording stations and the contributing earthquakes, which is finally followed by the methodology used to obtain baseline corrected data in a uniform and consistent manner. Two different schemes in common use for baseline correction are based on the application of the Ormsby filter without zero pads (Trifunac 1971) and that on the Butterworth filter with zero pads at the start as well as at the end (Converse and Brady 1992). To integrate the advantages of both the schemes, Ormsby filter with zero pads at the start only is used in the present study. A large number of typical example results are presented to illustrate that the methodology adopted is able to provide realistic velocity and displacement records with much smaller number of zero pads. The present strong motion database of corrected acceleration records will be useful for analyzing the ground motion characteristics of engineering importance, developing prediction equations for various strong motion parameters, and calibrating the seismological source model approach for ground motion simulation for seismically active and risk prone areas of India. [ABSTRACT FROM AUTHOR]

In this paper we investigated the feasibility of using the higher-frequency waveforms to characterize the seismicity in the Koyna–Warna region, globally well-known for its reservoir-triggered earthquakes. The local seismicity and source parameters have been extensively explored by many researchers using various types of data, e.g. analog data, broadband seismological data, and deep seismic sounding data. However, here we have utilized the seismological data recorded by 97 temporary 4.5-Hz geophones to locate the local seismicity in the region. We first investigated an appropriate 1-D P-wave velocity model through the inversion of P- and S-times. In order to compensate for the thick and laterally heterogeneous basalt, we incorporated the station corrections in the 1-D velocity inversion process. The station correction varies from + 0.09 to − 0.20 s for the P-waves, with clear demarcations of positive and negative station corrections corresponding to the northern and southern regions, respectively. The patterns of positive and negative station corrections suggested that the northern part is characterized by lower velocity, while slighter higher velocities present in the southern part of the study area are relative to the estimated 1-D velocity model. The earthquake locations are further improved by using the double-difference (hypoDD) approach, which enabled us to accurately map shallow subsurface seismogenic faults in the region. The data comprises 10,062 P-wave and 9049 S-wave catalog differential times from 348 earthquakes (1.0 ≤ ML ≤ 4.5). The precisely relocated earthquakes (ERH < 0.05 km ERZ < 0.5 km and rms < 0.02 s) form the three clusters of earthquakes in the NW–SE direction along the lineaments close to Warna Reservoir. The observed seismicity trends on the surface could be due to the vertical manifestations of faults or fractures. Their alignments are confirmed by the analysis of focal mechanism which exhibits a predominant normal type of faults with one nodal plane consistent with the main alignment of the seismic events. [ABSTRACT FROM AUTHOR]

In comparison to the east coast, the tsunami hazard for the west coast of India remains under-recognized, despite the impact in 1945 following a Mw 8.1 earthquake in the Makran subduction zone in the northern Arabian Sea. The previous occurrences of tsunamis in the Arabian Sea that would have a bearing on the west coast of India are being debated, including the question whether the Makran region has the potential to generate greater-magnitude earthquakes. With this in the backdrop, we present here the historical and geological evidence of a tsunami impact zone from a site on the Konkan Coast of western India. Located in the village of Kelshi, the impact zone is preserved within a coastal dune complex that also reveals occupation layers. This laterally extending 30–40-cm-thick zone, coinciding with a habitation level, displays varied sedimentary structures including scour-fill features, and is inter-layered with shells, at a height of ~ 3 m from the high-tide level. We attribute these sedimentary features to a tsunami flooding event that was contemporaneous with the transportation of shells, dated at 1508–1681 CE. The geological inference matches with the description by the Portuguese fleets of a sea disturbance in 1524 CE, reported from Dabhol, not far from Kelshi, and also from the Gulf of Cambay, located about 500 km to the north. Precluding submarine landslide scenarios, the modeling results suggest that the high impact in Kelshi could have been generated by a Mw ≥ 9 earthquake sourced in the Makran subduction zone. It is, however, intriguing how a Mw ≥ 9 earthquake in the Makran region finds no mention in the historical documentation. We underscore the need for fresh efforts along the Makran coast to reconstruct the tsunami recurrence history that would generate required validating constraints on the 1524 event, if it was indeed generated by a massive earthquake among other mechanisms. [ABSTRACT FROM AUTHOR]

We present a modern seismological study of the earthquake of 26 June 1941 in the Andaman Islands, the largest pre-2004 event along that section of the India-Burma plate boundary. Despite a large conventional magnitude ( M PAS = 8.1 ), it generated at best a mediocre tsunami for which no definitive quantitative reports are available. We show that the 1941 earthquake took place under the Andaman accretionary prism and consisted of a composite event, whose nucleating phase had a strike-slip mechanism incompatible with a data set of spectral amplitudes of mantle Rayleigh and Love waves. Combining this initial phase with a larger normal faulting mechanism can reconcile them with P-wave first motions, reports of subsidence on the eastern coast of the Andaman Islands and the small amplitudes of any putative tsunami. The small tsunami results from a combination of that mechanism and of a source located under the islands themselves and in shallow water, implying a reduction in amplitude under Green's law when transitioning to a deeper basin. [ABSTRACT FROM AUTHOR]

Seismic anisotropy of crust beneath the Shillong-Mikir Plateau and the surrounding regions of northeast India have been investigated with the help of splitting analysis of S-wave of local earthquakes. We estimate a total 83 pairs of splitting parameters ( Φ and δt) from 67 local shallow focus earthquakes (depth ≤ 32 km) recorded by the 10 broadband seismological stations operated in the study region. The results show delay times ranging from 0.02 to 0.2 s, which correspond to anisotropy up to 4%, suggesting significant strength of anisotropy in the study region. Fast polarization direction ( Φ) in the Shillong Plateau shows mostly NW-SE trend in the western part and NE-SW trend in the northern part. Φs near Kopili fault (KF) follows NW-SE trend. Φ at most of the stations in the study region is consistent with the local stress orientation, suggesting that the anisotropy is mainly caused by preferentially aligned cracks responding to the stress field. On the other hand, anisotropy observed near the KF is due to aligned macroscopic fracture related to strike-slip movement in the fault zone. [ABSTRACT FROM AUTHOR]

Northeast India bounded by latitudes 20°-30°N and longitudes 87°-98°E is one of the most seismically active areas in the world. This region has experienced several moderate-to-large-sized earthquakes, including the 12 June, 1897 Shillong earthquake ( M 8.1) and the 15 August, 1950 Assam earthquake ( M 8.7) which caused loss of human lives and significant damages to buildings highlighting the importance of seismic hazard assessment for the region. Probabilistic seismic hazard assessment of the region has been carried out using a unified moment magnitude catalog prepared by an improved General Orthogonal Regression methodology (Geophys J Int, 190:1091-1096, 2012; Probabilistic seismic hazard assessment of Northeast India region, Ph.D. Thesis, Department of Earthquake Engineering, IIT Roorkee, Roorkee, 2013) with events compiled from various databases (ISC, NEIC,GCMT, IMD) and other available catalogs. The study area has been subdivided into nine seismogenic source zones to account for local variation in tectonics and seismicity characteristics. The seismicity parameters are estimated for each of these source zones, which are input variables into seismic hazard estimation of a region. The seismic hazard analysis of the study region has been performed by dividing the area into grids of size 0.1° × 0.1°. Peak ground acceleration (PGA) and spectral acceleration ( S ) values (for periods of 0.2 and 1 s) have been evaluated at bedrock level corresponding to probability of exceedance (PE) of 50, 20, 10, 2 and 0.5 % in 50 years. These exceedance values correspond to return periods of 100, 225, 475, 2475, and 10,000 years, respectively. The seismic hazard maps have been prepared at the bedrock level, and it is observed that the seismic hazard estimates show a significant local variation in contrast to the uniform hazard value suggested by the Indian standard seismic code [Indian standard, criteria for earthquake-resistant design of structures, fifth edition, Part-I. Bureau of Indian Standards, New Delhi, 2002]. Not only holistic treatment of earthquake catalog and seismogenic zones has been performed, but also higher resolution in spatial distribution could be achieved. The COV maps have been provided with the strong ground-motion maps under various conditions to show the confidence in the results obtained. Results obtained in the present study would be helpful for risk assessment and other disaster mitigation-related studies. [ABSTRACT FROM AUTHOR]

The continued seismicity in Koyna-Warna region of the western part of Maharastra (India) and its relationship with subsurface structures, concealed below thick volcanic sequences, are studied in detail using gravity field along with newly available deep scientific drilling results. This seismically active zone is marked by a large conspicuous negative gravity anomaly, the causes of which are yet to be fully understood. Recent findings from the boreholes drilled in the Koyna (G upta et al. in Int J Earth Sci 104:1511-1522, ) and Killari seismic zones, both of which penetrated the thick Deccan volcanic cover and the underlying Archean crystalline basement, have motivated us to revisit the Bouguer gravity field over this region, using a newly developed finite element method of regional-residual separation. Our study reveals the presence of two thick low-density/low-velocity crustal zones below the Koyna-Warna region, the shallower one between 5 and 13 km depth and the deeper one between 35 and 43 km depth just above the Moho. Both of these zones appear to contain mantle-metasomatised and fractionated magmatic material, respectively. Interestingly, the hypocenters of all M ≥ 5 Koyna earthquakes occur within the upper low-velocity/low-density zone. We also suggest high-order crustal exhumation below this region, which led to the removal of the entire sedimentary and granitic upper crustal column. This process has brought denser mid-crustal lithological facies close to the surface. Quaternary uplifting and movement of fault blocks along the old as well as newly created fault planes seem to be still continuing. A paleo-rift may have existed beneath this region below which Moho temperatures (~600 °C) and mantle heat flow (~31 mW/m) are still high. [ABSTRACT FROM AUTHOR]

We model the spatial and temporal evolution of October 8, 2005 Kashmir earthquake's aftershock activity using the rate-and-state dependent friction model incorporating uncertainties in computed coseismic stress perturbations. We estimated the best possible value for frictional resistance ' Aσ', background seismicity rate ' r' and coefficient of stress variation 'CV' using maximum log-likelihood method. For the whole Kashmir earthquake sequence, we measure a frictional resistance Aσ ~ 0.0185 MPa, r ~ 20 M3.7+ events/year and CV = 0.94 ± 0.01. The spatial and temporal forecasted seismicity rate of modeled aftershocks fits well with the spatial and temporal distribution of observed aftershocks that occurred in the regions with positive static stress changes as well as in the apparent stress shadow region. To quantify the effect of secondary aftershock triggering, we have re-run the estimations for 100 stochastically declustered catalogs showing that the effect of aftershock-induced secondary stress changes is obviously minor compared to the overall uncertainties, and that the stress variability related to uncertain slip model inversions and receiver mechanisms remains the major factor to provide a reasonable data fit. [ABSTRACT FROM AUTHOR]

A visibility graph (VG) is a rather novel statistical method in earthquake sequence analysis; it maps a time series into networks or graphs, converting dynamical properties of the time series into topological properties of networks. By using the VG approach, we defined the parameter window mean interval connectivity time < T>, that informs about the mean linkage time between earthquakes. We analysed the time variation of < T> in the aftershock-depleted catalogue of Kachchh Gujarat (Western India) seismicity from 2003 to 2012, and we found that < T>: i) changes through time, indicating that the topological properties of the earthquake network are not stationary; and, ii) appeared to significantly decrease before the largest shock (M5.7) that occurred on March 7, 2006 near the Gedi fault, an active fault in the Kachchh region. [ABSTRACT FROM AUTHOR]

The 2004 Indian Ocean tsunami gave the opportunity to gather unprecedented tsunami observation databases for various coastlines. We present here an analysis of such databases gathered for 3 coastlines, among the most impacted in 2004 in the intermediate- and far field: Thailand-Myanmar, SE India-Sri Lanka, and SE Madagascar. Non-linear shallow water tsunami modeling performed on a single 4′ coarse bathymetric grid is compared to these observations, in order to check to which extent a simple approach based on the usual energy conservation laws (either Green's or Synolakis laws) can explain the data. The idea is to fit tsunami data with numerical modeling carried out without any refined coastal bathymetry/topography. To this end several parameters are discussed, namely the bathymetric depth to which model results must be extrapolated (using the Green's law), or the mean bathymetric slope to consider near the studied coast (when using the Synolakis law). Using extrapolation depths from 1 to 10 m generally allows a good fit; however, a 0.1 m is required for some others, especially in the far field (Madagascar) possibly due to enhanced numerical dispersion. Such a method also allows describing the tsunami impact variability along a given coastline. Then, using a series of scenarios, we propose a preliminary statistical assessment of tsunami impact for a given earthquake magnitude along the Indonesian subduction. Conversely, the sources mostly contributing to a specific hazard can also be mapped onto the sources, providing a first order definition of which sources are threatening the 3 studied coastlines. [ABSTRACT FROM AUTHOR]

We estimate frequency-dependent attenuation of P and S waves in Garhwal Himalaya using the extended coda normalization method for the central frequencies 1.5, 2, 3, 4, 6, 8, 10, 12, and 16 Hz, with earthquake hypocentral distance ranging from 27 to 200 km. Forty well-located local earthquake waveforms were used to study the seismic attenuation characteristics of the Garhwal Himalaya, India, as recorded by eight stations operated by Wadia Institute of Himalayan Geology, Dehradun, India, from 2007 to 2012. We find frequency-dependent P and S wave quality factors as defined by the relations Q = 56 ± 8 f and Q = 151 ± 8 f by fitting a power-law frequency dependence model for the estimated values over the whole region. Both the Q and Q values indicate strong attenuation in the crust of Garhwal Himalaya. The ratio of Q/ Q > 1 obtained for the entire analyzed frequency range suggests that the scattering loss is due to a random and high degree of heterogeneities in the earth medium, playing an important role in seismic wave attenuation in the Himalayan crust. [ABSTRACT FROM AUTHOR]