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An introduction to articles published within the issue is presented, including one on the seismic network, catalog of earthquakes from 1668 to the present, another on the source mechanisms of earthquakes, and one paper on severe hazards of liquefaction in Gujarat, India.

Different catalogues of earthquakes are available for the Indian region besides regional catalogues for the Gujarat region. The limitations of these and other historical catalogues have been discussed in this paper for earthquake hazards and other studies. Keeping in view that these historical catalogues have been successful in explaining the recent occurrence of a few earthquakes like Coimbatore (1900), Hyderabad (1983), Bangaluru (1985) and Jabalpur (1997), the study has been extended to the Gujarat region. In this paper, therefore, an attempt has been made to correlate the historical data of the felt earthquakes in the Gujarat region from all the available sources with instrumental data to examine the recurrence of the earthquakes. It has been found that in addition to the seismically active Kutch region and Narmada and Cambay rifts, there is a tendency for earthquakes to recur in many districts of Gujarat for which felt data from historical times is available. The seismicity map prepared by the Institute of Seismological Research (ISR), Gandhinagar based on the Seismic Telemetry (2006–2020) shows scattered seismicity over the whole of Gujarat region besides active seismicity in the Kutch region. This data when combined with that from the catalogue prepared by the National Centre of Seismology provides greater confidence for earthquake hazard studies It has also been found that the coastal regions of Saurashtra and south Gujarat are more prone to swarm types of earthquakes. [ABSTRACT FROM AUTHOR]

This paper reports results of mathematical modeling applied to the stress and strain in epicentral zones before and after the large earthquakes that occurred on June 22, 2002 in the Qazvin Province, northwestern Iran (Mw = 6.4) and the Gujarat, India earthquake of January 26, 2001 (M = 6.9). The modeling relied on a method for calculating stress and strain in a blocky elastic isotropic heterogeneous medium disturbed by a set of faults that are due to an external tectonic stress field. The boundary conditions were specified based on geological and seismological data. It has been shown that the epicenters of large crustal earthquakes occur in zones of high stress concentration at the ends of tectonic faults. Rupture occurs when the relationship between acting tectonic stresses satisfies the requirement σyy/σxx >3, thus connecting zones of high stress. The evolution of the aftershock process is controlled by the stress drop caused by a new rupture, while the resulting aftershock clusters are in spatial correlation with the stress drop zone. The new rupture propagates in the direction of dominant tectonic fault orientation in the region. We show a relationship to exist between rupture length and the possible retrospectively predicted location and magnitude of the earthquake depending on the elastic energy buildup and its possible release under specified structural tectonic conditions. [ABSTRACT FROM AUTHOR]

Seismic hazard assessment is the key tool for rational planning, safety and design of infrastructures in seismically vulnerable regions. Gujarat is the only state in peninsular India with the maximum seismic hazard of large shallow earthquakes originating from intra-plate seismicity. Probabilistic seismic hazard assessment (PSHA) of Gujarat is carried out in this paper. Three seismogenic sources, namely Kutch, Saurashtra and Mainland Gujarat, are considered, and seismicity parameters are estimated separately for each region taking into account the completeness of the available earthquake data. Peak ground acceleration (PGA) of the horizontal component and spectral acceleration at specific periods are considered as the intensity measures. Ground motion predictive equation chosen was reported to be based on simulated ground motions and verified against the strong motion records in the study region. Results are reported for the 17 major cities at the bedrock and also for the soil sites. Apart from hazard curves, 2475 and 475 years of return periods are considered for the PGA and uniform hazard spectra (UHS). The results are compared with the present recommendations of Indian Standards. Key observations include (1) Indian Standards underpredict PGA in the entire Gujarat when the soil sites are considered and in a few cities even at the bedrock; (2) amplification of PGA (or short period hazard) on account of soil sites should be included in the Indian Standard, which is currently absent; (3) shape of the UHS indicates that a separate amplification is required at the hyperbolic portion; and (4) ratio of 2475-475 years of PGA, which is considered 2.0 in Indian Standard, should be reduced to 1.5. Time-dependent recurrence model is also included in this paper and compared with conventional PSHA. General observations include that (1) hazard may increase significantly on account of time dependency; (2) this also influences the disaggregation and in turn the selection of ground motions; and (3) time since last earthquake significantly influences the extent of the effect of time dependency. [ABSTRACT FROM AUTHOR]

In this paper, we outline the overview of the problem of intraplate seismicity and summarize our studies aimed at identifying potential earthquake sources in three regions located in continental environments. We study the French Massif Central situated within the West-European platform, the Gujarat area located at the northwestern edge of the Indian shield, and northeast Egypt located in the northeastern part of the African continent. These regions exhibit different levels of seismic activity. The French Massif Central reveals the lowest rate of seismicity, northeast Egypt exposes a low-to-moderate seismicity, and Gujarat reveals moderate seismicity with rare occurrence of strong events. We use a phenomenological approach for identifying possible locations of earthquakes, which is based on the pattern recognition applied to morphostructural data. The approach hypothesizes the nucleation of strong earthquakes at morphostructural nodes forming at the intersections of morphostructural lineaments, which are delineated by morphostructural zoning. Nodes are characterized by the pertinent geophysical and geological parameters, on the basis of which pattern recognition algorithm pinpoints capable nodes, i.e., the nodes where events of a certain sizes may occur. Seismic hazard assessment of continental regions is impeded because their seismic history is normally very pure. In such a situation, information on capable nodes is a necessary input for seismic hazard assessment. The example of northeast Egypt shows the exploiting capable nodes to assess seismic hazard by the Neo-Deterministic Seismic Hazard Assessment Methodology. [ABSTRACT FROM AUTHOR]

The earthquakes have the most dominating societal and economic impact on the built environment. The earthquakes in an intraplate region are infrequent but often damaging. The uncontrolled urban growth in cities due to population explosion and migration makes it necessary to assess seismic hazards in an active region. It provides parameters for seismic safety and helps in disaster mitigation. The Kachchh Rift Basin (KRB) of western India is a seismically active intraplate region where many damaging earthquakes have occurred in the past (Mw 7.8 in 1819, Mw 7.6 in 2001). The KRB hosts many economic corridors and ports. Though the region has been put in a category with highest seismic hazard, the entire region is not prone to high hazards. The primary objective of the study is to integrate major attributes that influence seismic hazard on a GIS platform and prepare a multi-criteria-based hazard map by multi-criteria decision process named as analytical hierarchy process (AHP) developed by Saaty. In this study, the information about some of the attributes like peak ground acceleration (PGA), geology and geomorphology, and tsunami hazard is taken from published literature, whereas shear wave velocity to 30 m depth (Vs)30 and amplification factor were obtained through empirical relationships. The integration of these different attributes was performed, and weights were assigned depending on their contribution to the seismic hazard. The multi-criteria approach reveals that the southwestern part comprising of Kachchh mainland has a low hazard as compared to central and northern parts and almost 1 million people and around 0.18 million houses are exposed to moderate to high hazard. Large swaths of land are prone to liquefaction hazard. The corridor comprising of Bhuj, Bhachau and Rapar needs seismic microzonation. This macro level hazard map will be beneficial for the urban planners and government authorities to decide the areas, where seismic microzonation or site-specific studies are required that would help in mitigating earthquake disasters in the future. [ABSTRACT FROM AUTHOR]

On June 20, 2012, an M = 5.1 earthquake known as the Dholavira earthquake occurred in the epicentral region of the 2001 Bhuj earthquake (M = 7.6) in Kachchh region, Gujarat, India. In this study, pre-seismic ULF emissions from the Dholavira earthquake are investigated using high-resolution magnetic data from the digital fluxgate magnetometer at the Multi Parametric Geophysical Observatory (MPGO) in Desalpar, Gujarat, India, in the frequency band 0.001‒0.5 Hz. To reduce anthropogenic and atmospheric disturbances, we have included the midnight magnetic data (i.e. 18‒21 UT) from January 1 to July 31, 2012. The data is analyzed in five frequency bands in light of local seismicity, geomagnetic storms and lightning events. The planetary Kp index and the Disturbance storm time index (Dst) are also taken into account for the study. In order to distinguish seismo-magnetic signatures from global geomagnetic effects, a polarization analysis is applied to the magnetic data. The results showed that the maximum variability of the polarization ratio occurred before the Dholavira earthquake. In addition, the results of the Pc3 amplitude ratio (Z/H and Z/G) have also enhanced 7 days before this earthquake. To understand the dynamics of seismic processes, fractal dimensional analysis is also applied to magnetic data. The observed fractal dimensions increased slightly a few days before the Dholavira earthquake. Since geomagnetic storms and lightning have only a negligible influence during this period, the abnormal fluctuations in the observed ULF emissions can be correlated with the Dholavira earthquake. [ABSTRACT FROM AUTHOR]

During 2006–2016, a strong motion seismic network of twenty 3-component accelerogarphs was deployed by the National Geophysical Research Institute, Hyderabad, India, in the epicentral region of the 2001 Mw7.7 Bhuj earthquake. On 7 March and 6 April 2006, two earthquakes of Mw5.5 have occurred in the Kachchh seismic zone, which led to an excellent dataset from 16 to 18 strong-motion accelerograph sites. This dataset enabled us to estimate strong motion parameters for these earthquakes that allowed us to examine the suitability of the design response spectra of BIS (criteria for earthquake-resistant design of structures, part I—general provisions and buildings, Bureau of Indian Standards, 2002) in Kachchh, Gujarat. Our study reveals that the estimated normalized response spectra at strong-motion accelerograph sites, which are lying in tertiary formations or near a zone of geological contact between Jurassic and tertiary formations, exceeded the design response spectra at 0.07–0.2 s, correlating with complete collapse of low-rise buildings, water tanks and dams in the area during the 2001 Bhuj earthquake. While the normalized acceleration spectrum of hard sediment (rock site) is found to not exceeding the design spectrum, correlating with the lack of damage in the Mesozoic hill zone. The estimated normalized acceleration spectra at SMA sites lying on hard sediments of Jurassic/Mesozoic formations are found to be lower than the design spectra, which suggests that these stations are less hazardous. We also notice that spectral acceleration values at few sites lying on quaternary formations have exceeded the design spectra at 3–4 s, suggesting these sites hazardous for engineered reinforced structures like bridges. Our estimated mean pseudo velocity spectra show peaks at 0.2, 0.5, 1.2 and 6 s. The peak at 6 s probably can cause damages to the reinforced engineered structures while the peak at 1.2 s could be causing damages to the multistoried reinforced buildings, as observed during the occurrence of the 2001 Bhuj mainshock. Thus, our estimated normalized acceleration spectra show peaks exceeding the design spectra at shorter (0.07–0.2 s, 0.5 s, 1–1.2 s) as well as longer periods (3–4 s, 6 s), which could be attributed to local site effects. We also notice that the modeled normalized response spectra for the shallow March event shows a better correlation with the NGA West2 model for tectonically active regions of US while the modeled normalized spectra for the deeper April event suggest a better correlation with the NGA east model for the stable parts of USA. We propose that the design spectra for the Kachchh region, Gujarat, India, should be modified incorporating local site, path and source effects. [ABSTRACT FROM AUTHOR]

In the absence of strong motion records, ground motion during the 26th January, 2001 Kutch, India earthquake, has been estimated by analytical methods. A contour map of peak ground acceleration (PGA) values in the near source region is provided. These results are validated by comparing them with spectral response recorder data and field observations. It is found that very near the epicenter, PGA would have exceeded 0.6 g. A set of three aftershock records have been used as empirical Green's functions to simulate ground acceleration time history and 5% damped response spectrum at Bhuj City. It is found that at Bhuj, PGA would have been 0.31 g–0.37 g. It is demonstrated that source mechanism models can be effectively used to understand spatial variability of large-scale ground movements near urban areas due to the rupture of active faults. [ABSTRACT FROM AUTHOR]