S T G Raghukanth

Empirical approaches based on the available earthquake catalog are popular among engineers to estimate seismic hazard. The seismic activity on individual faults inferred from these approaches is associated with a large number of uncertainties, especially due to lack of data. This article proposes a mechanistic approach to quantify seismic and fault activity, thereby overcoming the difficulties in the existing empirical procedure. A finite element (FE) model for Indian Plate, with the geological ‘cratons’ is developed and subject to plate driving forces namely, the ridge push and the slab pull forces. The material properties and the thickness of both plate and cratons are obtained through inversion using the recently available Global Positioning System (GPS) data. The tectonic faults are modeled using the well-known ‘extended FE method (XFEM)’. The simulated strains at fault level are compared with the computed strain rates from the earthquake catalog. Further, the procedure to quantify fault activity is demonstrated for Gujarat, India.

Quantification of seismic activity is one of the most challenging problems faced by earthquake engineers in probabilistic seismic hazard analysis. Currently, this problem has been attempted using empirical approaches which are based on the regional earthquake recurrence relations from the available earthquake catalogue. However, at a specified site of engineering interest, these empirical models are associated with large number of uncertainties due to lack of sufficient data. Due to these uncertainties, engineers need to develop mechanistic models to quantify seismic activity. A wide range of techniques for modeling continental plates provides useful insights on the mechanics of plates and their seismic activity. Among the different continental plates, the Indian plate experiences diffused seismicity. In India, although Himalaya is regarded as a plate boundary and active region, the seismicity database indicates that there are other regions in the Indian shield reporting sporadic seismic activity. It is expected that mechanistic models of Indian plate, based on finite element method, simulate stress fields that quantify the seismic potential of active regions in India. This article explores the development of a finite element model for Indian plate by observing the simulated stress field for various boundary conditions, geological and rheological conditions. The study observes that the magnitude and direction of stresses in the plate is sensitive to these conditions. The numerical analysis of the models shows that the simulated stress field represents the active seismic zones in India.

This article explores an engineering approach to model the seismic activity in India. Finite element analysis is carried out to estimate the stresses and displacements in the Indian plate. Assuming the Himalayan boundary as fixed, the plate-driving forces are modelled as axial forces applied at the mid-oceanic ridge between the Indian and African plates. The effect of Aravali, Dharwar and Bundelkhand cratons on the stress patterns in the plate is also studied. The obtained results are validated to the extent possible with the available recorded seismicity data and measurements from Global Positioning System (GPS).

The seismic activity in the capital city of Delhi is a matter of concern in the design and safety of numerous infrastructure facilities such as buildings, pipelines, railway lines and heritage structures. The city is continuously exposed to several small and large earthquakes due to local and Himalayan earthquake sources. Determination of ground motion in this region is an important problem for engineers to estimate hazard due to future damaging earthquakes. This article discusses the application of the spectral finite element method to simulate ground motion time histories in and around Delhi for regional earthquakes. The regional ground motions are obtained using the SPECFEM3D Globe package considering the effect of topography, bathymetry and three-dimensional variations of material properties and ellipticity of the Earth. The method is demonstrated for two local earthquakes near Delhi by comparing the peak amplitudes, arrival times and duration of the simulated time histories with the available strong motion records. Further, a hypothetical large earthquake is assumed in the epicentral region and ground motions are simulated. The statistics of the obtained peak ground displacements and the contours of permanent ground residual displacements are presented for the epicentral region.

One of the most challenging problems in the estimation of seismic hazard is the ability to quantify seismic activity. Empirical models based on the available earthquake catalogue are often used to obtain activity of source regions. The major limitation with this approach is the lack of sufficient data near a specified source. The non-availability of data poses difficulties in obtaining distribution of earthquakes with large return periods. Such events recur over geological time scales during which tectonic processes, including mantle convection, formation of faults and new plate boundaries, are likely to take place. The availability of geometries of plate boundaries, plate driving forces, lithospheric stress field and GPS measurements has provided numerous insights on the mechanics of tectonic plates. In this article, a 2D finite element model of Indo-Australian plate is developed with the focus of representing seismic activity in India. The effect of large scale geological features including sedimentary basins, fold belts and cratons on the stress field in India is explored in this study. In order to address long term behaviour, the orientation of stress field and tectonic faults of the present Indo-Australian plate are compared with a reconstructed stress field from the early Miocene (20 Ma).