J. P. Narayan

Effects of P-Wave and S-Wave Impedance Contrast on the Characteristics of Basin Transduced Rayleigh Waves

Different aspects of body wave amplifications in a basin have been studied in detail in the past. However, the study on amplification of basin-transduced surface waves is very limited, although surface waves are more damaging as compared to body waves. This paper presents the effects of P-wave and S-wave impedance contrast (IC) at the basin-edge on the spectral amplification and complex mode transformation of basin-transduced Rayleigh (BTR) waves. BTR-waves, the acronym for basin-transduced Rayleigh waves, is used in the manuscript. Analysis of simulated results reveals a complex mode transformation of Rayleigh wave after entering the basin. The less dispersed horizontally polarised first mode and highly dispersed vertically polarised fundamental mode of BTR-waves were inferred in the basin. The first mode of BTR-waves is developed in a frequency bandwidth and the frequency content in that is very much dependent on the Poisson’s ratio and fundamental frequency of soil in the basin. An increase of phase velocity of first mode of BTR-waves with Poisson’s ratio is obtained. It is concluded that spectral amplification of BTR-waves depends on both the P-wave IC and S-wave IC at the basin-edge. An increase of the largest and average spectral amplifications of the horizontal component of the BTR-wave is obtained with the increase of both the P-wave IC and S-wave IC. On the other hand, an increase of the largest and average spectral amplification of the vertical component of the BTR-wave is obtained with the increase of P-wave IC, but such a systematic and significant effect is not inferred with the increase of S-wave IC at the basin-edge.

EFFECTS OF SOIL LAYERING ON THE CHARACTERISTICS OF BASIN-EDGE INDUCED SURFACE WAVES AND DIFFERENTIAL GROUND MOTION

This paper studies the effects of soil layering in the basin based on the characteristics of basin-edge induced surface waves and associated differential ground motion. Seismic responses of various basin-edge models were simulated using software based on parsimonious finite difference staggered grid approximation of 2.5D eiastodynamic wave equation. Seismic responses of various models with different number of soil/soft rock layers but for a fixed thickness of deposit, fundamental frequency and impedance contrast revealed a decrease of surface wave amplitude with an increase in the number of layers in the basin. Shifting of dominant frequency towards the higher values was obtained with an increase of number of layers. An increase of dispersion of surface waves with an increase of number of soil layers in the basin was observed. A minor increase of Rayleigh wave velocity with an increase of number of soil layers was also obtained, but in the case of Love wave it was almost negligible. Spectral analysis of the edge-induced surface waves revealed that the anomalous earthquake intensity may arise in a zone of width of 2.5–3.0 km, parallel to basin-edge and at an offset of 0.5–0.7 km from the edge. Maximum horizontal differential ground motion (HDGM) developed by Love wave (≈4.9×10−2) was more than that of Rayleigh wave (≈9.4×10−3). Large variation in HDGM caused by the surface waves was obtained with a change in the number of layers in the basin and maximum HDGM was observed when there were only two layers in the basin. It was inferred that the effect of soil layering in the basin was more on the Rayleigh wave as compared to the Love wave. Development of large HDGM near the basin-edge and its dependency on the number of soil layers reveals that basin-edge induced surface waves need special attention during seismic microzonation or seismic hazard prediction.

Run-up and Inundation Pattern Developed During the Indian Ocean Tsunami of December 26, 2004 Along the Coast of Tamilnadu (India)

The tsunami run-up, inundation and damage pattern observed along the coast of Tamilnadu (India) during the deadliest Indian Ocean tsunami of December 26, 2004 is documented in this paper. The tsunami caused severe damage and claimed many victims in the coastal areas of eleven countries, bordering the Indian Ocean. Along the coast of Indian mainland, the damage was caused by the tsunami only. Largest tsunami run-up and inundation was observed along the coast of Nagapattinam district and was about 10–12 m and 3.0 km, respectively. The measured inundation data were strongly scattered in direct relationship to the morphology of the seashore and the tsunami run-up. Lowest tsunami run-up and inundation was measured along the coast of Thanjavur, Puddukkotai and Ramnathpuram districts of Tamilnadu in the Palk Strait. The presence of shadow of Sri Lanka, the interferences of direct/receded waves with the reflected waves from Sri Lanka and Maldive Islands and variation in the width of continental shelf were the main cause of large variation in tsunami run-up along the coast of Tamilnadu.

Study of combined effects of sediment rheology and basement focusing in an unbounded viscoelastic medium using P-SV-wave finite-difference modelling

This paper consists of two parts. First, a fourth-order-accurate staggered-grid finite-difference (FD) program for simulation of P-SV-wave in viscoelastic medium is presented. The incorporated realistic damping is based on GMB-EK-model. The accuracy of program is validated by comparing computed phase-velocity and quality-factors with same based on GMB-EK-model and Futterman’s relations. The second part of paper presents the combined effects of sediment damping and synclinal basement focusing (SBT) on ground motion. The results reveal SBT focusing, mode conversion and diffraction of incident waves. The response of elastic SBT model reveals an increase of spectral amplification with increasing frequency. The viscoelastic response of SBT model reveals that a particular frequency may get largest amplification for a particular set of values for damping, focal-length and distance from tip of the SBT. This frequency-dependent amplification may explain mysterious damage reported in some past earthquakes if predominantly amplified frequency matches natural frequency of damaged structures.

2.5D Simulation of basin-edge effects on the ground motion characteristics

The effects of basin-edge and soil velocity on the ground motion characteristics have been simulated using 2.5D modeling. One of the most significant advantages of the 2.5D simulation is that 3D radiation pattern can be generated in a 2D numerical grid using double-couple shear dislocation source. Further, 2.5D numerical modeling avoids the extensive computational cost of 3D modeling. The responses of basin-edge model using different soil velocities revealed that surface waves were generated near the edge of the basin and propagated normal to the edge, towards the basin. Further, the results depict increase of amplification, duration and surface wave generation with the decrease in soil velocity.

SIMULATION OF RIDGE-WEATHERING EFFECTS ON THE GROUND MOTION CHARACTERISTICS

Seismic responses of weathered ridge model with various types of weathering materials/soils and thickness were simulated, using double-couple point shear dislocation sources to evaluate the significance of ridge-weathering effects on the ground motion characteristics. 2.5D finite difference modeling was adopted for simulating the ground motion using parsimonious staggered grid scheme. The analysis of responses of weathered and non-weathered ridge models reveals that surface waves were generated near the top of the ridge. The surface waves were not dominating on the top of the ridge but at some lower elevation. Results also revealed ground motion amplification with elevation. Maximum amplitude of ground displacement was observed on the top of the ridge, when it was not very much weathered, but the reverse was the case (surface waves were dominating near the base) when weathering velocity was more than three times lesser than the underlying rock. An important conclusion was drawn based on simulated results that the thickness of weathering in terms of wavelength (λ) play a vital role in the generation of very strong surface wave with long duration, i.e. when the thickness is equal to or more than λ/8. The decrease of weathering-velocity further increased the amplitude and duration of surfaces waves. On the other hand, surface waves caused by the ridge itself or by weathered materials having thickness less than λ/12 have amplitudes similar or somewhat more than the incoming waves and with much smaller duration. Therefore, special measure for weathering thickness and its velocity is recommended on the basis of simulated results during the construction of buildings on the ridge topography.

Computation of ground motion amplification in Kolkata megacity (India) using finite-difference method for seismic microzonation

This paper presents the ground motion amplification scenario along with fundamental frequency (F0) of sedimentary deposit for the seismic microzonation of Kolkata City, situated on the world’s largest delta island with very soft soil deposit. A 4th order accurate SH-wave viscoelastic finite-difference algorithm is used for computation of response of 1D model for each borehole location. Different maps, such as for F0, amplification at F0, average spectral amplification (ASA) in the different frequency bandwidth of earthquake engineering interest are developed for a variety of end-users communities. The obtained ASA of the order of 3–6 at most of the borehole locations in a frequency range of 0.25–10.0 Hz reveals that Kolkata City may suffer severe damage even during a moderate earthquake. Further, unexpected severe damage to collapse of multi-storey buildings may occur in localities near Hoogly River and Salt Lake area due to double resonance effects during distant large earthquakes.

Control of Structural and Electrical Transitions of VO2 Thin Films

Unstrained and defect-free VO2 single crystals undergo structural (from high-temperature tetragonal to low-temperature monoclinic phase) and electronic phase transitions simultaneously. In thin films, however, in the presence of unrelaxed strains and defects, structural (Peierls) and electronic (Mott) transitions are affected differently, and are separated. In this paper, we have studied the temperature dependence of structural and electrical transitions in epitaxially grown vanadium dioxide films on (0001) sapphire substrates. These results are discussed using a combined kinetics and thermodynamics approach, where the velocity of phase transformation is controlled largely by kinetics, and the formation of intermediate phases is governed by thermodynamic considerations. We have grown (020) VO2 on (0001) sapphire with two (001) and (100) in-plane orientations rotated by 122°. The (100)-oriented crystallites are fully relaxed by the paradigm of domain-matching epitaxy, whereas (001) crystallites are not relaxed and exhibit the formation of a few atomic layers of thin interfacial V2O3. We have studied the structural (Peierls) transition by temperature-dependent in situ X-ray diffraction measurements, and electronic (Mott) transition by electrical resistance measurements. A delay of 3 °C is found between the onset of structural (76 °C) and electrical (73 °C) transitions in the heating cycle. This temporal lag in the transition is attributed to the residual strain existing in the VO2 crystallites. With this study, we suggest that the control of structural and electrical transitions is possible by varying the transition activation barrier for atomic jumps through the strain engineering.

Spatial spectral amplification of basin-transduced Rayleigh waves

This paper presents the effects of impedance contrast (IC) on the basin-edge, sediment thickness and rheology of sediment in the basin on the spatial spectral amplification and complex mode transformation of the basin-transduced Rayleigh (BTR) waves. BTR-wave, the acronym for the BTR wave is used in the manuscript. Seismic responses of a 2D basin-edge model were simulated using a P-SV wave fourth-order spatial accurate time-domain finite-difference algorithm based on staggered-grid approximation of viscoelastic velocity-stress wave equations. The used algorithm is efficient enough to incorporate the frequency-dependent damping in the time-domain simulation. Analysis of the computed seismic responses revealed that the spatial spectral amplification of the horizontal and vertical components of the BTR-wave in the basin was very much dependent on the distance of the site from the basin-edge, impedance contrast at the basin-edge, sediment damping and the thickness of sediment in the basin. Furthermore, the development of different modes of the BTR-wave very much depends on the sediment thickness and IC at the basin-edge.

Study of role of basin shape in the site–city interaction effects on the ground motion characteristics

Abstract In this paper, the role of basin shape in the site–city interaction (SCI) effects on the ground motion characteristics is documented. The effects of city type and city density on the free-field motion are also documented. The seismic responses of various 2D basin models with different shapes as well as city type and city density were simulated using a fourth-order accurate staggered-grid viscoelastic SH-wave finite-difference algorithm. The analysis of simulated responses of various site–city models, ground motion perturbations and spectral amplifications revealed that basin shape plays an important role in SCI effects on the ground motion. Further, a considerable SCI effect was obtained at the double resonance. The city density seems to be a more prominent parameter of SCI, which largely reduces the ground motion amplitude in the city as compared to the height of buildings. It was also inferred that the existence of different types of buildings may result in a decrease in coherency of building response.