PHYSICS BASED STRONG GROUND MOTION SIMULATION AND ANALYSIS OF DYNAMIC RESPONSE OF URBAN LAYER

Abstract

The strong ground motion simulation is necessary to understand earthquakes’ physics as it will aid in controlling the disaster caused by the future earthquakes. The ground motion characteristics depend on the source, path, and local site effects. The recent research outputs have also revealed that complex interaction of the urban layer with underlying basin/sediment as well as the interaction of the individual building with the surrounding buildings modifies the response of buildings of the urban layer as well as free field motion within and outside the city/urban layer. This thesis improves the 3D viscoelastic fourth-order staggered-grid timedomain finite-difference (FD) code written by Narayan and Sahar (2014) by implementing state-of-the-art pseudo-dynamic rupture for the broadband strong ground motion (SGM) simulation for both the strike slip and reverse slip earthquakes. We demonstrate the improvements in the efficacy of the 3D-FD code by the implementation of perturbation of the peak-time of the source time function (STF), damage zone, and fault-roughness to the basic rupture model comprising of random distribution of slip, rake angle, peak-time as 0.13 of the rise time of STF as well as rupture arrival time and rise-time, both partially correlated with the slip distribution. Encouraging improvements in the radiation of seismic energy from the rupture plane and the reduction of coherency effect are obtained by using stochastic perturbation of peak-time (instead of 0.13 times the rise-time), adding the fault-roughness and damage zone separately to the basic rupture model. Finally, the enhanced 3D-FD code comprehends the pseudo-dynamic rupture model with a random distribution of slip, peak-time, and rise-time of the STF and rake, along with fault roughness and damage zone. An excellent match of the computed pseudo-spectral acceleration using the simulated ground motion with the same obtained using NGA-West2 GMPEs for a hypothetical earthquake of magnitude Mw=6.25 reveals the efficiency of enhanced 3D-FD code to appropriately radiate the seismic energy from rupture plane in a broadband as well as the obtained average Fault Normal/Fault Parallel (FN/FP) spectral ratio of the order of one (1.28) in the forward rupture direction for frequencies more than 0.8 Hz reflects the efficacy to reduce the coherency of high-frequency seismic radiations. We then simulate the 2004 Parkfield, California earthquake (Mw=6.0), and the obtained decent match of the simulated ground motion with the records on the hard-rock further validates the efficiency of the enhanced 3D viscoelastic FD code to model the rupture of strong/great earthquakes for the simulation of broadband SGM.We have simulated the Mw8.2 scenario earthquake on the Nahan segment of the western Himalaya as well as associated seismic hazard on a dense array in a 0-2.5 Hz frequency bandwidth at the basement level in the National Capital Territory Delhi. The simulated transverse component of velocity time history at the basement level at 158 locations of the NCT Delhi was transferred to the free surface taking in to account the rheological parameters of the sediment deposit above the quartzite basement. The simulated SGM were used to develop various thematic contour maps of earthquake engineering interest. The computed pseudo-spectral accelerations depict that buildings with natural period around 0.4s-2.5s may suffer with severe damage or collapse under double resonance condition since it is exceeding both the design basis and maximum credible earthquakes under double resonance condition. The obtained range of pseudo-spectral displacement (10-43 cm) in localities where sediment thickness is more than 225 m reveals a need of better procedure for an economical and safe design of the high-rise buildings under double/partial resonance condition. We quantified the site-city-interaction (SCI) effects on the responses of buildings of a city and free field motion under realistic earthquake loading for the economic development of a smart city. SH-wave responses of various homogeneous and heterogeneous cities situated on horizontal sediment layer as well as in 2D heterogeneous basins are simulated and analysed for different dynamic parameters of the buildings. The simulated SCI effects using realistic earthquake loading reveals a reduction of transfer function (TF) of buildings in a wide frequency bandwidth. This finding is conflicting with the reported splitting of bandwidth of the 𝐹0 𝑆𝐵 in the past SCI studies, carried out using simple plane incident wave-front with a single zero-phase wavelet. The obtained largest SCI effects on a building was highly dependent on the building type, city and basin heterogeneity in contrast to the general perception that it is maximum at the centre of city. It is also obtained that SCI effects are always beneficial to buildings when fundamental frequency of building on rock 𝐹0 𝑆𝑅 < 1.4𝐹0 𝐵 (𝐹0 𝐵 is the fundamental frequency of basin/sediment layer). The obtained reduction of 𝐹0 𝐶𝐵 of building of city as well as free field motion due to the effects of SCI corroborates with the past SCI studies. The increase of coupling between the buildings and basin due to an increase of building density causes an increase of SCI effects on the responses of both the buildings and free field motion. The SCI effects in the case of buildings with low damping are beneficial during an earthquake. It is recommended that the smart city should be homogeneous in nature and 𝐹0 𝑆𝑅 of buildings should be less than around 1.4 times the 𝐹0 𝐵 of the underlying basin/sediment deposit.Finally, the quantification of role of structural parameters and impedance contrast in the insulation and meta-capacities of the city for the Rayleigh wave as well as development of meta-blocks to create desired band-gaps at an earthquake engineering scale is also carried out. The Rayleigh wave and horizontally propagating SH-wave responses of the various considered homogeneous and heterogeneous city models with different heights, damping, widths in horizontal directions and impedance contrast between structure and half-space are simulated at the top of structure as well as at the free field after crossing the city. It is concluded that the structure act as a meta-structure for the Rayleigh waves and the meta-capacity of the city increases with increase of number as well as stiffness of structures and decrease in damping and impedance contrast. An increase of width of band-gaps at different longitudinal modes of vibrations of structure with decrease in impedance contrast is obtained. An increase of insulation capacity of a city for the Rayleigh waves with increase of number and width of structures and decrease in impedance contrast is obtained. It is concluded that meta-block can be developed using appropriate ceramic material taking into account the impedance of the halfspace to develop a desired band-gap for the Rayleigh waves. The feasibility of development of heterogeneous and homogeneous meta-cities for the Rayleigh waves is presented using metabehavior of structures and meta-blocks in the epicentral zone of shallow earthquake. Based on the inferred level of insulation and meta-capacities of city for the Rayleigh waves, it is recommended to consider the urban layer falling in the path of Rayleigh waves for the estimation of seismic hazard in the epicentral zone of shallow earthquakes.