Seismic Wave Propagation Through Layered Reduced Micropolar Medium

The ground motions recorded in the near-field regions of earthquakes reveal that, along with translational motions the rotational motions can severely damage structures. To capture these ground rotations more realistically, the present study makes a novel attempt to model the Earth medium as a horizontally layered reduced micropolar half-space. Layers in this medium are characterized by five material constants: Lame's constant (lambda), Eringen's shear modulus (mu), density (rho), rotational moment of inertia (j), and an additional material constant (kappa). Cylindrical coordinate system is used to analytically derive the Green's functions and obtain the response of the layered Earth medium subjected to a point earthquake force. The analytically obtained surface translations and rotations involve integrations over the wavenumber and frequency parameters and hence, are evaluated numerically. These solutions converge to corresponding classical elastic medium when the micropolar medium parameters obey the relations kappa MUCH LESS-THAN mu and 2 kappa rho j >>omega 2, where omega is the maximum frequency considered in the simulation. Thereafter, ground motions are simulated for several combinations of earthquake magnitudes and epicenter distances by modeling the Earth medium as an eight layered reduced micropolar half-space. Further, ground motions for the 2012 Wutai earthquake were simulated using the proposed model and compared with the response of classical elastic medium. The comparisons indicate that the surface rotations are significantly higher in the case of reduced micropolar medium. The proposed formulation can be extended to simulate ground motions for finite-fault seismic sources.