Abstract
We reconstructed a 2D seismic wavefield and obtained its propagation properties by using the seismic gradiometry method together with dense observations of the Hi-net seismograph network in Japan. The seismic gradiometry method estimates the wave amplitude and its spatial derivative coefficients at any location from a discrete station record by using a Taylor series approximation. From the spatial derivatives in horizontal directions, the properties of a propagating wave packet, including the arrival direction, slowness, geometrical spreading, and radiation pattern can be obtained. In addition, by using spatial derivatives together with free-surface boundary conditions, the 2D vector elastic wavefield can be decomposed into divergence and rotation components. First, as a feasibility test, we performed an analysis with a synthetic seismogram dataset computed by a numerical simulation for a realistic 3D medium and the actual Hi-net station layout. We confirmed that the wave amplitude and its spatial derivatives were very well-reproduced for period bands longer than 25 s. Applications to a real large earthquake showed that the amplitude and phase of the wavefield were well reconstructed, along with slowness vector. The slowness of the reconstructed wavefield showed a clear contrast between body and surface waves and regional non-great-circle-path wave propagation, possibly owing to scattering. Slowness vectors together with divergence and rotation decomposition are expected to be useful for determining constituents of observed wavefields in inhomogeneous media.
Highlights
The High-sensitivity seismograph network (Hi-net), which is operated by the National Research Institute for Earth Science and Disaster Resilience of Japan (Okada et al 2004), is one of the densest seismograph networks in the world
A three-dimensional numerical simulation of seismic wave propagation was performed by using the finite difference method with the Japan Integrated Velocity Structure Model (JIVSM) of Koketsu et al (2012) with a spatial extension to include an area not covered by the original model (Maeda et al 2014)
We compared the spatial derivatives between the numerical simulation and that estimated from the seismic gradiometry method
Summary
The High-sensitivity seismograph network (Hi-net), which is operated by the National Research Institute for Earth Science and Disaster Resilience of Japan (Okada et al 2004), is one of the densest seismograph networks in the world. It covers almost all of the Japanese islands with a nearly uniform station separation of about 20 km (Fig. 1). Seismic waves propagating across arrays have been visualized for various arrays (e.g., Obara et al 2005; Sheldrake et al 2002; Trabant et al 2012). By treating a seismic wave as a spatially varying field, Maeda et al (2014) detected a new sort of long-period scattered wave that originated from reverberation in the seawater column within a deep trench
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