IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS012 Poster presentation 2209
Gaussian-beam receiver function synthetics in a 3-D heteregeneous
medium with velocity discontiuities at the Japan subduction zone
Prof. Kazuro Hirahara
Geophysics, Graduate School of Science Kyoto University IASPEI
Takashi Tonegawa, Takuro Shibutani
We have constructed receiver functions (RFs) observed at high-density broadband and short-period
stations over the Japan Islands to map the Moho beneath the Japan Islands, the slab top and the
oceanic crust of the Pacific and the Philippine Sea slabs, and the lower boundary of the Pacific slab, 410
and 660 km velocity discontinuities. . The mapping, however, depends on the reference velocity model.
Only radial RFs (RRFs) have been employed, though the transverse RFs (TRFs) have considerable
amplitudes in relation to subducting slabs. We propose RF tomography which combines the travel time
tomography and the RF analyses to obtain the 3-D velocity structure including velocity discontinuities.
RF tomography requires iterative forward modeling of RFs in a 3-D heterogeneous medium. The recent
advancement of computer power enables us to synthesize short-period waveforms with SEM and FDM.
However, iterative synthesizing of RFs in RF tomography is still formidable. We extend a computer code
GBM3D (Sekiguchi,1992), which can treat only the waves from the source in the model space and the
plane waves incident vertically at the bottom. Therefore, we implement calculation of teleseismic
waveforms with arbitrarily incident angles in the followings. As in the original code, we assume the 3-D
heterogeneous velocity, Q and density structure in each layer which are interpolated by 3-D cubic spline
functions and the layer boundaries are represented by 2-D ones. We set the dense grids at the bottom
and calculate the slowness vectors and travel times at the grids from a teleseismic source, assuming a
1-D earth model outside of the modeling space. From each grid, we execute kinematic and dynamic raytracings
for a variety of phases, and sum up beams with Gaussian weights to synthesize waveforms at
stations. It has been pointed out that GBM has the several tuning parameters and seems to be less
reliable in some cases. For almost vertical incident teleseismic waves, calculations are stable if ray
beams with enough density are summed. Then RRFs and TRFs are calculated from the synthetic
waveforms. We present the modeling of RFs which include the Ps phases converted at the Moho and
the upper mantle discontinuities with comparisons of observed RFs observed at several regions. The
first is theKii Peninsula region where the Philippine Sea slab subducts steeply with a 3-D complex
configuration, and RF analyses (e.g., Yamauchi et al., 2003) have provided the structure of the slab top
and the subducting oceanic crust. The second is the Tohoku region, where RF analyses have clarified
the subducting oceanic crust overlying the Pacific slab and the lower boundary of the slab, and other
phases. The third is the 410 km and 660km discontinuities across and around the stagnant Pacific slab.
Keywords: receiver function, gaussian beam method, subducting slab