Abstract
AbstractCrustal configuration of the central Chugoku region with disposition of the Philippine Sea Plate (PHS) in this area are investigated through the receiver function approach using short-period Hi-net data. Images of the upper mantle discontinuities are also obtained. Restituted short-period receiver functions bring out discernible variations in average composition of the crust and its thickness in the study region. TheVp/Vsvalues in the study area are generally high, reaching values in excess of 1.85 at a few places. The central part of the study region showing the highestVp/Vsvalues is coincidentally a subregion of least seismicity, possibly bestowed with special subsurface structure. Migrated receiver function images, bothPsandPpsimages, unambiguously trace the NW subducting PHS taking a steeper plunge in the northwest part of the Chugoku region reaching depths of 70 km from its low dip disposition in the southeast. An excellent correlation of the subducting PHS with the hypocenters is also seen. We demonstrate that short-period data after restitution and application of appropriate low pass filters can indeed detect presence of the global 410-km and 660-km discontinuities and map their disposition reasonably well. Our migrated receiver functions image the deflections in the 410-km and 660-km discontinuities in an anti-correlated fashion on expected lines of Clapeyron slope predictions induced by subduction of the Pacific plate (PAC) beneath Japanese islands, though PAC itself is feebly traced but shows good correlation with slab seismicity.
Highlights
The Japanese islands are located in an acknowledged region of tectonic complexity encompassed by various convergent plate boundaries
To enable appreciate the quality of data used in determination of H -Vp/Vs values; we present slowness stack sections at a few individual stations (Figs. 3(a)–(f))
Recognising that the study region could generally be complex in nature owing to its unique tectonic setting, variation in crustal thickness and Vp/Vs are expected to be large
Summary
The Japanese islands are located in an acknowledged region of tectonic complexity encompassed by various convergent plate boundaries. To understand the above distribution of Poisson’s ratio (related to Vp/Vs) in the general context of seismicity of the study region, we selected earthquakes restricted to the study area (133◦E /34◦N/ 135◦E/ 35.5◦N) in the depth range 0–30 km with magnitudes in excess of 3.0 for the period 1950–2002 from the Japanese Meteorological Agency (JMA) database and plotted their locations (Fig. 7) on our Vp/Vs map.
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