Abstract
Abstract We present a novel receiver-function image of the upper mantle structure around the Japan subduction zone. To increase the amount of available waveform data containing the relatively lower frequency component, we examined whether the Hi-net tiltmeter recordings are usable for imaging the upper mantle discontinuities by comparing them with broadband seismograms in different frequency bands. We found that the two are comparable at a frequency band between 0.02 and 0.16 Hz. To make receiver functions from tiltmeter data, stacked vertical components of broadband seismograms were used as source-time functions. Since such source-time functions may include biases from local structure, we also produced regional stacked source-time functions. The receiver function with the above frequency band does not seem to be affected by local structure. In the images derived from the receiver-function gathers, we were able to visualize both the oceanic Moho and the lower slab boundary, which could be traced down to depths of 400 km and 600 km, respectively. These images also show an uplift of the 410-km discontinuity and a depression of the 660-km discontinuity in the regions that are probably affected by the cold subducting Pacific slab.
Highlights
Slab-induced depth variation of the seismic discontinuities in the upper mantle beneath the Japanese Islands has been well mapped by receiver function (RF) analysis based on broadband seismograms
Our RF image suggests that the oceanic Moho within the subducting slab can be seen to depth of 400 km by positive RF amplitude, and the lower slab boundary is detected down to a depth of 600 km by negative dipping RF amplitudes [Figs. 6(a) and (b)]
We report that high-density short-period network (Hi-net) tiltmeter recordings are available for imaging the upper mantle discontinuities in the Japan subduction zone
Summary
Slab-induced depth variation of the seismic discontinuities in the upper mantle beneath the Japanese Islands has been well mapped by receiver function (RF) analysis based on broadband seismograms (see Li et al, 2000; Tonegawa et al, 2005). The density of broadband stations throughout the Japanese Islands is still too low (140 stations in Japan) to investigate the relation between the slab and the 410 km and 660 km upper mantle discontinuities (hereafter called the ‘410’ and ‘660’, respectively). These two discontinuities are observed globally with velocity jumps of several percentages, and are usually interpreted as mineral phase changes (Ringwood, 1975). Since the dominant frequency of the Hi-net short-period station is around
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