Upper mantle imaging beneath the Japan Islands by Hi-net tiltmeter recordings

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.