Major crustal and Moho discontinuities in the Japanese Islands identified from receiver function imaging

Abstract:

Recent travel time inversion analyses have elucidated crustal structures of whole areas in the Japanese Islands. However, very few studies have paid attention to crustal velocity discontinuities due to the limitation of spatial resolution. In this study, we show the major crustal and Moho discontinuities in the Japanese Islands estimated from receiver function analyses. To identify the spatial changes of velocity discontinuities, we construct many vertical cross-sections of receiver function images transferred lapse-time from P onset to depth using one-dimensional velocity structure models estimated at each station. The velocity structures are estimated from a grid search method of receiver functions in this study. They are also used to interpret the depths of Moho discontinuity. As a result, we identified many seismic discontinuities in the crust and the uppermost mantle. There are distinct positive gradient just beneath the seismogenic zone of shallow in-land earthquakes, which corresponding a velocity boundary between the upper crust and the middle or lower crust. The intra-crust discontinuities in the northeastern Japan region imply heterogeneous structures horizontally. The depths of Moho discontinuity tend to increase in mountain regions with some undulations. However, the discontinuities where volcanic deep low frequency earthquakes occur are not clear and characterized by a low-velocity or negative velocity gradient area sandwiched between the positive velocity contrasts. The depths of Moho discontinuity interpreted from receiver functions are deeper than the estimation from tomographic imaging and travel time of the refracted seismic waves in several areas. This may suggest the existence of low-Vs and/or high-Vp/Vs materials in the crust-mantle boundary.