Topography of the 410 km and 660 km discontinuities beneath the Japan Sea and adjacent regions by analysis of multiple‐ScS waves

Abstract

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Due to the sparse distribution of seismic stations in the sea, previous studies mostly focus on mantle transition zone (MTZ) structures beneath continents or island arcs, leaving the vast area of the Japan Sea and Okhotsk Sea untouched. In this study, we analyzed multiple‐ScS reverberation waves, and a common‐reflection‐point stacking technique was applied to enhance consistent signals beneath reflection points. A topographic image of the 410 km and 660 km discontinuities is obtained beneath the Japan Sea and adjacent regions. One‐dimensional and 3‐D velocity models are adapted to obtain the “apparent” and “true” depth. We observe a systematic pattern of depression (~10–20 km) and elevation (~5–10 km) of the 660, with the topography being roughly consistent with the shift of the olivine‐phase transition boundary caused by the subducting Pacific plate. The behavior of the 410 is more complex. It is generally ~5–15 km shallower at the location where the slab penetrates and deepened by ~5–10 km oceanward of the slab where a low‐velocity anomaly is observed in tomography images. Moreover, we observe a wide distribution of depressed 410 beneath the southern Okhotsk Sea and western Japan Sea. The hydrous wadsleyite boundary caused by the high water content at the top of the MTZ could explain the depression. The long‐history trench rollback motion of Pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab.

Constraining the 410-km Discontinuity with Triplication Waveform

“…The turning points, the most sensitive regions of the triplicated ray paths, are below the Tatar Strait of Russia. Our derived interface at 400±5 km is consistent with the overall 0-10 km uplift of the 410-km discontinuity in this region observed with ScS reverberations (Wang et al 2017). Furthermore, our result is of higher resolution due to the smaller Fresnel zone for P wave at higher frequency (∼ 0.5 Hz).…”

Genesis of Intermediate‐Depth and Deep Intraslab Earthquakes beneath Japan Constrained by Seismic Tomography, Seismicity, and Thermal Modeling

“…At greater depths of 200-300 km, the sudden widening of the seismogenic zone (Figure 3c) possibly indicates that a new hydrous phase, for example, phase A, starts to break down between temperatures of 600-700 °C and triggers earthquakes due to the fluid-related embrittlement at the higher P-T regime (Figure 4; Omori et al, 2004). The deep seismicity between 300 and 380 km may be associated with continued dehydration of phase A (Omori et al, 2004), because the 410-km discontinuity is only elevated up to 395-km depth due to the cold slab (Wang et al, 2017), and therefore, the phase transformational faulting of metastable olivine to wadsleyite (Kirby et al, 1991) is not a viable mechanism for these seismicity. The DSZ width decreases in the depth range of 300-380 km compared to the width at depths of 200-300 km, with seismicity concentrated in a slab-normal depth range of 15-30 km (Figure 3d), possibly suggesting a wedge-shaped Phase A layer tapering away with increasing P-T toward the slab cold core (Figure 4).…”

Fine Structure of the 660‐km Discontinuity Beneath Southeastern China

“…Robust seismic studies of upper mantle discontinuities have been made both locally and globally. Long‐period SS/PP precursors (Deuss et al, ; Flanagan & Shearer, ; Lee & Grand, ; Schmerr & Garnero, , ; Shearer, ), ScS reverberations (Bagley et al, ; Courtier & Revenaugh, , ; Katzman et al, ; Revenaugh & Jordan, ; Wang et al, ), and P receiver functions (Agius et al, ; Ai et al, ; Wei & Chen, ) are the most popular techniques used. Most studies mainly focus on the topography of the discontinuities, inferring the interaction between the MTZ and cold slab or hot plume.…”