Abstract
We conduct waveform inversion to infer the three-dimensional (3-D) S-velocity structure in the lowermost 400 km of the mantle (the D′′ region) beneath the Northern Pacific region. Our dataset consists of about 20,000 transverse component broadband body-wave seismograms observed at North American stations for 131 intermediate and deep earthquakes which occurred beneath the western Pacific subduction region. We use S, ScS, and other phases that arrive between them. Resolution tests indicate that our methods and dataset can resolve the velocity structure in the target region with a horizontal scale of about 150 km and a vertical scale of about 50 km. The 3-D S-velocity model obtained in this study shows three prominent features: (1) prominent sheet-like lateral high-velocity anomalies up to $$\sim$$ 3% faster than the Preliminary Reference Earth Model (PREM) with a thickness of $$\sim$$ 200 km, whose lower boundary is $$\sim$$ 150 km above the core–mantle boundary (CMB). (2) A prominent low-velocity anomaly block located to the west of the Kamchatka peninsula, which is $$\sim$$ 2.5% slower than PREM, immediately above the CMB beneath the high-velocity anomalies. (3) A relatively thin ( $$\sim$$ 300 km) low-velocity structure continuous from the low-velocity anomaly “(2)” to at least 400 km above the CMB. We also detect a continuous low-velocity anomaly from the east of the Kamchatka peninsula at an altitude of 50 km above the CMB to the far east of the Kuril islands at an altitude of 400 km above the CMB. We interpret these features respectively as: (1) remnants of slab material where the bridgmanite to Mg-post-perovskite phase transition may have occurred within the slab, (2, 3) large amounts of hot and less dense materials beneath the cold Kula or Pacific slab remnants just above the CMB which ascend and form a passive plume upwelling at the edge of the slab remnants.
Highlights
IntroductionThe D′′ region is the lowermost several 100 km of the mantle immediately above the core–mantle boundary (CMB), and its base is in contact with the liquid iron alloy outer core
We detect a continuous low-velocity anomaly from the east of the Kamchatka peninsula at an altitude of 50 km above the core–mantle boundary (CMB) to the far east of the Kuril islands at an altitude of 400 km above the CMB. We interpret these features respectively as: (1) remnants of slab material where the bridgmanite to Mg-postperovskite phase transition may have occurred within the slab, (2, 3) large amounts of hot and less dense materials beneath the cold Kula or Pacific slab remnants just above the CMB which ascend and form a passive plume upwelling at the edge of the slab remnants
The D′′ region is the lowermost several 100 km of the mantle immediately above the core–mantle boundary (CMB), and its base is in contact with the liquid iron alloy outer core
Summary
The D′′ region is the lowermost several 100 km of the mantle immediately above the core–mantle boundary (CMB), and its base is in contact with the liquid iron alloy outer core. Lay and Helmberger (1983) and Young and Lay (1990) studied seismic structure in D′′ in particular regions beneath the circum-Pacific, especially beneath the Northern Pacific, and proposed 1-D S-velocity structure models, SLHO and SYLO, respectively, which have a positive velocity jump about 240 km above the CMB and a negative velocity gradient beneath the discontinuity. He et al (2014) suggested an 850-kmthick low-velocity anomaly surrounded by a 210-km-thick high-velocity anomaly in D′′ beneath Kamchatka on the basis of forward modeling of seismic waveforms. The number of earthquake sources used by the above studies is 10 for Lay and Helmberger (1983), 36 for Young and Lay (1990), 2 for He et al (2014), and 3 for Sun et al (2016), respectively, and waveform stacking or waveform forward modeling was used
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