(Guest post by Yuki Suzuki and Kenji Kawai (University of Tokyo))
The D′′ layer (Bullen 1949), the lowermost several hundred kilometers of the Earth’s mantle immediately above the core-mantle boundary (CMB), is the thermal boundary layer (TBL) of mantle convection in touch with the liquid iron alloy outer core. The D′′ layer, especially beneath subduction zones, provides clues for understanding the dynamics of the Earth’s mantle, because thermally and chemically distinct slab materials can perturb the temperature and mantle flow. Since the D′′ layer is the TBL at the base of the Earth’s mantle and the solidus of its constituent materials is thought to be close to the mantle geotherm, vertical and lateral variations of temperature and chemical composition associated with the Earth’s thermal evolution are expected (e.g., Garnero and McNamara 2008; Kawai and Tsuchiya 2009; Garnero et al. 2016). Hence, it is important to study the D′′ layer beneath subduction zones to investigate how slabs have subducted to the lowermost mantle.
Recently, dense seismic arrays like the USArray, which includes many transportable stations that have been moved eastward to cover the entire conterminous area of the U.S.A., are providing high quality data for high resolution imaging of localized regions of the D′′ layer. It allows us to infer the detailed S-velocity structure in the D′′ layer beneath the Northern Pacific subduction zone by analyses using a large number of waveforms for many earthquakes and stations.
Our group has developed methods for waveform inversion of a large amount of seismograms recorded at dense seismic array network such as USArray for more detailed seismic structure in the Earth’s mantle (Kawai and Geller 2010; Kawai et al. 2014). Recently, Kawai et al. (2014) inferred the 3-D S-velocity structure in D′′ beneath Central America and found a subducted cold slab surrounded by hotter TBL materials.
In Suzuki et al. (2016), we conduct waveform inversion to infer the 3-D SH-velocity structure in the lowermost 400 km of the mantle (including the D′′ region) beneath the Northern Pacific. Our dataset consists of about 20,000 transverse component broadband body-wave seismograms observed at North American stations (mainly the USArray stations) for 131 intermediate-depth and deep earthquakes which occurred beneath the western Pacific subduction region.
The 3-D SH-velocity model obtained in this study shows three prominent features:
(i) prominent sheet-like lateral high-velocity anomalies up to ~3 per cent faster than the Preliminary Reference Earth Model (PREM; Dziewonski and Anderson 1981) with a thickness of ~200 km, whose lower boundary is ~150 km above the core-mantle boundary (CMB).
(ii) A prominent low-velocity anomaly block located to the west of the Kamchatka peninsula, which is ~2.5 per cent slower than PREM, immediately above the CMB beneath the high-velocity anomalies.
(iii) A relatively thin (~300 km) low-velocity structure continuous from the low-velocity anomaly “ii” 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: (i) remnants of slab material where the bridgmanite (Mg-perovskite) to Mg-post-perovskite phase transition may have occurred within the slab, (ii,iii) large amounts of hot and less dense materials beneath the cold paleoslab remnants just above the CMB which ascend and form a passive plume upwelling at the edge of the slab remnants (See Figure 1).