16:00 〜 16:15
[SIT14-19] Waveform inversion for the 3D S-wave velocity structure in D″ beneath the Hawaiian Islands
キーワード：波形インバージョン、太平洋LLSVP、最下部マントル、D″
The D″ region is the thermal boundary layer at the lowermost several hundred kilometers of the mantle and plays an essential role in mantle dynamics. In D″, two large low-shear-velocity provinces (LLSVPs) are known to exist: one beneath the Pacific and another beneath Africa. Since their discovery in the 1980s, there has been debate on whether thelow shear wave velocity is mainly due to heterogeneity in temperature or chemical composition. Mantle circulation simulations have found that if mantle convection is driven solely by the effect of temperature, gatherings of small thermal plumes called “plume clusters” will be created, while large masses of chemical heterogeneity referred to as “thermochemical piles” will be formed if chemical heterogeneity plays a role in driving the circulation. Although many studies have carried out whole-mantle inversions and envisaged the seismic structure of the lowermost mantle, their horizontal resolution of around 1000 km is insufficient to distinguish between thermochemical piles and plume clusters. In a previous study, our group has used waveform inversion to infer the localized structure in the northwestern boundary region of the Pacific LLSVP at a horizontal resolution of around 250 km, finding that this part of the LLSVP seems to resemble plume clusters. However, whether the other parts of the Pacific LLSVP exhibit similar characteristics is yet to be examined.
In this study, we utilize over 5000 broadband body-wave seismograms observed at seismic stations in the circum-Pacific region and conduct waveform inversion to resolve the 3D S-wave velocity structure of the northern boundary region of the Pacific LLSVP. The seismograms are obtained from the IRIS datacenter and include data of seismic waves from deep- and intermediate-focus earthquakes recorded at epicentral distances of 70-100 degrees. We use the radial and vertical components in addition to the transverse component, incorporating the SKS phase into the inversion along with the S and ScS phases. A resolution of ~250 km horizontally and ~50 km vertically is achieved, allowing insights into the internal structure of the Pacific LLSVP.
In the presentation, we will show the inferred 3D S-wave velocity structure of our target region along with resolution checks conducted to validate those results and discuss whether the northern part of the Pacific LLSVP is closer to the picture of thermochemical piles or to that of plume clusters.
In this study, we utilize over 5000 broadband body-wave seismograms observed at seismic stations in the circum-Pacific region and conduct waveform inversion to resolve the 3D S-wave velocity structure of the northern boundary region of the Pacific LLSVP. The seismograms are obtained from the IRIS datacenter and include data of seismic waves from deep- and intermediate-focus earthquakes recorded at epicentral distances of 70-100 degrees. We use the radial and vertical components in addition to the transverse component, incorporating the SKS phase into the inversion along with the S and ScS phases. A resolution of ~250 km horizontally and ~50 km vertically is achieved, allowing insights into the internal structure of the Pacific LLSVP.
In the presentation, we will show the inferred 3D S-wave velocity structure of our target region along with resolution checks conducted to validate those results and discuss whether the northern part of the Pacific LLSVP is closer to the picture of thermochemical piles or to that of plume clusters.