摘要:地球深部地慢對於地球熱構造與化學性質的演化扮演極為重要的角色. 其中, 核幔
邊界對於地球磁場, 地球冷卻和地表之活山活動皆有密不可分得關係. 因此, 藉由
地球深部構造的探究可以進而了解地球演化過程. 地震學家過去發現位於地慢側之
D"不連續帶內, 其包含許多複雜的小尺度構造之側向變化 (10-100公里內). 例如,
位於深部地慢之熱構造與化學性質異常, 物態與物態之相變帶, 含鐵礦物相的電離
子位階變化, 非均向性, 富含氧的玄武岩層, 部分熔融等等, 其複雜度皆與上部地
慢不分軒輊. 我們認為, 藉由探究環太平洋D"不連續帶小尺度構造的成份來源, 可
以進而得知整個地球內部運作行為. 本計畫將會著重於環太平洋區域中深部地幔高
速異常區域與隱沒板塊以及與D"不連續帶之相互關係深入探討. 非常幸運地, 環太
平洋孕震帶富含充沛的中深度地震以及相對密集的全球地震測網, 其中包含台灣,
中國, 日本, 美國 和各地臨時地震測網, 其優秀的地震與測站覆蓋率, 提供最佳
的地震學研究環境. 如本研究計畫所述, 利用地震波波形異常的分析, 我們可以藉
由觀測波形資料反映出地球物理學中兩個重要的研究議題:1. 隱沒板塊與相變帶之
側向變化, 可能的核幔邊界之富含氧化鐵與部分熔融層之觀測, 地幔熱住與隱沒板
塊的相互關係. 2. 藉由在D"不連續帶附近之隱沒板塊遺跡之相關研究, 反映深部
地慢構造與地表地體構造的連結.
若要對於上述兩個問題有完整的了解, 利用高速運算電腦計算大量的模擬波形是相
當重要的. 我們將會使用陣列地震學分析技術, 圖形顯示卡加速之全波場模擬技術
, 以及目前發展中的全波形反演等方法, 在現有的速度模型基礎下, 解析更小尺度
, 更劇烈的速度變化異常, 並且利用新的模型計算理論地震波與觀測資料比較. 這
些新產生的模型, 我們可以估算速度異常構造的尺度與相關的地震參數, 將可用於
地球動力學之全球地幔對流的模擬中, 也可以提供相關數據用於礦物學相關研究當
中. 結合地震學,地球動力學以及礦物學的研究成果, 將可以顯著提升我們對隱沒
板塊, D"不連續帶, 超低速層以及大尺度低速構造與相關深部地幔動力學的演化的
了解.
Abstract: The lower mantle plays a fundamental role in the thermal and chemical
evolution of the planet. The boundary between the core and mantle is a
primary interface within the deep interior and has a fundamental
influence on the magnetic field, the cooling of the planet, and
volcanism at the Earth`s surface. Thus, this region may be key to
unlocking the planet`s past thermo-chemical evolution. Seismologists
have revealed that the mantle side of the core mantle boundary (D”) is
extraordinarily complex with a myriad of fine structure (e.g. ~10 km to
a few 100 km’s). Thermal and chemical heterogeneity, solid-solid phase
transitions, variations in the electronic state of Fe, anisotropy, a
possible oxide-enriched basal layer, and melting within the lower mantle
are probably all required in order to explain observed structure and
suggests that the lower boundary of the mantle is as complex as the
mantle`s top boundary. Determining the origin of the fine scale
structure of the lower mantle, in general D”, in particular, along the
circum-Pacific corridor, is key toward understanding how the solid Earth
works as a globally interconnected system. The role of subducted slabs
and the circum-Pacific lower mantle high velocity belt is particularly
complicated with a mixture of phase-changes and slab debris as discussed in this proposal. Fortunately, this ring around the Pacific contains
most of the Earth’s seismicity that is well recorded by global arrays;
arrays in Taiwan, China and Japan, the USArray and many flex-arrays are
now becoming available. As demonstrated in this proposal, the data set
from USArray and other surrounding networks display waveform complexity
at all ranges with remarkable coverage, which gives us the opportunity
to bring observational insights from seismic data to address two
important geophysical research topics: 1. Systematic study of the role
of slabs in the laterally-varying post-perovskite phase transition
(upper and lower boundaries), possible Fe-oxide and melt layers at the
CMB and plume activity beneath and at the edges of the oldest slabs. 2.
Lower mantle structure connecting the present surface subduction with
the sinking of slab remnants interacting with apparent up-welling flow
as one approaches D".
In order to get thorough understanding of those two topics, extensive
waveform modeling running on the high-performance super computer is
essential. We will use array seismology techniques, GPU-powered full
wavefiled simulation approach, and asymptotic full waveform inversion
technique (in development) to sharpen current tomographic models and
produce synthetic seismograms that match observed waveforms
complexities. The physical dimensions of these structures in the lower
mantle and their inferred seismic parameters can be used in geodynamic
models of mantle convection and explored in mineral physics experiments
at CMB conditions. These further mineral physics and geodynamic
experiments will allow for improvement in our understanding of the
connection between subducted slabs, the D” layer, ULVZs, and LLSVPs and
the dynamic evolution in the deep mantle.