利用有限頻寬走時殘差探討全球D'區域剪力波速度構造之非均質與非均向特性

Abstract

D”是位在地幔與地核交界處上方平均厚度約250公里的速度層。對於地幔對流系統而言，D”層被視為底部受熱且化學成分特殊的熱化邊界層，同時是向上湧升地幔熱柱起源地以及向下沉隱沒海洋板塊最終棲息的位置。地震學研究指出，此區域擁有複雜的側向速度非均質與非均向性。本研究量測1997年到2012年、規模大於5.8、震央距40度到145度之間的遠震於全球測站所記錄的剪力波走走時異常變化來探討D”的速度構造特性。 結合群集分析和波形交互相關法同時將相似波形分組和量測相對全球側向平均的一維速度模型ak135的走時殘差，並剔除低訊噪比的資料。然後將不同兩波相所量測到走時殘差再透過相同測站配對求得差分走時殘差，包括S(Sdiff)-SKS、ScS-S、ScS2-ScS和ScS3-ScS四種走時資料用來研究D”區域剪力波側向速度非均質性。結果指出在環太平洋地區，差分走時殘差以負值為主，代表此區域底下的D”層剪力波速度高於全球平均值，反映古老殘餘的太平洋隱沒板塊物質；而在太平洋中間與非洲地區，差分走時殘差殘差以正值為主，代表此區域底下的D”層速度較低，反映兩超大規模的地幔熱柱慢速構造的存在。而在太平洋東北側與西伯利亞地區，則觀測到差分走時殘差正負參雜的情形，顯示這些區域底下的D”存在小尺度複雜的速度構造。 透過量測的S(Sdiff)波相走時殘差進一步分析其SV和SH分量的走時差，即所謂的剪力波分離現象來研究D”的非均向性。經修正完上地幔非均向性影響後，顯示在震央距較小的測站SH和SV分量的走時差通常在±1秒內，並隨震央距拉長有逐漸增加的趨勢。在阿留申隱沒帶、加勒比海隱沒帶底下高速區，大多觀測到SH分量早到，主要解釋成在D”層平躺的殘餘板塊物質本身內部礦物快軸呈水平優選排列所造成。但考慮到Sdiff波的SV和SH有限頻寬走時異常對三維非均質速度構造敏感度不同亦會造成SH分量早到的視分離現象，因此此區域所觀察到的剪力波分離應同時受到D”本身彈性非均向性性質以及有限頻寬波傳效應所影響。而在太平洋南側與非洲西側的低速區，同時觀測到SV以及SH早到的現象。前人研究指出SH在低速D”區早到的情形與核幔邊界附近發生部分熔融岩漿受水平向的底層地幔流應力作用形成構造上的優選排列有關，但該模型無法解釋SV早到的現象。相反地，有限頻寬波形模擬顯示Sdiff波在慢速D”層傳遞時會造成SV早到，表示在部分慢速D”區域本身的彈性非均向性可能不大，使得SV和SH的走時分離主要是受到有限頻寬敏感度差異所控制。

The D’’ region which lies in the lowermost ~250 km of the mantle has long been postulated as a major thermo-chemical boundary layer in the earth’s dynamic evolution, where the upwelling plumes most likely originate and the downwelling cold slabs terminate. Numerous seismological investigations have found D” exist both strong velocity heterogeneity and anisotropy. In this study, we collect broadband waveforms from earthquakes with epicentral distances of 40-145o and magnitudes greater than 5.8 during 1997-2012. A cluster analysis with waveform cross correlation is employed to measure differential traveltime residuals of S(Sdiff)-SKS, ScS-S, ScS2-ScS, ScS3-ScS relative to predictions of the spherically-symmetric earth model ak135. Our results show the phase pairs passing through D” beneath the Circum-Pacific Rim generally have large negative residuals while those through D” beneath the Central Pacific and Africa have positive residuals. The residual variations are in agreement with large-scale degree 2 pattern of mantle convective circulation in the lowermost mantle as revealed from global tomographic images, in which the circum-Pacific ring of high velocities associated with the remnants of ancient subducted slabs surround the two antipodal large low-velocity regimes originating from the upwelling hot superplumes anchored at the core-mantle boundary beneath the central Pacific and African. Besides, the residuals from the paths through the Northeastern Pacific plate and Siberia give both positive and negative values within small sampling areas in D”, suggesting that these regions may exists complex, small-scale structural heterogeneity. The split times between the vertically (SV) and transversely (SH) polarized arrivals of S(Sdiff) phases after correcting for upper mantle anisotropy are further analyzed to investigate D” seismic anisotropy. We observe both positive and negative SV-SH split times in seismically slow regions; however, the proposed viable scenario in which partial melts produced at the base of the hot mantle plumes in D” have been aligned horizontally by the basal boundary flow can only explain advanced SH arrivals relative to SV. Positive SV-SH times (i.e., SH traveling faster) found in high-velocity regions are consistent with intrinsic LPO of anisotropic minerals in horizontally-lying slabs and apparent splitting resulting from finite-frequency sensitivity. Numerical waveform modeling has indicated that because of difference of the finite-frequency sensitivities, the SHdiff phase arrivals would advance and delay with respect to SVdiff while propagating through high- and low-velocity D” regions, respectively, similar to our observed apparent splitting between them. Both finite-frequency effects and elastic anisotropy may equally contribute to the observed splitting results.