Detecting the structure of the mantle transition zone in Japan subduction zone from the waveform triplications
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摘要: 本文基于中国数字地震台网记录的发生于日本北海道地区的一次中源地震的三重震相资料研究了日本海俯冲区地幔转换带的速度结构．结果表明，该区域P波速度结构与S波速度结构的一致性整体上较强．冷的西太平洋俯冲板块导致410 km间断面出现了10 km的抬升，660 km间断面出现了25 km的下沉；410 km和660 km间断面之上均存在与俯冲板块相关的高速层；660 km间断面下方存在厚度为65 km的低速异常．纵横波波速比vP/vS值在210—400 km深度范围内偏低，约为1.827，体现出海洋板块低泊松比的特征；在560—685 km深度范围内，该值偏高，约为1.831，可能预示地幔转换带底部含有一定量的水．Abstract: This paper applies the triplicated waveforms of an intermediate-depth earthquake at the Hokkaido of Japan, retrieved from the China Digital Seismograph Network, to mapping the structure of the mantle transition zone in Japan subduction zone. The results show that the P-wave velocity structure is consis-tent to S-wave velocity structure for the region as a whole. The cold subduction slab of the western Pacific Plate causes a 10 km uplift of the 410 km discontinuity and a 25 km depression of the 660 km discontinuity; atop the two discontinuities, there are high-velocity layers associated with the slab; below the 660 km discontinuity, there is a low-velocity anomaly with the thickness of 65 km. The seismic velocity ratio (vP/vS) shows a lower value (~1.827) zone at the depth range of 210—400 km, indicating the low Poisson’s ratio signature of the oceanic plate; and the velocity ratio shows a higher value (~1.831) zone at the depth range of 560—685 km, possibly implying the hydrous environment at the base of mantle transition zone.
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图 1 本文所用地震位置和台站分布图震源球标出了2011年10月21日中源地震的震中位置，黑色和红色三角形分别表示P波和S波三重震相所用台站，黑色和红色圆点分别为P波和S波射线拐点的地表投影；Ⅰ和Ⅱ为以方位角15°为间隔所划分的两个子区域；灰色线为板块边界(引自DeMets et al，1994)，黑色虚线为和达-贝尼奥夫带等深线(引自Gudmundsson，Sambridge，1998)
Figure 1. Location of the earthquake and stations used in the study The beach ball represents the epicentral location of the intermediate-depth earthquake on 21 October 2011. Black and red triangles denote the stations for the P- and S-wave triplications，respectively. Black and red dots are the surface projections of P and S-wave turning points，respectively. I and II label two sub-regions divided by the azimuthal interval of 15°. Gray lines indicate the plate boundaries(DeMets et al，1994). Contours shown in black dashed lines represent the Wadati-Benioff Zone(Gudmundsson，Sambridge，1998)
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图 2 中源地震P波三重震相原理图
(a)三重震相射线路径，包含了410上方的回折波(AB)、410之上的反射波(BC)、410下方的回折波(CD)、660之上的反射波(DE)和660下方的回折波(EF)；(b)三重震相的折合走时及波形图，图中字母A-F， M，O和O′是对走时曲线的标记，Δ表示震中距
Figure 2. Schematic illustration of the P-wave
triplication for an intermediate-depth earthquake (a)The ray paths for the triplication，including the turning wave above the 410 km discontinuity(hereafter as 410) (AB)，the reflection on the 410(BC)，the turning wave below the 410(CD)，the reflection on the 660 km discontinuity (hereafter as 660)(DE)and the turning wave below the 660(EF)；(b)The reduced travel times and synthetic waveforms for the triplication. The traveltime curves are marked with the symbols A-F，M，O and O′，and Δ represents epicentral distance
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图 3 地震事件震源深度重定位
(a)重定位所用台站分布；(b)震源球图解(Dziewonski et al，1981; Ekström et al，2012)；(c)图(a)中台站所记录到的观测波形；(d)pP-P相对时差的残差均方根随震源扰动深度的变化
Figure 3. The focal depth relocation of the earthquake event
(a)Distribution of the stations used in the relocation;(b)The focal mechanism(Dziewonski et al，1981;Ekström et al，2012);(c)Observed waveforms of the stations in Fig.(a);(d)Root mean square of time residuals of pP-P relative intervals versus focal depth perturbation
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图 4 Ⅰ和Ⅱ子区域P波波形拟合
图(a)和(b)分别为Ⅰ子区域观测和合成P波波形图，图(c)和(d)分别为Ⅱ子区域观测和合成P波波形图，图(e)为P波速度模型IASP91(黑线)和JSP(红线)．图(b)和(d)中合成波形为基于JSP模型合成，图(a)-(d)中黑色和红色折合走时曲线分别由IASP91和JSP模型计算所得
Figure 4. The P waveform fitting for sub-regions Ⅰand Ⅱ
Figs.(a)and(b)show observed and synthetic P waveforms for the sub-region Ⅰ，respectively，and Figs.(c)and(d)show those for the sub-region Ⅱ，respectively. Fig.(e)shows the P-wave velocity models IASP91(black line)and JSP(red line). Synthetic waveforms in(b)and(d)are calculated with the JSP model，and in(a)-(d)，black and red reduced traveltime curves are calculated with the IASP91 and JSP models，respectively
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图 5 Ⅰ和Ⅱ子区域S波波形拟合
图(a)和(b)分别为Ⅰ子区域观测和合成S波波形图，图(c)和(d)分别为Ⅱ子区域观测和合成S波波形图， 图(e)为S波速度模型IASP91(黑色线条)and JSS(红色线条). 图(b)和(d)中合成波形是由JSS模型合成，图(a)-(d)中黑色和红色折合走时曲线分别是由IASP91和JSS模型计算所得
Figure 5. The S waveform fitting for sub-regions Ⅰand Ⅱ
Figs.(a)and(b)show observed and synthetic S waveforms for the sub-region Ⅰ，respectively，and Figs.(c)and(d)show those for the sub-region Ⅱ，respectively. Fig.(e)shows the S-wave velocity models IASP91(black line)and JSS(red line). Synthetic waveforms in(b)and(d)are calculated with the JSS model，and in(a)-(d)，black and red reduced traveltime curves are calculated with the IASP91 and JSS model，respectively
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图 6 410和660之上的P波和S波反射点分布图
图(a)和(c)中黑色圆点分别为410和660之上的P波反射点，背景图像分别为GAP_P4模型(Obayashi et al，2013)在400 km和685 km深度的水平图像；图(b)和(d)中红色圆点分别为410和660之上的S波反射点，背景图像分别为S40RTS模型(Ritsema et al，2011)在400 km和685 km深度的水平图像
Figure 6. The location of P- and S-wave reflected points on the 410 and 660
In Figs.(a)and(c)，black dots are the P-wave reflected points on the 410 and 660，respectively; background images are the horizontal images from the GAP_P4 model(Obayashi et al，2013)at the depth of 400 km and 685 km，respectively. In Figs.(b)and(d)，red dots are the S-wave reflected points on the 410 and 660，respectively; background images are the horizontal images from the S40RTS model(Ritsema et al，2011)at the depth of 400 km and 685 km，respectively
表 1 地震事件震源参数表
Table 1 Source parameters of the earthquake event
年-月-日 时：分：秒 震源深度*/km 北纬/° 东经/° Mb 2011-10-21 08:02:37.62 188.0 43.8729 142.5315 6.0 *经本文重定位的震源深度 
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