Phase velocity maps of the Taiwan region from the ambient noise tomography of the cross terms in Green’s function tensors
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摘要: 基于我国台湾地区24个宽频带地震台记录到的长达18个月的三分量连续波形数据，采用交叉项互相关方法提取了瑞雷波的经验格林函数张量，进而反演获得了台湾地区6—22 s周期的瑞雷波相速度分布图像，较好地刻画了该地区的地壳速度结构。结果显示：短周期图像上的滨海平原、屏东平原等呈低速特征，西部丘陵、中央山脉和海岸山脉呈高速特征；低速区域随相速度周期的增大而逐渐东移；中长周期图像上，中央山脉南北部均呈低速且南侧的速度较北侧低，表明欧亚板块和菲律宾海板块碰撞后的物质从东北和西南两个方向被侧向挤出时南部可能较北部活跃；台中—南投地区在对应深度附近的高速异常，表明新生代时期的澎湖地台在南海北缘的拉张与碰撞演化过程中，保留了其相对稳定的性质。Abstract: Based on the three-component continuous waveform data recorded by the 24 broadband seismic stations in the Broadband Array in Taiwan for Seismology （BATS） from January 2016 to June 2017, the empirical Green’s function tensors of surface waves are extracted by the cross-term correlation method. Rayleigh wave phase velocity maps from the period 6 s to 22 s are subsequently derived from cross-terms of the Green’s function tensors, which made good description of the crustal velocity structure in Taiwan region. Our results show that the Coastal plain and the Pingtung basin exhibit low-velocity characteristics, while the Western Foothills, Central Range and Coastal Range exhibit high-velocity characteristics in the short-period band. The low velocity zone shifts eastward gradually with period increasing. On the medium and long periods, velocities in southern and northern segments of Central Range are significantly low, indicating the northward and southward material extrusion related to the lithospheric collision between the Eurasian Plate and the Philippine Sea Plate. Besides, the velocities in the southern segment of the Central Rang are slightly lower than that of the northern segment, which may imply the more active southward extrusion. In addition, the high-velocity anomaly near the related depth in Taichung-Nantou area may indicate that the Penghu platform retains its relatively stable state since the Cenozoic Era during the extension and collision evolution of the northern margin of the South China Sea.
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2021年5月22日青海果洛州玛多县发生MS7.4地震，震中位于（34.59°N，98.34°E），其震源机制解显示该地震为高倾角走滑型（张喆，许立生，2021）。玛多地震的发震构造为昆仑山口—江错断裂，是东昆仑断裂的一条分支断裂（王未来等，2021）。玉树地震台位于甘孜—玉树断裂附近。玛多地震震中和玉树地震台均位于巴颜喀拉次级地块内，玉树地震台位于巴颜喀拉地块的南边界。此次地震震中处于玉树地震台的NE方向，距巴颜喀拉地块北边界85 km （图1）。
玛多地震前玉树地震台井水温呈现异常变化。2021年3月15日9时玉树地震台井水温出现突降，截止到16日0时，最大降幅约0.002 6 ℃，之后水温逐步回返恢复，总体呈不规则的“V”型变化（图2）。2021年5月22日青海果洛州玛多县发生MS7.4地震，异常测项距震中220 km。
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图 2 2021年玛多MS7.4地震前玉树地震台井水温整点值观测曲线Figure 2. The observation curve of hourly well water temperature in Yushu seismic station before the 2021 Maduo MS7.4 earthquake玉树地震台水温观测井深105 m，井内套管下设深度100 m，水温观测仪器为SWY- Ⅰ 型数字水温仪，传感器到井口的距离为12.168 m，井孔岩芯为中生代侏罗纪浅成花岗岩。自2007年6月开始观测，数据稳定连续，高频波动明显，2012年至2013年间断性仪器故障，存在长时间的数据缺失（图3）。2017年11月仪器改造，更换为SZW- Ⅱ 型水温仪，数据连续稳定（图4）。
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图 3 2007—2017年玉树地震台井水温整点值观测曲线Figure 3. The observation curves of hourly well water temperature in Yushu seismic station from 2007 to 2017![]()
图 4 2018年以来玉树地震台井水温整点值观测曲线Figure 4. The observation curve of hourly well water temperature in Yushu seismic station since 20182021年3月15日玉树地震台井水温出现异常变化后观测人员进行现场异常核实。现场调查可知：仪器工作状态正常，附近无施工和灌溉抽水情况，基本排除了人类活动和自然环境造成的干扰（孙小龙等，2020）。对观测井和距观测井20 m的饮用泉进行取样，与2018年的取样结果进行对比分析，结果（表1）显示：两次采样结果均在“未成熟水”范围内，水化学类型均属HCO3-Ca型（张磊等，2019）；2021年饮用泉的
${\rm{HCO}}_3^{-} $ 浓度较2018年显著增大，考虑其水体溶解的CO2含量增加，可能指示区域存在断裂活动的迹象（康来迅等，1999）。表 1 2018年与2021年玉树地震台水样结果对比Table 1. Comparison of water sample results in Yushu seismic station in the year 2018 and 2021样品编号 各组分浓度/（mg·L−1） Ca2+ Mg2+ Na+ K+ HCO3− ${\rm{SO} }_4^{2 - }$ Cl− 2021年观测井 51.31 22.9 23.09 4.52 354.1 33.71 2.15 2021年饮用泉 72.88 27.16 22.07 4.51 423.1 39.48 2.45 2018年观测井 67.16 24.25 26.56 9.91 346 45.56 9.95 2018年饮用泉 75.29 23.68 22.84 10.07 340.5 49.9 9.73 玉树地震台井水温测项自2007年观测以来，一共出现7次异常，异常对应率为100%，对应于M5.0以上地震（图3，4），一般在异常出现后3个月内发震，地震分散在青藏高原内部（何案华等，2012；王博等，2016；杨晓霞等，2016）。为获得更为明确的时空强指示信息，以本次“突降—缓慢上升”的异常形态对震例进一步梳理，结果列于表2，可见：震例指示异常开始的三个月内青藏高原巴颜喀拉地块边界及其附近的M7.0以上地震（图5）（芦山MS7.0地震发生在观测数据断记期间，故未统计在内）。异常指标通过预报效能检验的R值评分为R＝0.61，R0＝0.45 （张国民等，2002）。因此在井水温异常上升恢复的过程中发生于2021年3月19日的西藏那曲MS6.1地震经研判认为非目标地震，异常仍需跟踪，而2021年5月22日玛多MS7.4满足异常所指示的时空强三要素特征。
表 2 玉树地震台井水温震例统计Table 2. Earthquake case statistics of water temperature in Yushu seismic station序号 异常起始时间 异常形态 异常幅度/℃ 持续时间/d 井震距km 对应地震 1 2008-03-15 突降—上升 0.022 18 643 2008年3月21日于田MS7.3，2008年5月12日汶川MS8.0 2 2010-01-19 突降—上升 0.029 24 46 2010年4月14日青海玉树MS7.1 3 2021-03-15 突降—上升 0.002 6 36 20 2021年5月22日玛多MS7.4 ![]()
图 5 玉树地震台对应于三次大地震的井水温异常整点值曲线Figure 5. The anomalous hourly value curve of well water temperature in Yushu seismic station corresponding to the three major earthquakes在地震孕育过程中，区域应力应变状态发生改变，在应力加载作用下，含水层岩体变形、相应的孔隙压力发生变化，导致井-含水层系统的水动力条件改变，进而引起井水微温度场发生改变（车用太等，1996；鱼金子等，1997；孙小龙，刘耀炜，2006）。玛多MS7.4为左旋走滑型地震，玉树地震台井水温测项位于其主动盘一侧，在孕震后期临近发震时，应力加载作用显著增强，可能造成水温测项所在区域的短期应力加载，微破裂大量发育，使不同含水层的地下水串通混合，进而水温快速下降，之后井-含水层系统趋于稳定，水温缓慢上升恢复。
综合异常核实和震例梳理结果显示，此次玉树地震台井水温异常信度较高，从时空强三要素较好地对应于2021年5月22日玛多MS7.4地震。通过对水温前兆异常的可能机理探讨分析，玉树地震台作为巴颜喀拉地块上地球物理场观测的构造敏感点，其井水温的异常变化对该地块上的地震孕震过程具有较好的短期指示意义，在未来震情跟踪过程中应予以重点关注。
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图 1 台湾地区的构造背景和基本构造单元
Figure 1. The tectonic settings and basic tectonic units in Taiwan region
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图 2 垂向（a）和交叉项（b）的互相关函数图像对比
Figure 2. The comparison of cross-correlation functions from vertical-vertical （a） and cross （b） terms
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图 3 垂向互相关CZZ与交叉项互相关CRZ的信噪比SNR对比
Figure 3. SNR comparison between the vertical-vertical correlation CZZ and the cross-term correlation CRZ
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图 4 基于垂向互相关CZZ （红色圆点）与交叉项互相关CRZ （绿色圆点）提取的瑞雷波频散曲线对比
Figure 4. The comparison of Rayleigh wave dispersion curves extracted from vertical-vertical correlation （red dots） and cross-term correlation （green dots）
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图 5 不同周期上的射线分布（三角形表示台站）
Figure 5. Distribution of ray paths for different periods （Triangles represent stations）
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图 6 交叉项互相关在不同周期上的相速度分辨率图像（a−h）和垂向互相关在12 s周期上的分辨率图像（i）
Figure 6. The phase velocity resolution maps of different periods based on cross-term correlation （a−h） and the resolution map based on vertical-vertical correlation for the period 12 s （i）
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