汶川<em> M</em><sub>S</sub>8.0地震前后远震PS转换波与P波到时差的变化特征

引用本文

宋春燕, 邵学钟, 马瑾. 汶川 M_S8.0地震前后远震PS转换波与P波到时差的变化特征 [J]. 地震地质, 2019,41(3): 726-742.
SONG Chun-yan, SHAO Xue-zhong, MA Jin. VARIATION OF THE TRAVEL TIME DIFFERENCE BETWEEN TELESEISMIC PS CONVERTED WAVE AND PP TRANSMITTED WAVE IN FOCAL REGION BEFORE AND AFTER WENCHUAN M_S8.0 EARTHQUAKE [J]. SEISMOLOGY AND GEOLOGY, 2019,41(3): 726-742. 复制到剪切板

DOI:10.3969/j.issn.0253-4967.2019.03.012
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汶川 M_S8.0地震前后远震PS转换波与P波到时差的变化特征

宋春燕^1,²⁾, 邵学钟²⁾, 马瑾²⁾

1)新疆维吾尔自治区地震局, 乌鲁木齐 830011

2)中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029

〔作者简介〕宋春燕, 女, 1978年生, 2017年于中国地震局地质研究所获固体地球物理专业博士学位, 高级工程师, 主要从事数字地震学和地震预测等方面的工作, E-mail: songchunyan1@163.com。

收稿日期: 2017-12-21

基金项目: 中国地震局地质研究所基本科研业务专项(IGCEA1815)、国家自然科学基金(41474051)和中国地震局地震科技星火计划项目(XH19050Y)共同资助

摘要

利用远震初至波穿越地壳、上地幔等速度界面时产生的一系列PS型透射转换波, 测定远震PS转换波与初至P波的到时差Δ t_PS= t_PS- t_P随时间的变化特征, 有可能监测孕震区转换界面以上有限的地层空间内介质物性的变化。选取2001—2012年作为研究时段, 通过四川地震台网的YZP和JJS 2个台站记录到的兴都库什、苏门答腊南部地区2组震中距变化<3°的远震数据, 得到2个台站在2008年5月12日汶川 M_S8.0地震前后记录的PS转换波与初至P波到时差Δ t_PS的变化特征。结果表明, 在2006年以前Δ t_PS有一个缓慢增大的过程; 汶川地震前约2a内Δ t_PS出现明显的降低过程, 最大降幅达0.2~0.3s, 超过测量误差4~5倍; 震前2~3个月低值有一定程度的回返。文中讨论了可能引起Δ t_PS变化的原因, 分析表明文中提出的远震转换波法在地震监测中有很好的应用前景, 值得进行进一步实验研究。

关键词: PS转换波与P波到时差; 汶川 M_S8.0地震; 地壳介质波速变化

中图分类号:P315.3⁺1 文献标志码:A 文章编号:0253-4967(2019)03-0726-17

VARIATION OF THE TRAVEL TIME DIFFERENCE BETWEEN TELESEISMIC PS CONVERTED WAVE AND PP TRANSMITTED WAVE IN FOCAL REGION BEFORE AND AFTER WENCHUAN M_S8.0 EARTHQUAKE

SONG Chun-yan^1,²⁾, SHAO Xue-zhong²⁾, MA Jin²⁾

1)Earthquake Agency of Xinjiang Uygur Autonomous Region, Urumqi 830011, China

2)State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China

Abstract

When P waves from distant earthquakes meet a velocity discontinuity in the earth's crust and upper mantle, they give rise to a series of converted PS waves besides PP refracted waves. It is possible to monitor the variation of the physical properties of the medium in the limited formation space above the transition zone of the seismogenic zone by measuring the time difference between the teleseismic PS converted wave and the first arrival P wave, that is, time-variation Δ t_PS= t_PS- t_P. The advantage of this method is that the transition point of the teleseismic source with similar source is relatively stable at the transition interface, and the accuracy of the measured relative time is high, and the change of the medium in a small range of the seismogenic zone above the conversion interface can be monitored.
This paper studies the variation of the travel time difference Δ t_PS in focal region before and after Wenchuan M_S8.0 earthquake. We select 2001 to 2012 as the research period, use teleseismic waveforms which occurred in the southern region of Sumatra and Hindu Kush area recorded by Sichuan station YZP and JJS. These teleseisms satisfy 5.0≤ M≤6.5, and their waveform signal-to-noise ratio is high with clear initial P-wave motion. The epicentral distance of teleseisms is less than 3 degrees. Then we obtain the variation of the travel time difference Δ t_PS between teleseismic PS converted wave and PP transmitted wave recorded during the study period in the two stations. The results show that there is a slow increasing trend of Δ t_PS before 2006, and an obvious low value process of Δ t_PS appeared in the period about 2 years before the Wenchuan earthquake. The maximum decline was about 0.2~0.3s, more than 4~5 times the measurement error. The low value has a certain degree of return about 2~3 months before the earthquake.
The change of arrival time difference indicates that the medium is in different states in different periods of seismogenic process. The sharp decrease in Δ t_PS from 2006 to January 2008 may be due to the strong disturbance caused by the stress accumulation of the medium. At this stage, the velocity of P wave and S wave increases with the increase of stress, and the increase of S wave velocity will result in the decrease of Δ t_PS. The change of Δ t_PS is greatly affected by S wave velocity, so Δ t_PS appears to decrease rapidly. Regarding the low value that has a certain degree of return about 2~3 months before the Wenchuan earthquake, the possible reason is that the release of stress is much higher than the accumulation of stress in meta-instability stage. At this stage, the velocity of S wave decreases and the decrease of S wave causes Δ t_PS to increase. Then, the Wenchuan earthquake of magnitude 8.0 occurred. It is shown that the teleseismic converted wave method in this paper can monitor the variation of medium's wave velocity before large earthquakes, and it has a good prospect in seismic monitoring and worth further experimental study.

Keyword: travel time difference between teleseismic PS converted wave and P wave; Wenchuan M_S8.0 earthquake; wave velocity variation of crustal medium

文章图片

0 引言

通过监测地震波速的变化了解地壳内部的应力积累情况从而预测地震, 是近代地震学研究早期就有的想法。国内外的地震学家采用各种地震波监测方法研究地震前后地壳介质的时变性质, 提出了测量波速、波速比变化(Aki et al., 1970; 李艳娥等, 2014, 2016)、剪切波分裂的变化(Crampin et al., 1990)、尾波Q值的变化(Jin et al., 1986)及利用重复的人工或天然震源研究波速变化(Reasenberg et al., 1974; Poupinet et al., 1984)等地震学方法, 取得了很多进展。

在使用天然地震直达波监测地震波速变化的方法中, 由于直达波的传播路径短, 震源参数测定的误差容易导致较大的波速测量误差。一些预期存在很大波速比变化的地震观测结果却给出了否定性的结论(McEvilly et al., 1974; Kanamori et al., 1975)。

利用重复地震, Poupinet等(1984)研究了美国加州地区Calaveras断层带的波速变化, 得到了一些较好的结果。近年来, 随着地震背景噪声成像技术的发展, 地震背景噪声也被作为重复震源用于地下介质变化的监测研究中(Shapiro et al., 2005; Yao et al., 2006; Bensen et al., 2007)。地震背景噪声可以弥补天然地震时空分布的局限性, 但是一方面通常需要叠加数月的噪声数据才能得到可靠的信号, 另一方面地震背景噪声源等因素的变化也可能影响对介质变化的测量(Zhan et al., 2013)。由于人工气枪震源等具有可重复性的优点, 可以很好地进行野外试验, 故被广泛用于监测地震波速变化的研究中, 所得速度变化的精度可达10^-3s(Reasenberg et al., 1974; 杨微等, 2010; 王宝善等, 2011, 2016)。人工震源在某种程度上克服了天然地震的不足, 但更大范围、更深部的波速变化尚未被检测出来。

本文提出利用远震PS转换波与P波到时差来监测地震前介质物性变化的方法。远震转换波测深法通常用于研究地壳、上地幔的构造(Kenneth et al., 1962; 邵学钟等, 1978, 1993, 2013; 刘启元等, 1985, 2005)。利用远震PS转换波与P波到时差随时间的变化来监测大震震源区附近地壳介质的动态变化, 在国外未见过类似研究, 国内仅进行过一些初步的试验研究。此方法的优点是震源相近的远震在转换界面上的转换点相对稳定不变, 测定相对到时的精度高, 能监测转换界面以上孕震区小范围内介质的变化情况。邵学钟等(2013)利用中国数字地震台网记录的远震转换波与P波到时差随时间的变化来研究及监测地壳介质的动态变化, 发现在1989年以前的2a内该到时差有0.1~0.2s左右的急剧下降, 这一异常被认为可能与1988年11月的云南澜沧-耿马地震的孕震过程有关。

2008年5月12日四川汶川发生M_S8.0地震, 造成了重大的人员伤亡和经济损失, 深入分析和研究震前的各种前兆异常现象, 探讨有效的地震预测方法具有重大的现实意义。国内外众多地学工作者对汶川地震给予了极大关注, 对其发震机理、破裂过程、地壳结构及形变、波速变化等方面都进行了研究, 取得了很多有意义的成果(张培震等, 2008; Chen et al., 2010; 刘志坤等, 2010; 王林瑛等, 2011; Froment et al., 2013; 李艳娥等, 2014, 2016)。本文以汶川M_S8.0地震为例, 研究地震前后由于介质变化引起的远震转换波到时差Δ t_PS的变化特征, 并讨论其在地震预测研究方面的应用前景。

1 方法及原理

假设介质中存在某些速度界面, P波从界面下侧入射, 透过界面时产生的PP型透射波(即初至P波)和PS透射转换波都将先后到达地表。PS波与原生P波的到时差Δ t_PS=t_PS-t_P可表示为

其中, H为转换界面深度; ${\overset{̅}{V}}_{P}$ 、 ${\overset{̅}{V}}_{S}$ 为转换界面以上介质中P波和S波的平均速度; C为P波射线参数。

利用固定地震台资料测定Δ t_PS随时间的变化时, 式(1)中转换界面深度H可看做常数。对于震中距Δ ≥ 30° 的远震, C $V_{P}^{2}$ ≪1, 故震中距的少量变化对Δ t_PS的影响很小。计算表明, 在远震情况下, 震中距变化5° ~10° 所引起的 Δ t_PS变化< 0.5% ~1.0%, 而震源深度对Δ t_PS的影响则更小。介质中 ${\overset{̅}{V}}_{P}$ 、 ${\overset{̅}{V}}_{S}$ 的变化对Δ t_PS有重要影响, 且 ${\overset{̅}{V}}_{S}$ 变化的影响比 ${\overset{̅}{V}}_{P}$ 变化的影响大得多, 前者约为后者的3倍(邵学钟等, 2013)。

在孕震区及其附近的一些固定测点上长期观测来自相对固定的远震震源区激发的地震波, 并测定各测点到时差Δ t_PS随时间的变化, 有可能实现对孕震区转换界面以上很有限的地层空间内介质的物理性质及波速变化的长期监测。

2 资料选取

本文主要研究汶川8.0级地震震源区及周边区域到时差Δ t_PS在大震前后的变化情况, 因此选取2001— 2012年的12a作为研究时段。在台站的选取上, 尽量选择靠近震源区且波形记录较好、信噪比高且在研究时段内地震记录较完整的台站。比较之后选择了四川数字地震台网的油榨坪台(YZP)和金鸡寺台(JJS)进行研究(图1), 2个台站位置分别为(30.86° N, 103.56° E)(YZP)和(31.00° N, 104.54° E)(JJS), 二者均位于断层下盘, 前者距主震震中约20km, 后者距主震震中约100km, 距余震带约50km。

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	图 1 文中选取的台站及汶川地震序列分布图Fig. 1 Distribution of selected stations and Wenchuan earthquake sequence.

远震事件的选择标准一般为: 1)30° ≤ Δ ≤ 90° , 且同组远震中最大震中距变化不超过2° ~3° ; 2)震源在地壳以下的上地幔中; 3)5.0≤ M≤ 6.5。在研究时段, 尤其是2006— 2008年间可选择地震发生较多的地区, 以便识别地震前后变化。最终选取兴都库什地区的中源地震和印尼苏门答腊南部地区的远震分别进行分析(图2), 同组远震震源参数的变化引起转换波到时差的测量误差 ≤ 0.5% 。在这种条件下, 超过测量误差的转换波到时差随时间的变化应能反映介质性质的动态变化。

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	图 2 文中选取的2组远震震中分布图Fig. 2 Distribution of selected teleseismic earthquakes.

3 数据处理

依据USGS地震目录, 选出信噪比较高且P波初动清晰的地震波形。所选择的波形中只有极个别的5.0级地震波形可用, 其它则信噪比较低, 震级> 5.3级的地震波形, 其初至P波才较为清晰; 在兴都库什地区选择的中源地震深度约200km, 深度差不超过50km。

对数字地震仪记录的三分量原始波形数据进行去均值、扣除仪器响应等预处理。由于地壳界面的埋深较浅, 即使自深度100km内的上地幔顶部界面传播而来的PS波, 其到时差≤ 10s, 因此本文使用P波前20s的数据进行分析。对P波分量进行频谱分析后, 对三分量记录进行周期为0.5~1.2s的低通滤波, 经处理后水平分量上转换波的信噪比明显提高。

为了消除P波对水平分量的干扰, 同时使转换波在水平分量上的投影达到最大, 对三分量记录进行了坐标轴旋转的变换。具体方法是: 先旋转SN方向坐标轴, 使之与P波入射面重合, 然后再旋转UD方向坐标轴, 使之与P波入射线方向重合, 从而由原始的SN、 EW、 UD三分量记录变为R、 T、 P(径向、切向和垂向)3个特征分量的记录。在P波射线坐标系下, P与P波出射射线方向重合; T垂直于P波入射面; R在P波入射平面内, 并与P波方向垂直。

YZP台的兴都库什和苏门答腊南部地区远震转换波P、 T、 R三分量记录如图 3所示, 在兴都库什地区挑选的中源地震深度约200km, P波初动尖锐, 符合要求的有24个地震。P波震相的振幅极大值对应横坐标0时刻, 纵坐标按远震事件发震时间排列, P、 T、 R 3个特征分量波形如图3a所示。转换波主要分布在R、 T分量上, 对应P波零振幅平面附近。远震转换波可分别在T分量和R分量上被识别出, 也可以同时在T、 R分量上识别出, 这取决于转换波的偏振方向。转换波震相最大振幅对应的时间坐标就是PS转换波与P波的到时差, 转换界面越深则转换波到时差越大。从图中可以看出, 初至P波有较好的相似性; 在T分量图中能识别出多组清晰的PS_H型转换波, 其中信噪比最高、最清晰的有2组, 其与初至P波的到时差约2.1s和6.03s; 在R分量图上可识别出多组优势的PS_V型转换波, 其中最为清楚、信噪比最高的只有1组, 其与初至P波的到时差约6.04s。

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	图 3 YZP台记录的来自兴都库什和印尼苏门答腊南部地区的远震转换波 a 兴都库什地区; b 印尼苏门答腊南部地区Fig. 3 Records of station YZP from southern Sumatra and the Hindu Kush teleseismic converted wave.

图3b为苏门答腊南部地区的远震转换波, 该地区震源深度在30~80km且P波初动尖锐的地震有30个。将P波震相的振幅极大值对应横坐标0时刻, 纵坐标按远震事件发震时间排列, P、 T、 R 3个特征分量波形如图3b所示。 T分量上最清楚的PS_H型转换波与初至P波的到时差约6.03s; R分量上与初至P波的到时差约6.04s的PS_V型转换波较为清晰, 其次还有与初至P波的到时相差约4.18s的转换波。

图 4为JJS台记录的兴都库什和苏门答腊南部地区远震转换波, 兴都库什地区符合要求的22个远震按发震时间顺序排列的P、 T、 R三分量波形如图4a所示。可以看出, 初至P波有较好的相似性。在T分量上能识别出2组清晰的PS_H型转换波, 其与初至P波的到时差约2.35s和5.8s; 对R分量上2组优势的PS_V型转换波进行分析, 发现其与初至P波的到时差约1.03s和5.81s。

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	图 4 JJS台记录的来自兴都库什和印尼苏门答腊南部地区的远震转换波 a 兴都库什地区; b 印尼苏门答腊南部地区Fig. 4 Records of station JJS from southern Sumatra and the Hindu Kush teleseismic converted wave.

苏门答腊南部地区符合要求的38个远震按发震时间顺序排列的P、 T、 R三分量波形如图4b所示。对T分量上2组清晰的PS_H型转换波进行分析, 其与初至P波的到时差约4.31s和5.69s; 对R分量上2组清晰的PS_V型转换波进行分析, 其与初至P波的到时差约5.63s和9.28s。

4 转换波到时差Δ t_PS的特征

在R、 T分量上可清楚看出, 汶川地震前Δ t_PS存在明显变化, 但各界面转换波之间也可能存在一定程度的相互干涉。为了更精确地测量转换波的到时差, 采用分离干涉波方法(邵学钟等, 2013), 得到各地震台站的Δ t_PS随时间的变化曲线。

4.1 YZP台远震转换波到时差Δ t_PS的特征

图 5和图 6分别是YZP台记录的兴都库什和苏门答腊南部地区远震PS转换波与初至P波到时差的变化曲线, 为了减少误差的影响, 对数据进行了三点滑动平均处理。选取的兴都库什地区的远震震中距YZP台约3 100km, 苏门答腊南部地区远震震中距YZP台约4 100km, 通过计算同组远震由震中距和震源深度的变化引起到时差的测量误差 ≤ 0.3% , 约0.02~0.04s。

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	图 5 YZP台记录的兴都库什地区远震在各界面Δ t_PS变化曲线Fig. 5 Curves of travel time difference variation for Hindu Kush earthquakes recorded at station YZP.

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	图 6 YZP台记录的苏门答腊地区远震在各界面Δ t_PS变化曲线Fig. 6 Curves of travel time difference variation for Sumatra earthquake recorded at station YZP.

2001— 2006年之间, 经过三点平均滑动处理后Δ t_PS的变化量不超过0.05s(图 5和图6), 进行滑动平均前的变化量也不超过0.1s。2006年底— 2008年2月Δ t_PS急剧降低, 最大降幅达0.2~0.3s, 超过测量误差的5倍, 故该异常是可信的, 其反映了地壳介质的变化。2009年之后Δ t_PS的变化量明显超过2006年之前, 最大幅度达0.1s, 可能与震后应力调整及余震的发生有关。

将YZP台记录的兴都库什地区远震在各界面的PS转换波与P波到时差曲线保持横坐标不变、按纵轴平移, 放入同一坐标系中进行比较。图 7是将YZP台兴都库什地区PS_H型与初至P波的到时差约2.1s和6.03s的转换波、 PS_V型与初至P波的到时差约6.04s的转换波沿纵轴平移后得到的结果。可以看出, Δ t_PS的异常变化曲线都有相似的形状, 且在2006年之前不同深度界面的到时差曲线有一个缓慢上升的过程, 最大增幅达0.06s; 2007年1月— 2008年1月到时差均出现明显的下降趋势, 下降幅度达0.2~0.3s, 自2008年2月开始到时差回返; 震后到时差处于波动状态, 最大变化幅度达0.11s。

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	图 7 YZP台兴都库什地区远震各界面Δ t_PS变化的比较Fig. 7 Comparison of curves of travel time difference variation for Hindu Kush earthquakes with each interface recorded by station YZP.

图 8为YZP台苏门答腊南部地区PS_V型与初至P波到时差约4.18s和6.04s的转换波、 PS_H型与初至P波到时差约6.03s的转换波沿纵轴平移后得到的结果, 图中曲线的形状相似。由图 8可以得出与兴都库什地区远震类似的结果, 不同深度转换波的到时差曲线在2006年之前有一个缓慢上升的过程, 最大增幅达0.05s; 2007年1月— 2008年1月到时差均出现急剧下降趋势, 最大降幅达0.2~0.3s, 自2008年3月开始到时差回返; 震后到时差处于波动状态, 最大变化幅度达0.1s。

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	图 8 YZP台记录的苏门答腊地区远震在各界面Δ t_PS变化的比较Fig. 8 Comparison of curves of travel time difference variation for Sumatra earthquakes with each interface recorded by station YZP.

YZP台2组远震资料均表明汶川地震前YZP台转换波到时差从2007年1月— 2008年2月的13个月内下降了0.2~0.3s, 2008年3月以后逐渐向均值附近恢复。下降量超过测量误差3~4倍以上, 故结果是可信的。

4.2 JJS台远震转换波到时差Δ t_PS的特征

图 9是JJS台记录的苏门答腊南部地区远震PS转换波与初至P波的到时差变化曲线及其经三点滑动平均处理后的结果。

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	图9 JJS台记录的兴都库什地区远震在各界面Δ t_PS变化曲线Fig. 9 Curves of travel time difference variation for Hindu Kush earthquakes recorded by station JJS.

图 10是JJS台记录的PS_H型与初至P波到时差约2.35s和5.81s的转换波、 PS_V型与初至P波到时差约4.32s和5.8s的转换波沿纵轴平移后得到的结果。可以发现其与YZP台类似, 不同深度的Δ t_PS曲线在2006年之前似乎有一个缓慢上升的过程, 最大增幅达0.06s; 2007年1月— 2008年1月到时差均出现急剧下降的趋势, 最大降幅达0.2~0.3s, 自2008年3月开始到时差回返; 震后到时差处于波动状态, 最大变化幅度为0.09s。

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	图10 JJS台记录的兴都库什地区远震在各界面Δ t_PS变化的比较Fig. 10 Comparison of curves of travel time difference variation for Hindu Kush earthquakes with each interface recorded by station JJS.

汶川地震前JJS台记录的兴都库什地区远震Δ t_PS急剧降低, 不同深度界面的降幅均超过测量误差的4~5倍, 故该异常是可信的, 这一过程与YZP台得到的结果一致。

图 11是JJS台记录的苏门答腊南部地区远震PS转换波与初至P波的到时差变化曲线及其经三点滑动平均处理后的结果。

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	图11 JJS台记录的苏门答腊地区远震在各界面Δ t_PS变化曲线Fig. 11 Curves of travel time difference variation for Sumatra earthquakes recorded by station JJS.

图 12是JJS台记录的PS_V型与初至P波到时差约5.63s和9.28s的转换波、 PS_H型与初至P波的到时差约4.31s和5.69s的转换波沿纵轴平移后得到的结果。可以看出, 不同深度的Δ t_PS曲线在2006年之前似乎有一个缓慢上升的过程, 最大增幅达0.06s; 2007年2月— 2008年2月到时差均出现急剧下降趋势, 最大降幅达0.2~0.3s, 自2008年3月开始到时差回返; 震后到时差处于波动状态, 变化最大幅度达0.12s。

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	图12 JJS台记录苏门答腊地区远震在各界面Δ t_PS变化的比较Fig. 12 Comparison of curves of travel time difference variation for Sumatra earthquakes with each interface recorded by station JJS.

将图7、 8、 10、 12的平均曲线绘制在一张图中(图13), 对比不同台站4组远震Δ t_PS的平均变化情况。从图中可看出4条曲线有相似的形态, 在2006年之前变化基本一致, 2006年之后分别出现了不同程度的低值变化, 最大变化量出现在2008年前后, 平均降幅约2.3s, 在2008年2月4条曲线均出现了回返。

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	图 13 YZP和JJS台分别来自兴都库什和苏门答腊地区的Δ t_PS平均变化量Fig. 13 Average variation of the travel time difference of converted wave recorded by station YZP and JJS.

5 讨论和结论

(1)四川数字地震台网的油榨坪台(YZP)和金鸡寺台(JJS)转换波资料的分析结果表明, 通过2个台站分别记录的2组远震得到的Δ t_PS随时间变化的平均曲线都有相似的变化特征, 充分说明异常的稳定性和可靠性。图 13代表了震前Δ t_PS变化的综合平均结果, 可以看出在2006年之前普遍出现Δ t_PS略微增加的现象, 但增加量不大, 仅能看出一个趋势。汶川地震前2a左右PS转换波的到时差Δ t_PS出现明显的减小过程, 震前约3个月低值有一定程度的回返。Δ t_PS下降幅度约0.2~0.3s, 超过测量误差4~5倍, 故这一负异常是可信的。地震后Δ t_PS有强烈的变动, 逐渐回至2001年左右的水平。

(2)Δ t_PS的变化曲线表明介质在孕震的不同时期具有不同状态。 2006年以前介质受到一定扰动, 使得Δ t_PS出现略微增大的趋势, 这种现象可有不同解释。根据前人的研究可知, 裂隙介质中水和流体的侵入可以引起 ${\overset{̅}{V}}_{P}$ 的增大和 ${\overset{̅}{V}}_{S}$ 的减少, 根据式(1)可知这2种因素都能引起Δ t_PS的增大。大量研究工作表明巴颜喀拉块体走向NE的边界断裂带表现出右旋走滑兼挤压的活动特征, 巴颜喀拉块体的E向运动存在自西向东的速度衰减, 其与四川盆地的相互作用造成壳幔物质的挤压并向上逆冲, 不排除侵入震源区断裂的可能。另外, 能量积累逐渐会聚到龙门山断裂带的中段, 震源区介质的物理性质也会发生变化(李勇等, 2006; Densmore et al., 2007; 张培震等, 2008; 江在森等, 2009; 李延兴等, 2009; 闻学泽等, 2011; 陈长云等, 2013)。流体的侵入会降低闭锁区断层的摩擦力, 有利于地震的发生。

Δ t_PS在2006年底— 2008年1月出现的大幅降低, 其原因可能是介质受应力积累的影响出现强烈扰动, 使得 ${\overset{̅}{V}}_{P}$ 、 ${\overset{̅}{V}}_{S}$ 增大。由式(1)可知, ${\overset{̅}{V}}_{P}$ 增大使Δ t_PS也相应增大, ${\overset{̅}{V}}_{S}$ 增大则会使Δ t_PS减小。但对于同样的变化量, ${\overset{̅}{V}}_{S}$ 的变化使得 Δ t_PS发生变化的量值要远大于 ${\overset{̅}{V}}_{P}$ 。由此可知, Δ t_PS的变化主要由 ${\overset{̅}{V}}_{S}$ 的变化而决定, 故Δ t_PS表现为快速减小。

Δ t_PS在汶川震前2~3个月有低值回返的现象, 其原因可能与马瑾等(2012, 2014)提出的临震前出现的介质亚失稳应力状态有关。在亚失稳准动态阶段, 应力的释放速率远高于应力的积累速率, 从而使介质的 ${\overset{̅}{V}}_{P}$ 和 ${\overset{̅}{V}}_{S}$ 出现一定程度降低, 对于同样的变化量, ${\overset{̅}{V}}_{S}$ 的变化引起Δ t_PS的变化比 ${\overset{̅}{V}}_{P}$ 大得多, ${\overset{̅}{V}}_{S}$ 降低会造成Δ t_PS增大回返。这个过程是不可逆的, 且将很快导致地震的发生。正确识别这一阶段对探索地震预测有重要意义。

(3)由式(1)可知, 到时差与转换界面深度、介质中P波、 S波速度及P波射线参数有关, 所选震源和震中距都较为接近, 震源时间函数的差异, 即入射初始P波波形的差异不会影响到时差, 信噪比的差异会对到时差产生一定影响。文中选取了转换波信噪比较高的记录, 信号水平是干扰背景的几倍, 有的记录在某些时段干扰较大, 但在有用信号的时段干扰较小, 则该记录仍然可用, 而在有用信号时段干扰确实较大的记录则被舍弃。与其它研究震前波速变化的方法相比, 利用远震PS转换波与P波到时差监测地壳介质动态变化的方法更便于实施, 受震源参数的影响较小, 测量的精度较高, 能直接监测转换界面以上孕震层的动态变化, 有较好的应用前景。该方法的局限性是需要挑选震源位置较为接近的远震记录(可选震中距相差5° 之内的远震)进行分析, 有时1a内能选出的有用远震不多, 限制了监测频次。考虑到本文中同一台站的不同组远震的Δ t_PS变化类似, 因此可多选几组远震同时进行监测以弥补样本量的不足, 从而可长期监测台站周围地壳介质的动态变化。

(4)在图 3和图 4中, 为了使PS波更清晰、直观, 对个别P波初动尖锐向下的波形的三分量同时进行了反向处理, 由于最终得到的结果是PS波和初至P波的到时差, 故该处理对结果没有影响。

(5)结果表明, 不同深度转换震相的到时差幅度相当, 并没有随着深度增加而发生明显变化。由公式可知, 到时差主要与介质的P波、 S波速度有关, 这说明浅部波速变化更为显著。范景辉等(2002)的实验研究表明, 岩石波速在低压区随压力升高迅速呈非线性增加, 随后则转变为近线性缓慢增加, 这主要源于岩石内部微裂隙闭合对波速的贡献。应力的增加使得壳幔物质挤压并向上逆冲, 侵入震源区断裂, 可能也是造成浅部波速变化的原因。

致谢本研究工作使用的主要软件由邵学钟研究员提供; 四川省地震局吴鹏为本研究的数据收集及处理提供了帮助。在此一并表示感谢!

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[20]	杨微, 葛洪魁, 王宝善, 等. 2010. 由精密控制人工震源观测到的绵竹5. 6级地震前后波速变化[J]. 地球物理学报, 53(5): 1149-1157. YANG Wei, GE Hong-kui, WANG Bao-shan, et al. 2010. Velocity changes observed by the precisely controlled active source for the Mianzhu M_S5. 6 earthquake[J]. Chinese Journal of Geophysics, 53(5): 1149-1157(in Chinese). [本文引用:1]
[21]	张培震, 徐锡伟, 闻学泽, 等. 2008. 2008年汶川8. 0级地震发震断裂的滑动速率、复发周期和构造成因[J]. 地球物理学报, 51(4): 1066-1073. ZHANG Pei-zhen, XU Xi-wei, WEN Xue-ze, et al. 2008. Slip rates and recurrence intervals of the Longmenshan active fault zone and tectonic implications for the mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China[J]. Chinese Journal of Geophysics, 51(4): 1066-1073(in Chinese). [本文引用:2]
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2013

0.0

陈长云, 任金卫, 孟国杰, 等. 2013. 巴颜喀拉块体东部活动块体的划分、形变特征及构造意义[J]. 地球物理学报, 56(12): 4125-4141.

CHEN

Chang-yun

, REN

Jin-wei

, MENG

Guo-jie

, et al. 2013. Division, deformation and tectonic implication of active blocks in the eastern segment of Bayan Har block[J]. Chinese Journal of Geophysics, 56(12): 4125-4141(in Chinese).

Based on the concept of active blocks, we analyze the major active faults and secondary faults, historical earthquakes, field geological investigation, and other data, then divide the eastern segment of Bayan Har block and its adjacent regions into four first grade blocks: Bayan Har block(I), South China block(II), Sichuan-Yunnan block(III), and West Qinling block(IV). According to GPS velocities, data of geophysical fields and geomorphological features, we further divide the Bayan Har block into three subblocks, Aba (I-1), Maerkang (I-2), and Longmenshan (I-3), and divide the Xiqinling block into two subblocks, Minxian (IV1) and Lixian (IV2), and then check the relative independence among those blocks by F-test. The modern translation and rotation rates of those blocks were calculated using the GPS data from the Crustal Motion Observation Network of China between 1999 and 2007. The results show that besides the eastward or southeastward translation the subblocks also rotate around the Euler pole; the slip rates for the eastern Kunlun fault, Ganzi-Yushu fault, and the Xianshuihe fault are much higher than that for the Longmenshan fault; the faults trending northwest or northwest-west show apparently left-lateral strike-slip and with a tensile characteristic, and those faults trending northeast show obviously right-lateral strike-slip and compression characteristics, except for the Chengxian-Taibai fault which shows left-lateral and compression characteristics. The eastward motion of Bayan Har block is accommodated mainly by the Longriba fault and Minjiang fault, and the right-lateral horizontal component of Longriba fault is obviously similar to 4.8+/-1.6 mm/a, and this may indicate that the eastward motion of Bayan Har block is discontinuous. The Longmenshan fault was divided into three segments by Maerkang subblock, Longmenshan subblock and Minxian subblock. The strain rate on the middle part of the Longmenshan fault is smaller than that on the southern part, and the deformation northwest of the fault shows that near the fault the strain rate becomes smaller, it maybe imply that high strain energy had accumulated before the earthquake, which promoted the surface rupture. The change from thrust motion to thrust and right strike-slip motion revealed by the surface ruptures and focal mechanism of aftershocks of Wenchuan earthquake may be related to the change of the stress on the middle part of the Longmenshan fault from southwest to northeast. The low velocity of Maerkang, Longmenshan, and Minxian subblocks relative to the South China block, the low strain rate and the tightly locked character of Longmenshan fault may determine the low slip rate of the Longmenshan fault.

Chen Chang-Yun 1,2 ;Su Jian-Feng 2 ;Ren Jin-Wei 3 ;Meng Guo-Jie 3 ;Yang Pan-Xin 3 ;Xiong Ren-Wei 3 ;Hu Chao-Zhong 3 ;Su Xiao-Ning 3 ;

基于活动块体的基本概念，综合对研究区内活动断裂带空间展布、地震活动性等资料的分析将巴颜喀拉块体东部及邻区划分为巴颜喀拉块体（I）、华南块体（Ⅱ）、川滇块体（Ⅲ）和西秦岭块体（IV）等4个一级块体.利用GPS形变场、地球物理场等资料结合F检验法，将巴颜喀拉块体划分为阿坝（I 1 ）、马尔康（I 2 ）和龙门山（I 3 ）3个次级块体，将西秦岭块体划分为岷县（IV 1 ）和礼县（IV 2 ） 2个次级块体.利用分布在各个块体内部的GPS测站，计算各活动块体及块体边界断裂带的运动变形特征.结果表明：各活动块体的整体运动包括平移和旋转运动；东昆仑断裂带、甘孜—玉树断裂带和鲜水河断裂带的滑动速率明显高于龙门山断裂带的滑动速率；巴颜喀拉块体东部走向北西或北西西的边界断裂表现出左旋拉张的特性；走向北东的边界断裂带，除成县—太白断裂带外，均表现出右旋走滑兼挤压的活动特征.巴颜喀拉块体的东向运动存在自西向东的速度衰减，衰减主要被龙日坝断裂带和岷江断裂带分解吸收，其中龙日坝断裂带的水平右旋分解非常明显，约为~4.8±1.6 mm/a，岷江断裂带的水平分解较弱.龙门山断裂带被马尔康、龙门山和岷县等次级块体分成南、中、北三段，龙门山断裂带中段上的主压应变率要明显小于龙门山断裂带南段上的应变率，其北西侧变形幅度从远离断裂带较大到靠近断裂带逐渐减小，表明其在震前已经积累了较高的应变能，有利于发生破裂滑动.汶川地震后，地表破裂带和余震分布揭示的断裂带运动性质自南西向北东由以逆冲运动为主，逐渐转为逆冲兼走滑的特征可能与龙门山断裂带中段所受主压应力方向自南西向北东的变化有关.马尔康、龙门山和岷县3个次级块体与华南块体之间较低的相对运动速度以及龙门山断裂带低应变率、强闭锁的特征都决定了汶川地震前龙门山断裂带低滑动速率的运动特征.

... 陈长云等, 2013) ...

2002

0.0

范景辉, 杨晓松. 2002. 岩石波速温度和压力系数的测量方法及其应用[J]. 地球物理学进展, 17(3): 525-532.

FAN

Jing-hui

, YANG

Xiao-song

. 2002. The measurements of rock elastic wave velocities at high temperature and high pressure and its implication for lithosphere seismic velocities[J]. Progress in Geophysics, 17(3): 525-532(in Chinese).

The measurements of rock elastic wave velocities at high temperature and high press provide us important constrains on the interpretation of seismic data. We review the methods for determine the partial derivatives of wave velocities with respect to pressure and temperature （（V/T） P and （V/P） T）, and intrinsic velocities （V 0） by experiments in situ, as well as some important factors （temperature, pressure, experimental flow） influencing the experimental results. Then we show the approaches to model elastic velocities of rocks corresponding with certain temperature and press condition and to compare them with velocity profiles observed by geophysical survey.

Laboratory of Tectonophysics, China Seismological Bureau, Institute of Geology, China Seismological Bureau, Beijing 100029, China

概述了高温高压实验技术在研究地球深部物质组成和结构等方面所起的作用,认为它是把地球物理模型同地球深部物质组成结合起来必不可少的一个中间环节.通过分析典型的试验结果,介绍了在实验室测定岩石波速温度系数(аV/аT)P、压力系数(аV/аP)T和岩石本征波速(V0)的方法,以及运用它们进行波速外推的原理.结合前人工作实例,阐明了将此技术与地质及地球物理研究结合,为地球物理测深资料的解释提供实验约束的研究思路.针对在前人工作中实验参数及实验结果存在差异的现象,着重探讨了实验条件和流程对实验结果的影响.认为,在用回归方法计算(аV/аP)T时,通常应在大于300 MPa的压力段上采集实验数据,以消除岩石内部先存微裂隙的影响;在计算(аV/аT)P时,为抑制热裂隙的产生,还应满足不小于1 MPa/℃的压力增量;另外,测定(аV/аT)P时,实验温度不宜太高,以避免引起岩石内部的矿物相变和物态变化.

... 范景辉等(2002)的实验研究表明, 岩石波速在低压区随压力升高迅速呈非线性增加, 随后则转变为近线性缓慢增加, 这主要源于岩石内部微裂隙闭合对波速的贡献 ...

2009

0.0

江在森, 方颖, 武艳强, 等. 2009. 汶川8. 0级地震前区域地壳运动与变形动态过程[J]. 地球物理学报, 52(2): 505-518.

JIANG

Zai-sen

, FANG

Ying

, WU

Yan-qiang

, et al. 2009. The dynamic process of regional crustal movement and deformation before Wenchuan M_S8. 0 earthquake[J]. Chinese Journal of Geophysics, 52(2): 505-518(in Chinese).

In this paper, the physical process of earthquake pregnant in the 10 years before Wenchuan 8.0 earthquake occurring in 2008 which reflected by regional crustal deformation is discussed, which includes the analysis of regional tectonic deformation, strain accumulation，the micro-dynamic process of crustal movement on large scale and co-seismic displacement fields. The Wenchuan 8.0 earthquake locates at the middle piece of Longman Mountain, which the horizontal relative movement is not obvious relative to steady South China block in the period of 1999～2004. From 2004 to 2007, small horizontal different movement (1.6 mm/a), especially dextral movement, shows in the middle piece of Longman Mountain. The east Bayan Har block locating at the northwest of Longman Mountain faults shows dextral shearing deformation and slowly pushing and squeezing deformation on large scale, which dextral deformation is about 2.1±0.2(10 -8 /a) and crustal shortening deformation is about 2.1±0.2(10 -8 /a). Based on the GPS geodesic time series, the relative compression movement in northeast direction on large scale enhances obviously from 2005, and the northeast direction movement of Qinghai-Tibet block relative to South China block enhance obviously too. Under the discussion to the special characteristic of strain accumulation before Wenchuan earthquake, we consider that the occurrence of this earthquake is the result of slowly crustal strain accumulation on large scale and in long period, which caused by the pushing and squeezing effect of Bayan Har block. Before earthquake, Longmen Mountain faults are in the locking status, and the earthquake pregnant area is in the deadlock stage which can not bring deformation caused by the elastic deformation limit. On the other hand, the crustal movement on large scale and the tectonic dynamic changing are beneficial to occurrence of Wenchuan earthquake.

1 Institute of Earthquake Science, CEA, Beijing 100036 2 Institute of Geology, CEA, Beijing 100029 3 Earthquake Administration of Sichuan Province, Chengdu 610000 4 Earthquake Administration of Hebei Province, Shijiazhuang 050021

利用GPS等观测资料研究了2008年汶川8.0级地震前的区域地壳运动与构造变形、应变积累，以及大区域地壳运动微动态变化过程，结合同震位移场分布等分析、讨论了汶川地震前近10年区域地壳变形的表象所反映的大震孕震最后阶段的物理过程.结果表明，发生汶川8.0级地震的龙门山断裂带1999～2004年相对稳定的华南地块无明显的相对运动，2004～2007年有一定程度的相对运动显示，以右旋活动为主，年速率为1.6mm/a.龙门山断裂带西北侧的巴颜喀拉地块东部为右旋剪切为主兼有推挤的大尺度缓慢变形状态，右旋扭动变形率为2.1±0.2(10 -8 /a)，地壳总体缩短变形率为-0.7±0.1(10 -8 /a).由GPS基准站资料计算的基线时间序列显示，2005年以来大尺度北东向地壳缩短的相对运动明显增强，青藏块体相对华南地块的北东向运动明显增强.对汶川大地震前应变积累的特殊性等问题进行了初步讨论，分析认为，汶川大地震的发生是四川盆地西缘的龙门山断裂带受到其西侧巴颜喀拉地块推挤导致大尺度、长时期、缓慢的地壳应变积累的结果.在发震前不仅龙门山断裂带为显著闭锁状态，且孕震区域可能存在地壳弹性变形趋于极限后难以发生变形的相持阶段，大尺度地壳运动与动力环境的动态变化对汶川大地震的发生有促进影响.

... 江在森等, 2009 ...

2016

0.0

... 李艳娥等, 2014, 2016)、剪切波分裂的变化(Crampin et al ...

... 李艳娥等, 2014, 2016) ...

2014

0.0

... 李艳娥等, 2014, 2016)、剪切波分裂的变化(Crampin et al ...

... 李艳娥等, 2014, 2016) ...

2009

0.0

李延兴, 张静华, 周伟, 等. 2009. 汶川 M_S8. 0地震孕育发生的机制与动力学问题[J]. 地球物理学报, 52(2): 519-530.

Yan-xing

, ZHANG

Jing-hua

, ZHOU

Wei

, et al. 2009. The mechanism and dynamics of the generation and occurrence for Wenchuan M_S8. 0 earthquake[J]. Chinese Journal of Geophysics, 52(2): 519-530(in Chinese).

On 12 May 2008, an M S 8.0 earthquake occurred in Wenchuan, Sichuan Province. The seismogenic fault is Yingxiu―Beichuan fault of the Longmenshan fault zone. Analysis of GPS velocity field before the earthquake discovers that the shortening velocity along the direction of about N107.5°E from west Bayan Har to the Longmenshan faults is 13.5 mm/a, the clockwise slipping velocity of the Longmenshan faults is 1.1 mm/a, the faults are locked. Sichuan Basin shows a little compressing deformation along the direction of about N107°E, and a little expanding deformation along the direction of SW. Co-earthquake displacement field shows that the earthquake may be caused by thrust of Bayan Har block along direction of SE and the downthrust of Sichuan Basin along the direction at NW occurred simultaneously. Analysis of the strain field discovers that the principal compressive and the principal tensile strain rates in the epicenter area are -30.840×10 -9 /a and 13.956×10 -9 /a respectively before earthquake, and the principal compressive strain axis is in a direction of N105.4°E which is consistent with the average direction at N107.5°E of the principal compressive stress axis from focal mechanism result. The earthquake fault trend and slipping direction obtained by the mechanism proposed in the paper are also consistent with the results of the focal mechanism. The interactions of the Indian, the Philippine Sea and the Pacific plates with the Eurasian plate are long-term acting force of accumulating elastic strain energy and generating Wenchuan great earthquake. Sumatra-Andaman great earthquake of Dec.2004 makes the interactions of Tibetan plateau and South China block to strengthen, which promoted occurrence of Wenchuan earthquake.

First Crust Monitoring and Application Center, CEA, Tianjin 300180, China

2008年5月12日四川省汶川县发生了 M S8.0强烈地震.发震断层是龙门山断裂带的映秀—北川断裂.分析震前的GPS速度场发现，从巴颜喀拉块体西部到龙门山断裂带沿大约N103°E方向的缩短速率为13.0 mm/a，龙门山断裂带的右旋走滑速率1.1 mm/a，断裂带处于闭锁状态.四川盆地沿大约N103°E方向有少量的压缩变形，而沿SW方向有少量的拉张变形.同震位移场显示，这次地震可能是巴颜喀拉块体SE向逆冲与四川盆地NW向俯冲同时发生的.应变场分析发现，震前震中区的主压与主张应变率分别为-30.840×10 -9 /a与13.956×10 -9 /a，主压应变轴N105.4°E与震源机制解得到的主压应力轴的方向N103°E一致.由本文提出的应力-应变机制得到的断层滑动方向和走向与地表破裂调查和震源机制解得到的结果一致.印度、太平洋和菲律宾海板块与欧洲板块的相互作用是龙门山断裂带积累弹性应变能和孕育汶川地震的长期作用力.苏门达腊大地震使青藏高原和华南块体的相互作用加强，促进了汶川地震的发生.

... 李延兴等, 2009 ...

2006

0.0

... 另外, 能量积累逐渐会聚到龙门山断裂带的中段, 震源区介质的物理性质也会发生变化(李勇等, 2006 ...

2005

0.0

... 刘启元等, 1985, 2005) ...

1985

0.0

刘启元, 邵学钟. 1985. 天然地震PS转换波的合成理论地震图研究[J]. 地球物理学报, 28: 291-302.

LIU

Qi-yuan

, SHAO

Xue-zhong

. 1985. Research on synthetic seismogram of PS converted wave in natural earthquake[J]. Chinese Journal of Geophysics, 28: 291-302(in Chinese).

In recent years, some important results have been obtained in the research of the crust and upper mantle structures by use of PS converted waves. In this paper the dynamic characteristics of the PS converted waves from a single discontinuity, thin layers and stack of thin layers are studied based on the generalized ray theory of the synthetic seismogram. The corresponding FORTEAN program was written by us. The effects of medium absorption for converted waves in the sedimentary layer and upper crust was discussed as well.The results indicate that the PS wave form generated at the single discontinuity, the thin layers and the stack of thin layers is similar to that of the primary P wave; the period of PS waves in the short-period seismogram is about 1.0 second which is related to the structures, but its variation is very small; the amplitude ratio of PS waves to P wave is 0.1-0.6 or more. The relative energy of PS waves increases in the region with sedimentary layers. The absorption of the medium has a strong effect on the PS wave's energy. Computational results in this study are roughly consistent with field observations. These provide a theoretical basis for identification of PS phases and interpretation of the data.

Institute of Geology, State Seismological Bureau

本文根据合成理论地震图的广义射线理论,并应用自行编写的FORTRAN计算程序,研究了单一界面、薄层和迭层界面上形成的PS转换波的动力学特征,讨论了沉积盖层及上部地壳介质吸收对转换波的影响。

... 刘启元等, 1985, 2005) ...

2010

0.0

刘志坤, 黄金莉. 2010. 利用背景噪声互相关研究化川地震震源区地震波速度变化[J]. 地球物理学报, 53(4): 853-863.

LIU

Zhi-kun

, HUANG

Jin-li

. 2010. Temporal changes of seismic velocity around the Wenchuan earthquake fault zone from ambient seismic noise correlation[J]. Chinese Journal of Geophysics, 53(4): 853-863(in Chinese).

We detected the variations of crustal seismic velocity around the M s 8.0 Wenchuan earthquake fault zone during March 2007-March 2009 from ambient noise cross-correlation by using continuous seismic data recorded by the broadband stations of Sichuan digital seismic network. The data processing procedure was divided into three phases: (1) single station data preparation including time domain normalization, spectral whitening, (2) cross-correlation and temporal stacking of 31-day daily cross-correlation functions (CCF), and (3) estimation of relative velocity change by measuring travel time shifts in frequency 0.1~0.5 Hz between the 31-day stacks and the reference empirical Green function, which was the stack of 14 months of CCF before the Wenchuan earthquake. The preliminary results revealed a sudden post-seismic velocity drop for the station pairs across the Longmenshan fault, the largest of which was more than 0.4%. The seismic velocity changes with time exhibited a spatial difference: in the southwest segment of the Longmenshan fault, the maximum value of velocity reduction occurred at the time immediately after the Wenchuan earthquake, whereas in the northeast segment of the fault velocity reduction appeared 1~4 months after the main earthquake. Furthermore, the spatial extent and magnitude of the post-seismic velocity change in the Sichuan Basin exceeded that in the mountainous regions west of the fault zone. The static stress change and near-surface physical damage caused by strong ground motion could not entirely explain the measurements in this study. We think the temporal changes of seismic velocity maybe related to the damages from fault ruptures and stress changes around the fault zones.

Institute of Earthquake Science，China Earthquake Administration，Beijing 100036，China

利用2007年3月至2009年3月四川数字地震台网的宽频带连续波形资料，通过计算地震背景噪声互相关提取台站对间的经验格林函数，在0.1～0.5 Hz频带下测量每天经验格林函数与参考经验格林函数的走时偏移,进而得到各台站对在该时段内的相对地震波速度变化.结果表明，2008年5月12日汶川 M s 8.0级地震造成了震源区地震波速度的急剧降低，最大降幅达0.4%；大致以安县为界，余震带西南部地区在汶川主震后波速降即达到最大值，而东北部地区的最大波速降一般出现在主震后的1~4个月，相对地震波速度变化的这种分段特性与地震序列的时空分布特征有较好的对应关系；在震源区外围的四川盆地也观测到了震后波速降低，而川西高原内部则没有出现显著的波速变化.进一步的分析和计算结果表明主震的静态应力变化和强地面运动引起的地表破坏都不能很好地解释震后波速的急剧降低，地震导致的断层区内部结构破坏和周边介质应力状态改变可能是波速变化的主要原因.

... 刘志坤等, 2010 ...

2014

0.0

马瑾, 郭彦双. 2014. 失稳前断层加速协同化的实验证据和地震实例[J]. 地震地质, 36(3): 547-561. doi: 103969/j. issn. 0253-4967. 2014. 03. 001.

Jin

, GUO

Yan-shuang

. 2014. Accelerated synergism prior to fault instability: Evidence from laboratory experiments and an earthquake case[J]. Seismology and Geology, 36(3): 547-561(in Chinese).

Identification of short-term and impending precursors, including the signal indicating earthquakes are inevitable, is one of focused issues in research of earthquake prediction. To explore this problem, modeling study of instability on a planar strike-slip fault was performed in the laboratory. It is based on the condition that the stress variation curves at a loading machine can reveal the stress state of the specimen and allow us to recognize its meta-instability stage. In terms of the advantage that the information from the loading machine can be compared with observations to the physical quantity of the sample, this work captures and analyzes the differences of temporal-spatial evolution processes of strain parallel and perpendicular to the fault in the stress linearity-off stage and meta-instability stage. The study suggests that a fault consists of relatively fragile and tough portions; the former usually are weakened first as expressed by pre-slip of the fault, and slow or small earthquakes indicative of beginning of strain release; while the latter become the locality of strain accumulation and fast instability finally, i.e. the future seismic source. The synergism process of a fault is actually a process of interaction between different portions of the fault. It is also a conversion from independent activities of each fault segment to synergism activity. The degree of synergism is an indicator of the stress state. It is based on the assumption that an earthquake results from sudden fast slip on a fault which relies on two primary conditions: one is the fault has a high synergism degree which facilitates connection between fault segments resulting in rapid slip of longer fault segments, and the other is sufficient strain is accumulated at some portions of the fault to overcome resistance of local tough portions on the fault. Usually the synergism process of a fault includes three stages: generation of strain-release patches, expansion and increasing of these patches, and mutual connection of strain-release areas. The first stage occurs when the stress curve deviates from linearity, strain variations of different portions of the fault begin to diverge, resulting in isolated patches of strain release and strain accumulation along the fault. In the second stage, related with the quasi-static instability of early meta-instability, those isolated areas of strain release increase and extend steadily. The third stage is equivalent to the late meta-instability that is a quasi-dynamic instability process when the sections of strain release on the fault accelerate to expand and strain levels of strain-accumulation areas accelerate to rise. The accelerated synergism begins at the time when the quasi-static state transforms into the quasi-dynamic state, of which the expansion mechanism of strain release segments changes, i.e. the expansion of isolated fault segments is replaced by connection between fault segments under interaction. At this time the fault is in a critical state and bound to generate earthquakes sooner or later. As a case study, based on the experimental results above and coupled with temporal-spatial evolution of earthquakes on the Laohushan-Maomaoshan Fault west of the Haiyuan Fault zone, this work analyzes the synergism process of the fault before the M 6.2 earthquake on 6 June 2000 in this region.

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China

地震是断层的快速错动，它有2个主要条件：一是断层协同化程度较高，一旦应力条件达到，能够迅速连接造成较长断层段的快速错动；二是断层上一些部位积累了足够高的应变，能够克服局部高强部位的错动阻力。地震短临前兆和必震信息识别是地震预报研究的焦点问题之一，为此开展了实验室平直断层失稳模拟研究。从应力变化曲线上可以判定标本所处的应力状态和识别亚失稳应力阶段。利用在实验室便于对压机信息和场上物理量的观测信息进行对比的优势，捕捉和比较应力偏离线性阶段和亚失稳阶段平行断层应变的时空演化过程中的差别。研究表明，由于断层上不同部位相互作用，断层各部位由独立活动逐渐转变为协同化活动，而断层活动协同化程度是判定断层所处应力状态的一个标志。断层活动协同化过程一般包括应变释放点产生、释放点的扩展和增多以及释放段之间相互连接3个阶段：第1阶段发生在偏离线性阶段，断层上不同部位的应变变化开始分化，出现孤立的应变释放区和积累区；第2阶段应变释放区的平稳扩展与亚失稳前期准静态失稳有关，断层上孤立应变释放区增多，并出现稳态扩展；第3阶段相当于亚失稳的后期，即准动态失稳阶段，断层上的应变释放区加速扩展，积累区应变水平加速提高。加速协同化开始于由准静态扩展向准动态扩展的转化，加速的机制是断层段间出现强相互作用。转化的本质在于扩展机制发生了变化，即由孤立断层段的扩展转变为断层段间在相互作用下的连接，这时断层进入发生地震的临界状态。根据实验结果，结合海原断裂带西侧的老虎山-毛毛山断裂地震活动的时空演化，分析了2000年6月6日 M L 6.2地震前该断层的协同化过程。

... t_PS在汶川震前2~3个月有低值回返的现象, 其原因可能与马瑾等(2012, 2014)提出的临震前出现的介质亚失稳应力状态有关 ...

2012

0.0

... t_PS在汶川震前2~3个月有低值回返的现象, 其原因可能与马瑾等(2012, 2014)提出的临震前出现的介质亚失稳应力状态有关 ...

2013

0.0

... 邵学钟等, 1978, 1993, 2013 ...

... 邵学钟等(2013)利用中国数字地震台网记录的远震转换波与P波到时差随时间的变化来研究及监测地壳介质的动态变化, 发现在1989年以前的2a内该到时差有0 ...

... t_PS有重要影响, 且 V̅S变化的影响比 V̅P变化的影响大得多, 前者约为后者的3倍(邵学钟等, 2013) ...

... 为了更精确地测量转换波的到时差, 采用分离干涉波方法(邵学钟等, 2013), 得到各地震台站的#cod#x00394 ...

1978

0.0

邵学钟, 张家茹, 杨小峰, 等. 1978. 利用地震转换波探测地壳、上地幔构造试验[J]. 地球物理学报, 21(2): 89-100.

SHAO

Xue-zhong

, ZHANG

Jia-ru

, YANG

Xiao-feng

, et al. 1978. An experimental study of the structure of the earth's crust and upper mantle by converted waves[J]. Chinese Journal of Geophysics, 21(2), 89-100(in Chinese).

When P waves from distant earthquakes meet a velocity discontinuity in the earth's crust and upper mantle, they give rise to a series of converted PS waves besides PP refracted waves. The difference of arrival times between such PS and PP waves can be used to determine the structures in the earth's crust and upper mantle.For the first time, method converted waves from distant earthquakes have been tried in the Shiahjin-Gautarng （夏津-高唐） region of western Shandung, Province and in the Changpirng （昌平） area west of Peking.Polarization filtering was employed to analyze the seismic data in order to increase the reliability in identifying the PS phases. As a result, the crustal structures in these two regions have been found and it is indicated that the main deep discontinuities thus obtained are essentially the same as those determined by deep seismic sounding experiments using explosions.

Institute of Geology, National of Seismological Bureau

当远震P波通过地壳、上地幔速度界面时,除了产生PP型折射波,还产生一系列PS型折射转换波.这类PS转换波与初至P波之间的到时差,可用来测定地壳和上地幔构造. 转换波测深法首次在我国鲁西夏津—高唐地区和京西昌平地区进行了试验应用.为提高转换波震相识别的可靠性,在资料分析中采用了偏振滤波的方法.工作结果获得了上述两地区的深部构造剖面图.转换波法与爆炸地震法所得结果之对比表明,两者的主要界面基本一致.

... 邵学钟等, 1978, 1993, 2013 ...

1993

0.0

邵学钟, 张家茹, 章思亚, 等. 1993. 邢台地震区深部构造背景的地震转换波探测和研究[J]. 地球物理学报, 36(5): 609-620.

SHAO

Xue-zhong

, ZHANG

Jia-ru

, ZHANG

Si-ya

, et al. 1993. The exploration and study of seismic converted waves in the deep tectonic setting of the Xingtai seismic area[J]. Chinese Journal of Geophysics, 36(5): 609-620(in Chinese).

During the period of 1982-1986 three seismological profiles with NW orientation going through Dongwang,Longyao and Baichikou strong earthquake source zones in Xingtai earthquake area have been conducted using the method of converted waves of earthquakes.The field observed data were interpreted with new packet of programs we have created in recent years for processing the data of converted waves.The phase correlation and tracing of converted waves were realized.As a result some more detailed cross-sections of deep structure and velocity models were constructed for the earthquake source zones.It has been found that the deep structures of earthquake source zones are characterized by the strong deformation of upper crust,the local thinening of middle crust,the local uplift of lower crust and upper mantle,the present of deep-sited faults,the local thickening of lower velocity layer in the lower crust with increased velocity ratio of P and S waves,and the decrease of velocity value in the upper mantle.It is suggested that the temparature in lower crust and upper mantle may be relatively high,and the occurrance of the earthquakes may be related with the intrusion of mantle material into the crust.

Institute of Geology, State Seismological Bureau, Beijing 100029

1982—1986年期间,在东汪、隆尧和百尺口邢台地区的强震震源区完成了3条近NW向地震转换波测深剖面。利用转换波数据处理新方法,实现了转换震相的相位对比追踪,得出了震源区较详细的深部构造剖面图和速度结构模型。发现强震震源区深部构造的特点为:上地壳强烈褶皱变形;中地壳急剧减薄;下地壳和上地幔局部上隆;存在一组深部超壳断裂;低速层急剧增厚,波速比增大以及上地幔波速偏低。表明研究区的下地壳和上地幔的温度可能偏高,其地震活动可能与上地幔物质的侵入作用有关。

... 邵学钟等, 1978, 1993, 2013 ...

2016

0.0

... 王宝善等, 2011, 2016) ...

2011

0.0

王宝善, 王伟涛, 葛洪魁, 等. 2011. 人工震源地下介质变化动态监测[J]. 地球科学进展, 26(3): 249-256.

WANG

Bao-shan

, WANG

Wei-tao

, GE

Hong-kui

, et al. 2011. Monitoring subsurface changes with active sources[J]. Advance in Earth Sciences, 26(3): 249-256(in Chinese).

To provide images of Earth interior and their temporal variations are main tasks for geophysicists. Comparing to our understanding of the static structure, we are still lacking in the understanding of the temporal variations of the subsurface. Recently, with the accumulation of observation data, repeated earthquakes and ambient noises are used to monitor subsurface changes. The resolutions and precisions of passive source monitoring are limited by the spatial and temporal distributions of sources. Monitoring subsurface changes with seismic wave generated by artificial sources naturally becames another goal to pursue. We systematically describe recent advances in active source monitoring at various scales. In this review, we also introduce cross-correlation based time delay estimation and its precision limitation. The precision of time delay estimation is limited by the repeatability of source, signal-to-noise ratio and bandwidth of recorded signals. And seismic sources with large capacity and high repeatability are preferred for active monitoring. Advantages and disadvantages of different sources are compared in this review. According to the comparison, we suggest that the airgun source is superior to other sources for the purpose of active monitoring. Techniques as waveform stacking, coda wave interferometry and their application in active monitoring are also introduced. Waveform stacking can be used to enhance the signal-to-noise ratio and facilitate the phase identification. Coda wave travels longer and is more sensitive to subsurface changes than direct waves. Therefore, coda wave interferometry technique can be used to detect subtle variations. With the development of techniques for signal excitation, recording and processing, it is now possible to conduct large scale active subsurface monitoring. Relevant researches have broad potential applications to such fields as natural disaster mitigation and resource exploration.

1.Institute of Geophysics, China Earthquake Administration， Beijing100081， China; 2. Earthquake Administration of Beijing，Beijing100080，China; 3. Earthquake Administration of Yunnan Province，Kunming650024, China

成像地球内部介质结构及其动态变化是地球物理学家的主要任务。相对于对地球介质静态结构的认识，对于地下介质性质随时间变化的了解明显不足。近年来随着观测资料的增加，已经开始利用重复地震和噪声等震源研究地下介质变化。受到源位置和时间等因素制约，基于地震和噪声等被动震源的观测结果的时间和空间分辨率有限。利用人工震源主动向地下发射地震波，进行地下介质监测自然成为一个重要的发展方向。系统地介绍了国内外利用人工震源进行不同尺度地下介质变化监测的研究,分析了不同震源的优缺点。根据分析和对比结果，认为在进行地下介质变化研究方面，气枪震源要优于其他震源。还介绍了相关的数据处理方法，包括互相关延时估计，尾波干涉和信号叠加等。目前用于大尺度地下动态监测的震源，接收和数据处理等主要技术已经基本发展成熟，利用人工震源进行地下介质大尺度监测成为可能，相关研究在减轻自然灾害和资源探查等领域具有广阔的应用前景。

... 王宝善等, 2011, 2016) ...

2011

0.0

... 王林瑛等, 2011 ...

2011

0.0

闻学泽, 杜方, 张培震, 等. 2011. 巴颜喀拉块体北和东边界大地震序列的关联性与2008年汶川地震[J]. 地球物理学报, 54(3): 706-716. doi: 103969/j. issn. 0001-573. 2011. 03. 010.

WEN

Xue-ze

, DU

Fang

, ZHANG

Pei-zhen

, et al. 2011. Correlation of major earthquake sequences on the northern and eastern boundaries of the Bayan Har block, and its relation to the 2008 Wenchuan earthquake[J]. Chinese Journal of Geophysics, 54(3): 706-716(in Chinese).

The 2008 Wenchuan earthquake occurred on the eastern boundary of the Bayan Har block. To understand better the regional dynamic environment and its relation to the earthquake, this research studies the kinematics and deformation of the fault systems of the northern and eastern boundaries of the block, as well as the correlation of occurrences of major earthquake sequences on the two boundaries, based on analyzing data from active tectonics, focal mechanism, GPS station velocity, earthquake rupture distribution and seismicity of historical major earthquakes. The result shows that, owing to the resistance of the South China block, the east- to southeast ward "extruding" movement of the Bayan Hat block, which is manifested by the active left-lateral strike-slip faulting along the northern boundary of the block, is transformed into thrust faulting and lateral shortening with strike-slip faulting on the eastern boundary of the block. This suggests that the "extruding" movement of the block has been "loading on" the eastern boundary remarkably. Such loading is also presented in the correlation of occurrences of major earthquake sequences on the two fault systems of the northern and eastern boundaries of the block. From the middle to late 19th century to 2001, a sequence of 8 major events ruptured about(2) /(3) of the whole length of the northern boundary of the Bayan Har block. At the same time, the sequence has the characteristics of accelerating strain release and gradually shortening intervals between events, suggesting that the movement of the block has been speeding up since 1902 at the latest. As a response to this speeding-up movement, a sequence of 3 major events have occurred on the eastern boundary of the block since about 1933, which has also the characteristics of accelerating strain release and gradually shortening intervals between events. The 2008 Wenchuan earthquake is the most-recent event in the response sequence. The factor that the occurrence of the response sequence of major earthquakes on the eastern boundary is delayed for decades at least behind the sequence on the northern boundary would be associated with the non-rigid motion of the block. In addition, it is found that the event's order-number versus time relationships of the two sequences of major earthquakes are helpful in analyzing the mid- and long-term seismic potentials for the northern and eastern boundaries of the Bayan Hat block.

[Wen Xue-Ze] China Earthquake Adm, Inst Earthquake Sci, Beijing 100036, Peoples R China.;[Wen Xue-Ze; Du Fang; Long Feng] Earthquake Adm Sichuan Prov, Chengdu 610041, Peoples R China.;[Zhang Pei-Zhen] China Earthquake Adm, Inst Geol, State Key Lab Earthquake Dynam, Beijing 100029, Peoples R China.

2008年汶川地震发生在巴颜喀拉块体的东边界.为了探讨区域动力学背景与该地震发生的关系,本文基于活动构造、震源机制解、GPS站速度、地震破裂展布以及历史大地震活动等资料分析巴颜喀拉块体北、东两个边界断裂系统的运动、变形以及大地震序列发生的关联性.结果表明:由于受到华南地块的阻挡,巴颜喀拉块体朝东-南东方向的 "逃逸"运动—表现为该块体北边界的左旋走滑断层作用—在该块体的东边界转换成为逆冲或缩短兼走滑断层作用,亦即该块体的运动对其东边界断裂系统具有显著的"加载"作用.这种加载作用也反映在该块体北和东两个边界大地震序列发生的关联性上.19世纪中-晚期至2001年期间,一个由8次事件组成的大地震序列使巴颜喀拉块体北边界总长度的2/3破裂,该序列同时具有加速应变释放以及事件的间隔逐渐缩短的特征,反映最晚自1902年以来该块体发生了加速运动.作为对这一加速运动的响应,巴颜喀拉块体东边界大约自1933年以来发生一个由3次事件组成的大地震序列,也显示出加速释放以及事件的间隔逐渐缩短的特征;2008年汶川地震正是该响应序列中的最新事件.东边界的响应型大地震序列的发生至少滞后于北边界的数十年,可能与该块体的非刚性运动有关.另外,这两个大地震序列的事件序次-时间关系有助于分析该块体北与东边界的中-长期地震危险性.

... 闻学泽等, 2011 ...

2010

0.0

杨微, 葛洪魁, 王宝善, 等. 2010. 由精密控制人工震源观测到的绵竹5. 6级地震前后波速变化[J]. 地球物理学报, 53(5): 1149-1157.

YANG

Wei

, GE

Hong-kui

, WANG

Bao-shan

, et al. 2010. Velocity changes observed by the precisely controlled active source for the Mianzhu M_S5. 6 earthquake[J]. Chinese Journal of Geophysics, 53(5): 1149-1157(in Chinese).

The stress status change and its exact measurement of the fault zone has been a hotspot in geophysics. After the Wenchuan earthquake, we carry out continuous monitoring across the range-front fault of the Longmenshan fault zone, using a 10-ton precisely controlled active source. An M s 5.6 earthquake happened at 02∶03 in the morning of June 30th 2009 near the experimental site. and the direct S wave velocity across the fault zone occurred a slight change by comparative analysis of data before and after the earthquake. Time delay is up to 5~9 ms, the S wave velocity decreased ~0.3%. The time delay was mainly caused by the stress change within the fault zone due to the earthquake.

Institute of Geophysics, China Earthquake Administration, Beijing 100081, China

地震危险区断层的应力状态变化及其精确测量一直是地球物理学关注的热点问题之一，主动震源探测技术的发展为解决上述问题带来了可能.汶川大地震后，结合汶川地震断裂带科学钻探，在四川省绵竹市九龙镇——跨越龙门山断裂带前山断裂，利用10 t精密控制人工震源进行了连续监测试验，在试验观测期间的2009年6月30日凌晨2时3分，当地发生了 M s 5.6级地震，地震前后观测数据的对比分析表明，该地震发生前后，穿过断裂带的直达S波走时发生了时延为5~9 ms的微弱变化，射线路径上的平均波速相对下降了约0.3%，该时延主要是由地震同震效应引起的断裂带内应力调整和物性变化造成的.所使用的精密控制人工震源性能稳定，适于开展断层监测.这一结果对于发展断裂带主动震源动态监测技术具有重要意义.

... 杨微等, 2010 ...

2008

0.0

张培震, 徐锡伟, 闻学泽, 等. 2008. 2008年汶川8. 0级地震发震断裂的滑动速率、复发周期和构造成因[J]. 地球物理学报, 51(4): 1066-1073.

ZHANG

Pei-zhen

, XU

Xi-wei

, WEN

Xue-ze

, et al. 2008. Slip rates and recurrence intervals of the Longmenshan active fault zone and tectonic implications for the mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China[J]. Chinese Journal of Geophysics, 51(4): 1066-1073(in Chinese).

The great Wenchuan earthquake of May 12, 2008 occurs on the Longmen Shan fault zone which forms a prominent section of the seismically very active seismic belt called the North-South Trending Seismic Belt by Chinese seismologists. There have been 3 earthquakes with magnitudes from 6 to 6 1 / 2 along the Longmen Shan fault zone during more then 2000 year documented history of the Sichuan province. Previous active faulting studies indicate slow (less than 3 mm/yr ) slip rate across the Longmen Shan fault zone. Why such a strong earthquake occurred in the Longmen Shan fault zone? What are the characteristics of the May 12 earthquake? What is the tectonic mechanism of the earthquake? On the basis of geological studies of the earthquake surface rupture zone and pre-earthquake GPS measurements in the region, we try to understand the questions mentioned above. The May 12 earthquake, 2008 is caused by displacement along the Yingxiu-Beichuan fault along which a more than 200 km long surface ruptures was formed. Another strand of the Longmen Shan fault zone, Guanxian-Jiangyou fault also ruptured as indicated by more then 60 km long surface ruptures. GPS measurements before the earthquake suggest that slip rate across the entire Longmen Shan fault zone does not excess about 2 mm/yr, and does not excess 1 mm/yr across individual fault strand. These data agree with seismogeological studies and historical seismicity of the Sichuan province. Using maximum coseismic displacement obtained from seismogeological studies and inversions from seismic waves, the recurrence intervals of great earthquakes, such as the 2008 Wenchuan earthquake, can be estimated to be 2000 ~6000 years. The Longmen Shan fault zone has a high dipping angle (more than 50°~60°) near surface and low-angle at depth (15~20 km). This kind of listric shape favors significant strain or energy accumulation to form great earthquake. The May 12 Wenchuan earthquake, 2008 is characterized by slow strain accumulation, long recurrence interval, and significant damage power. It is a new type of earthquake event that deserves further studies.

1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;2 Sichuan Seismological Bureau, Chengdu 610041, China

2008年5.12汶川大地震发生在中国大陆南北地震带中段.由于龙门山断裂带历史上只发生过3次6～6 1 / 2 级强震，而且其晚第四纪构造活动速率很低，以至于对其潜在地震危险性认识不足.为什么在龙门山地区突发大地震，该地震具有哪些特征？其成因机制是什么？本文在地震地质科学考察的基础上，利用震前的GPS观测结果，试图对上述问题进行一些初步的思考和探讨.结果表明，5.12汶川大地震是龙门山断裂带的映秀—北川断裂突发错动的结果，地表上形成200多公里长的地表破裂带；灌县—江油断裂在地震中也发生了破裂，形成的地表破裂带长达60多公里.震前的GPS观测表明，横跨整个龙门山断裂带的滑动速率不超过～2 mm/yr，单条断裂的活动速率不超过～1 mm/yr，与地震地质研究结果和历史地震记录相一致.利用地震地质考察和地震波反演得到的最大同震位移可以获得相当于5.12汶川大地震的强震复发周期为2000～6000年.龙门山断裂带发育在破裂强度很大的变质杂岩体中，断裂带本身在剖面上呈“犁形”或“铲形”结构，有利于能量积累，形成破坏性巨大的地震.所以，5.12汶川大地震是一次低滑动速率、长复发周期和高破坏强度的巨大地震，是一种值得高度重视和深入研究的新的地震类型.

... 国内外众多地学工作者对汶川地震给予了极大关注, 对其发震机理、破裂过程、地壳结构及形变、波速变化等方面都进行了研究, 取得了很多有意义的成果(张培震等, 2008 ...

... 张培震等, 2008 ...

1970

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... 国内外的地震学家采用各种地震波监测方法研究地震前后地壳介质的时变性质, 提出了测量波速、波速比变化(Aki et al ...

2007

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... Bensen et al ...

2010

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... Chen et al ...

1990

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... 李艳娥等, 2014, 2016)、剪切波分裂的变化(Crampin et al ...

2007

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2013

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1986

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1975

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1962

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... 远震转换波测深法通常用于研究地壳、上地幔的构造(Kenneth et al ...

1974

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... 一些预期存在很大波速比变化的地震观测结果却给出了否定性的结论(McEvilly et al ...

1984

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... Poupinet et al ...

... 利用重复地震, Poupinet等(1984)研究了美国加州地区Calaveras断层带的波速变化, 得到了一些较好的结果 ...

1974

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... , 1986)及利用重复的人工或天然震源研究波速变化(Reasenberg et al ...

... 由于人工气枪震源等具有可重复性的优点, 可以很好地进行野外试验, 故被广泛用于监测地震波速变化的研究中, 所得速度变化的精度可达10^-3s(Reasenberg et al ...

2005

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... 近年来, 随着地震背景噪声成像技术的发展, 地震背景噪声也被作为重复震源用于地下介质变化的监测研究中(Shapiro et al ...

2006

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2013

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... 地震背景噪声可以弥补天然地震时空分布的局限性, 但是一方面通常需要叠加数月的噪声数据才能得到可靠的信号, 另一方面地震背景噪声源等因素的变化也可能影响对介质变化的测量(Zhan et al ...