山东庙岛群岛地区P波三维速度结构反演与2017年震群的发震构造分析

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李霞, 陈时军, 张正帅, 戴宗辉, 李小晗, 卢仲斌. 山东庙岛群岛地区P波三维速度结构反演与2017年震群的发震构造分析[J]. 地震地质, 2020,42(5): 1188-1204.
LI Xia, CHEN Shi-jun, ZHANG Zheng-shuai, DAI Zong-hui, LI Xiao-han, LU Zhong-bin. INVERSION OF P-WAVE THREE-DIMENSIONAL VELOCITY STRUCTURE AND ANALYSIS OF SEISMOGENIC STRUCTURE OF EARTHQUAKE SWARM IN 2017 IN MIAODAO ISLANDS, SHANDONG PROVINCE[J]. SEISMOLOGY AND GEOLOGY, 2020,42(5): 1188-1204. 复制到剪切板

DOI:10.3969/j.issn.0253-4967.2020.05.010
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山东庙岛群岛地区P波三维速度结构反演与2017年震群的发震构造分析

李霞, 陈时军, 张正帅, 戴宗辉, 李小晗, 卢仲斌

山东省地震局, 济南 250014

〔作者简介〕李霞, 女, 1975年生, 高级工程师, 2005年于山东大学获计算机科学与技术专业工学学士学位, 主要从事数字地震学应用及地震综合预测研究, 电话: 0531-58622041, E-mail: cyfm@163.com。

收稿日期: 2020-01-09 修回日期: 2020-03-05

基金项目: 中国地震局地震科技星火计划项目(XH19027)和山东省地震局重点基金(JJ1701)共同资助

摘要

2017年3月3日、 9月2日山东庙岛群岛地区发生2次显著震群, 短时间内记录到2 000余次地震, 其中 M_L4.0以上地震4次, 是1970年以来该区域最强的地震事件。文中揭示了庙岛群岛地区地壳的深部速度结构特征, 探讨了震群发生的孕震环境。利用2008年以来庙岛群岛及附近地区的地震观测资料, 采用双差层析成像方法反演获得了庙岛群岛地区的P波三维速度结构及地震精定位结果。水平速度结构显示, 庙岛群岛地区地壳的中上层P波速度结构横向不均匀性明显, 胶东半岛北部海陆交会地区5km和16km深度层的介质速度高于北侧海槽水道区域, 同时研究区10km和13km深度层的速度整体偏高, 结构较稳定。速度结构剖面显示, 大竹山岛震群发生在倾角较陡的2个低速体的夹层间, 北长山岛震群则发生在小体积高速体边缘。结合研究区断层活动、区域应力累积水平及地震活动特征, 分析认为这2次震群活动主要是在区域应力增强(调整)的背景下, 局部介质的不均匀性和区域断层的低应力摩擦引发的能量释放。

关键词: 庙岛群岛; 双差层析成像; P波三维速度结构; 地震精定位

中图分类号:P315.2 文献标志码:A 文章编号:0253-4967(2020)05-1188-17

INVERSION OF P-WAVE THREE-DIMENSIONAL VELOCITY STRUCTURE AND ANALYSIS OF SEISMOGENIC STRUCTURE OF EARTHQUAKE SWARM IN 2017 IN MIAODAO ISLANDS, SHANDONG PROVINCE

LI Xia, CHEN Shi-jun, ZHANG Zheng-shuai, DAI Zong-hui, LI Xiao-han, LU Zhong-bin

Shandong Earthquake Agency, Jinan 250014, China

Abstract

On March 3, 2017, an earthquake swarm of M_L 4.5 occurred near Dazhushan Island in Miaodao archipelago, Shandong Province, as of December 31, 2017, 2 453 aftershocks were recorded, including 46 earthquakes of M_L3 and above and 4 earthquakes of M_L4 and above. It is the most active and frequent earthquake swarm activity in Miaodao Islands area in the regional network records. On September 2, 2017, another earthquake swarm of M_L3.0 occurred near Beichangshan Island, about 15km away from the south of Dazhushan Island. More than 300 earthquakes were recorded, including one earthquake of M_L3 or above. According to the seismic data, two earthquake swarms were also recorded near Daheishan Island and Tuoji Island in Miaodao archipelago from February to March 1976. It is believed that these two swarms may be the “precursory earthquake swarms” of Tangshan strong earthquake with M=7.8 in 1976. The differences in spatial location, energy release and focal depth between the two swarm events are very similar to those in 2017. Therefore, in this paper, the three-dimensional velocity structure of P-wave in Miaodao archipelago area and the results of seismic precise relocation are obtained by using the double-difference tomography method, and the deep structural environment factors of the preparation of the earthquake swarms and the differences in the characteristics of the earthquake swarms are analyzed in combination with the fault activity and medium characteristics.
The velocity structure provides important information related to earthquake location and focal medium, and provides important basis for understanding the background of earthquake preparation and the mechanism of earthquake occurrence. Based on the observation report data of Shandong and Liaoning seismic networks, this paper selects 4 766 seismic events recorded clearly from January 2008 to December 2017 in Miaodao archipelago and nearby areas, and excludes the data with the difference of P-wave and S-wave travel time and time distance curve larger than 5s. After the difference grouping of earthquake events, 4 555 events recorded by 65 stations are finally selected for double-difference tomography inversion, and there are 26 430 P-wave absolute arrival data, 513 299 difference arrival data, 26 356 S-wave absolute arrival data and 508 482 difference arrival data. Limited by geographical conditions, the ray density is dense in the south and sparse in the north. After repeated test and selection of inversion parameters and model recovery test, high-resolution P-wave three-dimensional velocity structure image and high-precision earthquake positioning results are obtained in Miaodao archipelago, Shandong Province, and the following conclusions are obtained:
(1)The results of seismic precise relocation show that the convergence of seismic distribution near Miaodao islands is good, the NW direction zonal distribution of earthquake swarm activity is obvious, and the focal depth is mainly concentrated in the middle and upper crust. The characteristics of swarm activity show group occurrence in a short period of time, and there are obvious differences in the form of expression: the swarm near Tuoji Island has deep focal point, high frequency, large release energy, and wide distribution of focal area; the swarm activity characteristics near Daheishan Island and Beichangshan Island are just the opposite.
(2)The horizontal velocity structure shows that the lateral heterogeneity of velocity structure exists in every depth layer, which reflects the unbalanced uplift of crystalline basement and the zone filling of igneous rock. The velocity structure of the shallow crust is in good agreement with the known geological structure; the middle and shallow layers clearly reveal the basement uplifting area and the subsidence zone on both sides of Miaodao Islands; the velocity structure of the middle and lower layers is obviously affected by the deep faults and magmatic activities, and near the Dazhushan Island-Weihai North Fault, it shows obvious low-velocity characteristics, while the northern sea area of Jiaodong Peninsula is characterized by independent high-velocty abnormal blocks.
(3)The velocity structure profile shows that there is a certain correlation between the activities of faults and earthquake swarm and the velocity structure of P wave. There is an obvious low velocity region in the middle and upper crust in the profile passing through the Dazhushan Island earthquake swarm, and the earthquake swarm is nearly vertical and layered scattered in the relatively high velocity medium between the upper and lower low velocity bodies or near the velocity conversion zone, which is consistent with the characteristics of the Dazhushan Island-Weihai North Fault. The profile across the Beichangshan Island earthquake swarm reveals that there are small high-speed bodies in the middle and upper layers of the crust, and the earthquake swarm occurs at the edge of the high-velocity body.
(4)Earthquake swarm often occurs in places with dense fracture distribution, relatively weak medium and low strength. Based on the analysis of the characteristics of earthquake swarm and the three-dimensional velocity structure of P-wave in Miaodao archipelago area, it is considered that the two significant earthquake swarm activities in 1976 and 2017 are the energy release caused by the inhomogeneity of local medium and the low stress friction of regional fault during the process of regional background stress enhancement(adjustment).

Keyword: Miaodao Archipelago; Double-difference tomography; three-dimensional P-wave velocity structure; precise earthquake relocation

文章图片

0 引言

庙岛群岛位于胶东半岛北部、渤海海峡南岸, 由一系列岛屿组成, 在构造上位于渤海凹陷带的东侧, 是胶辽古隆起的结合部位。其出露地层为上元古界蓬莱群浅变质岩, 部分岛屿见新生界第四纪基性火山喷发岩和松散沉积物以及中生代岩浆侵入岩。这一区域除基底长期隆起外, 主要受NNE向的郯庐断裂带和NW向的张家口-蓬莱断裂带控制, 此外NEE向的聊考-广齐断裂通过海底的繁杂构造也向此延伸, 形成了海、陆交互的复杂构造特征(图 1)。地震资料表明, 庙岛群岛及周边地区的地震活动强烈, 发生过数次强烈地震, 包括1548年蓬莱附近海域7.0级和1597年渤海7.0级地震。

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图 1 研究区及附近地区的断层分布和历史地震分布(放大图据刘忠亚等, 2016改绘)
红色方框为本文的研究区, 黑色实心圆为地震震中; 黑色实线为断层: F₁ 庙岛陆坡断裂; F₂、 F₃ 栖霞-蓬莱断裂系; F₄ 长岛-芝罘岛断裂; F₅ 神道口断裂; F₆ 大竹山岛-威海北断裂; F₇ 钦岛断裂Fig. 1 The distribution of faults and historical earthquakes in the study region and its adjacent areas (adapted after LIU Zhong-ya et al., 2016 for the enlarged drawing).

本文以现代中、小地震活动频繁的庙岛群岛南部地区作为研究区, 主要包括砣矶岛以南的大竹山岛, 南、北长山岛, 大、小黑山岛及庙岛等11个岛屿所在的胶东半岛北部海域。这一区域分布的断裂主要为NW向张家口-蓬莱断裂带东段的蓬莱-威海断裂系, 由南向北依次为长岛-芝罘岛断裂(F₄)、神道口断裂(F₅)、大竹山岛-威海北断裂(F₆)和钦岛断裂(F₇)(图 1)。海域声波探测显示, 这些次级断裂的走向为290° ~310° , 倾角大, 断面近直立, 晚第四纪以来主要表现出正断活动的特点(王志才等, 2006)。 NE向的F₂、 F₃断裂被认为是栖霞-蓬莱断裂系在海域的延伸, 被NW向断裂切割错断, 平面形态短小细碎, 其与NW向断裂交会的区域小地震活动密集。 2017年3月3日大竹山岛附近就发生了M_L4.5震群, 截至2017年12月31日共记录到余震2 453次, 其中M_L3以上地震46次, M_L4以上地震4次, 是庙岛群岛地区有区域台网记录以来活动强度最高、发震频次最多的一次震群活动。 2017年9月2日在大竹山岛南侧约15km的北长山岛附近又发生了M_L3.0震群, 共记录地震300余次, 其中M_L3以上地震1次。地震资料显示, 1976年2— 3月在庙岛群岛的大黑山岛和砣矶岛附近也记录了2次震群活动。有研究认为, 这2次震群可能是1976年唐山7.8级强震的 “ 前兆性震群” (魏光兴等, 1980)。而这2次震群在空间位置、释放能量及震源深度等方面的差异与2017年发生的2次震群十分相似, 故将庙岛群岛地区短时间内发生成组、高频次震群活动的构造环境作为本文的研究目标。

地震层析成像是利用大量地震观测数据反演研究区三维速度结构的一种方法, 主要包括模型参数化、三维射线追踪及反演等。利用接受函数、面波层析成像及噪声层析成像等方法可确定区域的地壳上地幔结构、分析地壳深部的孕震环境(何正勤等, 2009; 郭震等, 2015; 李敏娟等, 2018; 张风雪等, 2018)。双差层析成像方法则被广泛应用于水库、火山、活动断裂带等小尺度、精细化的三维速度结构反演(Chiarabba et al., 2003; 王长在等, 2013; Panayotopoulos et al., 2014; Zhang et al., 2014; 韩晓明等, 2018)。利用层析成像方法对环渤海及渤海海峡开展研究的结果表明, 渤海海峡的速度结构存在明显的非均匀性, 海峡北部的地壳速度较高, 结构完整, 而海峡南部的地壳速度偏低, 地震活动频繁。 (李志伟等, 2006; 胥颐等, 2016)。但由于研究范围较大、剖面位置多侧重渤中凹陷且分辨尺度较大等原因, 上述资料难以清晰地展示庙岛群岛地区的深部构造特征。尤其是蓬莱-威海断裂对研究区地壳结构产生了怎样的影响、庙岛群岛地区震群多发与地壳速度结构存在怎样的关联等问题, 尚待深入研究。 2017年爆发的震群活动和及时架设的流动观测为开展速度结构研究提供了大量地震波走时资料, 对提高模型空间分辨率具有重要意义。本文利用双差层析成像方法反演得到了庙岛群岛地区地壳P波三维速度结构, 同时给出了地震精定位结果, 并结合研究区断层和介质特性分析了孕育震群及产生震群特征差异的深部构造环境因素。

1 方法简介

双差层析成像方法(Zhang et al., 2003)是在双差定位法(Waldhauser et al., 2000)与传统地震层析成像技术相结合的基础上发展起来的, 其思路为: 假定相邻2个地震事件到同一台站的路径相似, 对这2个相邻事件的到时残差作差得到双差数据, 利用双差数据反演速度结构。具体而言, 对于给定的震源和台站, 从震源i到地震台站k的地震波走时可以表示为

$T_{k}^{i} = τ^{i} + \overset{k}{\int_{i}} u d s$ (1)

其中, τ ⁱ是地震i的发震时刻, u是地震波的慢度矢量, ds是路径积分元。对走时方程(1)进行一阶泰勒展开, 得到地震波观测到时与理论到时之差 $r_{k}^{i}$ 的线性方程:

$r_{k}^{i} = \overset{3}{\sum_{l = 1}} \frac{\partial T_{k}^{i}}{\partial x_{l}^{i}} Δ x_{l}^{i} + Δ τ^{i} + \overset{k}{\int_{i}} δu d s$ (2)

其中, Δ $x_{l}^{i}$ (l=1, 2, 3)为震源位置在3个坐标轴方向上的扰动, Δ τ ⁱ为地震i的发震时刻扰动, δ u为地震波的慢度扰动。若符合组对条件的地震i和j均被台站k所记录到, 则这2次地震的走时残差之差(即双差)可表示为

$r_{k}^{i} - r_{k}^{j} = \overset{3}{\sum_{l = 1}} \frac{\partial T_{k}^{i}}{\partial x_{l}^{i}} Δ x_{l}^{i} + Δ τ^{i} + \overset{k}{\int_{i}} δu d s - (\overset{3}{\sum_{l = 1}} \frac{\partial T_{k}^{j}}{\partial x_{l}^{j}} Δ x_{l}^{j} + Δ τ^{j} + \overset{k}{\int_{j}} δu d s)$ (3)

之后通过求解震源位置与速度结构的耦合方程, 可实现三维速度结构、震源的相对位置和绝对位置的反演。在反演过程中通过 “ 节点法” 进行模型参数化, 利用 “ 伪弯曲法” 进行射线追踪实现正演计算, 并借助带阻尼的最小二乘法, 使双差走时数据残差、绝对走时数据残差的二范数系统构成目标函数最小化, 进而反演得到精细三维速度结构。

2 数据选取和反演模型的建立

2.1 数据选取

依据庙岛群岛震群分布特点, 选取(37.8° ~38.3° N, 120.45° ~121.05° E)为本文的研究范围(图 1), 为提高数据质量, 计算时选用了更大范围(37° ~39° N, 119° ~122° E)的地震走时资料和观测台站。研究区位于渤海海峡南岸, 其南部的胶东半岛地震台站分布密度较高, 岛屿上布设有长岛台、北隍城台及大竹山岛台, 监控能力可达M_L1.0; 其北侧的渤海海峡地震活动偏弱, 为增加射线密度, 吸收了辽宁台网辽东湾的部分观测资料。依据山东及辽宁地震台网观测报告, 选取2008年1月— 2017年12月记录清晰的4 766个地震事件, 剔除P波和S波时距曲线差值> 5s的数据(图2b), 经过地震事件的差分组对, 最终选取了由65个台站记录的4 555个事件用于层析成像反演, 参加反演的P波绝对到时数据有26 430条, 差分到时数据有513 299条, S波绝对到时数据有26 356条, 差分到时数据有508 482条。受地理条件限制, 射线密度呈现南密北疏的格局。地震和台站分布见图2a。

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	图 2 a 地震和台站间射线分布图; b 地震P波和S波走时-震中距关系曲线Fig. 2 Rays between earthquakes and seismic stations(a)and traveltime-distance relation of earthquakes waves(b).

2.2 初始速度模型的建立

反演采用的初始速度模型参考了穿过渤海海域的海-陆地震联合观测剖面和渤海海峡及周边地区的地震层析成像结果(Hao et al., 2013; Liu et al., 2015; 汪晟等, 2017)。表1给出了P波的初始速度结构, 海平面以上的部分与第一层速度相同, P波与S波速度的比值为1.732。在分层速度模型中用5km深度界面反映地壳浅部和中、新生带沉积层的速度, 13km深度界面反映结晶基底附近的速度, 22km深部界面反映上、下地壳分界面的速度变化, 另外依据庙岛群岛附近的初始震源深度增加10km和16km深度界面。

表1 庙岛群岛的P波初始速度结构 Table1 Initial P-wave velocity structure of the Miaodao archipelago

2.3 反演参数的选取及网格划分

由于双差层析成像方法采用带阻尼的LQSR算法, 阻尼因子通过对速度变量的调节作用控制速度图像的平滑度和反演结果的误差, 因此选取合理的阻尼因子和光滑权重对反演结果的稳定性至关重要。本文采用权衡分析法(Eberhart-Philips, 1986; Lin et al., 2010), 分别以10~1 000、 0.01~100作为阻尼因子和光滑权重的测试范围, 首先将通过不同组合的阻尼因子和光滑权重反演计算获得的速度均方根和走时残差均方根绘制归一化的L曲线(图 3), 权衡选取适合所有曲线曲率的最大阻尼值作为最佳速度阻尼因子。然后固定已选取的速度阻尼因子, 再对所选范围的光滑权重重新进行反演, 认为速度图像的平滑度与反演误差折中对等时的光滑权重为最佳权重值。经过反复计算, 最终确定的阻尼因子和光滑权重的最优值分别为200和10。

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	图 3 光滑权重和阻尼因子的权衡曲线Fig. 3 Trade-off curves of smoothing and damping weight parameters.

根据庙岛群岛地区的地震空间分布特征, 经过对多种网格节点划分方式的尝试和分辨率测试结果的对比分析, 最终选取0.1° × 0.1° 的网格间隔, 深度方向的网格节点分别为0、 5km、 10km、 13km、 16km、 20km、 25km、 30km、 40km(图 4)。

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	图 4 反演网格分布及1976年以来的震群分布黑色实心圆为1976年的震群, 红色实心圆为2017年的震群, 蓝色空心圆为2008— 2017年的区域地震Fig. 4 Tomography grid and earthquake sequences distribution since 1976.

3 反演结果与讨论

3.1 结果的分辨率及可靠性分析

在层析成像的研究中, 对反演分辨率的评价尤其重要。我们采用棋盘格检验(Humphrey et al., 1998)方法对速度结构反演的可靠性和空间分辨能力进行测试。首先, 在初始一维速度模型的基础上添加± 5%的速度扰动, 形成正负相间顺序排列的三维网格模型, 即检测板模型。然后, 利用实际地震射线在该三维网格模型上做正演计算得到理论走时数据集, 再对理论走时数据集及初始速度模型做反演计算, 将反演得到的三维速度结构与检测板模型的相似程度进行对比评估。相似程度越高, 则解的可靠性越高、分辨能力越强。图 5 给出了检测板模型和不同深度的恢复检测结果, P波的速度在5~16km的深度节点基本能恢复正、负相间的网格变化, 砣矶岛以南地区的恢复程度较好。由于反演使用的是天然地震, 庙岛群岛北部渤海海峡地震稀疏, 距离辽东湾观测台站较远, 因此研究区北侧的射线密度低, 测试效果不理想。整体来看, 在5km和16km深度, 砣矶岛以北的地区基本无法恢复, 10km和13km深度的分辨能力相对最好, 除砣矶岛西北区域外, 其它地区的恢复程度都较好, 20km深度基本无法恢复。因此, 本文在讨论研究区的速度结构时, 主要分析砣矶岛以南地区5~16km深度层的P波三维速度结构特征。

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	图 5 检测板测试模型及不同深度剖面的棋盘测试结果Fig. 5 Checkerboard resolution tests at different depths.

3.2 重定位结果

经过15次迭代, 双差层析成像方法同时给出了研究区3 555个地震的精定位结果。由重定位前、后的走时残差均方根直方图(图 6)可见, 初始定位的残差大部分为0.05~0.25s。重定位后的残差绝大多数分布在0.03s以内, 平均残差由重定位前的0.136s降低为0.016 9s, 地震定位的精度得到了显著改善。

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	图 6 重定位前后走时残差均方根直方图Fig. 6 Histograms of travel time residual RMS before and after relocation.

重定位后的地震空间分布显示(图7a), 震群事件的收敛性较好, 地震活动沿NW向条带状分布明显。大竹山岛震群沿大竹山岛-威海北断裂分布, 震群活动随时间推移向S逐步偏移。从深度剖面看(图7b, c), 震群活动在约7km深度出现间断面, 分为上、下2部分。上层部分的地震呈水平状分布, 震源深度集中在3~6km范围, 地震震级普遍较小, 下层部分的震源深度分布在7~18km的深度范围, 且较大地震多位于该震源深度范围的底部。从地震活动的时间进程看, 下层部分的地震发生时间早于上层部分, 即震群活动的发展过程分为2个阶段, 下层地震活动起主要作用, 在其引起的应力调整作用下, 原震群位置略偏S的地壳浅层也发生了一丛小地震活动。北长山岛震群分布在神道口断裂南侧, 地震频次偏低, 分布范围较小, 深度剖面显示该震群震源深度集中在5~10km的范围(图7d, e)。

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	图 7 重定位后地震空间分布及震源深度剖面图黑色圆圈为2008— 2016年地震, 彩色圆圈为2017年震群活动Fig. 7 Spatial distribution and focal depth profiles of earthquakes after relocation.

3.3 P波三维速度结构反演结果分析

图 8 为反演得到的不同深度层的P波速度结构, 同时将发生在各层上、下2.5km深度范围的地震垂直投影在剖面上。研究区内凹陷和隆起交错, 规模不等的构造断裂交会伴生, 因此不同深度层均显示出明显的横向速度不均匀性, 也在一定程度上反映出了海-陆相接的构造关系, 即北长山岛以南地区的P波速度明显高于其北侧的海槽水道区域, 并在10km和13km深度层与砣矶岛北侧的高速区相连, 将渤中凹陷和北黄海盆地分隔。研究区为胶辽隆起的一部分, 整体处于 “ 南升北降” 的微变之中, 南侧胶东半岛及附近海域的基底为太古代和元古代的中— 深变质岩系, 长期的构造隆起使其缺失古生代— 三叠纪地层, 致使结晶基底埋深较浅, 故而波速偏高(黄永华等, 2007; 潘素珍等, 2015)。北侧各岛之间水道的掘蚀洼地分布着大量的火成岩侵入体, 在诸岛海岸附近分布有NE、 NW向断层活动迹象, 在陡峭的海蚀带, 节理发育, 石英岩呈碎裂状态(丁东, 1994)。

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	图 8 研究区不同深度的水平层析成像结果Fig. 8 Horizontal slices of velocity at different depths by tomoDD.

一般认为断裂是高速区和低速区的分界。从5km深度层的速度结构看, 钦岛断裂F₇和长岛-芝罘岛断裂F₄之间的南砣矶岛水道区域出现NE向展布、不连续的明显低速区, 随着深度增加到10km、 13km, 上述低速区范围明显缩小, 仅在大竹山岛-威海北断裂F₆两侧及长岛-芝罘岛断裂F₄西段的大黑山岛附近存在小范围低速区域。这些浅层较大范围的低速区可能与渤海海峡中生代及第四纪经历的多次火山活动相关, 海底火山杂岩以深大断裂为通道上涌至地壳浅部或喷溢出地表, 并在凹陷区域沉积了巨厚的碎屑岩(滕吉文等, 1997; 陈晓辉等, 2011)。南长山岛附近的小范围高速区在延伸至10km深度时沿NE向快速扩大, 与砣矶岛北侧的NE向狭长高速区被大竹山岛-威海北断裂F₆及其附近的松散构造所分隔; 至13km深度层大竹山岛-威海北断裂对速度结构的分割效应减弱, 高速区逐渐向NE方向扩展并与砣矶岛附近的高速区域连成一体, 形成庙岛群岛东侧P波速度明显高于其西侧的横向速度分块格局, 显示出渤海凹陷与北黄海盆地在中上地壳内的差异性升降活动。 16km深度层的速度分布与10km和13km深度层的速度结构差异较大, 砣矶岛南侧近SN向的狭长高速区由大竹山岛-威海北断裂F₆沿线较大面积的低速区域替代, 最低波速达5.4km/s。地震投影显示, 2017年3月震群活动中较大地震的震源深度多位于16km左右, 可见这一低速区域的存在可能是研究区中等地震多发的因素之一。在这一深度层, 南、北长山岛附近的高速区明显收缩并向大黑山岛及莱州湾附近偏移, 是2017年9月北长山岛震群震源深度集中在10km以内的原因所在。神道口断裂F₅东南端与NE向断裂交会部位出现小范围的高速区, 诸如此类体积较小的高速体在5~16km深度剖面均有投影, 可能与该区中新世晚期的火山活动导致基性岩浆沿断裂进行的上涌活动有关(黄朋等, 2007; 欧光习等, 2010)。另外, 栖霞-蓬莱断裂系F₂西侧出现了1个明显的低速投影, 这一低速投影位于庙西火山带, 与该地区新构造运动强烈且形式多样、 NW向和NE向2组断裂共轭交错和地壳介质相对脆弱有关。

整体来看, 10km、 13km深度层的P波速度相对较高, 地壳介质的完整性较好, 这一结果与较大范围地震层析成像所揭示的庙岛群岛及附近地区在中地壳速度相对稳定的结果一致(李志伟, 2006; 汪晟等, 2017)。 2017年3月大竹山岛震群在8~13km的深度分布的地震相对较少也是对这一结果的佐证。而在5km和16km深度层, 大竹山岛-威海北断裂附近的P波低速特征明显, 说明研究区地壳介质结构在横、纵方向上均有显著差异。

图 9 给出研究区P波速度的垂向剖面, 其中剖面AA'基本沿庙岛群岛轴线分布, 纵贯长岛-芝罘岛断裂F₄、神道口断裂F₅和大竹山岛-威海北断裂F₆; 剖面BB'和CC'分别穿过大竹山岛震群和北长山岛震群。为便于对比, 将剖面两侧10km范围内的地震投影在相应剖面上。由南向北, AA'剖面上中、上地壳速度结构的横向起伏变化显著, 剖面的中间位置为南砣矶岛水道, 分布着的大量火成岩侵入体及大竹山岛-威海北断裂附近的岩石破碎带, 两侧分别为凹陷到隆起的过渡地带, 其中南侧胶东半岛北部至北长山岛的上元古界结晶基底抬升明显。纵穿的3条断裂基本位于高速区与低速区的分界线附近或速度连续面的阶变处, 地震活动大部分发生在高、低速转换带附近。精定位结果显示, 震源深度由南向北逐渐加深, 2017年3月发生的震群活动能清晰反映出大竹山岛-威海北断裂的倾角较大、断面近直立的特征。

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	图 9 庙岛群岛地区P波深度剖面的速度结构图像 F₁长岛-芝罘岛断裂; F₂神道口断裂; F₃大竹岛-威海北断裂Fig. 9 Vertical cross-section of V_p in Miaodao archipelago area.

BB'剖面与大竹山岛-威海北断裂平行。 P波速度剖面显示, 断裂下方的中、上地壳内分布着面积较大、形状不规则的连续低速区, 该低速区分为NW和SE 2个中心区域且NW方向的低速区埋深较深, 位于5km以下。 2个中心低速区与16km深度的低速区之间夹裹着较薄的相对高速层。 2017年3月发生的大竹山岛震群近直立分布在上、下2个低速区之间的高速夹层内或速度转换边界, 并表现出明显的分层特征。一般认为, P波速度偏低, 则表示岩体结构破碎或岩体裂缝中存在液态或气态物质, 故抗压强度较弱, 易形成释放地震能量的通道。此次震群中几次显著地震的震源机制解的一致性较好, 均为走滑机制, 其中NW向节面走向约为320° , 与大竹山岛-威海北断裂的走向(300° )基本一致(高银鸿等, 2018; 李铂等, 2019), 而震群的最大主压应力方向为98° , 与庙岛群岛地区近EW向的构造应力场方向一致(郑建常等, 2018)。这表明, 在构造应力增强的作用下, 走向与其近于平行的大竹山岛-威海北断裂F₆易发生水平错动, 形成走滑机制的地震。因此, 基于震源机制解及速度结构特征, 分析认为2017年3月大竹山岛震群的发震构造为大竹山岛-威海北断裂F₆。另外, 1548年在大竹山岛震群震源区的SE侧曾发生7.0级地震, 发震断层很可能是NW向的蓬莱-威海断裂(晁洪太等, 1995)。依据该断裂附近的速度剖面结果推测, 此次7级地震很可能与断裂下方中地壳内存在的低速块体有一定关联, 低速体内的矿物部分熔融或脱水, 具有塑性特征, 本身不能积累较大的应变能, 但可以对周围脆性的应力状态起到放大作用(刘国栋等, 1992)。

CC'剖面位于长岛-芝罘岛断裂与神道口断裂之间。 P波速度结构的横向变化分界明显, 北长山岛以西为连续的低速区, 最深可达约10km, 北长山岛以东的P波速度整体偏高, 在地表浅层及中地壳分布有小规模的高速体。推测高、低速度间断面为穿过南长山岛北部的南城断裂, 该断裂的陆域长度为5km, 总体走向为N20° E, 倾角为55° ~80° , 为倾向NW的逆断裂(葛孚刚等, 2010)。 2017年9月2日北长山岛震群分布在南城断裂附近的小体积高速块体边缘, 这些小体积的高速体可能是基性火山岩侵入体(侯贵廷等, 2003), 具有较高的波速比和较低的剪切模量, 但由于体积偏小, 不易积累较大的应变能, 因此以小地震群的形式使能量趋向于四周相对软弱的部位释放。

3.4 对2017年和1976年2组震群相关性的讨论

庙岛群岛南部地区于2017年和1976年春季分别发生了2组地震学特征一致的震群活动(魏光兴等, 1980; 申金超等, 2019)。这种重复事件的发生需要具有一定的构造场所和应力背景条件。庙岛群岛附近的NE向郯庐东支断裂和NW向蓬莱-威海断裂在强震的孕育和发生过程中呈共轭关系, 但在第四纪中期后均活动较弱, 仅蓬莱-威海断裂系中的少数次级断层在晚更新世、全新世时有活动。中、小地震的震源机制解及震群活动特征同样表明蓬莱-威海断裂带对渤海海域的现代地震活动具有控制作用。 1976年砣矶岛震群和2017年3月大竹山岛震群均位于蓬莱-威海断裂的次级断裂— — 大竹山岛-威海北断裂附近的海槽凹地(图 4), 这一区域次级断裂密集交会, 中、上地壳以低速特征为主, 故震群释放应变能大、震源深度较深; 1976年大黑山岛震群和2017年9月北长山岛震群位于胶东北部结晶基底抬升最高, 中、上地壳结构稳定、速度偏高的区域, 除NW向的长岛-芝罘岛断裂外, 还分布着许多规模较小的NE向微小断裂, 故震群规模小、震源浅(图 9)。以上是产生2组震群中不同位置震群特征差异的构造原因。

统计结果显示, 胶东半岛的震群、小地震序列群体活动出现后的1a内, 华北东部发生5.0级以上地震的对应率为66.7%(李冬梅等, 2011), 典型的震例为1976年唐山7.8级地震前庙岛群岛的砣矶岛和黑山岛发生的震群活动(魏光兴等, 1980)。地震资料显示, 唐山7.8级地震时, 整个华北地区处于地震能量释放的活跃时段, 仅1966— 1976年间环渤海地区就发生7级以上地震4次, 在这种背景应力场强调整或联动的过程中, 地壳介质较为松散、应力承载能力偏弱的庙岛群岛地区以震群的形式参与了应变能的释放过程似乎不难理解。对应力积累水平的测算结果表明, 当前渤海海峡处于中低水平的应力积累中, 断层活动性弱, 地壳较为稳定(姜迪迪等, 2013; 于磊, 2017), 同时, 华北地区6级以上中强地震长期平静, 2017年大竹山岛和北长山岛震群的应力降也低于区域背景水平(周少辉等, 2018)。这些现象表明, 虽然2017年庙岛群岛的震群活动具有前兆性质(高银鸿等, 2018), 但区域背景应力处于累积过程中, 不具备短时间发生中强地震的应变能条件。基于上述2种背景应力状态, 认为1976年和2017年庙岛群岛附近发生的震群活动是在区域应力增强的背景下, 局部介质的不均匀性和断层的低应力摩擦引发的能量释放过程。

4 结论

本文利用双差层析成像方法反演得到了山东庙岛群岛地区较高分辨率的P波三维速度结构及高精度地震定位结果, 并得到以下结论:

(1)精定位结果显示, 庙岛群岛附近震群活动的NW向带状分布明显, 震源深度主要集中在中、上地壳。震群活动存在短时间内成组发生的特点, 并且在表现形式上存在明显差异: 砣矶岛附近的震群震源深、频次高、释放能量大, 震源区分布范围广; 大黑山岛— 北长山岛附近的震群活动特征恰好与此相反。

(2)水平速度结构显示(图 8), 速度结构的横向不均匀性在各个深度层都存在, 反映了结晶基底的不均衡抬升和火成岩的成带灌入。地壳浅层的速度结构与已知的地质构造吻合较好; 中浅层清晰地揭示出庙岛群岛的基底隆升区与两侧的下沉带; 中、下层的速度结构受深大断裂及岩浆活动影响明显, 大竹山岛-威海北断裂F₆附近为显著的低速特征, 胶东半岛北部海域则为相互独立的高速异常块体。

(3)速度结构剖面显示, 断裂、震群活动与P波速度结构具有一定的相关性。穿过大竹山岛震群的剖面在中、上地壳的低速特征明显, 震群近直立分层散布在上、下2层低速体之间的相对高速介质内, 与大竹山岛-威海北断裂F₆的特征相符。穿过北长山岛震群的剖面在地壳的中、上层出现体积较小的高速异常体, 震群发生在高速体边缘。

(4)震群往往发生在裂隙分布较密, 介质相对脆弱、强度偏低的特殊区域。对庙岛群岛地区震群的特征及P波三维速度结构的分析认为, 1976年和2017年2组显著震群活动均是区域背景应力调整过程的表现形式。

致谢中国科学技术大学张海江教授提供了tomoDD程序; 审稿专家提出了宝贵的修改建议。在此一并表示感谢!

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1995

0.0

... 0级地震, 发震断层很可能是NW向的蓬莱-威海断裂(晁洪太等, 1995) ...

2011

0.0

... 陈晓辉等, 2011) ...

1994

0.0

... 北侧各岛之间水道的掘蚀洼地分布着大量的火成岩侵入体, 在诸岛海岸附近分布有NE、 NW向断层活动迹象, 在陡峭的海蚀带, 节理发育, 石英岩呈碎裂状态(丁东, 1994) ...

2018

0.0

... )基本一致(高银鸿等, 2018 ...

... 这些现象表明, 虽然2017年庙岛群岛的震群活动具有前兆性质(高银鸿等, 2018), 但区域背景应力处于累积过程中, 不具备短时间发生中强地震的应变能条件 ...

2010

0.0

... , 为倾向NW的逆断裂(葛孚刚等, 2010) ...

2015

0.0

郭震, 陈永顺, 殷伟伟. 2015. 背景噪声面波与布格重力异常联合反演: 山西断陷带三维地壳结构[J]. 地球物理学报, 58(3): 821—831.

GUO

Zhen

, CHEN

Yong-shun

, YIN

Wei-wei

. 2015. Three-dimensional crustal model of Shanxi graben from 3D joint inversion of ambient noise surface wave and Bouguer gravity anomalies[J]. Chinese Journal of Geophysics, 58(3): 821—831(in Chinese).

The main objective of this study is to obtain a model that can explain the surface wave data and Bouguer gravity anomaly simultaneously and also to alleviate the non-uniqueness of surface wave inversion. We applied a novel non-linear joint inversion technique using both surface wave and gravity anomalies to obtain high resolution 3D shear-wave velocity crustal model of Central North China Craton including Shanxi graben. The crustal model helps us to understand the crust and mantle dynamics and the evolution history of the Shanxi rift.#br#We bandpass filtered WGM2012 global Bouguer gravity anomaly data to remove the data with wavelength less than 50 km and greater than 200 km. We then projected the gravity data and surface wave data from ambient noise tomography to a Cartesian coordinate system with grid spacing 50 km×50 km. The joint inversion was performed in this coordinate system. The inversion volume is much larger than the target region. This parameterization strategy was chosen to minimize the likelihood edge effects from gravity modelling. We used Crust 2.0 global model as our initial model in the inversion. An approximate non-linear P velocity and density relationship is derived from combing Birch's law and Nafe-Drake's curve. For each iteration of inversion, the sensitivity kernel of surface wave and gravity data were recalculated and updated until the inversion converged. We did several experiments to obtain an optimized weighting parameter to balance the influences of both datasets. A surface-wave-only inversion was also performed to compare the results of joint inversion.#br#Both shear wave velocity models from joint inversion and surface-wave-only inversion can fit surface wave data well though only the joint inversion provides an acceptable fit to the gravity data. Gravity misfits decrease from >30 mGal to ~3.4 mGal after joint inversion. The velocity anomalies from both inversions all show a remarkable correlation with surface geology and upper crustal structures beneath Shanxi rift. The Lüliang mountain region and Datong volcanic region all show relatively high shear wave velocity in the upper crust. Meanwhile, the basins in the southern rift show slow velocities. The southern part of the Shanxi rift is slower than the northern part in the upper crust. The Cenozoic extension started from the northern rift and then passed on to the southern rift. However, in the mid-and-lower crust beneath the Datong volcanic region, the surface-wave-only inversion resolves much slower shear wave velocity compared to the joint inversion model. The most pronounced difference occurs in the mid-and-lower crust beneath the Lüliang mountain region. The joint inversion reveals a slow velocity anomaly with amplitude about 2%, while the surface-wave-only inversion model shows a fast velocity.#br#The Datong volcanic region, north part of the Shanxi rift, might have experienced mantle upwelling since the Cenozoic time. The hot and buoyant mantle heated the upper most mantle and the crust, which caused the large-scale mid and lower crust slow velocity and negative gravity anomaly in the Datong volcanic region. The slow velocity anomaly beneath the Lüliang mountain region is also related to the mantle upwelling beneath the Datong volcanic region. The Cenozoic mantle upwelling might control the formation and evolution of the Shanxi rift.

1. Department of Geophysics, Peking University, Beijing 100871, China; 2. Shanxi Earthquake Bureau, Taiyuan 030000, China

讨论了利用面波与布格重力异常联合反演三维地壳速度结构的新方法,并利用该方法联合反演获得山西断陷带地壳S波速度结构.通过建立速度与密度之间的经验关系,利用非线性迭代反演方法获得最终速度模型.结果显示,联合反演获得的速度模型可以同时提高对面波及重力数据的观测拟合程度,而面波单独反演得到的速度模型则无法很好的拟合重力观测数据.相比较,联合反演速度模型中的大同火山区中下地壳的低速异常幅值小于面波单独反演模型中低速异常体的幅值.联合反演速度模型结果揭示,吕梁山地区在中下地壳存在低速异常,并且和北部的大同火山区低速异常相连接,说明可能导致新生代以来大同火山区岩浆活动的上地幔构造活动(上地幔局部上涌,地幔柱)可能对山西断陷带的形成和构造活动起到了一定的控制作用,并且导致了吕梁山地区中下地壳的低速异常.

... 郭震等, 2015 ...

2018

0.0

... 韩晓明等, 2018) ...

2009

0.0

何正勤, 叶太兰, 丁志峰. 2009. 华北东北部的面波相速度层析成像研究[J]. 地球物理学报, 52(5): 1233—1242.

Zheng-qin

, YE

Tai-lan

, DING

Zhi-feng

. 2009. Surface wave tomography for the phase velocity in the northeastern part of North China[J]. Chinese Journal of Geophysics, 52(5): 1233—1242(in Chinese).

By using the surface waves recorded by 197 broadband stations of the temporary seismic array in North China during Dec. 2006 and June, 2007, the dispersion curves for the period between 8 and 75 s along 435 paths were obtained by the two-station method. The phase velocity images with a resolution of 30~50 km for 33 periods were obtained by using the algorithm developed by Ditmar and Yanovskaya. We analyzed the phase velocity images for four typical periods. These images revealed the heterogeneity of the crustal and upper mantle velocity structure in the northeastern part of North China. The phase velocity image for the period of 10 s clearly sketches the border of the sub-tectonic units between the mountain and basin areas in North China. The phase velocities are very different between the mountain and basin areas. Strong earthquakes ( M ≥6) mostly located at the transition zones of higher and lower velocity anomalies in the images for the period of 16 s. The large-scale ruptures were also near the edges of the transition zones. In the center of the research area, a remarkable gradient zone of the phase velocity was in the NE orientation in the image for the period of 26 s. The location of the gradient zone of the phase velocity is close to the gradient zone of the gravity, which implies the crust velocity structure along one side of the gravity gradient zone has a lager difference from that along the other side.

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

本文用华北临时地震台阵197个宽频带台站2006年12月～2007年6月记录的面波资料，用双台法测定了435条路径上周期8～75 s的基阶瑞利波相速度频散曲线.通过Ditmar & Yanovskaya 方法反演得到33个周期分辨率为30～50 km的相速度分布图像.分析研究了4个具有代表性周期的相速度分布图像，这些图像揭示了华北东北部地壳上地幔速度结构的横向非均匀性质.10 s周期的相速度分布图像清晰地勾绘出了华北东北部盆地与山区及其内部次级构造单元的边界，平原与山区的相速度存在明显的差异；华北东北部6级以上的强震大都分布在16 s周期高相速度与低相速度急剧变化的过渡带上，规模较大的断裂也大部分位于过渡带的边界附近；在研究区域中部，26 s周期的相速度出现一个明显的呈北东向展布的相速度梯度带，该梯度带与重力梯度带位置相近，说明该重力梯度带两侧的地壳速度结构差异较大.

... 利用接受函数、面波层析成像及噪声层析成像等方法可确定区域的地壳上地幔结构、分析地壳深部的孕震环境(何正勤等, 2009 ...

2003

0.0

侯贵廷, 钱祥麟, 史謌, 等. 2003. 渤海中部磁性层析成像的初步研究[J]. 北京大学学报(自然科学版), 39(1): 112—117.

HOU

Gui-ting

, QIAN

Xiang-lin

, SHI

, et al. 2003. Magnetic tomography in the central Bohai Sea of China[J]. Acta Scientiarum Naturalium Universitatis Pekinensis, 39(1): 112—117(in Chinese).

The central Bohai Sea is one of the thinest lithosphere in eastern China where strike-slip and extension faults are very intensive with intensive magmatic activity, including the volcanic rocks and intrusion, which show the conspicious high positive magnetic anomalies. The spatial distribution, shape and property of high magnetic anomalies in the central Bohai Sea are inversed by the magnetic tomography, based on the analysis of regional geology and seismic sections in the Bohai basin.

1 Department of Geology, Peking University, Beijing, 100871； 2 Institute of Geology and Geophysics, Academia Sinica, Beijing, 100101

渤海中部是中国东部岩石圈最薄的地区之一，走滑作用和伸展作用十分强烈，同时伴随强烈的岩浆活动，发育火山岩和侵入岩，在本区表现为一些明显的磁异常高。通过对渤海中部的磁化强度数据进行层析成像分析，结合渤海湾盆地的区域地质和地震剖面的分析，反演引起渤海中部磁异常高的地质体的空间分布形态和性质。

... 2017年9月2日北长山岛震群分布在南城断裂附近的小体积高速块体边缘, 这些小体积的高速体可能是基性火山岩侵入体(侯贵廷等, 2003), 具有较高的波速比和较低的剪切模量, 但由于体积偏小, 不易积累较大的应变能, 因此以小地震群的形式使能量趋向于四周相对软弱的部位释放 ...

2007

0.0

黄朋, 付永涛, 李安春, 等. 2007. 山东蓬莱大黑山岛碱性火山岩地球化学特征[J]. 海洋与湖沼, 38(1): 91—96.

HUANG

Peng

, FU

Yong-tao

, LI

An-chun

, et al. 2007. Geochemical characteristics of Daheishan Island alkalic volcanic rocks from Penglai, Shand ong Province[J]. Oceanologia Et LimnologiaSinica, 38(1): 91—96(in Chinese).

The Daheishan Island (37°58′10″N, 120°37′13″E) is a small island of the Miaodao Archipelago in Bohai Sea of China. The island features with alkalic volcanic rocks. These rocks bear important information of the interaction between the crust and the mantle in the area and of the geological evolution of Chinese marginal seas. However, the rocks are poorly studied with no citable reference. In this case, the flesh volcanic rocks were sampled from outcrop at the northwest of Daheishan Island on 2001. The contents of major elements and isotopic compositions, and K-Ar ages were analyzed with the Philips PW 1400 X-Ray fluorescence spectrometry (XRF), Finnigan—MAT261 mass spectrometer and MM12OOB inert gases mass spectrometer, at the Institute of Geology and Geophysics(IGG), Chinese Academy of Sciences(CAS), on 2002. Geological data of chemical composition, K-Ar age and Nd-Sr-Pb imprints were obtained from these rocks. By analyzing these data, the authors concluded that: the volcanic formation of Daheishan Island belongs to the petrological series of sodic-alkalic basalt, and the volcanism can be divided into two phases (>8Ma and 2 O and Na 2 O. The depth of magma chamber became deeper as magmatism developed. Mantle composition of the chamber area included end members of mainly PREMA and then DM in mantle composition. Furthermore, the melting of MORB of the Pacific Plate probably provided the magmatic matter in the chamber area.

庙岛群岛中的大黑山岛出露碱性火山岩，对研究壳-幔过程及黄、渤海演化具重要意义。文中作者对大黑山岛火山岩的常、微量元素、K-Ar年龄及Nd-Sr-Pb同位素体系进行测试确定。结果表明，岩石属钠质碱性玄武岩系列，岩浆岩喷发可分为8Ma前和8Ma后两个阶段；岩石常量元素含量除K2O、Na2O外差异小；随着火山岩的演化，岩浆源区深度变大；大黑山火山岩Nd-Sr-Pb同位素比值与夏威夷洋岛玄武岩相似，其岩浆的地幔端元组分为PREMA+DM，又以PREMA为主，并有MORB物质混入。

... 神道口断裂F₅东南端与NE向断裂交会部位出现小范围的高速区, 诸如此类体积较小的高速体在5~16km深度剖面均有投影, 可能与该区中新世晚期的火山活动导致基性岩浆沿断裂进行的上涌活动有关(黄朋等, 2007 ...

2007

0.0

... 三叠纪地层, 致使结晶基底埋深较浅, 故而波速偏高(黄永华等, 2007 ...

2013

0.0

姜迪迪, 江为为, 胡卫剑. 2013. 环渤海地区地壳结构及其动力学特征研究[J]. 地球物理学进展, 28(4): 1729—1738.

JIANG

Di-di

, JIANG

Wei-wei

, HU

Wei-jian

. 2013. Study of crustal structure and geodynamic characteristics around the Bohai Sea area[J]. Progress in Geophysics, 28(4): 1729—1738(in Chinese).

Bohai Sea region is both the earthquake-prone areas of eastern Chinese mainland and China's focused earthquake monitoring areas, meanwhile it has been developed and put into production around the Bohai Sea area including a series of oilfield, like the Liaohe, Jidong, Dagang, Shengli and the Bohai Sea, which shows the Bohai Sea region has been the focus of geology and Geophysics. In this paper, combined with the artificial seismic profiles, we apply the harmonic progression method to inverse the Moho depth of the region with gravity data, and use the results of geology, gravity, magnetism, seismic tomography, artificial seismic exploration, geothermy, and stress to research the geophysical field, the crustal structure and geodynamics of the study area, which is hoped to be helpful to understand the deep structure Geodynamics, oil and gas resources around the Bohai Sea area. Through the the inversion results and analysis of dynamic characteristics, we get the following understandings: 1) Bouguer gravity anomalies of the study area change from negative anomalies to positive anomalies, when they are from land to sea, which reflects the general characteristics of the crustal thickness thinning gradually from west to east. Magnetic anomalies in the northern region of the study area change more dramatically, alternated with positive and negative number. Most of the waters show negative magnetic anomalies. The main trend of gravity and magnetic anomalies is NE, NNE.;2) The thickness of the crust around the Bohai Sea area ranges from 31km to 40km, and the general trend is thinning from west to east, which coincides with the results of seismic tomography; 3) Kinetic mechanism of the Bohai Basin should be that the multi-phase tectonic stress field caused by remote effects of the horizontal movement of the surrounding plates has made the multi-direction stretch and tension, and what made the formation of the Bohai Bay Basin should be alternately affected by four periods and two kinds of dynamic actions.

1. Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China

环渤海地区是我国大陆东部强震的多发区,也是我国地震重点监测地区,环绕渤海地区已经开发并投产了包括辽河、冀东、大港、胜利以及渤海海域在内的一系列油田,因此环渤海地区一直是我国地质与地球物理学科的研究重点.本文利用重力数据结合人工地震探测剖面应用调和级数法对该地区的莫霍面深度进行了反演计算,并应用地质、重、磁、地震层析成像、人工地震探测、地热、应力等方面的成果资料,对研究区的地球物理场、地壳结构、地球动力学特征进行了探讨,希望对环渤海地区的深部结构、地球动力学、油气资源等研究有所帮助.通过反演计算结果和动力学特征的分析,得到了以下的认识:1)该地区的布格重力异常主要特征为从陆地向海洋,异常从负异常向正异常变化,反映了地壳厚度自西向东逐渐变薄的总体特征.磁异常在研究区北部地区变化较为剧烈,呈正负交错状.大部分海域地区表现为负磁异常场区.重、磁异常走向主要为NE、NNE向;2)研究区地壳厚度在31~40 km,总体趋势为自西向东逐渐减薄,通过与地震层析成像结果比较,两个结果十分吻合;3)渤海盆地动力学机制应是周边各个板块水平运动的远程效应所造成的多期构造应力场作用下多方向的伸展、拉张而成,渤海湾盆地的形成是四个时期、两种动力作用交替影响的结果.

... 对应力积累水平的测算结果表明, 当前渤海海峡处于中低水平的应力积累中, 断层活动性弱, 地壳较为稳定(姜迪迪等, 2013 ...

2019

0.0

... 李铂等, 2019), 而震群的最大主压应力方向为98#cod#x000b0 ...

2011

0.0

... 7%(李冬梅等, 2011), 典型的震例为1976年唐山7 ...

2018

0.0

... 李敏娟等, 2018 ...

2006

0.0

... (李志伟等, 2006 ...

... 整体来看, 10km、 13km深度层的P波速度相对较高, 地壳介质的完整性较好, 这一结果与较大范围地震层析成像所揭示的庙岛群岛及附近地区在中地壳速度相对稳定的结果一致(李志伟, 2006 ...

1992

0.0

... 依据该断裂附近的速度剖面结果推测, 此次7级地震很可能与断裂下方中地壳内存在的低速块体有一定关联, 低速体内的矿物部分熔融或脱水, 具有塑性特征, 本身不能积累较大的应变能, 但可以对周围脆性的应力状态起到放大作用(刘国栋等, 1992) ...

2016

0.0

2010

0.0

... 欧光习等, 2010) ...

2015

0.0

... 潘素珍等, 2015) ...

2019

0.0

... 申金超等, 2019) ...

1997

0.0

滕吉文, 张中杰, 张秉铭, 等. 1997. 渤海地球物理场与深部潜在地幔热柱的异常构造背景[J]. 地球物理学报, 40(4): 468—480.

TENG

Ji-wen

, ZHANG

Zhong-jie

, ZHANG

Bing-ming

, et al. 1997. Geophysical fields and background of exceptional structure for deep latent mantle plume in Bohai Sea[J]. Chinese Journal of Geophysics, 40(4): 468—480(in Chinese).

Bohai Sea is located at eastern edge of mainland between 37°07'-41°0° N and 117° 33'- 121°10' E. They are composed of three groups of faults in NNE-NE, EW and NW direction, which cross Tanlu fault. These three groups of faults are consistent with three groups of mantle uplift and convergence in the Bohai center depressed.Bohai and its neighbouring region are the main area of Seismicity and Mesozoic,Cenozoic magmatic activities area. The crustal depth is 28-29km, and the upper mantle is uplift. The isothermal Curie Surie surface is 12km deep and has high heat flow value （56-77mW/m2）. There is approximate circle low velocity body existing in the upper mantle near the Miao island, until 120km deep there is low velocity zone existing. The faults are the channel of heating and gaseous state substances. We have comprehensively rcseached deep structure and deep internal processes as well as geophysical field responsibility and formation through Gravity, Airbornmagnetism,Paleomagnetism, Earthquake, Gcothermism, Tectonic stree field, crustal and upper mantle structure and seismic tomography in Bobal Sea and its around region. We advance a new argument: Bobal Sea is not only a tectonic background of deep heating,but also a devoloping mantle plume.

Institute of Geophysics, Academia Sinica, Beijing 100101, China

渤海是中国东部陆缘的一个裂谷型盆地，通过重力、航磁、古地磁、天然地震、地热、应力场、地壳与上地幔结构及地震层析成像等资料，对渤海湾及其周边地带的深部结构、地球物理场效应和深层物理过程进行了综合研究．这一盆地是由NNE－NE向、近EW向和NW向3组断裂组成，这3组断裂与3组地幔隆起带基本相对应，并在渤中坳陷交汇、渤海内部的现代构造运动以水平构造应力场作用下的走滑运动为主要特征，渤海区内地壳厚度仅28-29km，上地幔向上隆起，等温居里面埋深浅（为12km），并为较高区热流值区（56-77mW／m2）．该区岩石层厚度变化显著，庙岛西部中地壳中有一近似圆形的低速体，在120km深度仍为低速异常区．深大断裂可为深部热物质与气态物质上涌的通道．通过综合研究，提出渤海湾很可能是一个潜在的、尚在发展中的地幔热柱．

... 这些浅层较大范围的低速区可能与渤海海峡中生代及第四纪经历的多次火山活动相关, 海底火山杂岩以深大断裂为通道上涌至地壳浅部或喷溢出地表, 并在凹陷区域沉积了巨厚的碎屑岩(滕吉文等, 1997 ...

2017

0.0

... 汪晟等, 2017) ...

2013

0.0

王长在, 吴建平, 房立华, 等. 2013. 玉树地震震源区速度结构与余震分布的关系[J]. 地球物理学报, 56(12): 4072—4083.

WANG

Chang-zai

, WU

Jian-ping

, FANG

Li-hua

, et al. 2013. The relationship between wave velocity structure around Yushu earthquake source region and the distribution of aftershocks[J]. Chinese Journal of Geophysics, 56(12): 4072—4083(in Chinese).

Using travel time data from 21 temporary seismic stations and the permanent stations of Qinghai Seismological Network, we obtained precise relocation of earthquake sequence and 3D seismic velocity structure around Yushu focal area by double-difference tomography. To ensure the accuracy of the phases, the seismic events downloaded from the Data Management Center of China National Seismic Network were re-processed. The result of aftershock relocation shows that the earthquake sequence has a banding distribution along NW in both sides of the fault, which reveals horizontal lineations of hypocenters that define the narrow regions on the fault where stress is released by brittle failure. At the northwest end the aftershocks are distributed not only along the Yushu-Garze fault, but also along the direction vertical to the fault. It reveals an intersecting fault. The result of 3D seismic velocity structure showed that shallow velocity structure has good correlation with surface geology. The velocity structure of middle crust shows that Bayan Har block is characterized by high velocity, Qiangtang block is imaged as a low velocity region. The result of earthquake location and seismic velocity structure shows that the high-low velocity anomaly has certain control action to the aftershock distribution. The mainshock occurred in the transition zone between low and high velocity bodies. Most aftershocks appear to be distributed on the periphery of the high-velocity body. There are rarely earthquakes in the high-velocity body. The high-velocity body reflects the more brittle and competent parts of the crust, which are capable of sustaining greater seismogenic energy. The seismogenic energy in the high-velocity body was released into the surroundings after the mainshock, which leads to many aftershocks in the surrounding areas.

Wang Chang-Zai 1 ;Wu Jian-Ping 1 ;Fang Li-Hua 1 ;Wang Wei-Lai 1 ;

利用玉树震区21个应急流动地震台站和青海省地震台网固定地震台站的观测数据，采用双差层析成像方法，对2010年4月14日至6月15期间发生的地震进行了重定位，并反演得到了玉树地震震源区的三维速度结构.重定位结果揭示余震主要沿NW向成窄带状分布在断层的两侧，表明脆性破裂应力释放主要集中于一个狭窄的区域内.在西北端，余震偏离玉树—甘孜断裂分布，在SW向也有分布，推测可能与南西向次级断裂有关.双差层析成像得到的速度结构在浅部与地表地质构造相一致，中上地壳的速度结构显示巴颜喀拉地块为高速异常，羌塘地块为低速异常.玉树地震余震分布与特定的速度结构存在相关性：主震发生在高低速过渡带偏高速体的一侧，余震主要分布在高速体外围，高速体内部几乎没有余震分布.一般说来，中上地壳的高速体通常具有较高的强度，可以积累较强的孕震能量.主震发生后，高速体内积累的弹性能量向周边释放，可能是导致高速体周边余震发生的主要原因.

... 王长在等, 2013 ...

2006

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王志才, 邓起东, 晁洪太, 等. 2006. 山东半岛北部近海海域NW向蓬莱-威海断裂带的声波探测[J]. 地球物理学报, 49(4): 1092—1101.

WANG

Zhi-cai

, DENG

Qi-dong

, CHAO

Hong-tai

, et al. 2006. Shallow-depth sonic reflection profiling studies on the active Penglai-Weihai fault zone offshore of the northern Shand ong peninsula[J]. Chinese Journal of Geophysics, 49(4): 1092—1101(in Chinese).

Shallowdepth sonic reflection profiling survey has been conducted on the offshore of the north of the Shandong peninsula, and the late Quaternary offset features of the PenglaiWeihai fault zone are documented from more than ten shallow profiles. The fault zone passes through the offshore between the Changdong islands and the Dazhudao island, and consists of a group of NW trending faults, which are dominated by normal and reverse slips with strikeslip component. The PenglaiWeihai fault zone is characterized by the obvious Quaternary activity， which can be divided by the TaocunDongdoushan fault in the North Yellow Sea into two segments, the ChangdaoYantai segment and the YantaiWeihai segment from east to west, respectively. The western ChangdaoYantai segment is a late Pleistocene active segment, where the uppermost fault points discovered in the the survey are buried at more than 30m bellow the sea bottom, and offset the early to middle late Pleistocene sedimentary layers, and no Holocene offset has been found. The eastern YantaiWeihai segment is a mid Pleistocene segment on which no late Pleistocene offset has been found. The PenglaiWeihai fault zone is closely related to the occurrence of history strong earthquakes and modern small earthquakes, among them the 1548 Bohai strait MS7 earthquake occurred on its western segment and the 1948 Weihai MS6 earthquake on its east segment. The intersection regions of the PenglaiWeihai fault zone with NE striking fault zones are considered to be the likely places for the future medium to strong earthquakes.

1 Shandong Earthquake Administration, Jinan 250014, China 2 Institute of Geology, China Earthquake Administration, Beijing 100029, China

在山东半岛北部近海海域进行了多条剖面的声波探测，初步查明了蓬莱－威海断裂带的基本活动特征.该断裂带由一系列北西走向的次级断层组成，主体在长岛至大竹岛之间的海域，多数断层以正断兼有走滑运动为主，部分断层具有逆冲运动性质.断裂带在第四纪活动明显，以北东向桃村-东陡山断裂在海域的延伸段为界，蓬莱-威海断裂带可分为两段：长岛-烟台段和烟台-威海段.西段长岛-烟台段为晚更新世活动段，多数次级断层上断点埋深位于海底以下30m，明显错断了中更新世地层，部分断层明显错断了晚更新世早期地层，有些断层达到晚更新世地层中部，但没有发现全新世地层错动现象.东段烟台-威海段为中更新世活动段，没有发现晚更新世地层错断现象.蓬莱-威海断裂带对历史地震和现代小震活动具有明显控制作用，其中1548年7级地震发生于断裂带西段，1948年6级地震发生于断裂带东段.蓬莱-威海断裂带与北东向断裂交汇区是中强地震发生的有利部位.

... , 倾角大, 断面近直立, 晚第四纪以来主要表现出正断活动的特点(王志才等, 2006) ...

1980

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... (魏光兴等, 1980) ...

... 4 对2017年和1976年2组震群相关性的讨论庙岛群岛南部地区于2017年和1976年春季分别发生了2组地震学特征一致的震群活动(魏光兴等, 1980 ...

... 8级地震前庙岛群岛的砣矶岛和黑山岛发生的震群活动(魏光兴等, 1980) ...

2016

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... 胥颐等, 2016) ...

2017

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... 于磊, 2017), 同时, 华北地区6级以上中强地震长期平静, 2017年大竹山岛和北长山岛震群的应力降也低于区域背景水平(周少辉等, 2018) ...

2018

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... 张风雪等, 2018) ...

2018

0.0

... , 与庙岛群岛地区近EW向的构造应力场方向一致(郑建常等, 2018) ...

2018

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... 于磊, 2017), 同时, 华北地区6级以上中强地震长期平静, 2017年大竹山岛和北长山岛震群的应力降也低于区域背景水平(周少辉等, 2018) ...

2003

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... 双差层析成像方法则被广泛应用于水库、火山、活动断裂带等小尺度、精细化的三维速度结构反演(Chiarabba et al ...

1986

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... 本文采用权衡分析法(Eberhart-Philips, 1986 ...

2013

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... 2 初始速度模型的建立反演采用的初始速度模型参考了穿过渤海海域的海-陆地震联合观测剖面和渤海海峡及周边地区的地震层析成像结果(Hao et al ...

1988

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... 我们采用棋盘格检验(Humphrey et al ...

2010

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... Lin et al ...

2015

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... Liu et al ...

2014

0.0

... Panayotopoulos et al ...

2000

0.0

... , 2003)是在双差定位法(Waldhauser et al ...

2014

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... Zhang et al ...

2003

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... 1 方法简介双差层析成像方法(Zhang et al ...