利用时移层析成像方法分析2014年云南景谷MS6.6地震震源区的P波速度变化

doi:10.3969/j.issn.0253-4967.2021.06.012

摘要/Abstract

摘要：

为了获得2014年景谷 M_S6.6 地震发生前后10a间震源区高空间分辨率的P波速度变化, 文中基于2008年1月1日—2017年12月31日由云南区域数字地震台网所记录的景谷地震震源区的地震资料, 首先采用双差层析成像方法联合绝对到时和相对到时反演了景谷地震震源区高分辨率的三维P波速度结构, 反演结果表明景谷地震的余震序列分布于P波高速异常区及低速异常区的交界处, 与澜沧江断裂有所相交的断裂处于低速异常区, 这可能与断层中的流体有关。然后采用基于双差层析成像的时移层析成像方法得到了不同时间段之间的P波速度变化的时空分布, 并结合已有的地质与地球物理研究成果, 对P波速度的变化特征及其机制进行了探究, 得到几点认识: 1)景谷主震震中附近浅层深度的P波速度最大降幅为0.2%, 在景谷主震发生2个月后出现, 主要受岩石破坏影响所致。2)5~15km深度处整体存在P波速度上升条带区域, 推测该区域为高强度、高阻介质的脆韧性转换带, 不受主震发生的影响。在2014年12月6日 M_S5.8 及 M_S5.9 余震发生后, 余震分布方向发生了明显变化, 震源深度加深, 脆韧性转换带受其影响使得P波速度下降了3.8%。3)震后约3a, P波速度上升并超过震前水平, 可能在震源区的愈合过程中还包含了2018年9月8日云南墨江 M_S5.9 地震发生前的应力积累过程。

关键词: 景谷地震, 时移层析成像, 双差层析成像, P波速度变化

Abstract:

Various studies have reported on temporal changes of seismic velocities in the crust before and after earthquake. New time-lapse seismic tomographic scheme based on double-difference tomography can measure the temporal changes of seismic-wave velocities in the Earth and can offer a higher spatial resolution. The result is less affected by different data distribution and quality in different time periods. On October 7, 2014, an M_S6.6 earthquake occurred in Jinggu County, Pu'er City, Yunnan Province, and then on December 6, 2014, two strong aftershocks with magnitude M_S5.8 and M_S5.9 occurred successively. In order to obtain the high spatial resolution P-wave velocity changes in the hypocenter region of the 2014 Jinggu M_S6.6 earthquake, firstly, we used the seismic data in the hypocenter region of the Jinggu earthquake recorded by the Yunnan digital seismic network from January 1, 2008 to December 31, 2017 to invert the high-resolution three-dimensional P-wave velocity structure in the hypocenter region of the Jinggu earthquake by combining the absolute and relative arrival times using the double-difference tomography method. The inversion results show that the aftershock sequence is distributed at the junction between P-wave high-velocity anomaly area and low-velocity anomaly area. This may be the reason why the depth distribution of aftershocks is shallow in NW and deep in SE, and the number of aftershocks decreases fast in NW and slow in SE. The faults that intersect the Lancangjiang Fault are in the low-velocity anomaly zone, so the low velocity anomaly may be related to the fluid in the faults. Then, according to the technical route, this 3D velocity structure was taken as the initial model to invert for the 3D velocity structure of the five periods, and the 3D P-wave velocity structures of the five periods were obtained by using double differential tomography. Finally, the three-dimensional P-wave velocity model of the five periods was taken as the initial model and the new time-lapse tomography was used to obtain the spatial and temporal distribution of the P-wave velocity changes between different periods. In addition, combining the results with the existing geological and geophysical research results, the characteristics and mechanism of P-wave velocity changes are explored, our results indicate that:
(1)The maximum decrease in P-wave velocity at the shallow depth near the epicenter of the main earthquake is 0.2%, which occurred two months after the main earthquake and was caused mainly by rock failure.
(2)There is a P-wave velocity rising zone at a depth of 5km to 15km which is not affected by the rupture of the main earthquake. The existence of this zone caused the P-wave velocity change in the focal area to be discontinuous in depth. It is speculated that the reason for the existence of this zone is that there is a brittle-ductile transition zone with high-strength and high-resistance medium at this depth range. After the occurrence of the M_S5.8 and M_S5.9 aftershocks on December 6, the direction of distribution of aftershocks changed significantly, and also the focal depths showed a deepening trend. The distribution of the two strong aftershocks and their aftershocks were mainly located in the brittle-ductile transition zone, thus affecting the medium within a depth range of 5 to 15km, resulting in decrease of P-wave velocity with a 3.8%decline. It shows that the two strong aftershocks above magnitude 5 have an impact on the brittle-ductile transition zone, and the occurrence characteristics of aftershocks are usually consistent with the characteristics of P-wave velocity change.
(3)About three years after the Jinggu main earthquake, the amplitude of P-wave velocity increase is much larger than that of the previous P-wave velocity drop in the focal area. The P-wave velocity exceeded the pre-earthquake level. This indicates that the area experienced not only a post-earthquake seismic velocity recovery process, but also other physical processes. Combining with the results on strain field change obtained by the GPS data, it is inferred that the significant increase of P-wave velocity in this area is attributed to the superposition between the P-wave velocity increase due to the stress accumulation before the September 8, 2018, Yunnan Mojiang M_S5.9 earthquake and the post-earthquake seismic recovery process. So the P-wave velocity increase in this area is a complex process.

Key words: Jinggu earthquake, time-lapse seismic tomography, double-difference seismic tomography, P-wave velocity changes

中图分类号:

P315.2

曹颖, 钱佳威, 黄江培, 张国权, 付虹. 利用时移层析成像方法分析2014年云南景谷M_S6.6地震震源区的P波速度变化[J]. 地震地质, 2021, 43(6): 1563-1585.

CAO Ying, QIAN Jia-wei, HUANG Jiang-pei, ZHANG Guo-quan, FU Hong. P-WAVE VELOCITY CHANGES IN HYPOCENTER REGION OF THE 2014 JINGGU M_S6.6 EARTHQUAKE USING TIME-LAPSE TOMOGRAPHY BASED ON DOUBLE-DIFFERENCE TOMOGRAPHY[J]. SEISMOLOGY AND EGOLOGY, 2021, 43(6): 1563-1585.

图/表 15

图 1 研究所用台站、地震、网格节点及活动断裂分布 a 所用台站分布; b 地震和网格节点分布。红色圆点表示地震, 黑色 “×”表示网格节点, 黑色实线表示断裂, 虚线框为研究区域。F1澜沧江断裂; F2 窝拖寨断裂; F3南谷断裂; F4永平盆地东缘断裂; F5威远江断裂; F6益香-赵家沟断裂F7景谷-云仙断裂; F8无量山断裂; F9普文断裂。地震数据为本文重定位数据, 断裂构造数据引自毛泽斌等(2019)

Fig. 1 Distribution of stations, earthquakes, grid nodes and faults in this study.

图 2 P波走时曲线

Fig. 2 Curve of P-wave travel times.

表1 5个时间段的数据

Table1 Data of five periods

时间段	时间窗	台站数 /个	地震数目 /个	P波绝对到时 /个	P波相对到时 /个
P1	2008.01.01—2014.10.06	27	1 755	10 906	214 105
P2	2014.10.07—2014.10.17	20	2 161	18 131	258 019
P3	2014.10.18—2014.12.05	24	2 041	18 093	274 936
P4	2014.12.06—2014.12.31	20	1 849	17 062	264 584
P5	2015.01.01—2017.12.31	34	1 867	12 839	236 444

图 3 5个时间段的P波二维射线分布图 a 时间段1; b 时间段2; c 时间段3; d 时间段4; e 时间段5

Fig. 3 Distribution of 2-D P-wave ray paths for the five periods.

表2 一维P波速度模型

Table2 1D P wave velocity structure

深度/km	P波速度/km·s^-1
0	4.60
2	5.30
5	5.70
7	5.90
10	6.00
12	6.07
15	6.10
22	6.40
31	7.30
40	7.90

图 4 研究区不同深度处的P波速度结构和棋盘测试结果 a 不同深度处的P波速度结构; b 不同深度处的棋盘分辨率测试结果。黑色圆点表示地震震中, 白色五角星为 MS6.6 主震震中,白色圆点为 MS5.8 和 MS5.9 余震震中。F1澜沧江断裂; F2窝拖寨断裂; F3南谷断裂; F4永平盆地东缘断裂; F5威远江断裂;F6益香-赵家沟断裂; F7景谷-云仙断裂; F8无量山断裂。断裂构造数据引自毛泽斌等(2019)

Fig. 4 Distribution of P-wave velocity structure and checkerboard resolution test at different depth in the study area.

表3 5个时间段反演前后到时差的均方根残差变化

Table3 The RMS residuals between observed and predicted differential travel times based on initial 3D model and final 3D model for the five periods

时间段	P1	P2	P3	P4	P5
初始3D模型/s	0.624	0.241	0.182	0.168	0.377
最终的3D模型/s	0.214	0.046	0.048	0.050	0.052
下降百分比/%	65.7	80.9	73.6	70	86.2

图 5 5个时间段不同深度处的P波速度棋盘测试结果 a—e依次对应时间段P1—P5

Fig. 5 Checkerboard resolution test of P-wave velocity for the five periods.

图 6 5个时间段不同深度处的P波速度结构和地震分布 a—e依次对应时间段P1—P5。黑色圆点为地震震中, 白色五角星为 MS6.6 主震震中, 白色圆点为 MS5.8 和 MS5.9 余震震中, 白色实线表示DWS>100的区域。F1澜沧江断裂; F2窝拖寨断裂; F3南谷断裂; F4永平盆地东缘断裂; F5威远江断裂; F6益香-赵家沟断裂; F7景谷-云仙断裂; F8无量山断裂。断裂构造数据引自毛泽斌等(2019)

Fig. 6 Distribution of P-wave velocity structure and earthquakes at different depths for the five periods.

表4 不同时间段的数据

Table4 Data of different periods

时间段	所使用的地震数目/个	所用的台站数/个	P波到时差/个	平均空间距离/km
P2相对于P1	3 913	38	45 844	10.9
P3相对于P2	4 202	20	593 378	3.0
P4相对于P3	3 890	20	438 637	3.1
P5相对于P4	3 716	34	247 451	8.7

表5 不同时间段反演前后到时差的均方根残差变化

Table5 The RMS residuals between observed and predicted differential travel times based on initial 3D model and final 3D model for the different periods

时间段	P2相对于P1	P3相对于P2	P4相对于P3	P5相对于P4
初始3D模型/s	0.524	0.075	0.069	0.266
3D速度变化/s	0.124	0.010	0.011	0.047
下降百分比/%	76.3	86.7	84.1	82.3

图 7 不同时间段之间在不同深度处的棋盘测试结果 a 时间段2相对于时间段1; b 时间段3相对于时间段2; c 时间段4相对于时间段3; d 时间段5相对于时间段4

Fig. 7 Recovered checkerboard models for different periods.

图 8 不同时间段之间在不同深度处的P波速度变化和地震分布 a 时间段2相对于时间段1; b 时间段3相对于时间段2; c 时间段4相对于时间段3; d 时间段5相对于时间段4。黑色圆点表示地震震中, 白色五角星为 MS6.6 主震震中, 白色圆点为 MS5.8 和 MS5.9 余震震中, 红色实线表示DWS>500的区域, DWS<500的区域被阴影化。F1澜沧江断裂; F2窝拖寨断裂; F3南谷断裂; F4永平盆地东缘断裂; F5威远江断裂F6益香-赵家沟断裂; F7景谷-云仙断裂; F8无量山断裂。断裂构造数据引自毛泽斌等(2019)

Fig. 8 Distribution of temporal P-wave velocity changes and earthquakes at different depths for different periods.

图 9 不同时间段之间的P波速度变化沿垂直剖面AA'和BB'的分布图(剖面位置见图1b) a P2相对于P1; b P3相对于P2; c P4相对于P3; d P5相对于P4。红色实线表示DWS>500的区域, DWS<500的区域被阴影化, 绿色五角星为 MS6.6 主震震源, 绿色圆形为 MS5.8 和 MS5.9 余震震源, 黑色圆点表示地震

Fig. 9 Distribution of temporal P-wave velocity changes along the cross sections AA', BB' for different periods.

图 10 景谷地震序列分布图与震源区电性结构及岩石强度模型(改自王烁帆等, 2019) a 景谷地震余震分布图(Wang et al., 2018); b、 c 余震在2个剖面上的分布(Wang et al., 2018); d 景谷地震震源区电性结构模型(改自程远志等, 2016), 其中白色实线表示岩石强度随深度的变化曲线(数据源自孙玉军等, 2013)

Fig. 10 The hypocenter map of the Jinggu earthquake sequence and the electrical structure and rock strength model of the source area(after WANG Li-fang et al., 2019).

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