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

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