DEEP STRUCTURAL CHARACTERISTICS IN THE XINGTAI SEISMIC ZONE REVEALED BY PRECISE RELOCATION OF SMALL EARTHQUAKES AND TOMOGRAPHY

doi:10.3969/j.issn.0253-4967.2023.06.005

Abstract

Abstract:

From March 8^th to 29^th, 1966, five earthquakes(M≥6)occurred in the Xingtai area, with the M_S6.8 earthquake on March 8^th and the M_S7.2 earthquake on March 22^nd being the most severely damaged. The Xingtai earthquake resulted in over 8 000 deaths and the economic losses up to 1 billion yuan. The Xingtai earthquake has opened the scientific practice of earthquake prediction in China and is a milestone in the development of earthquake science in China.

Based on previous research results, there is a deep fault beneath the Xingtai earthquake area, which is the energy source of earthquakes, while there is a relatively independent fault system in the shallow part, which is generally recognized by scholars. However, the divergence regarding the seismogenic structure of the Xingtai earthquake mainly focuses on the unclear coupling relationship between the deep and shallow structural systems in the seismic area. The structural relationship between deep seismic faults and the shallow Xinhe Fault system requires new evidence to determine. In addition, previous scholars have proposed the viewpoint of “Newly generated Fault”, which can better explain the rupture characteristics of the Xingtai earthquake, but it still needs to be supported by the inversion results of the seismic rupture process based on the three-dimensional crustal fine structure. There are many small earthquakes in the Xingtai area. Deep structural information can be obtained using small earthquake data. Especially after 2009, the significant improvement in earthquake positioning accuracy in North China has made it possible to obtain new insights into deep structures. By locating small earthquakes, the spatial distribution and motion characteristics of faults are characterized, and seismic travel time tomography reveals the deep crustal velocity structure characteristics of the earthquake area. Combining previous geophysical exploration results, conducting deep and shallow structural analysis is of great significance for studying the spatial distribution, motion characteristics, and coupling relationship between deep and shallow structural systems of the fault system in the study area. The continuous aftershocks after the 1966 M_S7.2 earthquake in Xingtai, Hebei Province, have provided favorable conditions for conducting studies on deep tectonic structures in the region.

In this paper, based on the observation data of the Hebei seismostation from 1991 to 2021, we obtained the precise position results of 9 644 earthquakes in Xingtai and its neighboring area using the double-difference positioning method, and depicted the spatial patterns of deep ruptures. Based on the observation data of the North China Mobile Seismic Array from 2006 to 2008, 38 578 P-wave arrivals were used to obtain high-resolution travel time tomography results in the study area. This study shows that there are strong lateral heterogeneities in the velocity structure of the crust in the study area, with obvious low-velocity anomalies in the upper crust and high-velocity anomalies in the middle and lower crusts between the Xinhe Fault and the Yuanshi Fault, and the Xingtai earthquake is located at the junction of the high- and low-velocity anomalies, which has the medium conditions for accumulating large amounts of strain energy and is prone to rupture and stress release. The general trend of the dense zone of small earthquakes in the Xingtai earthquake area is relatively consistent with that of the eastern boundary of the high- and low-velocity anomalies. It is assumed that the deep and shallow fractures spreading along the eastern boundary of the high- and low-velocity bodies have been connected up and down and that the boundary of the anomalies is also a part where velocity changes are relatively strong and easily lead to seismic rupture; the results of various seismic and geological surveys have revealed that a deep major rupture that cuts through the entire crust exists beneath the Xingtai earthquake zone, with SE tendency and the upper breakpoint located near Dongwang, and the Xingtai earthquake prompted the deep and shallow pre-existing ruptures to connect from top to bottom.

Key words: Xingtai earthquake, precise relocation of small earthquake, tomography, deep structure, decollement structure

摘要：

1966年河北邢台 M_S7.2 地震发生后小地震持续不断, 为开展该地区的深部构造研究提供了便利条件。文中基于河北省地震台网1991—2021年间的观测数据, 采用双差定位法获得了邢台及其邻区9 644次地震的精定位结果, 清晰地刻画出深部断裂的空间形态; 结合华北流动地震台阵2006—2008年的观测数据, 采用38 578个P波到时数据, 获得了研究区高分辨率走时层析成像结果。研究表明: 研究区的地壳速度结构存在较强的横向不均匀性, 新河断裂与元氏断裂之间的上地壳存在明显的低速异常, 中下地壳存在高速异常, 邢台地震位于高、低速异常体交界处, 该位置具备积累大量应变能的介质条件, 易发生破裂, 释放应力; 邢台震区小地震密集带与高、低速异常体东边界的总体展布趋势比较一致, 推测沿高、低速体东边界展布的深、浅部断裂已上下贯通, 速度异常体的边界位置同样也是速度变化相对强烈的部位, 易发生地震破裂; 综合多种地震、地质调查成果, 揭示出邢台震区下方存在一条贯通整个地壳的深大断裂, 倾向SE, 上断点位于东汪附近, 邢台地震促使深浅部的先存断裂上下贯通。

关键词: 邢台地震, 小地震精定位, 层析成像, 深部构造, 滑脱构造

LIU Kang, YANG Ting, LI Hong-guang, FANG Li-hua, SONG Jian. DEEP STRUCTURAL CHARACTERISTICS IN THE XINGTAI SEISMIC ZONE REVEALED BY PRECISE RELOCATION OF SMALL EARTHQUAKES AND TOMOGRAPHY[J]. SEISMOLOGY AND GEOLOGY, 2023, 45(6): 1328-1348.

刘亢, 杨婷, 李红光, 房立华, 宋键. 小地震精定位与层析成像揭示的邢台地震区深部构造特征[J]. 地震地质, 2023, 45(6): 1328-1348.

Figures/Tables 10

Fig. 1 Distribution of faults and historical earthquakes(M≥5.0)in the study area.

Fig. 2 Distribution of stations.

Fig. 3 Relationship t of epicentral distance and travel time.

Fig. 4 P-wave initial velocity model.

Fig. 5 Precision positioning results of small earthquakes in the study area.

Fig. 6 Histograms of the focal depth distributions after earthquake relocation.

Fig. 7 Distribution of epicenters after relocation and vertical profiles in the Xingtai earthquake region.

Fig. 8 P-wave velocity disturbance image at the depth of 1~45km in the study area.

Fig. 9 Vertical profiles of P-wave velocity disturbance in the study area.

Fig. 10 Deep seismic reflection profiles in Xingtai earthquake region.

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