Abstract:

The Yilan-Yitong fault zone(YYFZ)is the western branch of the two major branches in the northeastern segment of the Tanlu fault zone(TLFZ). Influenced by the subduction of the western Pacific Plate, the northern section of this fault(the Tangyuan-Luobei section and the Fangzheng-Tangyuan section)is seismically active. In particular, the Tangyuan-Luobei section experiences frequent moderate and small earthquakes, while the Fangzheng-Tangyuan section has a background of strong earthquakes of magnitude 7 and above. A scientific understanding of the seismic mechanisms in this area is of significant reference value for analyzing the seismic hazards of the northern section of the Yilan-Yitong Fault.

This study determins the source mechanism solutions, calculates the regional GPS strain field, extracts point deformation anomalies, and computes the relative movement speed changes of the blocks on both sides of the northern section of the Yilan-Yitong fault. It also considers the effects of M7 and above strong earthquakes from the Japan Trench on positive loading(superposition) of co-seismic and post-seismic Coulomb stress and the post-seismic viscoelastic relaxation effect, providing a comprehensive analysis of the strain field status and seismic mechanisms in the northern section of the Yilan-Yitong fault.

The results indicate that:

(1)The strain field characteristics of the northern section of the Yilan-Yitong fault are characterized by NE-directed compression and NW-directed extension. The direction of the regional strain field is consistent with that of the background stress field, indicating that the currently observable strain field is controlled by the background stress field.

(2)The relative movement speed on both sides of the Yilan-Yitong fault in the Tangyuan-Luobei section has increased, and the corresponding seismic activity has intensified, indicating that the earthquake activity in this area is closely related to fault movement.

(3)The long-term trend of the water pipe tilt instrument, the gravity tidal factor, and the vertical oscillation tidal factor are roughly synchronous with the abnormal time period of relative block movement, suggesting that the elastic properties of the underground medium change due to tectonic movements, essentially reflecting changes in the internal stress field of the medium. The regional point deformation, gravity tidal factor, and abnormal changes in the strain field are correlated with seismic activity.

(4)The time series of the maximum shear strain parameter shows a continuous increase in the maximum shear strain in the region, indicating strong shearing effects in this area. Similar to the time series of the volumetric strain parameter, the amplitude of the maximum shear strain parameter during this active earthquake period significantly exceeded the limit. The over-limit of volumetric strain and maximum shear strain parameters may be anomalous phenomena related to seismic activity.

(5)Due to the western Pacific plate subduction, differential motion occurs between the Sanjiang Basin and the Xiaoxing’anling Uplift flanking the Tangyuan-Luobei section. While the strong tectonic activity of the main fault facilitates energy release, making it less prone to large earthquakes, the NW-striking secondary faults perpendicular to it are more fragmented. These secondary faults are prone to generate slip under shear stress, producing strike-slip type shallow moderate and small earthquakes. This may explain the frequent moderate and small earthquakes in the Tangyuan-Luobei section. The seismogenic process is accompanied by changes in the physical properties of the medium, reflecting alterations in the crustal stress field, as evidenced by significant annual and trend changes in point deformation and anomalies in the M2 wave tidal factor.

(6)The results of the co-seismic static Coulomb failure stress indicate that the northern section of the Yilan-Yitong fault is located in a region of positive Coulomb triggering stress from the subduction-type strong earthquakes offshore Honshu, Japan. The Tonghe section is the main area affected by co-seismic Coulomb failure stress, and strong earthquakes facilitate seismic events on the northern section of the Yilan-Yitong fault.

Although the impact of single strong earthquakes from the Japan trench is limited, considering the high frequency of M7 and above earthquakes and a recurrence period of approximately one year, the long-term cumulative effects over thousands of years cannot be ignored. The strong seismic activity on the northern section of the Yilan-Yitong fault primarily originates from local tectonic stress field actions, as well as the frequent subduction-type strong earthquakes from the offshore Honshu region of Japan that trigger positive loading (superposition) and post-seismic Coulomb stress effects on the northern section of the Yilan-Yitong fault zone. Due to relatively few deep earthquakes in the Fangzheng-Tangyuan section, stress is not sufficiently released, leading to the formation of stress accumulation areas that trigger strong earthquakes; this may be one of the important factors for strong earthquakes occurring in the Fangzheng-Tangyuan section.

Key words: the northern section of Yilan-Yitong Fault, GPS strain field, source mechanism solution, static coulomb rupture stress

摘要：

依兰-伊通断裂是郯庐大断裂东北段两大分支中的西支, 其北段地区(汤原—萝北段、 方正—汤原段)地震活动较为活跃。具体表现为汤原—萝北段中小地震频发, 而方正—汤原段具备发生7级以上强震的背景。文中基于依兰-伊通断裂北段震源机制解、 GPS应变场, 定点形变和重力潮汐因子异常, 以及日本本州近海俯冲带强震对依兰-伊通断裂静态库仑应力影响等计算结果, 分析并讨论了依兰-伊通断裂北段的应变状态及发震机制。研究结果表明: 依兰-伊通断裂北段应变场受背景应力场控制, 呈NE向挤压与NW向拉张特征。区域应变场、 定点形变及重力潮汐因子异常与地震活动之间存在一定相关性。在西太平洋板块俯冲作用下, 依兰-伊通断裂汤原—萝北段两侧的三江断陷盆地相对小兴安岭隆起产生运动差异, 在NW向次级断裂群区域岩石易错动破裂, 引发中小规模浅震活动。依兰-伊通断裂区域构造应力场、 日本本州近海俯冲带强震同震和震后库仑应力正向加载触发(叠加)和震后黏弹性弛豫等共同作用, 可能是推动方正—汤原段发生强震的重要因素之一。

关键词: 依兰-伊通断裂北段, GPS应变场, 震源机制解, 库仑破裂应力