Abstract:

Conjugate faults are a pair of faults developed under the identical regional tectonic stress fields with cross-cutting structures and opposite shear senses. They have been applied to restore the ancient regional tectonic stress fields, and the mechanics of local crust during its formation can be reflected by their dihedral angle. The ~60° intersecting conjugate fault occurs under brittle environment as proposed by the Anderson theory, while the 110° intersecting conjugate fault could be formed under the conditions of ductile environment as explained by maximum effective moment(MEM)criterion. In addition, there is another kind of conjugate faults with ~90° intersecting angle, which have been observed globally, but the mechanism of their formation still remains unsolved.
Conjugate faults have been intensively studied using traditional geological methods and laboratory rock experiments. Interferometric synthetic aperture radar(InSAR), as an important geodetic mapping tool with an unprecedented precision and spatial resolution, provides a potential for investigating conjugate faults by exploring three-dimensional geometric structures. In this study, we investigated the 2019 M_w≥6.4 Philippines earthquake sequence as an example to link the present deformation characteristics of the ruptured conjugate faults to the regional tectonic stress.
From October to December 2019, four M_W≥6.4 earthquakes occurred in Mindanao, Philippines. The epicenters were located in the Philippine Sea plate, at the junction of the Eurasian plate, the Pacific plate and the Indian Ocean plate. Affected by three-sided subduction, the plate boundaries are almost convergent boundaries with active tectonic movement and frequent seismic activities. The target earthquake sequence occurred in Mindanao where the Philippine Sea plate collided with the Sunda plate. According to the GCMT earthquake catalog, this earthquake sequence shows similar focal mechanisms to the eight M_W≥5.0 earthquakes in the study area before this earthquake sequence from 1992, which will have certain implications for the research on local mechanical background.
This study collected both C-band Sentinel-1 TOPS and L-band ALOS-2 SAR images in ascending and descending tracks to retrieve surface deformation of the earthquake sequence. Four Sentinel-1 interferograms and three ALOS-2 interferograms were obtained using an InSAR open source package: GMTSAR. Based on the latest global atmospheric model, ERA5, the atmospheric phase delay correction was conducted, and the standard deviations(SDs)of the used Sentinel-1 and ALOS-2 interferograms before and after correction were reduced from 1.94cm and 3.55cm to 1.93cm and 3.46cm, respectively. The improved InSAR deformation products were used for earthquake fault modelling with a geodetic inversion package PSOKINV, which is based on the elastic half-space dislocation model, also called “Okada Model”. The obtained faults were further divided into several sub-faults with small patch-sizes to determine the accumulated distributed slip. The predicted interferograms from the obtained slip models can fit the original interferograms well, and the SDs of the residuals of Sentinel-1 and ALOS-2 interferograms were 1.55cm and 3.36cm, respectively, which were lower than the noise levels of the original InSAR data.
The inversion results show that the four earthquakes mainly resulted from the ruptures of one dextral strike-slip fault(F₁)of strike 48.8°, dip 74.5° and slip angle -174.1°, and the other sinistral strike-slip fault(F₂)of strike 318.2°, dip 68.9° and slip angle 9.6°. The surface intersection of the two faults is nearly orthogonal, while the minimum spatial rotation angle between the two slip vectors is 29.28°. The latter indicates that two slip vectors are not completely conjugate in the seismological sense. The angle bisector of F₁ and F₂ is basically consistent with the azimuth of the regional principle compressive stress derived from seismic data, which also agrees with the horizontal components of the GPS velocities observed in the island. Given that the oblique direction of converging between the Philippine Sea and Sunda plates, a clear rotation of the regional stress conditions could have happened across the Philippine strike-slip fault.
Furthermore, 4790 aftershocks in the study area from October to December 2019 recorded by the local seismic network show that the aftershocks are evenly distributed above a depth of 31km, which is the depth of the Moho based on previous studies. Therefore, the seismogenic faults of the earthquake sequence could have extended to the Moho boundary, indicating that it is likely that they may have formed in the ductile mechanical environment originally. The Coulomb stress change(CSC)analysis indicates that the rupture of one branch of the conjugate faults can release stress on the both fault planes in the vicinity of their interaction, and pose positive CSC in the far fields simultaneously, in which CSC on itself is larger. Meanwhile, combined with 14 sets of conjugate faults collected globally in this study, L-shaped characteristics of the conjugate faults turn to be common. The phenomenon having different rupture lengths and slip magnitudes for each fault branch in a set of conjugate faults is likely related to the significant variations of the fault physical properties.

Key words: orthogonal conjugate faults, 2019 Philippines earthquake sequence, InSAR coseismic deformation, regional tectonic stress field

摘要：

共轭断裂是恢复区域构造应力场方位的主要地质学证据之一, 其交角大小可能反映了当地的力学环境。交角约90°的情况是其中重要的一类, 但目前对其认知仍较为有限。为研究此类断层的构造指示意义, 文中以2019年10—12月菲律宾4次M_W≥6.4地震序列为例, 采用InSAR空间测量学方法, 精细研究了该过程的地表形变特征, 进而确定了该地震断层系统的几何参数。结果显示, 这4次地震发生在走向为48.8°、 倾角为74.5°的右旋走滑断裂与走向为318.2°、 倾角为68.9°的左旋走滑断裂上, 可见2条断裂具有正交共轭的特征。经计算, 2条断层主要滑动矢量间的最小旋转角达29.28°, 在地震学意义上并不完全共轭。根据区域应力场结果, 该共轭系统的剪裂角平分线方向与区域压应力的方位基本一致。余震分布显示发震断层延伸到莫霍面边缘, 具备了成断层时潜在的韧性剪切条件。库仑应力模拟显示在共轭断裂系统中1支上的滑动过程可同时造成2支在交叉处一致的应力卸载作用, 表明共轭系统中的2条断层对区域应力场卸载具有等效性。在全球范围内, 搜集获取的共轭断层系统活动序列显示以“L”型展布的几何特征为主, 且呈现2支断层发育不平衡的现象, 该差异性很可能与断层之间的物性差异有关。

关键词: 正交共轭断裂, 2019年菲律宾地震序列, InSAR同震形变, 区域构造应力场