Abstract:

On January 23, 2024, an M_S7.1 earthquake occurred in Wushi, Xinjiang, China. The Wushi M_S7.1 earthquake was a thrust-dominated rupture with a strike-slip component, trending nearly NE, dipping NW, and exhibiting a relatively steep dip angle. Most of the M_S≥4.0 aftershocks in the Wushi sequence also displayed thrust or thrust-dominated strike-slip rupture mechanisms. The aftershocks were primarily distributed along the northeastern segment of the Maidant-Shayilam Fault, generally extending in a NE-NEE direction, with a distribution scale of approximately 62km. In terms of the aftershock activity, the M_S5.8 strongest aftershock on February 25 can be regarded as a significant temporal marker in the sequence's evolution: before February 25, aftershock activity was intense, with maximum seismic activity levels ranging from M_S5.0 to M_S6.0; afterward, aftershock activity noticeably weakened, maximum seismic activity levels dropping to the range of M_S4.0 to M_S5.0. Specifically, among the 50 M_S≥4.0 aftershocks recorded until April 30, 2024, 45 occurred before February 25, including all 10 M_S≥5.0 strong aftershocks. A similar pattern was observed in the temporal variation of focal mechanisms-before February 25, the focal mechanisms showed good consistency, whereas afterward, their consistency deteriorated. The stronger aftershocks with M_S≥4.0 in Wushi sequence exhibited significant temporal clustering characteristics. In the early post-seismic stage before February 25, the vast majority of M_S≥4.0 stronger aftershocks occurred during spring and neap tidal periods, displaying clear triggering features associated with the semimonthly tide. During this period, although aftershock activity gradually weakened and earthquake frequency decreased, the timing of larger aftershocks still indicated modulation by factors with a periodicity of “day” or its multiples, closely related to diurnal or semidiurnal tidal triggering.
Focusing on the tidal triggering phenomenon of larger aftershocks in the Wushi sequence, this study calculates the tidal normal stress, tidal shear stress, and tidal Coulomb failure stress changes(ΔCFS) on both nodal planes of the focal mechanism solutions for all M_S≥4.0 earthquakes in the sequence before and after the events. Based on the results of these calculations, we analyze the tidal triggering characteristics of the occurrence times of larger earthquakes in the sequence. The study primarily focuses on two aspects: one aspect examines the statistical characteristics of tidal triggering, specifically the triggering patterns of larger aftershocks in relation to spring-neap tides and semidiurnal tides. Another aspect is the mechanical relationship between tidal triggering and fault motion, investigating which tidal stress component(under different tidal force conditions)plays a dominant role in triggering aftershock activity in the Wushi sequence. In our study, ΔCFS>0 serves as the fundamental prerequisite for possible tidal triggering of seismic activity and is a necessary condition for determining tidal influence. Building upon this criterion, we further explore whether the triggering of seismic rupture is primarily due to enhanced tensional effects(increased normal stress), enhanced shear effects(increased shear stress), or a combination of both. The analysis aims to clarify the dominant tidal stress mechanism driving aftershock activity in the Wushi earthquake sequence.
The results indicate that the more vigorous aftershock activity of Wushi sequence during spring tide periods may be related to the progressive increase in tidal ΔCFS on the mainshock rupture plane(Nodal Plane I). In contrast, the fluctuations in stronger aftershocks during neap tide periods may be associated with the peak and subsequent decline of tidal ΔCFS on the auxiliary focal mechanism plane(nodal Ⅱ). From the perspective of tidal stress components, the fluctuations of M_S≥4.0 stronger aftershocks during syzygy spring tides(new/full moons) are primarily driven by the combined effects of: maximum tidal normal stress(σ_n) and contemporaneously increasing tidal shear stress(τ_n) on nodal I. This suggests that the fracture planes of the triggered earthquakes are likely closer to nodal I of the mainshock's focal mechanism. Conversely, the stronger aftershock activity during quadrature neap tides(first/third quarter moons)is mainly influenced by the combined effects of the maximum tidal shear stress(τ_n) and contemporaneously increasing tidal normal stress(σ_n) on nodal Ⅱ, implying that the rupture planes of these triggered earthquakes may be closer to nodal II of the mainshock's focal mechanism.
The findings further reveal that 70% of M_S≥4.0 earthquakes in the Wushi sequence exhibit significant semi-diurnal tidal triggering characteristics, with 64% of these events showing tidal triggering effects on both nodal planes of their focal mechanisms. This phenomenon may be attributed to the unique geometric relationship between the nodal planes of the thrust-type ruptures in the Wushi M_S7.1 mainshock and most M_S≥4.0 aftershocks, where the two nodal planes share similar or nearly identical strikes but dip in opposite directions. It should be noted that tidal triggering signals on both nodal planes do not imply simultaneous rupture on both planes. Further analysis indicates that the semi-diurnal tidal shear stress component, superimposed along the fault slip direction, serves as the dominant tidal triggering factor for M_S≥4.0 earthquakes in the Wushi sequence, with 60% of the triggered events being influenced by tidal shear stress(τ_n). Regarding the temporal relationship between earthquake occurrence and tidal stress, 94% of the semi-diurnally triggered M_S≥4.0 earthquakes occurred either during the peak tidal stress phase(within ±1.5 hours of the peak stress)or the adjacent stage following the peak stress.
It is noteworthy that the relatively relaxed and fractured structural and medium environment of the aftershock zone, combined with the critically high stress state immediately after the mainshock, along with the cumulative “slip-promoting” effect resulting from the co-directional superposition of tidal stresses and tectonic stresses, may be the possible reasons for the pronounced tidal triggering characteristics displayed by the early strong aftershocks in the Wushi earthquake sequence.

Key words: Wushi earthquake sequence, tidal triggering of seismic activity, tidal stress components, spring-neap tides, semi-diurnal tides

摘要：

2024年1月23日新疆乌什发生 M_S7.1 地震。乌什 M_S7.1 地震为逆冲型破裂, M_S≥4.0的余震大多发生在2024年2月25日 M_S5.8 最大余震之前的早期阶段。余震分布尺度约62km, 沿迈丹-沙依拉姆断裂东北段大体上呈NE-NEE向展布。乌什序列M_S≥4.0余震的发生时间具有显著的半月潮大、 小潮触发特征。进一步研究显示, 乌什序列大潮时段的较强余震活动, 主要与主震破裂面节面 Ⅰ 上潮汐ΔCFS持续增强趋于峰值的变化过程有关, 而小潮时段的较强余震起伏则可能与震源机制节面 Ⅱ 上潮汐ΔCFS自峰值时段开始的转折下降有关。从潮汐应力分量具体来看, 乌什序列M_S≥4.0较强余震在半月潮朔、 望大潮时段的起伏, 主要缘于节面 Ⅰ 上的最大潮汐正应力σ_n及同时期持续增大的潮汐剪应力τ_n共同作用的影响; 而上、 下弦小潮时段的较强余震活动, 则主要缘于最大潮汐剪应力τ_n及同时期持续增大的潮汐正应力σ_n两者共同作用的影响, 被触发地震的发震断层的产状与主震震源机制的节面 Ⅱ 可能较为接近。研究结果还显示, 乌什序列70%的M_S≥4.0地震具有显著的半日潮触发特征, 其中64%震例的震源机制2个节面均显示受半日潮触发影响, 这与乌什 M_S7.1 地震及多数M_S≥4.0余震逆冲型破裂特殊的震源机制节面关系(2个节面走向大体一致、 倾向相反)有关。沿断层滑动方向叠加的半日潮剪切应力是导致乌什地震序列较强地震发生的主要潮汐触发因素, 在所有受半日潮触发的M_S≥4.0地震中, 60%的震例缘于潮汐剪应力的触发影响。从发震时刻来看, 94%的地震发生在潮汐应力分量峰值时段(峰值前后1.5h内)及紧邻的峰后时段。余震区相对松弛、 破碎的构造和介质环境, 震后短时期不稳定的临界高应力状态, 加之潮汐应力与构造应力“同向促滑”的叠加效应, 是乌什序列早期阶段较强余震活动显示明显的潮汐触发特征的可能原因。

关键词: 乌什序列, 地震活动的潮汐触发, 潮汐应力分量, 半月潮, 半日潮