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

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