Focal depth is an important parameter for the study of regional seismicity and seismic hazard. Accurate focal depths can provide valuable references for seismic hazard assessment and seismogenic mechanism research. However, it is a challenge to determine an accurate focal depth for earthquakes that occur in regions with sparse seismic networks. Traditional methods relying on seismic wave （e.g. the P and S phases） arrival times are severely limited by network density, resulting in low measurement accuracy. Nonetheless, utilizing information such as seismic wave amplitudes, spectra, and depth phases, even in sparse seismic networks, can facilitate the accurate determination of focal depths.
In recent years, the sPL depth phase method has been widely utilized for determining the focal depths of local small and moderate earthquakes. The travel time of the sPL depth phase is primarily related to the focal depth and almost independent of the epicentral distance. Therefore, utilizing the sPL depth phase method not only avoids the compromise between the origin time and focal depth of earthquakes but also effectively reduces measurement errors induced by velocity models. Moreover, the CAP （cut-and-paste） method is a full waveform inversion method with significant advantages in determining focal depths for moderate earthquakes.
On 2 December, 2023, at 01:36:33 Beijing time, an earthquake of MS5.0 （local magnitude is ML5.3） occurred in Mangshi, Yunnan Province, followed by four ML≥3.5 aftershocks. Different institutions have reported significant disparities in the determined focal depth of the Mangshi MS5.0 mainshock. Hence, it is necessary to reassess the focal depth of the MS5.0 mainshock by using more regional seismic waveforms and different methods. Based on the broadband waveform data from the Yunnan Seismic Network and two regional velocity models, this study employed the CAP method to invert the focal mechanisms and focal depths of the mainshock （MS5.0） and four aftershocks （ML≥3.5） in the Mangshi earthquake sequence. Additionally, we employed the sPL depth phase to further determine the focal depths. Our research results indicate that: the Mangshi MS5.0 mainshock is characterized by a strike-slip fault with a significant normal component. The optimal double-couple focal mechanism solution is as follows: fault plane I: strike 89°, dip 78°, rake −20°; fault plane Ⅱ: strike 183°, dip 70°, rake −167°. The four ML≥3.5 aftershocks exhibit strike-slip with thrust or pure thrust mechanisms, the optimal double-couple focal mechanism solutions for these aftershocks all feature fault planes trending northeast （NE）, and the strike, dip, and rake angles of the average plane for the four ML≥3.5 aftershocks are approximately 247°, 65°, 26°, respectively. This orientation of the average plane is consistent with the distribution of the double-difference relocated aftershocks and the orientation of the maximum seismic intensity axis of the Mangshi earthquake sequence. Furthermore, the focal mechanisms of both the mainshock and the four aftershocks of ML≥3.5 have complex orientations of the P and T axes, indicating the presence of a complex stress regime within the source region. It is plausible to speculate that the rupture of the MS5.0 mainshock may have triggered nearby faults （with different fault planes） due to regional stress adjustments, resulting in significant differences in the focal mechanism types between the MS5.0 mainshock and the subsequent four ML≥3.5 aftershocks. Additionally, by using the CAP method, the optimal focal depth of the MS5.0 mainshock in Mangshi is determined to be 7 km, while the focal depths of the four ML≥3.5 aftershocks range from 5 to 7 km. On the other hand, the focal depth of the MS5.0 mainshock in Mangshi is estimated to be 7 km, and the focal depths of the four aftershocks are all approximately 5 km by utilizing the sPL depth phase method. The consistency between the focal depths determined by both two methods （with a difference of less than 2 km） indicates that the Mangshi earthquake sequence mainly occurred in the shallow part of the upper crust.
Considering that the epicenter of the Mangshi earthquake is located within the Longjiang Reservoir area, we statistically analyze the relationship between the ML≥1.0 seismic events and water level changes in the Longjiang Reservoir area from 2010 to 2024 to depict the characteristics of seismic activities in the reservoir area after the reservoir impoundment. Our study results indicate that seismic events with ML≥3.0 in the Longjiang Reservoir area are closely related to reservoir water levels, except for one ML3.1 earthquake, which occurred during a period of low annual water level on June 8, 2013, all other ML≥3.0 earthquakes in the reservoir area occurred during periods of high annual water levels. Among them, the largest earthquake, with a magnitude of ML4.2, occurred on September 24, 2011, during a period of high water levels after the impoundment of the Longjiang Reservoir.
The seismic activities in the Longjiang Reservoir area can be divided into four stages: the first stage is from January 2010 to February 2016, during which seismic activities of ML≥1.0 earthquakes were relatively calm （monthly frequency less than 10 times）, with a calm period of about six years; the second stage is from January 2016 to December 2017, during which the water level of the reservoir changed drastically, and the seismic activities were relatively active, with monthly frequencies of ML≥1.0 seismic activity ranging from a dozen to several dozen times; the third stage is from January 2018 to September 2023, during which the seismicity with magnitude above ML1.0 was relatively calm （monthly frequency less than 10 times） by a period of about six years; the fourth stage is from October 2023 to December 2023, during which the reservoir was at a high water level and its water level changed drastically followed the Mangshi MS5.0 earthquake on December 2, suggesting that there may be a certain correlation between the water level changes in the Longjiang Reservoir and the occurrence of this MS5.0 earthquake.
Given that this earthquake occurred within the Longjiang Reservoir area, as well as factors such as high water levels, shallow hypocentral distribution, and conspicuous discrepancies in the focal mechanism solutions of the main and aftershocks, this study tentatively hypothesizes that the infiltration of fluids into pre-existing fault fractures with potential for generating moderate to strong earthquakes within the reservoir area may have facilitated the occurrence of this MS5.0 mainshock. Additionally, the rupture of the mainshock was likely to induce adjustments of the local stress field, triggering slip along nearby NE-trending faults, and resulting in the predominant NE-oriented distribution of aftershocks in this seismic source area.
Mountain tunnels in western China are mostly located in regions with high seismic intensity and complex geological and topographic conditions, where active faults are densely distributed. Many tunnels are constructed in near-fault strong earthquake zones. For example, in the ongoing construction of the Sichuan-Tibet Railway, tunnels account for as much as 70% of the total route, and 54 active faults are distributed along and near the alignment. Numerous seismic investigations have shown that near-fault ground motions have a significant destructive impact on tunnel structures. Notable cases include the 1999 Chi-Chi earthquake in Taiwan, the 2008 Wenchuan earthquake in Sichuan, the 2016 Kumamoto earthquake in Japan, and the 2022 Menyuan earthquake in Qinghai, all of which resulted in severe tunnel damage.
The long-period velocity pulses commonly present in near-fault ground motions are considered key factors contributing to structural damage. However, due to the limited availability of strong earthquake observation records, especially near engineered structures, near-fault pulse-type ground motion records remain scarce. As a result, many existing studies have relied on synthetic ground motion methods to generate seismic time histories for analysis. Currently, research on the seismic response of mountain tunnels under near-fault pulse-type ground motions remains limited, particularly concerning the mechanisms through which pulse characteristics influence tunnel seismic responses, which are still not well understood.
On the other hand, topographic amplification effects induced by mountain terrain also significantly affect tunnel seismic responses. Studies have shown that mountain topography can amplify peak ground velocity （PGV） due to multiple reflections of seismic waves on the surface, leading to localized wave focusing effects and significant spatial variability within the mountain. However, most existing studies on mountain tunnel seismic responses commonly adopt full-space or half-space site models, neglecting topographic effects. Additionally, variations in the incident wave angle can alter the propagation path and energy distribution of seismic waves, further increasing the spatial variability of seismic damage.
This study comprehensively considers the coupled effects of near-fault ground motion characteristics and topographic amplification and proposes a systematic research framework. First, artificial synthesis techniques are employed to generate near-fault pulse-type ground motions. The high-frequency components of two representative recorded ground motions serve as the basis, while the low-frequency components are fitted using an equivalent pulse model. By adjusting pulse parameters, ground motions with different spectral characteristics are synthesized. Second, based on an indirect boundary element method-finite element method（IBEM-FEM）coupled method, a cross-scale efficient analysis was achieved, bridging the amplification effects in kilometer-scale complex sites to the dynamic damage of tunnel cross-sections at the centimeter scale, thereby revealing underlying seismic response patterns. Finally, taking pulse amplitude, pulse period, and pulse number as key parameters, the influence of near-fault pulse-type ground motions with different incident angles on tunnel seismic responses is examined, revealing the damage and failure patterns of tunnel linings.
The results indicate that tunnel lining damage severity increases with pulse amplitude. Compared with directivity effects, fling-step effects are more sensitive to changes in pulse amplitude, and oblique incidence combined with high-amplitude pulses can cause more severe structural damage. Tunnel damage severity decreases as pulse period increases, with directivity effects being more sensitive to pulse period variations. Under oblique incidence conditions, tunnel structural damage severity increases with the number of pulses.
For seismic design of mountain tunnels in near-fault regions, the combined impact of near-fault pulse effects and topographic amplification should be carefully considered, and tunnel seismic strategies should be optimized. Firstly, the locations of seismic wave focusing and amplification within the mountain should be identified and avoided to reduce the adverse effects of localized seismic motion amplification on tunnel structures. Secondly, for high-amplitude pulse effects, it is recommended to strengthen the seismic design of tunnel linings, particularly in areas where fling-step effects are prominent, to mitigate the damage caused by extreme ground motions. Additionally, during the design process, the influence of seismic wave incidence angle should be fully considered, and reinforcement measures for tunnel linings should be implemented accordingly to enhance tunnel seismic performance under complex wave propagation conditions.
Geological CO2 sequestration （GCS） is an important means for human society to alleviate greenhouse gas effects and achieve the so-called ‘dual carbon goals’, namely the ‘carbon peeking’ and ‘carbon neutrality’. However, injecting CO2 into geologic formations on a large scale or at excessive rates may cause overpressure in the injected formation, which may in turn alter the shear strength of the intersected faults or preexisting fractures, posing environmental risks to GCS engineering by triggering ground surface deformation and/or induced seismicity. The pore pressure buildup caused by fluid injection and production is an important cause for ground deformation and induced seismicity. Evaluation of the geomechanical effects caused by CO2 injection is an indispensable part of GCS risk assessment. Based on the inclusion theory and Green’s function method, the existing analytical solutions for evaluating the geomechanics of a single fault has been expanded, to enables it to evaluate geomechanical effects associated with large-scale CO2 injection into reservoirs penetrated by three faults. A modular PYTHON-scripted utility tool composed of tens of functions was developed based on the analytical solution proposed in this study, which enables rapid assessment of the distribution of the fault slip patches and the maximum size of the fault slip patch. The maximum moment magnitude of induced earthquakes can be estimated for various injection scenarios and site settings. The major controlling factors of the seismic risks can be identified as well. Based on the analysis of case studies, the following conclusions are drawn: Injection causes horizontal displacement to concentrate on the burial depths of the fault that simultaneously contacts the reservoir and the surrounding rock （i.e., the overlying and underlying formation units）, while vertical displacement varies uniformly with depth. Due to lateral confinement, injection causes the reservoir to expand in both the caprock and baserock directions simultaneously. Injection results in a negative horizontal strain increment and a positive vertical strain increment within the reservoir. The horizontal normal strain is significantly concentrated in the surrounding rock below the hanging wall of the fault and the overlying surrounding rock above the hanging wall of the fault. While the vertical normal strain follows a similar pattern to the horizontal normal strain, it has opposite signs. The expansion of the reservoir causes the rocks inside the reservoir to be subject to compressive normal stress. The horizontal normal stress only has a positive range in the surrounding rock below the reservoir on the upper wall of the fault and the surrounding rock above the reservoir on the lower wall of the fault, while the vertical normal stress shows stress concentration in the surrounding rock area near the intersection of the fault and the reservoir, indicating that the surrounding rock in these areas is subject to tensile stress. CO2 injection leads to the concentration of shear stress increment near the four singularities of the fault. The risks of on-fault seismicity （i.e., the maximum fault slip size and the maximal moment magnitude） are relevant to the distance of the fault to the injection center. The major controlling geomechanical parameters of the induced seismicity include the Biot coefficient, Poission’s ratio, and the initial stresses and the initial pore pressure.
Top Read
Notice
Links
Hot Search
