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 M_S5.0 （local magnitude is M_L5.3） occurred in Mangshi, Yunnan Province, followed by four M_L≥3.5 aftershocks. Different institutions have reported significant disparities in the determined focal depth of the Mangshi M_S5.0 mainshock. Hence, it is necessary to reassess the focal depth of the M_S5.0 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 （M_S5.0） and four aftershocks （M_L≥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 M_S5.0 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 M_L≥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 M_L≥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 M_L≥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 M_S5.0 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 M_S5.0 mainshock and the subsequent four M_L≥3.5 aftershocks. Additionally, by using the CAP method, the optimal focal depth of the M_S5.0 mainshock in Mangshi is determined to be 7 km, while the focal depths of the four M_L≥3.5 aftershocks range from 5 to 7 km. On the other hand, the focal depth of the M_S5.0 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 M_L≥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 M_L≥3.0 in the Longjiang Reservoir area are closely related to reservoir water levels, except for one M_L3.1 earthquake, which occurred during a period of low annual water level on June 8, 2013, all other M_L≥3.0 earthquakes in the reservoir area occurred during periods of high annual water levels. Among them, the largest earthquake, with a magnitude of M_L4.2, 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 M_L≥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 M_L≥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 M_L1.0 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 M_S5.0 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 M_S5.0 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 M_S5.0 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.