地震学报2025，Vol.47Issue(5)：611-631,21.DOI:10.11939/jass.20240026
2023年云南芒市MS5.0地震震源深度测定及其发震机理探讨
Focal depth determination of the MS5.0 earthquake in Mangshi,Yunnan,2023,and discussion on its seismogenic mechanism
摘要
Abstract
Focal depth is an important parameter for the study of regional seismicity and seis-mic hazard.Accurate focal depths can provide valuable references for seismic hazard assess-ment and seismogenic mechanism research.However,it is a challenge to determine an accu-rate 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.There-fore,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 with MS5.0(local mag-nitude is ML5.3)occurred in Mangshi,Yunnan Province,followed by four ML≥3.5 after-shocks.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 ofthe mainshock(MS5.0)and four aftershocks(ML≥3.5)in the Mangshi earthquake se-quence.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 fault component.The optimal double-couple focal mechan-ism solution of the mainshock is as follows:strike 89°,dip 78°,rake-20° for fault plane Ⅰ;strike 183°,dip 70°,rake-167° for fault plane Ⅱ.The four ML≥3.5 aftershocks exhibit strike-slip with thrust or pure thrust mechanisms,the optimal double-couple focal mechanism solu-tions for these aftershocks all feature fault planes trending northeast(NE),and the strike,dip,and rake 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 major axis of seismic intensity for the Mangshi earthquake sequence.Furthermore the focal mechanisms of both the main-shock and the four aftershocks of ML≥3.5 have complex orientations of the P and T axes,in-dicating the presence of a complex stress regime within the source region.It is plausible to spec-ulate 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 charac-teristics 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 reser-voir area occurred during periods of high annual water levels.Among them,the largest earth-quake,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 March 2016 to December 2017,during which the wa-ter 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)over a period of almost 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 mainshock and aftershocks,this study tentatively hypo-thesizes that the infiltration of fluids into pre-existing fault fractures with potential for generat-ing 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 ad-justments of the local stress field,triggering slip along nearby NE-trending faults,and result-ing in the predominant NE-oriented distribution of aftershocks in this seismic source area.关键词
芒市地震序列/震源深度/震源机制解/sPL深度震相/发震机理Key words
Mangshi earthquake sequence/focal depth/focal mechanism solutions/sPL depth phase/seismogenic mechanism分类
天文与地球科学引用本文复制引用
顾慧冬,姜金钟,李姣,常玉巧,杨跃文,王光明,张帅,姚远..2023年云南芒市MS5.0地震震源深度测定及其发震机理探讨[J].地震学报,2025,47(5):611-631,21.基金项目
云南省地震局科技专项(2021ZX04,2023ZX03)、"云南地震重点危险区密集台阵观测系统建设"项目和云南省地震局"地震机理与孕震环境研究"创新团队项目共同资助. （2021ZX04,2023ZX03）
