Study on the Physical Process and Seismogenic Mechanism of the Yangbi MS 6.4 Earthquake in Dali,Yunnan Province

The Ms 6.4 earthquake occurred on May 21,2021 in Yangbi County,Dali Prefecture,Yunnan Province,which was the largest earthquake after the 2014 Jinggu Ms 6.6 earthquake,in western Yunnan.After the earthquake,the rapid field investigation and earthquake relocation reveal that there was no obvious surface rupture and the earthquake did not occur on pre-existing active fault,but on a buried fault on the west side of Weixi–Qiaohou–Weishan fault zone in the eastern boundary of Baoshan sub-block.Significant foreshocks appeared three days before the earthquake.These phenomena aroused scholars'intensive attention.What the physical process and seismogenic mechanism of the Yangbi Ms 6.4 earthquake are revealed by the foreshocks and aftershocks?These scientific questions need to be solved urgently.

Field survey and analysis on near-fault severely damaged high-speed railway bridge in 2022 M6.9 Menyuan earthquake

The 2022 M6.9 Menyuan earthquake caused severe damage to a high-speed railway bridge,which was designed for high-speed trains running at speeds of above 250 km/h and is located right next to the fault.Bridges of this type have been widely used for rapidly constructing the high-speed railway network,but few bridges have been tested by near-fault devastating earthquakes.The potential severe impact of the earthquake on the high-speed railway is not only the safety of the infrastructure,trains and passengers,but also economic loss due to interrupted railway use.Therefore,a field survey was carried out immediately after the earthquake to collect time-sensitive data.The damage to the bridge was carefully investigated,and quantitative analyses were conducted to better understand the mechanism of the bridge failure.It was found that seismic action perpendicular to the bridge’s longitudinal direction caused severe damage to the girders and rails,while none of the piers showed obvious deformation or cracking.The maximum values of transverse displacement,out-of-plane rotation and twisting angle of girders reached 212.6 cm,3.1 degrees and 19.9 degrees,respectively,causing severe damage to the bearing supports and anti-seismic retaining blocks.These observations provide a basis for improving the seismic design of high-speed railway bridges located in near-fault areas.

破坏性地震事件中微博阅读量分析——以2021年云南漾濞MS 6.4地震为例

统计2021年云南漾濞MS6.4地震前后云南省地震局发布的相关微博的阅读量,分析微博的受关注程度,以期为地震应急宣传工作提供参考。分析发现,有关政府部门抗震救灾工作措施的微博阅读量最高,微博的阅读量与地震序列的震级水平呈正相关。

Vertical deformation before and after the 2022 Menyuan Ms6.9 earthquake and analysis of earthquake precursors

This study analyzed the vertical deformation before and after the 2022 Menyuan Ms6.9 earthquake in Qinghai Province,China,using leveling profiles across faults measured from Minle County in Gansu Province to Menyuan County in Qinghai Province.Our results suggest the following:(1)The amplitude of regional vertical differential motion near the Sunna-Qilian and Lenglongling faults within the Qilian Shan increased before the 2022 Menyuan earthquake.It was accompanied by the emergence of high gradient deformation zones.Deformation at the Tongziba cross-fault leveling site near the Sunan-Qilian fault was considerable.In contrast,deformation at the Daliang cross-fault leveling site near the stepover region(adjacent to the epicenter)between the Lenglongling and Tuolaishan faults was minor.After 2018,vertical deformation at the Tongziba site notably accelerated,while that at the Daliang site was insignificant.(2)After the 2022 Menyuan earthquake,140—150 mm of subsidence deformation occurred near the Daliang site,while the Tongziba site did not experience significant deformation.(3)Vertical deformation before and after the 2022 Menyuan earthquake conforms with the elastic-rebound theory,and the evolution of pre-earthquake deformation was consistent with the strike-slip fault deformation pattern at different seismogenic stages,i.e.,the relative motion near the locked fault in the late seismogenic stage gradually weakened.The characteristics of strain accumulation and release derived from the vertical deformation before and after the Menyuan MS6.9 earthquake help understand the deformation process of earthquake preparation and earthquake precursors.

Synthetic aperture radar interferometry—based coseismic deformation and slip distribution of the 2022 Menyuan MS6.9 earthquake in Qinghai,China

On January 8,2022,a 6.9 magnitude earthquake occurred in Menyuan County,Qinghai Province,with the epicenter located at the intersection of the Tuolaishan Fault and the Lenglongling Fault,which are part of the Qilian—Haiyuan fault zone.This study investigated the sliding characteristics and seismic mechanism of the earthquake to understand the activity and seismic risk of the fault on the northeastern margin of the Qinghai—Tibet Plateau.This paper analyzed Sentinel-1 synthetic aperture radar images to obtain the coseismic deformation field of the earthquake,which was then used to invert the slip distribution of the seismogenic fault and the coseismic Coulomb stress on the surrounding faults caused by the earthquake.It was found that the earthquake was primarily characterized by sinistral strike-slip movement.Along the satellite line of sight,the south wall of the fault had a maximum deformation of 0.62 m,and the north wall had a maximum deformation of 0.48 m.The coseismic slip distribution results indicated that the maximum slip of the earthquake was 4.51 m,and the moment magnitude was MW6.7.The Coulomb stress analysis showed that the 2016 Menyuan earthquake promoted the occurrence of the 2022Menyuan earthquake.

Gravity variations before the Menyuan Ms6.4 earthquake

In order to study the relationship between gravity variation and Menyuan Ms6.4 earth- quake, gravity variation characteristics in mid-eastern of Qilian Mountain were analyzed based on the 2012-2015 relative gravity datasets. The results indicated that the gravity changes in mid-eastern of Qilian Mountain increased gradually, while gravity changes around Menyuan remarkably. Besides, great positive-negative gravity changing gradients appeared along the Lengiongling Fault which was located at the north of Menyuan, and the 2016 Menyuan Ms6.4 earthquake occurred near the junction of positive and negative gravity changes.

Representative value of cross-fault in the northeastern margin of the Qinghai-Tibet block and case analysis of the 2016 Menyuan Ms6.4 earthquake

The equation for determining cross-fault representative value is calculated based on hanging wall and foot wall reference level surfaces. The cross-fault data reliability are analyzed base on the stability of reference datum and observation points, thereby facili- tating plotting of the representative value curves after removing interference. The spatial and temporal characteristics of fault deformation abnormalities before the 2016 Menyuan Ms6.4 earthquake, as well as the fault-movement characteristics reflected by representa- tive value, are summarized. The results show that many site trends had changed 1-3 years before the Menyuan Ms6.4 earthquake in the Qilian Fault, reflecting certain background abnormalities. The short-term abnormalities centrally had appeared in the 6 months to 1 year period before the earthquake near and in the neighborhood of the source region, demonstrating a significantly increased number of short-term abnormalities. Many sites near and in the neighborhood of the source region had strengthened inverse activities or had changed from positive to inverse activities in the most recent 2-3 years, which reflect stress-field enhancements or adjustment features.

Gravity changes and crustal deformations before the Menyuan,Qinghai Ms6.4 earthquake of 2016

In this study, the relative gravity data(2012 e2015), GPS data-derived horizontal deformation(2011 e2014) and the background vertical deformation from the leveling measurements(1970 e2011) in the northeastern margin of Tibetan Plateau were processed to systematically analysis the mechanism of temporalespatial patterns and the relationship with Menyuan Ms6.4 earthquake. It can be summarized in the following: 1) The regional gravity changes, the GPS and the vertical deformational showed an intense spatial relationship: the gravity increased along with the direction of horizontal movement, and decreased with the crustal uplift and vice versa, which reflected the inherited characteristics of geotectonic activities. 2) The crustal deformations were closely related to the active faults. The contour lines of gravity changes and vertical deformation were generally along with the Qilian-Haiyuan fault(strike is NWW), and the crustal horizontal deformation showed left-lateral strike slip motion near the Qilian-Haiyuan fault. 3) Menyuan Ms6.4 earthquake occurred in the high negative gravity variation area and a high gradient formed in regions, positive and negative variation of gravity amount to 110 m Gal.Specifically, a borderline of positive and negative gravity located in the south of epicenter along the north edge of Qilianshan fault and Lenglongling fault, as well as the vertical and/or horizontal deformation is intensely. The extrusion deformation, surface compression rate and gravity changes were obvious near the epicenter of 2016 Menyuan Earthquake.

Analyzing gravity anomaly variations before the 2016 Ms6.4 earthquake in Menyuan,Qinghai with an interpolation/cutting potential field separation technique

We evaluated 2011-2015 mobile relative gravity data from the Hexi monitoring network that covers the epicenter of the 2016 Menyuan Ms6.4 earthquake, Qinghai Province, China and examined the spatiotemporal characteristics of the gravity field at the focal depth. In addition, we assessed the regional gravity field and its variation the half-year before the earthquake. We use first different interpolation algorithms to build a grid for the gravity data and then introduce potential field interpolation-cutting separation techniques and adaptive noise filtering. The results suggest that the gravity filed at the focal depth of 11.12 km separated from the total gravity field at about -400-150 ×10^-8 m/s^2 in the second half of 2015, which is larger than that in the same period in 2011 to 2014 （±30×10^-8 m/s^2）. Moreover, at the same time, the gravity field changed fast from September 2014 to May 2015 and May 2015 to September 2015, reflecting to some extent material migration deep in the crust before the Menyuan earthquake.

Characteristics of regional crustal deformation before 2016 Menyuan Ms6.4 earthquake

On January 21, 2016, a strong earthquake with a magnitude of Ms6.4 happened at Menyuan, Qinghai Province of China. In almost the same place, there was another strong earthquake happened in 1986, with similar magnitude and focal mechanism. In this paper, we analyze the characteristics of regional crustal deformation before the 2016 Menyuan Ms6.4 earth- quake by using the data from 10 continuous Global Positioning System （GPS） stations and 74 campaign-mode GPS stations within 200 km of this event： （a） Based on the velocity field from over ten years GPS observations, a regional strain rate field is calculated. The results indicate that the crustal strain rate and seismic moment accumulation rate of the Qilian- Haiyuan active fault, which is the seismogenic tectonics of the event, are significantly higher than the surrounding regions. In a 20 km~ 20 km area around the seismogenic region, the maximum and minimum principal strain rates are 21.5 nanostrain/a （NW-SE extension） and -46.6 nanostrain/a （NE-SW compression）, respectively, and the seismic moment accumulation rates is 17.4 Nm/a. The direction of principal compression is consistent with the focal mechanism of this event. （b） Based on the position time series of the continuous GPS stations for a time-span of about 6 years before the event, we calculate the strain time series. The results show that the dilatation of the seismogenic region is continuously reduced with a ＂non-linear＂ trend since 2010, which means the seismogenic region has been in a state of compression. However, about 2-3 months before the event, both the dilatation and maximum shear strain show significant inverse trends. These abnormal changes of crustal deformation may reflect the non-linear adjustment of the stress-strain accumulation of the seismogenic region, when the accumulation is approaching the critical value of rupture.

Field source characteristic of gravity variation in Hexi region before Menyuan Ms6.4 earthquake based on the Euler deconvolution

This study adopted the Euler deconvolution method to conduct an inversion and interpretation of the depth and spatial distribution pattern of field source that lead to gravity variation. For this purpose, mobile gravity data from four periods in the Hexi region between 2011 and 2015 were obtained from an observation network. With a newly established theoretical model, we acquired the optimum inversion parameters and conducted calculation and analysis with the actual data. The results indicate that one is the appropriate value of the structure index for the inversion of the mobile gravity data. The inversion results of the actual data showed a comparable spatial distribution of the field source and a consistent structural trend with observations from the Qilian-Haiyuan Fault zone between 2011 and 2015. The distribution was in a blocking state at the epicenter of the Menyuan earthquake in 2016. Our quantitative study of the field source provides new insights into the inversion and interpretation of signals of mobile gravity variation.

Regional fault deformation characteristics before and after the Menyuan Ms6.4 earthquake

This study analyzes data regarding cross-fault deformations within the seismogenic zone of the 2016 Qinghai Menyuan Ms6.4 earthquake and its surrounding area. The results showed that the tendency anomaly sites near the epicenter had relatively long anomaly durations prior to the earthquake, while sudden-jumping anomaly sites started to increase in the middle eastern Qilian Mountains approximately a year before the earthquake and continued to increase and migrate towards the vicinity of the epicenter two to six months before the earthquake. Intensive observations a few days after the earthquake indicated that abnormal returns and turns before the earthquake were significant, but all had small amplitudes, and the coseismic effect was generally minor. In addition, the post-seismic tendency analysis of individual cross faults in the Qilian Mountain fault zone revealed an accelerating thrust tendency at all cross-fault sites in the middle Qilian Mountains after the 2008 Wenchuan Ms8.0 earthquake. This indicates that the Wenchuan mega-earthquake exerted a great impact on the dynamic environment of the northeastern margin of the Qinghai-Tibet plate and significantly enhanced the extrusion effect of the Indian plate on the middle Qilian Mountains, generating favorable conditions for the occurrence of Menyuan thrust earthquakes.

Analysis of Abnormal Characteristics of Regional Crustal Deformation before the Menyuan MS6.4 Earthquake by GPS Continuous Data

In order to study the characteristics of crustal deformation around the epicenter before the 2016 M_S6. 4 Menyuan earthquake,the GPS continuous stations of the period from 2010 to 2016 were selected according to the observation data of the tectonic environment monitoring network in Chinese Mainland. The deformation characteristics of the crust before the earthquake were discussed through inter-station baseline time series analysis and the strain time series analysis in the epicentral region. The results show that a trend turn of the baseline movement state around the epicenter region occurred after 2014,and the movement after 2014 reflects an obvious decreasing trend of compressional deformation.During this period,the stress field energy was in a certain accumulation state. Since the beginning of 2014,the EW-component linear strain and surface strain rate weakened gradually before the earthquake. It shows that there was an obvious deformation deficit at the epicentral area in the past two years,which indicates that the region accumulated a high degree of strain energy before the earthquake. Therefore,there was a significant background change in the area before the earthquake. The results of the study can provide basic research data for understanding the seismogenic process and mechanism of this earthquake.

Dynamic evolution of crustal horizontal deformation before the Ms6.4 Menyuan earthquake

An Ms6.4 earthquake occurred in the Menyuan county of Qinghai Province on Jan 21, 2016. In order to recognize the development of horizontal deformation and distinguish precursory deformation anomalies, we obtained coordinates time series, velocity and strain model around the seismic zones from processing of continuous observations from 2010 and 6 times of surveying Global Positioning System （GPS） data since 2009. The results show that, before the earthquake, the eastern segmentation of the Qilian tectonic zone where the Lenglongling Fault located is in strong crustal shortening and compressional strain state with dilatational rates of -15 to -25 ppb. The Lenglongling Fault has a strike-slip rate of 3.1 mm/a and a far-field differential orthogonal rate of 7 mm/a, while differential rate is only 1.2 mm/a near the fault, which reflects its locking feature with strain energy accumulation and high seismic risks. Dynamic evolution of deformation model shows that preevent dilatational rates around the seismic zones increases from 15 ppb/a to -20 ppb/a with its center moving to the source areas. Time series of N components of G337 station, which is 13.7 km away from the Lenglongling Fault, exhibit a 5 mm/a acceleration anomaly. Time series of base-station QHME （in Menyuan） displays a reverse acceleration from the end of Sep. to Dec., 2016 when it comes to a largest deviation, and the accumulative displacement is more than 4 mm and the value reverse till the earthquake. In our results, coseismic displacement of N, E, U components in QHME site are 3.0 mm, 3.0 mm, -5.4 mm, respectively. If we profile these values onto the Lenglongling Fault, we can achieve a 1.1 mm of strike slip and 4.1 mm updip slip relative to the hanging wall.

Seismogenic structure of the 2016 Ms6.4 Menyuan earthquake and its effect on the Tianzhu seismic gap

On January 21, 2016, a strong earthquake with a magnitude of Ms6.4 occurred at Menyuan, Qinghai Province of China. In almost the same region, there was another strong earthquake happened in 1986, with similar magnitude and focal mechanism. Based on comprehensive analysis of regional active faults, focal mechanism solutions, precise locations of aftershocks, as well as GPS crustal deformation, we inferred that the Lenglongiing active fault dips NE rather than SW as suggested by previous studies. Considering the facts that the 2016 and i986 Ms6.4 Menyuan earthquakes are closely located with similar focal mechanisms, both of the quakes are on the north side of the Lenglongling Fault and adjacent to the fault, and the fault is dipping NE direction, we suggest that the fault should be the seismogenic structure of the two events. The Lenglongling Fault, as the western segment of the well-known Tianzhu seismic gap in the Qilian-Haiyuan active fault system, is in a relatively active state with frequent earthquakes in recent years, implying a high level of strain accumulation and a high potential of major event. It is also possible that the Lengiongiing Fault and its adjacent fault, the Jinqianghe Fault in the Tianzhu seismic gap, are rupturing simultaneously in the future.

Apparent stress as an indicator of stress meta-instability:The 2021 M_(S)6.4 Yangbi earthquake in Yunnan,China

Investigating spatiotemporal changes in crustal stress associated with major earthquakes has implications for understanding seismogenic processes.However,in individual earthquake cases,the characteristics of the stress after it reaches its maximum value are rarely discussed.In this study,we use the 2021 M_S6.4 Yangbi earthquake in Yunnan,China and events of magnitudes M_L≥3.0 occurred in the surrounding area in the previous 11 years to investigate the spatiotemporal evolution of apparent stress.The results indicate that apparent stress began to increase in January 2015 and reached a maximum in January 2020.Apparent stress then remained at a high level until October 2020,after which it declined considerable.We suggest that the stress was in the accumulation stage from January 2015 to January 2020,and entered the meta-instability stage after October 2020.During the meta-instability stage,the zone of decreasing stress expanded continuously and the apparent stress increased around the Yangbi earthquake source region.These features are generally consistent with the results of laboratory rock stress experiments.We propose that apparent stress can be a good indicator for determining whether the stress at a specific location has entered the meta-instability stage and may become the epicenter of an impending strong earthquake.

2021年5月云南漾濞MS6.4地震序列发震构造及破裂过程初探

北京时间2021年5月21日21时48分36秒,云南省大理州漾濞县发生MS6.4地震。利用云南数字地震台网2021年5月18日至8月22日的震相报告,采用双差地震定位法,对漾濞MS6.4地震序列进行重新定位。重新定位结果显示序列呈NW向优势分布,破裂长约20 km,宽约7 km,对重新定位结果进行误差分析,水平方向定位误差约为0.8 km,垂直方向定位误差约为1.0 km,定位结果具有较好的稳定性。依据震中分布的走向将序列划分为NW向的主断层与NNW向的分支断层,主断层存在较为明显的分段现象,分支断层呈雁列状分布。根据小震丛集性发生在大震断层面及其附近的原则,利用重新定位后的小震震源位置反演得到漾濞MS6.4序列主断层走向约320°,倾角约89°,深度范围3~13 km。根据拟合得到的断层在地表的投影位置,推测本次地震的发震断层为维西-乔后断裂西侧的草坪断裂。基于断层滑动量分布识别出3个凹凸体,结合序列时空演化特征,分析了漾濞MS6.4地震序列的破裂过程,结果显示断层中段的凹凸体发生初始破裂,触发相邻的凹凸体发生主震,随后破裂沿断层走向传播,最终导致相邻的数个凹凸体全部发生破裂。

漾濞6.4级地震前后震源机制一致性时空演化特征

利用CAP方法反演2021年漾濞6.4级地震震源机制,并基于云南地区2015~2022年M≥3.0地震震源机制解结果,采用叠加应力场反演方法反演漾濞6.4级地震前后不同时段的应力张量方差空间分布特征;采用云南地区2013~2022年M≥3.0地震震源机制解结果分析震源区附近震源机制一致性参数的时序变化特征。结果表明:漾濞6.4级地震性质为右旋走滑型,矩震级M_(W)6.03,矩心深度5.8 km,节面Ⅰ走向39°、倾角75°,滑动角-16°;节面Ⅱ走向133°、倾角75°、滑动角-164°。应力张量方差空间演化特征显示,漾濞6.4级地震震源区附近应力张量方差经历了一个低值-显著升高-震前下降形成新低值-震后显著升高的变化过程。应力张量方差时间演化特征显示,漾濞6.4级地震震源区附近应力张量方差在地震前1~2 a达到最高值,后持续下降,下降至最低值后发生转折并趋势回升,整体呈正“V”字型,地震发生在正“V”字型转折后趋势回升阶段。应力张量方差的时空演化特征具有一致性。

GPS station short-term dynamic characteristics of micro displacement before Menyuan M6.4 earthquake

Continuous observation data from 24 GPS stations are selected in the area （33.0°N-41.0°N, 95.0°E-105.0°E） for this study （the period is from Jan. 1, 2015 to Jan. 20, 2016）. Three components, NS, EW and UD, of the daily solutions are filtered by the Hilbert-Huang transform （HHT） with frequency band of 5.787×10^-7-7.716 ×10^-8 Hz （20-150 days in period）. And short-term dynamic characteristics of micro displacement before Menyuan M6.4 earthquake are studied by using the temporal dependencies and cross spectrum analysis. The results show that before the earthquake the horizontal undulatory motions are higher than the average level in the series data which indicate the disturbance feature of regional stress before the earthquake. Three GPS stations on Qinghai-Tibet Plateau with their setting perpendicular to the seismogenic fault have consistent movement. The increase of amplitude of the horizontal micro motion observed before the quake is conducive to the earthquake occurrence. However, we could not be sure if the undulatory motion triggered the earthquake. It is quite necessary to build more GPS continuous observation stations and optimize the monitoring network so as to improve the understanding of the shortterm dynamic crustal variation before earthquake.

Rupture process of the January 8, 2022, Menyuan M 6.9 earthquake

After the occurrence of destructively strong earthquakes, rapid acquisition of the source rupture process can provide important reference information for post-earthquake disaster relief and aftershock trend determination.An M 6.9 earthquake occurred in Menyuan County, Qinghai Province on January 8, 2022. The epicenter is located in the seismic gap in the middle section of the Haiyuan fault belt. Such a typical strong earthquake was taken as an example to investigate the rupture process of strong earthquakes. Three days after the earthquake, the InSAR(Interferometric Synthetic Aperture Radar) coseismic deformation field was obtained by Sentinel radar, indicating that the surface ruptured obviously. The southern block of the earthquake faces towards the satellite about 95 cm along the LOS(line of sight) direction, and the northern block is away from the satellite by ~ 74 cm, consistent with the characteristic of left-lateral strike-slip motion. In this study, InSAR coseismic deformation data and farfield waveform data were used to jointly invert the earthquake rupture process, and a four-segment finite fault model was constructed by referring to the surface deformation. The inversion results show that the focal depth of the Menyuan earthquake is about 7 km, and the strike of the seismogenic fault is 89.0°, 104.0°, 119.0°and 131.0°from west to east, respectively. It is a high-dip left-lateral strike-slip earthquake event lasting about 14 s. The rupture propagation mode is a bilateral extension. The maximum slip along the fault is about 380 cm, and the seismic moment magnitude is 6.7. The surface rupture length is about 24 km, which is consistent with that measured in the field survey. The detailed seismic source model can provide basic data for the aftershock trend determination and seismic risk analysis of the adjacent active faults.