宫悦,龙锋,赵敏,杨鹏,王宇玺,梁明剑,乔慧珍,王宇航. 2024. 2022年6月10日四川马尔康MS6.0震群序列时空演化特征. 地震学报,46(2):173−191. doi: 10.11939/jass.20230104
引用本文: 宫悦,龙锋,赵敏,杨鹏,王宇玺,梁明剑,乔慧珍,王宇航. 2024. 2022年6月10日四川马尔康MS6.0震群序列时空演化特征. 地震学报,46(2):173−191. doi: 10.11939/jass.20230104
Gong Y,Long F,Zhao M,Yang P,Wang Y X,Liang M J,Qiao H Z,Wang Y H. 2024. The spatio-temporal evolution characteristics of the MS6.0 Barkam earthquake sequence in Sichuan on June 10,2022. Acta Seismologica Sinica46(2):173−191. doi: 10.11939/jass.20230104
Citation: Gong Y,Long F,Zhao M,Yang P,Wang Y X,Liang M J,Qiao H Z,Wang Y H. 2024. The spatio-temporal evolution characteristics of the MS6.0 Barkam earthquake sequence in Sichuan on June 10,2022. Acta Seismologica Sinica46(2):173−191. doi: 10.11939/jass.20230104

2022年6月10日四川马尔康MS6.0震群序列时空演化特征

The spatio-temporal evolution characteristics of the MS6.0 Barkam earthquake sequence in Sichuan on June 10,2022

  • 摘要: 2022年6月10日马尔康MS6.0震群序列发生在松岗断裂与龙日坝断裂交会处,呈震群型活动特征。本文以四川地震台网目录为基础,对马尔康地震序列的参数特征开展研究,取该序列三次MS5.0地震之后各一个小时,作为 Ⅰ , Ⅱ , Ⅲ 共计三个阶段,进行对比分析。由于较大地震后短时间内目录遗漏的余震较多,为增大研究所需的地震样本量,首先采用模板匹配法进行微小地震识别,以补充完备目录,并利用识别的地震目录及台网目录分别计算马尔康MS6.0地震序列的b值、p值等参数。计算结果显示,相比于第 Ⅱ 和 Ⅲ 阶段,第 Ⅰ 阶段具有显著的低b值(0.59),随着时间的推移,序列b值逐渐上升,后两个阶段分别为0.84和0.86。第 Ⅰ 阶段低b值的结果反映了此阶段孕震区应力水平较高。另外,第 Ⅰ 阶段序列的p值为0.76,明显低于后两个阶段的1.81和1.64,反映出第 Ⅰ 阶段序列频次衰减速度较慢,应力释放不充分,而后两个阶段刚好相反,表明不同阶段序列的时间演化特征存在差异。综合分析认为,MS5.8地震是MS6.0地震的前震。序列西支与东支的参数计算结果呈现不一样的特征,可能与MS5.8前震序列发生在西支有关。

     

    Abstract:
    According to the China Earthquake Networks Center, at 00:03 on June 10, 2022, local time, an earthquake with MS5.8 (ML6.3) struck Barkam City (32.27°N, 101.82°E), Aba Prefecture, Sichuan Province, with a focal depth of 10 km. Subsequently, at 01:28 local time, another MS6.0 (ML6.5) earthquake occurred in the same location (32.25°N, 101.82°E), with a focal depth of 13 km. These two earthquakes were located in the southeast part of the Bayan Har block, approximately two kilometers apart. According to the definition of earthquake sequence type, the magnitude difference between the two earthquakes ΔM=0.2 constitutes a swarm type earthquake, which is hereinafter referred to as “MS6.0 Barkam earthquake swarm”. The MS6.0 Barkam earthquake swarm exhibited numerous minor earthquakes, up until 23:00 on June 30, 2022, a total of 4 821 earthquakes of magnitude 0 and above were recorded, including seven earthquakes with magnitude ranging from MS3.0 to MS3.9, three earthquakes with magnitude ranging from MS4.0 to MS4.9, two earthquakes with magnitude ranging from MS5.0 to MS5.9, and one MS6.0 earthquake. The largest aftershock recorded was an earthquake with MS5.2 (ML5.6) at 03:27 local time on June 10. Considering the significant event of an earthquake with MS5.8 preceding the occurrence of the main shock with MS6.0, it is important to investigate the characteristics of the MS5.8 earthquake sequence as a foreshock and the sequence features before and after the main shock. Existing seismological methods indicate that earthquake sequences can intuitively reflect differences in tectonic stress fields, seismic structures, and the seismogenic environment medium. Moreover, the rupturing properties of the main shock often influence the evolution of aftershock sequences. This study utilizes seismic data from the Sichuan regional seismic network, based on parameters such as the b-value and p-value of the sequence, in combination with regional structure and focal mechanism solution parameters of the strong aftershock, to investigate the spatio-temporal evolution characteristics of the Barkam MS6.0 earthquake swarm sequence and to explore the sequence evolution characteristics before major earthquakes in Barkam.
    Regional geological settings and M≥5.0 historical earthquakes
    The Barkam region is situated in the eastern part of the southeastern margin of the Bayan Har block within the Songpan-Garze orogenic belt. The Longriba fault in the studied area divides the eastern part of the Bayan Har block into two parts, including the secondary Longmenshan block on the east and the Aba secondary block on the west. The Longmenshan block primarily features the NE-trending Longmenshan fault zone, the nearly SN-trending Minjiang fault, and the Huya fault. On the other hand, the Aba secondary block, features a series of large-scale strike-slip faults trending NW that have exhibited activity in the Late Quaternary period. These faults, along with the Garze-Yushu fault, the Xianshuihe fault, and the east Kunlun fault at the southern and northern boundaries of the Bayan Har block, collectively constitute the tectonic framework of the Bayan Har block. The Barkam MS6.0 earthquake swarm occurred near the intersection of the NW-trending Songgang fault and the NE-trending Longriba fault within a relatively complex fault structure. The Songgang fault is approximately 100 kilometers in length, with a maximum width of about 300 meters, trending between 320° and 330°, dipping to the northeast with an angle of 50° to 70°. It extends from the northern slope of Mengbi Mountain on the south side of Barkam, along a NW direction, and disappears near Zoigê. This fault exhibits relatively complex activity, featuring characteristics of multiple periods of activity. Its southern segment is a Late Pleistocene active fault, and historical records indicate that it has experienced three MS≥5.0 earthquakes, with the largest MS6.0 earthquake occurred on October 8, 1941 in Heishui area. The northern segment of the fault does not exhibit obvious surface activity since the Late Pleistocene. Apart from the Maerkang MS6.0 earthquake swarm, no MS≥5.0 earthquake was shown in historical records. Minor earthquakes are relatively active in the central part of the fault, concentrated in deep-seated dense activity along the NW direction. This reflects the existence of a northwest-trending ruptured surface and suggests that the northern segment of the fault possesses potential seismic risk. The Longriba fault is considered to be resulted from the strong obstruction of the southeastern movement of the Bayan Har block from the South China block during the Late Cenozoic, and it constitutes the backthrust-overthrust tectonic system of the Longmenshan structural belt, bearing the role of crustal deformation on the eastern edge of the Qinghai-Xizang Plateau since the Late Cenozoic. This fault is a new fault primarily characterized by right-lateral strike-slip movement in the NE direction, consisting mainly of two parallel branch faults: the Longriba fault and the Maoergai fault. It exhibits a series of distinct fault landforms and demonstrates Late Quaternary activity. Historical records indicate that the Longriba fault has not experienced any MS≥5.0 earthquakes. The M≥5.0 earthquakes that occurred within a 50 km range from the epicenter were all earthquake swarms. There were a total of two swarms, which took place on September 26 and November 6, 1969, with MS5.1 and MS5.3 in Aba, and on September 5 and November 8, 1970, with MS5.5 and MS5.5 in Rangtang.
    Basic situation of the sequence
    Using the multi-stage positioning method to locate the Barkam MS6.0 earthquake sequence. The results indicate that the sequence is situated to the northeastern side of the Songgang fault, with an overall parallel distribution along the NW-SE direction, forming two branches running in an east-west direction, parallel to the Songgang fault. The western branch spans approximately 12 km in length and 3 km in width, while the eastern branch extends about 15 km in length and 2 km in width, with a separation of roughly 2 km between the two. The sequence experienced a total of three MS≥5.0 earthquakes, with the MS5.8 earthquake occurred closer to the eastern side of the western branch, the MS6.0 earthquake located within the eastern branch, and the largest aftershock of MS5.2 situated to the east of the eastern branch. Prior to the MS6.0 earthquake, seismicity was primarily distributed along the western branch, while after the MS6.0 earthquake, the activity shifted to the eastern branch, indicating the spatial migration over time. Additionally, the distribution of seismicity is more scattered in the western branch and more concentrated in the eastern branch, suggesting potential differences in the seismicity patterns between the two branches and indicating that the sequence does not occur on a single fault structure, but rather on different branch faults.
    Using the CAP (cut and paste) method to calculate the focal mechanism solutions of the MS5.8, MS6.0, and some MS≥3.5 earthquakes of the sequence. The results revealed that the MS5.8 earthquake had best double-couple solutions with strike 324°, dip 76°, and slip 0° for nodal plane I, while strike 234°, dip 90°, and slip 166° for nodal plane Ⅱ . The MS6.0 earthquake had best double-couple solutions with strike 329°, dip 90°, and slip −3° for nodal plane I, and strike 58°, dip 87°, and slip −180° for nodal plane Ⅱ . Both main shocks and the focal mechanisms of the larger aftershocks exhibited consistent strike-slip motion, which is consistent with the regional predominance of reverse and strike-slip faulting. The strike of nodal plane I in the NW direction is consistent with the strike of the Songgang fault near the epicenter. Furthermore, the focal mechanism solutions for some MS≥3.5 events of the Barkam MS6.0 earthquake sequence indicated a concentrated depth distribution ranging from 5 to 8 km (Table 1). Combining the results of precise positioning and the study of the sequence’s seismogenic structure by Long et al (2023), it is suggested that the seismogenic structure of the Barkam MS6.0 earthquake swarm is complex, which is resulted from the simultaneous activity of several faults of different scales. These faults are located close to the Songgang fault, not exposed at the surface, and may be connected to the Songgang fault at depth, indicating that the seismogenic structure may be a concealed branch fault of the Songgang fault. It is noteworthy that there are traces of a NE-oriented distribution within the sequence, implying that the seismicity in this area may exhibit conjugate rupture characteristics.
    Early sequence parameter evolution characteristics
    The early temporal evolution of the sequence encapsulates the processes of nucleation and stress changes. The analysis of the temporal evolution characteristics is helpful to understand the mechanism and development of the sequence. In commonly used statistical seismological models, the regression parameter b value in the Gutenberg-Richter (G-R) relationship and the p value in the modified Omori’s law carry specific physical properties and are widely considered as statistical quantities characterizing the sequence. The G-R relationship is expressed as lgNabM, where b value represents the maximum likelihood solution b=\mathrmlg\mathrme/ ( \overlineM-M_\mathrmC ) , a value represents the overall level of seismic activity, N represents seismic frequency, MC represents the minimum completeness magnitude, \overlineM represents the average magnitude, and lge=0.4343. The rock fracture experiments indicate that the b value decreases with the increase of stress level. A smaller b value reflects higher regional stress levels, while a larger b value indicates lower stress levels. So the b value, as a means of assessing regional stress accumulation levels, has been widely used in seismic hazard assessment and post-earthquake trend analysis. Laura and Stefan (2019) studied the b values of the earthquake sequences of the AmatriceNorcia MW6.2, MW6.6 in central Italy on August 24 and October 30, 2016; the Kumamoto MW6.5, MW7.3 sequence in Japan; and the Tohoku MW7.3, MW9.0 sequence in Japan. They concluded that the b value of the foreshock sequence would significantly decrease. Jiang et al2021) found that the b value of the Yongping MS6.4 earthquake sequence in Yunnan showed a decrease followed by fluctuation before and after the main shock, reflecting the intense stress adjustment state in the sequence’s continuous process. Wang et al2023) discovered the phenomenon of b value decreasing after the foreshock and increasing after the main shock, clearly reflecting the development process of the foreshock-main shock-aftershock sequence. In the actual calculation process, calculation error of the b value is obtained through the construction of a bootstrap process. Additionally, assuming N=1, M corresponds to the theoretical maximum earthquake magnitude Mmax, expressed as Mmaxa/b. The expression of the modified Omori’s law is nt) =K/ ( t+c ) ^p , where nt) represents the number of aftershocks at time interval t after the main shock, K represents the aftershock occurrence rate, and p is referred to as the aftershock frequency decay coefficient, representing the rate of sequence decay. A larger p value indicates faster decay, while a smaller p value suggests slower decay, typically varying between 0.9 and 1.5. Its variation characteristics may be related to the uneven structure, temperature changes, and stress accumulation levels in the crust. Discretizing frequency statistics over time periods can result in information loss, thus we obtained the p value and other coefficients in cumulative frequency form. The final integrated form of the modified Omori’s law is: when p≠1, N ( t ) = Kc^1- p- ( t + c ) ^1- p / ( p - 1 ) , and when p=1, Nt)=Kln(c+t). In the actual inversion process, both equations are simultaneously calculated, and the fitting error is computed to select the smaller error value as the final p value calculation result. Due to the small sample size, conventional least squares methods may lead to local optimal solutions, hence genetic algorithms were used to solve the parameters. This study attempts to calculate the b value, p value, and other sequence parameters to identify evidence of the MS5.8 serving as a foreshock to the MS6.0 in the Barkam earthquake sequence.
    In this sequence, three MS≥5.0 events occurred within three hours of the sequence’s onset, indicating rapid sequence development. When we attempted to analyze the evolution characteristics between these three events, the short time intervals resulted in the signals of smaller earthquakes submerged in strong seismic waves, leading to incomplete aftershock records and insufficient statistical sample size. In order to increase the required seismic sample size for the study, a template matching method was used to detect missed earthquakes in the data from June 10th. Using 764 seismic events recorded by the network as templates, events with a correlation coefficient R≥0.85 were identified as individual earthquake events, and DBSCAN method was employed for earthquake correlation, ultimately obtaining 1 713 precise earthquake records, which is 2.2 times as much as the number of templates used. Since the identified earthquakes were similar to the template earthquakes in terms of location and nearly identical in terms of source mechanism solutions, the magnitudes of the missed earthquakes can be obtained by comparing their maximum S-wave amplitudes with the corresponding templates. The supplemented missed earthquakes mainly occurred before 10:00 a.m. , indicating significant interference of strong seismic waves in the manual identification of earthquakes, and also demonstrating the effectiveness of template matching in picking up missed earthquakes. The GFT method was used to compare and analyze the network catalog and the supplemented earthquake catalog before 10:00 a.m. , calculating the minimum completeness magnitudes, showing ML1.8 and ML1.3, respectively, proving the effectiveness of the template matching method in supplementing small magnitude earthquakes.
    On June 10, three seismic events of MS≥5.0 divided the seismic sequence into different stages. To ensure data comparability and control variables, each stage was defined as Ⅰ , Ⅱ , and Ⅲ , with each stage beginning one hour after each of the three MS≥5.0 events. For each stage, the Gutenberg-Richter-Frohlich (GFT) method was used to calculate the minimum completeness magnitude (MC) of the seismic sequence. Events above the MC were selected, and their b values, Mmax values, and p values were calculated. The analysis aimed to identify potential differences in stress states and seismic activity characteristics across the different stages and to understand the early evolution patterns of the sequence. We have listed the amount of data used and the calculation results for each stage in Table 2. From the table, it can be seen that the sample sizes for each stage (above the completeness magnitude) were all over 50, with 69, 57, and 66 events, respectively, ensuring robust calculation results. The MC values for the three stages were ML2.1, ML2.3, and ML1.7, indicating a significant impact of strong earthquakes on short-term monitoring capabilities, with the degree of impact varying with the magnitude. Compared to stages Ⅱ and Ⅲ , stageⅠexhibited a significantly lower b value (0.59). Over time, the b value gradually increased, with the subsequent two stages being 0.84 and 0.86, respectively. The low b value of stage Ⅰ suggests a high stress level in the seismic zone, indicating that the MS5.8 earthquake could be considered as a foreshock of the MS6.0 earthquake. The Mmax values also demonstrated differences across the stages, with an Mmax value of ML5.0 in stage Ⅰ , the highest among the three stages. The subsequent stages showed a gradual decrease of Mmax from ML4.2 to ML3.7, reflecting the potential foreshock nature of the seismic sequence in stage Ⅰ .
    The a value of stage Ⅰ (3.1) was not significantly different from that of stage Ⅲ (3.2), but was lower than that of stage Ⅱ (3.6), suggesting that the a value alone cannot distinguish between foreshocks and aftershocks. When calculating the p value for events above the MC for each stage, the p value for stage Ⅰ was 0.76, significantly lower than those of the subsequent two stages, which were 1.81 and 1.64, respectively. This suggested that the seismic sequence in stage Ⅰ exhibited slower decay and insufficient stress release, while the subsequent two stages were opposite, possibly indicating differences in the temporal evolution between the foreshock sequence and the aftershock sequence.
    Conclusions
    The spatial distribution characteristics of the Barkam MS6.0 earthquake swarm in the region indicated a significant spatial migration feature, suggesting that the sequence was not a simple single rupture event, but a complex seismic sequence. Given the presence of two branches in the sequence, the evolution characteristics over time were discussed separately for the east and west branches. Calculations for the west branch yielded MC of ML1.5, b value of 0.71, and p value of 0.81, while the corresponding parameters for the east branch were ML1.6, 0.93, and 0.94, respectively. The differences in the parameters between the west and east branches reflected that the two branches were formed not by a single fault structure. The lower b and p values in the west branch indicated insufficient stress release, a high stress level, and slow decay, possibly related to the occurrence of the foreshock sequence.
    In summary, the seismic cluster of the MS6.0 event in Barkam on June 10, 2022 occurred near the intersection of the Songgang fault and the Longriba fault, indicating a complex fault structure. The seismic cluster exhibited rich small earthquakes, slow overall decay, and segmental spatial characteristics. Several conclusions were drawn from the analysis:
    1) The Barkam MS6.0 earthquake swarm occurred near the intersection of the NW-trending Songgang fault and the NE-trending Longriba fault, indicating a complex fault structure. The precise positioning results showed that the sequence was located along the NE direction of the Songgang fault, with an overall NW-SE orientation and parallel east and west branches. The east and west branches exhibited spatial migration features and different spatial distribution patterns. The source mechanisms of the larger earthquakes in the sequence were consistent, all being strike-slip type, which aligns with the regional background and the nature of the Songgang fault. It is speculated that this sequence was caused by several different-sized faults, which are close to the Songgang fault, not exposed at the surface, and may be connected to the Songgang fault at depth, representing concealed branch faults of the Songgang fault.
    2) Using template matching, 1 713 precise earthquake events were identified, which was 2.2 times as much as the number of templates used. This reflected the significant interference of strong seismic waves on manual earthquake identification and demonstrates the effectiveness of template matching in detecting missed earthquakes.
    3) By dividing the sequence into three stages following each MS5.0 event, the b and p values were compared and analyzed. The results showed a significantly lower b values in stage Ⅰ , reflecting a higher stress level in the seismic zone during this stage. The M5.8 earthquake can be considered as a foreshock before the MS6.0 earthquake. Additionally, the p value for stage Ⅰ was significantly lower than the subsequent two stages, indicating differences in the temporal evolution between the foreshock sequence and the aftershock sequence.
    4) When analyzing the east and west segments separately, we found that the west segment exhibited lower b and p values, indicating insufficient stress release, high stress level, and slow decay, possibly related to the occurrence of the MS5.8 foreshock sequence in this segment.
    5) In summary, the comprehensive analysis suggests that the MS5.8 earthquake was a foreshock of MS6.0 event in Barkam. Temporally, the MS5.8 earthquake sequence before the main shock exhibited characteristics of low b and p values. Spatially, the west segment of the MS5.8 earthquake sequence also showed lower b and p values, indicating slow decay of the foreshock sequence of the Barkam MS6.0 earthquake swarm and the presence of insufficient stress release and high stress levels in the foreshock area.

     

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