Abstract
According to the China Earthquake Networks Center, on June 10, 2022, at 00:03 local time, a M5.8 （ML6.3） earthquake occurred in Maerkang City （32.27°N, 101.82°E）, Aba Prefecture, Sichuan Province, with a focal depth of 10km. Subsequently, at 01:28 local time, another M6.0 （ML6.5） earthquake occurred in the same location （32.25°N, 101.82°E), with a focal depth of 13km. These two earthquakes were located in the southeast part of the Bayan Har block, approximately 2 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 “M6.0 Maerkang earthquake swarm”. The M6.0 Maerkang earthquake swarm exhibited numerous minor earthquakes, as of 23:00 on June 30, 2022, a total of 4 821 earthquakes of magnitude 0 and above were recorded, including seven earthquakes with magnitudes ranging from M3.0 to M3.9, three earthquakes with magnitudes ranging from M4.0 to M4.9, 2 earthquakes with magnitudes ranging from M5.0 to M5.9, and one earthquake with a magnitude ranging from M6.0 to M6.9. The largest aftershock recorded was a M5.2 （ML5.6） earthquake at 03:27 local time on June 10. Considering the significant event of a M5.8 earthquake preceding the occurrence of the main M6.0 earthquake, it is important to investigate the characteristics of the M5.8 sequence as a foreshock and the sequence features before and after the main M6.0 earthquake. 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 Maerkang M6.0 earthquake swarm sequence and to explore the sequence evolution characteristics before major earthquakes in Maerkang. The Maerkang region is situated in the eastern part of the southeastern margin of the Bayan Har block, within the Songpan-Ganze orogenic belt. The Longriba fault in the study area divides the eastern part of the Bayan Har block into two parts, includes the secondary Longmen Mountain block on the east and the Aba secondary block on the west. The Longmen mountain block primarily features the NE-trending Longmen mountain 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 Ganzi-Yushu fault, the Xianshui river 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 Maerkang M6.0 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 at an angle of 50° to 70°. It extends from the northern slope of Mengbi Mountain on the south side of Maerkang, along a NW direction, and disappears near Ruoergai. 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 earthquakes of magnitude M≥5.0, with the largest M6.0 earthquake occurring 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 M6.0 earthquake swarm, no earthquakes of magnitude M≥5.0 occurred in historical records. Minor earthquakes are relatively active in the central part of the fault, concentrated in deep-seated dense activity, and are distributed 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 capabilities. The Longriba fault is considered to have formed as a result of the strong obstruction of the southeastern movement of the Bayan Har block by 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 earthquakes of magnitude M≥5.0. The earthquakes of M≥5.0 that occurred within a 50 km range of the Maerkang M6.0 earthquake swarm were all earthquake swarms. There were a total of two swarms, which took place on September 26 and November 6, 1969, with magnitudes of M5.1 and M5.3 in Aba, and on September 5 and November 8, 1970, with magnitudes of M5.5 and M5.5 in Rangtang. Using the multi-stage positioning method to locate the Maerkang M6.0 earthquake sequence. The results indicated that the sequence is situated on 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 M≥5.0 earthquakes, with the M5.8 earthquake occurring closer to the eastern side of the western branch, the M6.0 earthquake located within the eastern branch, and the largest aftershock of M5.2 situated to the east of the eastern branch. Prior to the M6.0 earthquake, seismic activity was primarily distributed along the western branch, while after the M6.0 earthquake, the activity shifted to the eastern branch, indicating spatial migration over time. Additionally, the distribution of seismic activity is more scattered in the western branch and more concentrated in the eastern branch, suggesting potential differences in the seismic activity patterns between the two branches and indicating that the sequence is not occurring 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 M5.8, M6.0, and some M≥3.5 earthquakes of the sequence. The results revealed that the main shock of the M5.8 earthquake had best double-couple solutions, with strike 324°, dip 76°, and slip 0° for fault plane I, while strike 234°, dip 90°, and slip 166° for fault plane Ⅱ. The main shock of the M6.0 earthquake had best double-couple solutions, with strike 329°, dip 90°, and slip −3°for fault plane I, and strike 58°, dip 87°, and slip −180° for fault plane Ⅱ. Both main shocks and the focal mechanisms of the larger aftershocks exhibited consistent strike-slip motion, consistent with the regional predominance of reverse and strike-slip faulting. The strike of fault 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 M≥3.5 events of the Maerkang M6.0 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 Maerkang M6.0 earthquake swarm is complex, resulting from the simultaneous activity of several faults of different scales. These faults are located close to the Songgang fault, are 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 seismic activity in this area may exhibit conjugate rupture 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: lgN＝a－bM, where “b” value represents the maximum likelihood solution: b=\frac\mathrmlg\mathrme\overlineM－M_\mathrmc , with “a” value representing the overall level of seismic activity, “N” representing seismic frequency, “MC” representing the minimum completeness magnitude, “ \overlineM ” representing 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, and vice versa. So the “b” value, which 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 Amatrice-Norcia MW6.2, MW 6.6 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 al （2021） found that the “b” value of the Yunnan Yongping MS6.4 earthquake sequence 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 al （2023） 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, the “b” value calculation error is obtained through the construction of a bootstrap process. Additionally, assuming N＝1, “M” corresponds to the theoretical maximum earthquake magnitude "Mmax", expressed as: Mmax＝a/b. The expression of the modified Omori’s law is: n（t） ＝\fracK （ t+c ） ^p , where “n（t）” represents the number of aftershocks at time “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 ） ＝ K\frac1p － 1c^1－ p－ （ t ＋ c ） ^1－ p , and when p＝1, N（t）＝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 M5.8 serving as a foreshock to the M6.0 in the Malang earthquake sequence. In this sequence, three M≥5.0 events occurred within three hours of the sequence's onset, indicating rapid sequence development. When attempted to analyze the evolution characteristics between these three events, the short time intervals resulted in strong seismic waves masking the signals of smaller earthquakes, 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 2.2 times 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 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 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. At June 10, three seismic events of M≥5.0 earthquakes divided the seismic sequence into different stages. To ensure data comparability and control variables, each stage was defined as stage Ⅰ, stage Ⅱ, and stage Ⅲ, with each stage beginning one hour after each of the three M≥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, stage Ⅰ exhibited 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 M5.8 earthquake could be considered as a foreshock of the M6.0 earthquake. The Mmax values also demonstrated differences across the stages, with stage Ⅰ having an Mmax value of ML5.0, 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 results indicated that 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 showed the opposite, possibly indicating differences in the temporal evolution between the foreshock sequence and the aftershock sequence. The spatial distribution characteristics of the Maerkang M6.0 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 not caused 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 M6.0 event in Maerkang 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 Maerkang M6.0 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 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 M5.0 event, the “b” and “p” values were compared and analyzed. The results showed that stage Ⅰ had significantly lower “b” values, reflecting a higher stress level in the seismic zone during this stage. The M5.8 earthquake can be considered as a foreshock before the M6.0 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, 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 M5.8 foreshock sequence in this segment. 5） In summary, the comprehensive analysis suggests that the M5.8 earthquake was a foreshock of M6.0 event in Maerkang. Temporally, the M5.8 earthquake sequence before the main shock exhibited characteristics of low “b” and “p” values. Spatially, the west segment of the M5.8 earthquake sequence also showed lower “b” and “p” values, indicating slow decay of the foreshock sequence of the Maerkang M6.0 earthquake swarm and the presence of insufficient stress release and high stress levels in the foreshock area.
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