The research team, led by Li Chuanyou of the State Key Laboratory of Earthquake Dynamics and Forecasting at the Institute of Geology, China Earthquake Administration, together with colleagues including Sun Kai, Li Junjie, Li Yanbao, and Liang Mingjian of the Earthquake Administration of Sichuan Province, conducted systematic field investigations around both epicenters. Their work combined geological mapping of surface deformation, documentation of earthquake environmental effects such as landslides and rockfalls, analysis of focal mechanism solutions, and relocation of aftershock sequences. This multi-pronged approach is essential in mountainous terrain like western Sichuan, where dense vegetation, steep topography, and limited road access make it difficult to trace active faults at the surface, and where the seismogenic structure of an earthquake often must be inferred indirectly from the pattern of damage and the distribution of aftershocks at depth.

The Lushan earthquake carried particular scientific weight because of its location and its history. The event occurred in the southwestern section of the Longmenshan fault zone, the same thrust belt that produced the catastrophic 2008 Wenchuan earthquake, a magnitude 7.9 disaster that killed nearly 90,000 people. It also occurred near the source area of the 2013 magnitude 7.0 Lushan earthquake, and from the moment the ground began shaking in June 2022, scientists debated two questions: was the new event simply an aftershock of the 2013 mainshock, and did it rupture within the so-called seismic gap that has long been identified between the Wenchuan and Lushan rupture zones? The Dayi gap, as it is sometimes called, is regarded by many researchers as a segment of the Longmenshan belt that has accumulated significant stress without releasing it, making it a candidate for a future large earthquake.

To resolve these questions, the team documented the distribution of coseismic effects across the epicentral region. Their surveys revealed a pronounced asymmetry. On the southeastern side of the epicenter, landslides and rockfalls triggered by the shaking were sporadic and small in scale. On the northwestern side, by contrast, the effects were far denser and larger, with the most severe concentration of landslides and secondary geologic impacts lining the Donghe River valley from the north of Baoxing County down to Yanjing township. This kind of asymmetric damage pattern is a classic diagnostic tool in earthquake geology: because shaking intensity and ground failure tend to be strongest on the hanging wall of a thrust fault and in the direction of rupture propagation, the distribution of landslides can point investigators toward the orientation and dip of the fault plane that slipped at depth.

When the field observations were combined with the focal mechanism solution of the mainshock and the relocated aftershock sequence, a coherent picture emerged. The seismogenic structure of the 2022 Lushan earthquake, the researchers conclude, is most likely a northwest-directed back-thrust fault situated between the Yanjing-Wulong fault and the Shuangshi-Dachuan fault. This structure is interpreted as a small secondary fault of the Shuangshi-Dachuan fault system rather than one of the primary through-going thrusts of the Longmenshan belt itself. The finding matters because it refines the long-running debate over the architecture of the southwestern Longmenshan zone, where multiple imbricated thrust sheets stack against the eastern margin of the Tibetan Plateau. It also bears directly on the seismic gap question: if the 2022 event broke a minor back-thrust rather than the main gap-forming structure, then the hazard represented by the gap itself may remain largely unaddressed by this moderate earthquake, a conclusion with significant implications for hazard assessment in the densely populated Sichuan Basin margin.

Nine days later and far to the northwest, the Maerkang earthquake swarm posed an entirely different set of puzzles. The magnitude 6.0 main event of the swarm occurred in the northwestern segment of the Songgang Fault, an area that has received comparatively little research attention and where interpretations of fault activity have varied widely. Unlike a classic mainshock-aftershock sequence, a swarm consists of many earthquakes of comparable size occurring over days to weeks without a single dominant event, a behavior often associated with complex fault geometry, fluid involvement, or an immature fault zone that has not yet developed a single smooth rupture surface. The Songgang Fault’s northwestern segment, the study finds, fits this description well: it is an immature section with a complicated internal structure, which helps explain why the swarm behaved the way it did.

One of the most intriguing results from Maerkang concerns the surface deformation itself. Damage to buildings, ground fissures, and landslides triggered by the swarm did not align along the main trace of the Songgang Fault, as one would expect if the primary fault had ruptured. Instead, the deformation formed a linear belt located east of, and nearly parallel to, the main fault trace. This offset pattern was a decisive clue. When the team integrated it with aftershock relocations and focal mechanism solutions, the conclusion became clear: the earthquake rupture did not occur along the main Songgang Fault at all, but along a branching fault situated to its east. In other words, even in a region where the mapped master fault appears to be the obvious suspect, the actual seismogenic structure turned out to be a subsidiary strand that had escaped detailed characterization, precisely because it is less prominent in the landscape and less studied in the literature.

Taken together, the two investigations point to a unifying interpretation. Both the magnitude 6.1 Lushan earthquake and the magnitude 6.0 Maerkang swarm occurred on secondary faults, and both are best understood as expressions of the same regional tectonic engine: the ongoing, unified movement of the Bayan Har block as it is squeezed eastward by the collision between India and Eurasia. The Bayan Har block has been responsible for a remarkable series of large earthquakes over the past two decades, including the 2008 Wenchuan event, the 2010 Yushu earthquake, the 2013 Lushan earthquake, the 2021 Maduo earthquake, and now the 2022 pair. As the block moves, stress is transferred to its boundaries and to the network of smaller faults that accommodate deformation within and around it, and the new study demonstrates that these smaller structures can fail in moderate earthquakes even when the major boundary faults remain locked.

The practical implications for seismic hazard assessment in western Sichuan are considerable. First, the results underscore that hazard maps built around major fault traces alone may underestimate the risk posed by unmapped or poorly characterized secondary faults, which are clearly capable of producing damaging magnitude 6 events. Second, the finding that the 2022 Lushan earthquake likely did not rupture the main seismic gap structure suggests that the gap remains a source of concern, and that stress in the southwestern Longmenshan belt may not have been meaningfully relieved by the recent event. Third, the Maerkang results show that immature fault zones with complex branching geometry can produce swarm-like sequences whose surface signatures are misleading, complicating rapid post-earthquake response and the siting of critical infrastructure such as dams, bridges, and hydropower stations, several of which are planned or operating along the rivers of this region. The authors’ field documentation, including the detailed mapping of landslide distributions along the Donghe River valley and the deformation belt east of the Songgang Fault, provides a valuable baseline dataset for engineers and planners.

More broadly, the study is a reminder of how much remains to be learned about the fine structure of active fault systems, even in regions that have been intensively studied after major disasters. The researchers acknowledge that the northwestern Songgang Fault segment, in particular, has seen limited prior investigation, and their work highlights the value of deploying field teams quickly after significant events, while surface evidence is fresh and before erosion, construction, and repair erase the record. By combining traditional geological fieldwork with modern seismological tools such as aftershock relocation and focal mechanism analysis, the team has shown that the earthquakes of western Sichuan cannot be understood fault by fault in isolation; they must be read as chapters in a single, ongoing story of crustal deformation driven by the Bayan Har block. As that block continues to move, the secondary faults it activates will remain an essential, and sometimes dangerous, part of the region’s seismic future.

Subject of Research: Seismogenic structures of the 2022 Lushan earthquake and Maerkang earthquake swarm in western Sichuan, China

Article Title: Seismogenic structures of the 2022 MS6.1 Lushan earthquake and the MS6.0 Maerkang earthquake swarm in western Sichuan, China

Article References: Chuanyou, L., Kai, S., Junjie, L., Yanbao, L., & Mingjian, L. (2026). Seismogenic structures of the 2022 MS6.1 Lushan earthquake and the MS6.0 Maerkang earthquake swarm in western Sichuan, China. Natural Hazards, 122(19), Article 640. https://doi.org/10.1007/s11069-026-08375-4

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08375-4

Keywords: Lushan earthquake, Maerkang earthquake swarm, seismogenic structure, Longmenshan fault zone, Songgang Fault, Bayan Har block, secondary faults, aftershock relocation, focal mechanism, landslides, seismic gap, western Sichuan