Active block boundaries represent areas where significant crustal stress accumulates, leading to concentrated tectonic deformation and frequent seismic activity. These boundaries are crucial for understanding the patterns of strong earthquakes within mainland China. The China Seismic Experimental Site, located in the Sichuan-Yunnan region, is a key area of tectonic deformation caused by the collision and convergence of the Indian and Eurasian plates. This region plays a vital role in transferring tectonic stress between western China and adjacent plates.

This comprehensive study analyzes the integrity, three-dimensional characteristics, hierarchy, and tectonic activity of blocks within the Sichuan-Yunnan region, following established schemes and criteria for defining active block boundaries. After detailed research, the major active fault zones in the region have been divided into three primary active block boundary zones and sixteen secondary boundary zones.

A new reference scheme was developed by considering several factors, including the historical distribution of strong earthquakes, the hierarchical patterns of earthquake frequency and magnitude, spatial variations in present-day deformation as revealed by GNSS data, and deep crustal differences indicated by gravity data and velocity structures. The Jinshajiang-Honghe Fault, Ganzi-Yushu-Xianshuihe-Anninghe-Zemuhe-Xiaojiang Fault, and Longmenshan Fault are identified as the primary active block boundary zones, while faults such as the Lijiang-Xiaojinhe, Nantinghe, and Longriba faults are classified as secondary boundary zones.

Through an integrated analysis of seismic activity, current deformation patterns, fault sizes, deep crustal structures, and paleoseismic data, the study estimates that the primary boundary zones have the potential to generate earthquakes of magnitude 7.5 or greater, while the secondary boundary zones could produce earthquakes of magnitude 6.5 or greater.

The expansion of geophysical exploration, including shallow and deep earth data, has allowed for a transition in the study of active tectonics from surface-focused to depth-focused, from qualitative to quantitative, and from two-dimensional to three-dimensional analysis. By integrating multiple data sources, i.e. regional geology, geophysics, seismicity, and large-scale deformation measurements, this study presents a more refined delineation of active blocks in the Sichuan-Yunnan region.

The new delineation scheme provides a scientific basis for future mechanical simulations of interactions between active blocks in the Sichuan-Yunnan Experimental Site. It also offers a framework for assessing the probability of strong earthquakes and evaluating seismic hazards. The purpose of this study is to re-analyze and refine the delineation of active block boundaries using high-resolution, coordinated data while building on previous research.

In summary, the Sichuan-Yunnan region’s primary fault zones are divided into three primary and sixteen secondary active block boundary zones. The study concludes that primary boundary zones are capable of generating magnitude 7.5 or greater earthquakes, while secondary zones can produce magnitude 6.5 or greater earthquakes. While the current block delineation scheme offers a valuable foundation, further discussion and refinement of certain secondary boundary zones are needed as detection and observational data improve. This study provides an essential framework for analyzing the dynamic interactions between active blocks, identifying seismogenic environments, and assessing seismic risks in the Sichuan-Yunnan region.

The data of active fault structure and three-dimensional(3D)fault models is essential for seismic risk analysis. With more and more requirement for complex 3D fault models, the demand for data sharing and related research increases dramatically. A web-based display system for three-dimensional fault models would improve data sharing and user experience. Moreover, constructing such a web-based system is also an important issue for data sharing.

The 3D active fault models are built in a data modeling platform, while the web display system is constructed by the geographic information system(GIS)platform. Because the data structure, type, and content between data modeling and GIS platforms are different, the following questions are critical, for example, how to migrate 3D model data from the modeling platform to the GIS platform?and can the migrated data present the right attributions?In this paper we used the Web AppBuilder of ArcGIS 10.6 Enterprise Edition to build a Web prototype system to display 3D fault models of the China Earthquake Science Experimental Field(Sichuan-Yunnan region). The system implemented the basic functions of a 3D Web application and successfully tested the 3D scene display scheme, user interaction mode, and data migration scheme.

The prototype system adopted a local scene, which can easily switch between the above-ground and underground viewing angles of the scene. The scene included 2D fault surface traces, 3D fault models, and earthquakes with or without focal depth. After data fusion, the 3D fault models were classified and displayed with active age, having a good visual fusion effect with 2D fault data. Earthquakes with or without focal depth were displayed in different colors. The earthquakes without focal depth were uniformly displayed at 17km depth according to the average focal depth of the earthquakes with focal depth. So the earthquakes without focal depth can be highly consistent with other elements in the 3D scene.

The user interface interaction mode in the 3D scene of the prototype system was consistent with the common interaction mode of 2D map applications in the following aspects: 1)map browsing; 2)Navigation menu; 3)Geographical inquiry; and 4)Functional interactive tools. The system interface was simple, clear, logical, and unified. Users were easily acquainted with the three-dimensional scene interface according to the two-dimensional map interaction experience. It conformed to the user interface interaction principles of simple, consistent, predictable, and easy feedback.

The prototype system had the basic functions of 3D scene browsing, zooming in and out, 3D object attribute viewing, geographic query, base map switching, layer control, legend, and distance measurement. However, the prototype system needed further development and more complex functions such as data attribute table browsing, space selection, and space query.

This paper presented a data migration scheme from the modeling platform to the GIS platform. The data migration of this scheme can be divided into four steps: data format conversion, coordinate system conversion, 2D and 3D attribute information mapping, and 3D data attribute table construction. After transforming the data format and coordination system from the modeling platform to the GIS platform, 2D and 3D data fusion should be carried out to make 3D data and 2D data have the same attribution. The format conversion and coordinate system conversion steps can be automatically completed in batches. Otherwise, mapping the 2D and 3D attribute information and building the 3D data attribute table need manual handling.

In summary, this paper presents a data migration scheme from the modeling platform to the GIS platform. Practice in reality shows that only after conversing data format and coordination system from the modeling platform, the 2D and 3D data fusion steps are caplable of ensuring a better visual integration of them. The Web-based prototype system of displaying 3D fault models of the China Seismic Experimental Site implements the basic functions of 3D scene application and tests the fused 2D and 3D data visualization. It is friendly and open to users, with a great demonstration significance.

The May 21, 2021 Yangbi M_S6.4 earthquake occurred at the western boundary of the Chuandian tectonic block in southeast Tibetan plateau. The structural background is complex, with multiple active faults distributed around the epicenter area. Focal mechanism and seismic waveform inversion reveal that this earthquake is right-lateral strike-slip type with a NW-trending rupture plane. This accords with the strike and motion directions of the Weixi-Qiaohou and Red River faults along the western boundary of the Chuandian block.
We made a careful field investigation along the Weixi-Qiaohou Fault and around the epicenter area, and did not find any obvious earthquake surface rupture. But we observed a NW-trending ground fissure zone near the epicenter area to the west of the Yangbi County. This zone is divided into two sections, the Yangkechang-Paoshuitian section in the northwest and the Xiquewo-Shahe section in the southwest. These sections have a length of 2.5~3km and 3~3.5km, respectively, and are separated by a ~6km gap. They are characterized by NW-trending ground fissures with a width of several meters to tens meters. The formation of these fissures is inferred to be related to the tectonic movement under the ground, and the fissures have the following features: 1)they are not affected by the topography and cut the slope and range upward; 2)they are continuous and concentrated in a zone with a strike of NW 310°~320°, which is consistent with the belt of aftershocks and differs from the gravity fissures that usually have no regular strikes; 3)they usually have a plane dipping towards upslope(southwest), opposite to the valley; 4)they present shear property, not tensional. This zone thus is interpreted to be the surficial expression of the seismogenic fault of the Yangbi M_S6.4 earthquake.
Moreover, satellite image and field observation suggest that a~30km long linear structure with a NW strike traverses the epicenter area, which may suggest an undiscovered fault. Relocation of small earthquakes shows that the aftershocks are concentrated in a NW-trending belt that is consistent with the linear structure. Furthermore, the fissure zone lies in the northeast side of the aftershock belt, which suggests that the earthquake fault dips SW. Such a dip direction coincides with that of the observed fissure plane, and also agrees with the results from the focal mechanism and InSAR inversion. Both the focal mechanism and the waveform inversion result suggest that the Yangbi earthquake is a right-lateral strike-slip type, which is consistent with the type of the observed ground fissures. No displacement is observed on the fissures, with is also consistent with the InSAR inversion results that suggest the rupture did not break the surface. In addition, there is no coseismic deformation observed along the Weixi-Qiaohou Fault, which may indicate this fault did not move during this earthquake.
Based on our field investigation, in combination with the focal mechanism, aftershock distribution, and InSAR and GNSS inversion results, the seismogenic fault for this Yangbi M_S6.4 earthquake is believed to be a NW-trending(310°~320°)fault with a length of~30km, named as the Yangkechang-Shahe Fault. According to the location, size, and motion of the fault, it is suggested that the Yangkechang-Shahe Fault is a secondary fault of the Weixi-Qiaohou fault system. This fault has a slightly SW-dipping plane, and is dominated by right-lateral strike-slip motion, which may be a younger fault developed during the westward expansion of the western boundary of the Chuandian block.

The elevation evolution history of the southeastern Tibet Plateau is of great significance for examining the deformation mechanism of the plateau boundary and understanding the interior geodynamic mechanics. It provides an important window to inspect the uplift and deformation processes of the Tibet Plateau, and also an important way to test two controversial dynamic end-element models of the Plateau boundary. In recent years, some breakthroughs have been made in the study of paleoaltitudes in the southeastern Tibet Plateau, which allows us to have a clearer understanding of its evolution process and dynamic mechanism. By reviewing and recalculation of the latest achievements of paleo-altitude studies of the basins in the southeastern Tibet Plateau from north to south, including the Nangqian Basin, Gongjue Basin, Mangkang Basin, Liming-Jianchuan-Lanping Basin, Eryuan Basin, Nuhe Basin and Chake-Xiaolongtan Basin, we discuss the surface elevation evolution framework of the Cenozoic geomorphology and dynamics in the southeastern Tibet Plateau. The results show as follows:
(1)There was an early Eocene-Oligocene quasi plateau with an altitude of at least 2.5km from the north to middle of the southeastern Tibet Plateau(north of Dali), while the surface elevation in the south(south of Dali to Yunnan-Guizhou Plateau)was relatively low, even close to sea level. Until Miocene, the north to middle of the southeastern Tibet Plateau reached the present altitude, while the southern part of the Tibet Plateau showed a differential surface uplift trend, which established the present geomorphologic pattern. But it cannot be completely ruled out that this trend was probably caused by the accuracy of the calculation results.
(2)The quantitative constraints on the uplift process of the southeastern Tibet Plateau during Cenozoic provide certain constraints for the dynamic mechanism of geomorphic evolution in the southeastern Tibet Plateau. The northern and central parts of the southeastern Tibet Plateau can be well explained by the plate extrusion model. In this model, the collision and convergence between India and Eurasia plate or Qiangtang block and Songpan-Ganzi block resulted in the shortening and thickening of the upper crust in the region, and making the early stage(early Eocene)surface uplift. Subsequently, due to delamination or the continuous convergence between the Qiangtang block and the Songpan-Ganzi block resulting in the shortening and thickening of the crust, the plateau continued to grow northward and rose to its present altitude around Miocene. In the Eocene, the area from the south of the southeastern Tibetan plateau to the Yunnan-Guizhou Plateau mainly showed a low altitude. It seems that it may be in the peripheral area not affected by the shortening and thickening of the upper crust during the early stage India-Eurasia plate collision or plate extrusion and escape. In addition, as proposed by the lower crustal channel flow model, the lower crust material made the low-relief upland surface extending thousands of kilometers in the region uplift gradually towards the southeast, which seems to explain the low elevation landform of the region in the early stage, but it could not explain the whole uplift process of the southeastern Tibet Plateau. Therefore, a single dynamic model may not be able to perfectly explain the Cenozoic complex uplift process of the southeastern Tibet Plateau, and its process may be controlled by various dynamic processes.
(3)According to the paleoaltitude reconstruction results, if most areas of the ancient southeastern Tibet Plateau, especially the area to the north of Jianchuan Basin, had been uplifted in a certain scale and became part of the early plateau in the early Cenozoic, and reached to the current surface altitude around Miocene, the widely rapid surface erosion in this area since Miocene probably would be a continuous lag response to the finished surface uplift process, and the lag time may correspond to the sequential response process of surface uplift, the decline of river erosion base level and the gradual enhancement of river erosion capacity. Therefore, it is not proper to regard the rapid denudation and rapid river undercutting as the starting time of plateau uplift, as proposed in the previous thermochronological study.

The Xigaze fore-arc basin is adjacent to the Indian plate and Eurasia collision zone. Understanding the erosion history of the Xigaze fore-arc basin is significant for realizing the impact of the orogenic belt due to the collision between the Indian plate and the Eurasian plate. The different uplift patterns of the plateau will form different denudation characteristics. If all part of Tibet Plateau uplifted at the same time, the erosion rate of exterior Tibet Plateau will be much larger than the interior plateau due to the active tectonic action, relief, and outflow system at the edge. If the plateau grows from the inside to the outside or from the north to south sides, the strong erosion zone will gradually change along the tectonic active zone that expands to the outward, north, or south sides. Therefore, the different uplift patterns are likely to retain corresponding evidence on the erosion information. The Xigaze fore-arc basin is adjacent to the Yarlung Zangbo suture zone. Its burial, deformation and erosion history during or after the collision between the Indian plate and Eurasia are very important to understand the influence of plateau uplift on erosion.
In this study, we use the apatite fission track(AFT)ages and zircon and apatite(U-Th)/He(ZHe and AHe)ages, combined with the published low-temperature thermochronological age to explore the thermal evolution process of the Xigaze fore-arc basin. The samples' elevation is in the range of 3 860~4 070m. All zircon and apatite samples were dated by the external detector method, using low~U mica sheets as external detectors for fission track ages. A Zeiss Axioskop microscope(1 250×, dry)and FT Stage 4.04 system at the Fission Track Laboratory of the University of Waikato in New Zealand were used to carry out fission track counting. We crushed our samples finely, and then used standard heavy liquid and magnetic separation with additional handpicking methods to select zircon and apatite grains.
The new results show that the ZHe age of the sample M7-01 is(27.06±2.55)Ma(Table 2), and the corresponding AHe age is(9.25±0.76)Ma. The ZHe and AHe ages are significantly smaller than the stratigraphic age, indicating suffering from annealing reset(Table 3). The fission apatite fission track ages are between(74.1±7.8)Ma and(18.7±2.9)Ma, which are less than the corresponding stratigraphic age. The maximum AFT age is(74.1±7.8)Ma, and the minimum AFT age is(18.7±2.9)Ma. There is a significant north~south difference in the apatite fission track ages of the Xigaze fore-arc basin. The apatite fission track ages of the south part are 74~44Ma, the corresponding exhumation rate is 0.03~0.1km/Ma, and the denudation is less than 2km; the apatite fission track ages of the north part range from 27 to 15Ma and the ablation rate is 0.09~0.29km/Ma, but it lacks the exhumation information of the early Cenozoic. The apatite(U-Th)/He age indicates that the north~south Xigaze fore-arc basin has a consistent exhumation history after 15Ma.
The results of low temperature thermochronology show that exhumation histories are different between the northern and southern Xigaze fore-arc basin. From 70 to 60Ma, the southern Xigaze fore-arc basin has been maintained in the depth of 0~6km in the near surface, and has not been eroded or buried beyond this depth. The denudation is less than the north. The low-temperature thermochronological data of the northern part only record the exhumation history after 30Ma because of the young low-temperature thermochronological data. During early Early Miocene, the rapid erosion in the northern part of Xigaze fore-arc basin may be related to the river incision of the paleo-Yarlungzangbo River. The impact of Great Count Thrust on regional erosion is limited. The AHe data shows that the exhumation history of the north-south Xigaze fore-arc basin are consistent after 15Ma. In addition, the low-temperature thermochronological data of the northern Xigaze fore-arc basin constrains geographic range of the Kailas conglomerate during the late Oligocene~Miocene along the Yarlung Zangbo suture zone. The Kailas Basin only develops in the narrow, elongated zone between the fore-arc basin and the Gangdese orogenic belt.
The southern part of the Xigaze fore-arc basin has been uplifted from the sea level to the plateau at an altitude of 4.2km, despite the collision of the Indian plate with the Eurasian continent and the late fault activity, but the plateau has been slowly denuded since the early Cenozoic. The rise did not directly contribute to the accelerated erosion in the area, which is inconsistent with the assumption that rapid erosion means that the orogenic belt begins to rise.

The generation, abandonment and preservation of terraces formed in active tectonic areas are important to the analysis of the role of the tectonics and climate along the temporal variations, so it appears significant as how to use the effective quantitative methods to extract and accurately depict these terraces. The increasingly convenient acquisition of high-precision topographic data has greatly promoted the advancement of quantitative research in geoscience, making it possible to analyze mid-micro-geomorphic features on a large scale, especially by studying the temporal and spatial evolution of tectonic deformation through accurate capture of micro-geomorphic features. Over the past decade, the rapid development of LiDAR(Light Detection and Ranging)technology has provided unprecedented opportunity to access high-precision topographic data(up to centimeter in vertical and horizontal directions). However, its relatively high cost and relatively complex data processing techniques limit its widespread application in the field of earth sciences. In recent years, with the continuous innovation and advancement of topographic measurement technology, the three-dimensional structure of motion reconstruction technology(Structure from Motion, SfM)has gradually been introduced into the field of digital topographic photogrammetry due to its rapid advantage in providing quick, convenient and cost-effective methods for obtaining high-density geospatial point data. This method thus shows great potential for providing high resolution topographic data with comparable resolution and precision. Therefore, with the acquisition of more and more high-resolution terrain data in recent years, it is an important development trend to explore automated or semi-automated quantitative geomorphological analysis methods. R language, as an excellent programming language, has not been used in the geology and geomorphology, although is widely applied in medicine and meteorology based on its powerful capability of statistician and graphic visualization. In this paper, we focus on the Yellow River multi-terraces formed to the east of the Mijia Shan, which belongs to the Jingtai-Hasi Shan segment of the Haiyuan Fault. With the analysis and visualization of the high-resolution topographic data collected from the SfM in the environment of the R language, we implement the semiautomatic classification and mapping of the Yellow River multi-terraces. The method identifies 20 terraces with different elevation. Our results also imply that the younger terraces have better continuity and elongation, and the older terraces have more deformation, which can be demonstrated from their gradually notable semi-parabolic shape. Besides this, it also suggests the diverse evolution stages of the Yellow River terraces. Our study indicates that R language is expected to become an efficient tool of statistics and visualization of the high-resolution topographic data.

The bedrock scarps are believed to have recorded the continuous information on displacement accumulation and sequence of large earthquakes. The occurrence timing of large earthquakes is believed to be correlated positively with the exposure duration of bedrock fault surfaces. Accordingly, cosmogenic nuclides concentration determined for the bedrock footwall can offer their times, ages, and slip over long time. In general, multiple sites of fault scarps along one or even more faults are selected to carry out cosmogenic nuclide dating in an attempt to derive the temporal and spatial pattern of fault activity. This may contribute to explore whether earthquake occurrence exhibits any regularity and predict the timing and magnitude of strong earthquakes in the near future. Cosmogenic nuclide ³⁶ Cl dating is widely applied to fault scarp of limestone, and the height of fault scarp can reach as high as 15~20m. It is strongly suggested to make sure the bedrock scarp is exhumed by large earthquake events instead of geomorphic processes, based on field observation, and data acquired by terrestrial LiDAR and ground penetration radar (GPR). In addition, it is better for the fault surface to be straight and fresh with striations indicating recent fault movement. A series of bedrock samples are collected from the footwall in parallel to the direction of fault movement both above and below the colluvium, and each of them is~15cm long,~10cm wide, and~3cm thick. The concentrations of both cosmogenic nuclide ³⁶ Cl and REE-Y determined from these samples vary with the heights in parallel to fault scarps. Accordingly, we identify the times of past large earthquakes, model the profile of ³⁶ Cl concentration to seek the most realistic one, and determine the ages and slip of each earthquake event with the errors. In general, the errors for the numbers, ages, and slips of past earthquake events are ±1-2, no more than ±0.5-1.0ka, and ±0.25m, respectively.