Calderas, large basin-shaped landforms created by massive explosive eruptions, leave behind “pot-like” structures that can provide essential insights into the history and processes of volcanic development and associated hazards. The Changbaishan Tianchi caldera, located on the Sino-North Korean border in eastern Jilin Province, China, is one of the best-preserved large Cenozoic composite active volcanoes in China. This caldera, close to the Wangtiane and Baotaishan volcanoes to the south and southeast, sits atop a basalt plateau, reaching a peak elevation of 2 749m. Its formation involved multiple phases of overflow eruption activities, followed by caldera collapse due to explosive eruptions and pressure loss within the crustal magma chamber during the late Pleistocene. Over time, glaciers and flowing water have sculpted its surroundings, creating U-shaped valleys along the caldera rim. The structure and formation processes of its paleocone have thus attracted significant attention.

In this study, we drew from reconstruction techniques applied to similar calderas globally. Starting with a focus on the volcanic cone profile, we identified large-scale stratovolcanoes with symmetrical cone shapes akin to Changbaishan Tianchi for comparison. Using high-resolution stereo imagery, we extracted a Digital Elevation Model(DEM)with remote sensing software. From these DEMs, we performed detailed topographic analysis, calculating and statistically modeling geomorphological parameters, which allowed us to develop a three-phase empirical model of cone topography. Applying a moving surface algorithm in MATLAB, we generated surface equations for each volcano profile, revealing quantitative relationships between pixel position, coordinates, and elevation in 3D geographic space. We then used ArcGIS's Kriging interpolation method to create a DEM of the reconstructed cone of Changbaishan Tianchi volcano, allowing us to approximate the original cone structure.

The results estimate the original Changbaishan Tianchi cone reached a height of 4, 100m, with a crater diameter of about 390m and a depth of 170m. The cone displayed a funnel-like structure at the summit, with slopes characteristic of stratovolcanoes. The inner edge of the cone had a relatively uniform slope, while the upper outer edge was steep, averaging 27°, and the lower outer slope angle decreased to an average of 18.5°. These parameters align with typical stratovolcano profiles. The explosive eruptions and subsequent cone collapse are estimated to have led to a volume loss of approximately 28.92km³.

This paleocone reconstruction of Tianchi volcano enhances our understanding of the history of the development and evolution of Tianchi volcano, contributing valuable data for reconstructing similar caldera cones and examining eruption mechanisms within the Changbaishan volcanic field. Moreover, this study provides critical information for analyzing the geological history of Tianchi volcano, including the formation of glacial landforms and processes related to eruptions and natural disasters.

Changbaishan-Tianchi volcano(CBS-TC), located in Jilin Province on the border between China and North Korea, is the largest composite volcano around China, which is still active. The eruption stages of this large Quaternary composite volcano can be roughly divided into 2.0~1.48Ma shield forming stage, 1.48~0.05Ma cone forming stage and the explosive eruption stage since 50000 years ago. Its great eruption activities(the Millennium Eruption)from 946AD to 947AD and magmatic disturbances from 2002 to 2005 have attracted great attention of the government and scholars.

Predecessors have done a lot of researches on Tianchi volcano, including its eruption periods, distribution of eruptive products, disaster assessment and so on. Geophysical data show that there are anomalies in the lower part, indicating the existence of magma chambers or conduits, but the accurate boundary and depth of magma chambers need to be further explored. The study of petro-geochemistry shows that the products of shield forming stage of Tianchi are mainly potassic trachy-basalts. The MgO^# of these basic magma is lower than that of the primary magma in Northeast China, indicating that they are the evolved magma undergoing the process of fractional crystallization. In the past, the cone forming stage was considered to have the characteristic of “bimodal” eruptions, that is, the cone forming eruptions of high SiO₂ trachytic/comenditic magma was accompanied by the low SiO₂ basaltic magma, which formed small cinder cones on the edifice. In recent years, some drilling data show that there are thick basaltic trachy-andesite and trachy-andesite strata under the cone, indicating that the products of the cone forming stage of Tianchi include early basaltic trachy-andesite, medium trachy-andesite and late trachyte. Their SiO₂ and Na₂O+K₂O contents are increasing with the degree of evolution. Since the late Pleistocene, Tianchi volcano has entered the stage of explosive eruptions with strong caldera forming effect. The eruptive products are mainly comenditic/trachytic airborne pumice, ignimbrite and so on. However, there are still many disputes about the magmatic evolution of CBS-TC, especially the evolution process from basalt to trachy-andesite, trachyte and comendite. In this study, we did abundant field geological investigation and collected rock samples of each eruptive stage of CBS-TC, and carried out whole-rock geochemical analysis. The results show that major elements of these samples have continuous linear trends with increasing of SiO₂ content in magma, and the distribution of rare earth elements and trace elements is also consistent, which indicates a continuous evolution process. Meanwhile, compared with intermediate-basic magma, the trachyte and comendite magma in Tianchi has a characteristic of high Th/La and ⁸⁷Sr/⁸⁶Sr values, indicating that the magma has also experienced assimilated contamination by crustal materials. In order to verify this fractional crystallization with assimilation(AFC)process of Tianchi magma, the author uses petro-thermodynamic simulation(MELTS model)to calculate the magma evolution. The condition parameters used in the simulation include temperature, pressure, oxygen fugacity, water content, etc. Those parameters are considered as close as possible to the real situation in the magma system. The conditions of pressure and water content are still controversial, which are limited by this simulation. It is found that the evolution of Tianchi magma tends to have occured under the conditions of low pressure(2kbar)and high water content(≥0.5wt%), and about 10% granitic assimilates were mixed in the late stage of evolution, which is consistent with the previous research on the location of magma chambers and melt inclusions. The simulation results are consistent with the trends of tested major elements of Tianchi volcano. To sum up, we found that besides fractional crystallization, assimilation and contamination of shallow crustal granite also play an important role in the evolution of basalt to comendite.

In this paper, the magmatic evolution of Tianchi volcano has been studied systematically, during which the method of petro-thermodynamic simulation combined with geochemical analysis is used. A series of new understandings have been obtained, including the eruption sequence, magmatic evolution, and contamination processes of Tianchi volcanic rocks. This analysis procedure provides a certain reference for the future study. The conclusions help to better understand this largest active volcano in China, and provide new ideas for interpretation of volcanic monitoring data, which helps prevent volcanic disasters. The study also provides references for the regional construction planning of the government.

The Longtan reservoir is located in Tian'e County, Guangxi Zhuang Autonomous Region, southwestern China on the upper reaches of Hongshui River, the main stream of the Pearl River. The dam of the reservoir is 200m high, and the maximum water depth can be up to 194m as the water level reaches 400m. The reservoir storage capacity is 27.3 billion cubic meters, so it is a typical high-dam reservoir with large storage capacity. Terrain of the reservoir is high in the west and low in the east. The reservoir is located at the confluence of the Hongshui River, Buliu River, Nanpan River, Beipan River, Mengjiang River and Caodu River. The construction of Longtan hydropower station officially started in July 2001, and the reservoir impoundment was on September 30, 2006. The power station is equipped with 9 sets of 700 000kW water turbine generator units, with a total installed capacity of 6.3 million kW and an average annual generating capacity of 18.7 billion kW·h. So its storage and hydropower capacity rank third only to the world-famous Three Gorges hydropower project and the ultra-large hydropower project in Xiluodu of Jinsha River in China. Seismicity enhanced rapidly in the reservoir area after the impoundment. More the 5 000 earthquakes have been recorded so far, with the maximum magnitude of M_L4.8, which occurred on September 18, 2010. The earthquakes are mainly concentrated in the deep water area where fault zones run through. Assuming the seismogenic fault can be simulated by a plane and most small earthquakes occur nearby the fault plane, the information of seismogenic fault can be obtained by the hypocenter location parameters of small earthquakes.

Northwest Guangxi is located in the Youjiang fold belt and the Hunan-Guangxi fold belt of secondary structure unit of South China fold system. The South China fold was miogeosyncline in the early Paleozoic, the Caledonian fold returned and transformed into the standard platform, and the Indosinian movement ended the Marine sedimentary history, which laid the basic structural framework of this area. Since the neotectonic period, large areas have been uplifted intermittently in the region and Quaternary denudation and planation planes and some faulted basins have been developed. Affected by the strong uplift of Yunnan-Guizhou plateau, the topography of the region subsides from northwest to southeast, with strong terrain cutting and deep valley incision. Paleozoic carbonate rocks and Mesozoic clastic rocks are mainly exposed on the earth's surface, and its geomorphology is dominated by corrosion and erosion landforms. The dating results show that most of the structures in northwest Guangxi are middle Pleistocene active faults, and the movement mode is mainly stick-slip. According to the seismogeological research results of the eastern part of the Chinese mainland, the active faults of the middle Pleistocene have the structural conditions for generating earthquakes of about magnitude 6. In the northwest Guangxi, the crustal dynamic environment and geological structure are closely related to Sichuan and Yunnan regions. Under the situation that magnitude 6 earthquakes occurred successively in Sichuan and Yunnan region and magnitude 7 earthquakes are poised to happen, the risk of moderately strong earthquakes in the northwest Guangxi region cannot be ignored. Based on the analysis of deep structure and geophysical field characteristics, it is concluded that the Tian'e-Nandan-Huanjiang area in the northwestern Guangxi is not only the area with strong variation of the Moho surface isobath, but also the M_L3.0 seismic gap since September 2015, and the abnormal low b value area along the main fault. Regions with these deep structural features often have the conditions for moderately strong earthquakes. The paper systematically analyzes the spatial and temporal distribution features and mechanism of regional gravitational field and horizontal crust movement and further studies and discusses the changes of regional gravitational field, crustal horizontal deformation and interaction between geologic structure and seismic activity based on 2014-2018 mobile gravity measurements and 2015-2017 GPS observation data in the northwestern Guangxi. The results show that:1)On July 15, 2017, a M_S4.0 earthquake in Nandan happened near the center of four quadrants of changes of gravity difference, and the center of abnormal area is located at the intersection of the Mulun-Donglang-Luolou Fault, the Hechi-Nandan Fault and the Hechi-Yizhou Fault. The dynamic graph of differential scale gravitational field reflects the gravity changes at the epicenter before and after the Nandan earthquake, which is a process of system evolution of "local gravity anomaly to abnormal four-quadrant distribution features → to earthquake occurring at the turning point of gravity gradient zone and the zero line to backward recovery variation after earthquake". Meanwhile, according to the interpretation of focal mechanism of the Nandan earthquake, seismogram and analysis of seismic survey results, the paper thinks that the four-quadrant distribution of positive and negative gravity, which is consistent with the effect of strike-slip type seismogenic fault before Nandan earthquake, demonstrates the existence of dextral strike-slip faulting; 2)The pattern of spatial distribution of gravitational field change in northwestern Guangxi is closely related to active fault. The isoline of cumulative gravity generally distributes along Nandan-Hechi Fault and Hechi-Yizhou Fault. The gravity on both sides of the fault zone is different greatly, and gradient zone has influences on a broad area; the spatial distribution of deformation field is generally featured by horizontal nonuniformity. Tian'e-Nandan-Huanjiang area is located at the high gradient zone of gravity changes and the horizontal deformation surface compressional transition zone, as well as near the intersection of Hechi-Yizhou Fault, Hechi-Nandan Fault and Du'an-Mashan Fault; 3)The geometric shape of gravitational field in northwestern Guangxi corresponds to the spatial distribution of horizontal crustal movement, which proves the exchange and dynamic action of material and energy in the region that cause the change and structural deformation of fault materials and the corresponding gravity change on earth's surface. The recent analysis of abnormal crustal deformation in northwestern Guangxi shows that Tian'e-Nandan-Huanjiang is a gradient zone of abnormal gravity change and also a horizontal deformation surface compressional transition zone. It locates at the section of significant change of Moho isobaths, the seismicity gap formed by M_L3.0 earthquakes and the abnormal low b-value zone. According to comprehensive analysis, the region has the risk of moderately strong earthquake.

On July 31^st, 2016, an earthquake of M_S5.4 occurred in Cangwu County, Guangxi Zhuang Autonomous Region, which is the first M_S ≥ 5.0 earthquake in coastal areas of southern China in the past 17a. The moderate earthquake activities have come into a comparatively quiet period in coastal areas of southern China for decades, so the study about the Cangwu M_S5.4 earthquake is very important. However, differernt research institutions and scholars have got different results for the focal depth of the Cangwu M_S5.4 earthquake. For this reason, we further measured the focal depth by using CAP method and sPL phase method.
sPL phase was first put forward by Chong in 2010. It is often observed between P and S wave of continental earthquakes with epicentral distance of about 30km to 50km. The energy of sPL phase is mainly concentrated on the radial component. Arrival time difference between sPL phase and direct P wave is insensitive to epicentral distancs, but increases almost linearly with the increase of focal depth. Based on these characteristics and advantages, sPL phase method is chosen to measure the focal depth of Cangwu M_S5.4 earthquake in the paper.
First of all, we selected the broadband waveform data through seismic stations distributed mainly in Guangxi and adjacent provinces from Data Management Centre of China National Seismic Network and Guangxi Earthquake Networks Center. And an appropriate velocity model of Cangwu area was constructed by the teleseismic receiver function method. Then, the focal mechanism and focal depth of Cangwu M_S5.4 earthquake were determined by using the CAP(Cut and Paste)method. Next, we compared the synthetic waveforms simulated by F-K forward method of different focal depth models with the actual observed waveforms. According to the difference of arrival times between sPL and Pg phases, we finally obtained the focal depth of Cangwu earthquake. The results show that the focal depth is 11km measured by CAP method and 9km by sPL phase method. Based on the focal mechanism solution, isoseismal shapes, aftershocks distributions and investigation on spot, we conclude that the Cangwu M_S5.4 earthquake is a left-lateral strike-slip earthquake which occurred in the upper crust. Our preliminary analysis considers that the seismogenic structure of Cangwu earthquake is a north-northwest branch fault, and the control fault of this earthquake is the Hejie-Xiaying Fault.

Based on the mobile gravity observation data in 2014-2016 in Guangxi and its adjacent areas, this paper systematically analyzed the changes of regional gravity field and its relation to the M_S5.4 Cangwu, Guangxi earthquake on July 31, 2016, and combined with GPS observation data and seismic geological survey results, discussed the temporal and spatial distribution characteristics of the changes of regional gravity field and its mechanism. The results show that:(1) Before and after the M_S5.4 Cangwu earthquake, the gravity anomaly changes near the earthquake area were closely related to the major faults in space, which reflects the crustal deformation and tectonic activities that caused the surface gravity change along the seismogenic fault in the period of 2014-2016; (2) The gravity changes near the epicenter before and after the M_S5.4 Cangwu earthquake showed an evolution process in which the positive gravity anomaly zone changed to the negative gravity anomaly zone, a gravity gradient belt appeared along NNE direction and the earthquake occurred in its reverse change process; (3) The epicenter of the M_S5.4 Cangwu earthquake located both near the gravity gradient belt and in the zero transition zone of the surface strain gradient and the edge of the high maximum shear strain rate area, the observational fact further proved that the dynamic image of gravitational field and deformation field have important instruction significance to the location prediction of strong earthquakes; (4) in recent years, the gravity dynamic change in northwestern Guangxi presented a four-quadrant distribution pattern, and there is the risk of generating earthquake of magnitude about 5 in the center of the quadrants.

Continuous MODIS/Terra satellite thermal infrared remote sensing data of the Jinggu M_S6.6 earthquake area from July 2014 to January 2015 is collected, and after cloud-removing, the thermal infrared data between 5:00a.m.-7:00a.m. Beijing Time, which is the best period for observation, is selected to perform land surface temperature data retrieval and analyze the temporal evolution of land surface temperature anomalies before and after the earthquake, as well as the relationship between abnormal spatial distribution and active fault. The impacts of non-structural factors such as topography, landform and seasonal weather of the earthquake zone on land surface temperature anomalies are discussed. The result shows that: a)there was thermal infrared anomalous temperature increase appearing near the epicenter two months before the M_S6.6 Jinggu earthquake and there was a certain correspondence between the anomalous temperature increase and earthquake occurrence time. The significant temperature increase happened in the first half of the month, reached its peak 7 days before the earthquake, and dropped rapidly after the earthquake. At the same time, there was also anomalous temperature increase to a certain extent appearing about half month before the strong aftershocks of magnitude 5.8 and 5.9; b)Through the correlation analysis of non-structural factors such as topography, landform and seasonal weather of the earthquake zone, it is found that the structural "temperature increase" before the Jinggu M_S6.6 earthquake was the information indicating the anti-season change of temperature increase in the earthquake zone; c)The anomalous temperature increase was cross-developed from the epicenter along the NS-NE trending conjugate faults, which is consistent roughly with the NNE-SSW predominant direction of the maximum principal stress of the regional tectonic stress field. After full consideration of the influence of non-structural factors such as topography, landform and seasonal weather on the abnormal temperature increase, it is inferred that this thermal infrared temperature increase is possibly a short-imminent anomaly before the earthquake.

In this study, the continuous thermal infrared data of MODIS/Terra satellite remote sensing of the Yutian M_S7.3 earthquake area from January 2014 to February 2014 are collected, and by cloud-removing, the thermal infrared data between 4:00am-6:00am in Beijing time which is the best period for observation to conduct land surface temperature data retrieval, are selected. Time evolution process of land surface temperature anomalies before and after the earthquake is analyzed as well as the relationship between abnormal spatial distribution and active fault. Then, the impact of non-structural factors such as topography of earthquake area, seasonal climate, rain and snow on land surface temperature anomalies is discussed. The result shows that: a)There was phenomenon of thermal infrared abnormal temperature increase appearing near the epicenter area one month before the Yutian earthquake, and there was a certain correspondence between the abnormal temperature increase and earthquake occurrence time. The significant temperature increase happened in the first half of the month, while 5 to 6 days before the earthquake, the abnormal increase reached its peak, and the temperature dropped rapidly after the earthquake. b)Through the relative analysis of non-structural factors such as earthquake area's topography and landform, seasonal weather, rain and snow, the anti-seasonal structural "temperature increase" signals were discovered, and the rain and snow had a certain degree of influence on the abnormal temperature increase of the earthquake area. c)Due to the complex and sensitive fault structures of the valleys and basins at the southwest of the epicenter, the thermal infrared abnormal temperature increase usually starts from this area and gradually migrates to the epicenter along the faults. d)Abnormal temperature increase zone presents strip distribution which is in consistent with the NE-striking main fault zone. After full consideration of the influence of non-structural factors on the abnormal temperature increase, it was inferred that this thermal infrared temperature increase could be a short-imminent precursor before the earthquake.

In order to know to what degree can elastic wave velocity be influenced by effective stress changes in rock medium at the bottom of reservoir, and the quantitative relationship between changes of elastic wave velocity, pore pressure and effective stress during the process of reservoir water body load-unloading and water infiltration, pre-works have been done on the geological structures and hydrogeologic conditions in the Zipingpu reservoir, Sichuan Province, and its adjacent areas in detail, and a mathematical model was deduced which could describe rock-mass distortion and stability influenced by liquid seepage in porous rock media during the process of reservoir water body load-unloading and water infiltration. On this basis the paper makes a further study to establish a quantitative mathematical model to describe the relationship between elastic wave velocity and effective stress, and also taking Zipingpu reservoir as an example, calcutates the dynamic changes of pore pressure, additional elastic effective stress, and elastic wave velocity using finite element method. The simulation results show that: the change in incremental value of elastic wave velocity during the process of reservoir water body load-unloading and water infiltration is not obvious, the maximum amplitude is only about ±0.013 km/s, and changes mainly concentrate in local areas above 5km depth under the reservoir bottom. The change in incremental value of elastic wave velocity is associated with changes of additional effective stress, similar to reservoir water-level fluctuation in curve shapes. Incremental value of elastic wave is related to location, depth, and additional effective stress of different observation points. The velocity change of P-wave is greater than S-wave at the same observation point. The time sequence changes of elastic wave velocity obtained by this paper are generally similar to the relative velocity variations of the Zipingpu reservoir region obtained by other scholars using seismic ambient noise method, but the variation range is slightly different.

With the advances in simulation techniques and understanding of geodynamic processes,numerical simulation is likely to play an increasingly important role in the research of seismic hazard analysis and earthquake prediction.In this paper,on the basis of the paper "A preliminary study on the application of numerical simulation methods to earthquake prediction research(Ⅰ)",the possible application of uncoordinated deformation analysis,Coulomb stress changes and earthquake probability modeling to the study of earthquake prediction is further discussed.When rock deforms from the elastic into the yield stage,the system is in a critical unstable state,the rock movement may deviate from the normal track and become complicated.The study results show that,before Wenan earthquake(M_S 5.1)on July 4,2006,GPS velocity was well consistent with the numerical simulation speed in most areas of North China,while there were some differences in some regions,especially in the northeast of the North China Plain block,where big inconsistency in movement characteristics occurred,resulting perhaps from the preparation of Wenan earthquake.Research on earthquakes triggered by Coulomb stress change is a focus problem now.Numerical simulation may play an important role in the analysis of Coulomb stress changes.By constructing three-dimensional dynamic model,the effect of various factors on the value and distribution of Coulomb stress change can be simulated,and more realistic results can be obtained.By numerical simulation of Coulomb stress changes to seismic activities beneath Sichuan Zipingpu reservoirs,it is found that with the increase of reservoir water storage time,the pore pressure diffusion in the effective additional stress field will be gradually expanded to the range of more than 10km underground.The regional effective additional stress field and seismic activities show different characteristics in several typical regions.The United States Southern California Earthquake Center has tried to study the earthquake probability as research objectives.It is worthy of referencing in China's earthquake research.Computer simulation of synthetic earthquake catalog is an effective way to solve the lack of data.The future direction of development should be a more realistic three-dimensional dynamic model,taking into account the multi-field coupling between heat,fluid and etc. ,improving hardware and software conditions and shortening the calculation time step,obtaining more complete information on fault movement,and simulating the fault activities.

It is the first time that several profiles of acoustic survey have been conducted across the Qianliyan Fault in the northern part of the South Yellow Sea. According to the difference of the late Quaternary fault activities revealed by the acoustic survey,the Qianliyan Fault may be divided by the Chaoliandao Fault into two segments. The southern segment is not active since the late Pleistocene,no offset in late Pleistocene strata has been observed on the acoustic profiles to the southeast of Rizhao. The northern segment is active in the late Pleistocene,normal faults are observed on the acoustic profiles west of Qianliyan island and offset the middle and upper part of the late Pleistocene layers,but no offset has been found in the latest Pleistocene and Holocene strata. In addition,it is found that the Qianliyan Fault offsets the late Pleistocene strata in the sea region 15km east of Shidao,Rongcheng County. In summary,the northern segment of the Qianliyan Fault is active in the latest Pleistocene in the sea region from the west of Qianliyan Island to the east of Shidao. Besides the Qianliyan Fault,other faults develop in the region to the southeast of the Qianliyan Island,which show obvious late Pleistocene active evidence. Therefore,the relative uplift status of the block where the Qianliyan Island lies on may be related to the late Pleistocene tectonic activities including that of the Qianliyan Fault. Both modern and history seismicity are relatively week in the region along the Qianliyan Fault. Since the establishment of the seismograph network in this region,no earthquake with magnitude equal or larger than 5and no small earthquake clusters have occurred along the Qianliyan Fault and in the surrounding area. The only historic earthquake is the 1932 M6(1/2) southern Yellow Sea earthquake which occurred in the sea region 28km southeast of the Qianliyan Fault. It is obviously that the seismicity is not concordant with the late Pleistocene active features of the Qianliyan Fault. Above all,the Qianliyan Fault is over 100km long from the Qianliyan Island to the offshore near Shidao and it is obviously active in the latest Pleistocene,the probability of the occurrence of a some M6.5 earthquake along the Qianliyan Fault should be considered in the practice of earthquake prediction and seismic hazard analysis.