The Fujian area is located tectonically at the southeastern margin of the South China continent, which consists of three sub-blocks, the northwest Fujian block, the southwest Fujian block and the east Fujian block. This region is the forefront of the interaction between the Eurasian plate and the Philippine Sea plate. Geologically, the Fujian area has undergone a complex tectonic evolution process, and the huge intrusive-volcanic rocks formed by multi-stage tectonic changes were widely exposed in this region. Since the inversion of the crustal three-dimensional P-wave velocity structure was important for understanding the tectonic evolution process and the deep seismogenic environment in the region, a lot of research work has been carried out in Fujian area, including seismic body wave tomography, ambient noise surface wave tomography and artificial seismic profiles. Although some important features of the crustal velocity structure in this region had been obtained by natural seismic body wave or ambient noise surface wave imaging, the grid lateral resolution was relatively poor(generally above 0.5° horizontally), which made it difficult to constrain effectively the detailed features of the fault zone velocity structure in this region. For example, the Fu'an-Nanjingfault zone, as an important fault zone in the region, which controlled the magmatic intrusion activities before the Mesozoic, the features of its deep velocity structure have been rarely revealed. Although the resolution of artificial seismic profiles was high, it covered a relatively limited detection range in this region.

In this paper, 3203 natural local earthquakes were selected using the observation reports of Fujian seismic network from 1999 to 2021 and integrating some data from neighboring provinces, which includes both 76423 absolute arrival time data and 389021 P-wave relative arrival time data from131 seismic stations. The test results of checkboard showed that the northwest Fujian block had poor recovery at all depths due to the limited internal seismic ray coverage, most areas of the southwest Fujian block had good recovery at all depths, and the east Fujian block could been recovered at all depths except for its northern region which had poor recovery at 0km, 25km and 30km depth. Under this resolution condition, the three-dimensional crustal P-wave fine velocity structure in Fujian region was obtained. The arrival time residual conforms to a Gaussian distribution before and after the inversion. The travel time residuals of the seismic phases were mainly distributed in the range of -1.5 to 1.5s before the inversion, and these travel time residuals of the seismic phases were mainly distributed in the range of -0.5 to 0.5s after this inversion. The travel time residuals were reduced significantly and were more concentrated around 0. Using the velocity structure obtained from the inversion and combining with the geological structure and geophysical field characteristics of this region, the tectonic implications which may be related to these features of velocity structure in the region were discussed. The main results are as follows:

(1)In the near-surface shallow layer, the P-wave low-velocity feature is mainly correlated better with the NW-trending faults, such as the Nanri island fault, Meizhou bay fault, Yong'an-Jinjiang fault and Jiulong river fault. This may be related to the relatively young activity age and more fragmented shallow parts of the NW-trending faults. The lateral variation of velocity is small in the middle and upper crust at 5km and 10km depths relative to other depths, but there is a relatively high velocity zone of P velocity in northeastern Fujian area.

(2)The P-wave velocity structure shows generally a relatively low velocity feature at 15~25km depth within the southwest Fujian block, especially in the south of the Yong'an-Jinjiang fault zone. Although the range distribution of this low velocity anomaly is relatively large, the magnitude of the anomaly is not large, and the upper crust and the bottom of the lower crust in the southwest Fujian block do not show this anomalous feature. On the other hand, the magnetotelluric sounding of the middle and lower crust of this block shows a high resistivity and the receiver function shows a low Poisson's ratio, this suggests that the low-velocity feature of this block is not caused by partial melt or ductile shear zone, but may be mainly caused by the more quartz-rich composition of the regional crust.

(3)There exist two P-wave low velocity anomalies in the middle-lower crust of the East Fujian block, which are below the two high thermal anomalous area of the geothermal heat flow in this region. It may suggest that the formation of these two relative low velocity anomalies may be related to the transformation of the coastal area into an extensional environment and the upwelling of deep mantle materials caused by the high-angle retraction of the Paleo-Pacific plate in the late Yanshanian period.

(4)The P-wave velocity features show that the velocity at the two sides of the Fuan-Nanjing fault zone is different obviously in the middle and lower crustal depths. This may imply the Fu'an-Nanjing fault has a certain control on the distribution of crustal velocity structure in the region, which is consistent with its deep characteristics of cutting the Moho interface which reflected by the Bourg gravity anomaly and aeromagnetic anomaly, which further confirms that it is a major deep fault zone in the region.

The Tienshan orogenic belt is one of the most active intracontinental orogenic belts in the world. Studying the deep crust-mantle structure in this area is of great significance for understanding the deep dynamics of the Tienshan orogen. The distribution of fixed seismic stations in the Tianshan orogenic belt is sparse. The low resolution of the existing tomographic results in the Tienshan orogenic belt has affected the in-depth understanding of the deep dynamics of the Tienshan orogenic belt. In this paper, the observation data of 52 mobile seismic stations in the Xinjiang Seismic Network and the 11 new seismic stations in the Tienshan area for one-year observations are used. The seismic ambient noise tomography method is used to obtain the Rayleigh surface wave velocity distribution image in the range of 10~50s beneath the Chinese Tienshan and its adjacent areas (41°~48° N, 79°~91° E). The joint inversion of surface wave and receiver function reveals the S-wave velocity structure of the crust and uppermost mantle and the crustal thickness below the station beneath the Chinese Tienshan area(41°~46° N, 79°~91° E). The use of observation data from mobile stations and new fixed seismic stations has improved the resolution of surface wave phase velocity imaging and S-wave velocity structure models in the study area.
The results show that there are many obvious low-velocity layers in the crust near the basin-bearing zone in the northern Tienshan Mountains and the southern Tienshan Mountains. There are significant differences in the structural characteristics and distribution range of the low-velocity zone in the northern margin and the southern margin. Combining previous research results on artificial seismic profiles, receiver function profiles, teleseismic tomography, and continental subduction simulation experiments, it is speculated that the subduction of the Tarim Basin and the Junggar Basin to the Tienshan orogenic belt mainly occurs in the middle of the Chinese Tienshan orogenic belt, and the subduction of the southern margin of the Tienshan Mountains is larger than that of the northern margin, and the subduction of the eastern crust is not obvious or in the early subduction stage. There are many low-velocity layers in the inner crust of the Tienshan orogenic belt, and most of them correspond to the strong uplifting areas that are currently occurring. The thickness of the crust below the Tienshan orogenic belt is between 55km and 63km. The thickness of the crust(about 63km)is the largest near the BLT seismic station in the Bazhou region of Xinjiang. The average crustal thickness of the Tarim Basin is about 45km, and that of the Junggar Basin is 47km. The S-wave velocity structure obtained in this study can provide a new deep basis for the study of the segmentation of the Tienshan orogenic belt and the difference of the basin-mountain coupling type.

Yingjiang area is located in the China-Burma border,the Sudian-Xima arc tectonic belt,which lies in the collision zone between the Indian and Eurasian plates.The Yingjiang earthquake occurring on May 30th,2014 is the only event above M_S6.0 in this region since seismicity can be recorded.In this study,we relocated the Yingjiang M_S5.6 and M_S6.1 earthquake sequences by using the double-difference method.The results show that two main shocks are located in the east of the Kachang-Dazhuzhai Fault,the northern segment of the Sudian-Xima Fault.Compared with the Yingjiang M_S5.6 earthquake,the Yingjiang M_S6.1 earthquake is nearer to the Kachang-Dazhuzhai Fault.The aftershocks of the two earthquakes are distributed along the strike direction of the Kachang-Dazhuzhai Fault (NNE).The rupture zone of the main shock of Yingjiang M_S6.1 earthquake extends northward approximately 5km.The aftershocks of two earthquakes are mainly located in the eastern side of the Kachang-Dazhuzhai Fault with a significant asymmetry along the fault,which differ from the characteristics of the aftershock distribution of the strike-slip earthquake.It may indicate that the Yingjiang earthquakes are conjugate rupture earthquakes.The non-double-couple components are relatively high in the moment tensor.We speculate that the Yingjiang earthquakes are related to the fractured zone caused by the long-term seismic activity and heat effect in the deep between Kachang-Dazhuzhai Fault and its neighboring secondary faults.Aftershock distribution of the Yingjiang M_S6.1 earthquake on the southern area crosses a secondary fault on the right of the Kachang-Dazhuzhai Fault,suggesting that the coseismic rupture of the secondary fault may be triggered by the dynamic stress of the main shock.

We integrated two-month phase data recorded by Yunnan Seismic Network, Zhaotong Seismic Network, Qiaojia Seismic Array and temporal stations deployed around the Ludian earthquake source region and relocated the aftershock sequence of the Ludian earthquake. The locations of 1 750 aftershocks were determined using double-difference location algorithm. The relocation result shows that the aftershock distribution has two predominant directions, to the southeast and southwest, and shows itself as an asymmetric conjugate shape. The lengths of the two aftershock strips are about 16km. The angle between the two strips is about 100°. Aftershock distribution shows that the seismogenic fault of the Ludian earthquake is a high-angle strike-slip fault. The mainshock is located at the middle at southwest of the two aftershock strips. Early aftershocks are distributed mainly along the NW-SE direction, perpendicular to the Zhaotong-Ludian Fault. The aftershocks located to the southwest of the mainshock may be triggered by the mainshock. According to the aftershock distribution and its relations with neighboring faults, focal mechanism of the mainshock, the long axis orientation of seismic intensity map, and distribution of landslides, we speculate that the seismogenic fault is the Baogunao-Xiaohe Fault. There are significant differences not only in seismic activity, deep velocity structure, but also the block movement direction and rate on both sides of the Baogunao-Xiaohe Fault. The northward expansion of aftershock activity may be blocked by the high-velocity anomaly zone located on the north side of the Baogunao-Xiaohe Fault.

Using the absolute relocation method, we relocate 1593 earthquakes from the seismic waveform data recorded by the dense temporary seismic array from August 2011 to August 2012, which is deployed in the southern segment of the north-south seismic belt. Then seismic traveltime tomography method is applied to obtain the 3-D crustal P-wave velocity structure in the western Yunnan area. The inversion results indicate that high-velocity anomaly extends to the middle crust from the surface in the Panzhihua area, with the high velocity on its west merging into a large-scale high-velocity block. We speculate that the high-velocity block plays a certain impediment to southward escaping of the Tibetan plateau materials, which caused the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block. In the south of the Jinshajiang-Red River Fault, low-velocity anomaly exsists in the lower crust of the Tengchong block and Baoshan block. Since it is located in the subduction boundary between the Indian plate and Eurasian plate, we consider that the low-velocity anomaly may be caused by the high temperature derived from the upper mantle due to the Indian plate's eastward subduction. Jinshajiang-Red River Fault is the southern boundary of the Sichuan-Yunnan block. The high-velocity crustal structure and the weak seismic activities in the middle segment of the Red River Fault(Midu to Red River areas)imply that this region is locked currently. However, as the boundary between the blocks, it should be one of the key monitoring areas in the future.

The crustal and upper mantle S-wave velocity structures in Changbaishan volcanic region were obtained from surface wave tomography and teleseismic receiver function modeling.In Changbaishan region,the S-wave velocity shows a thin lithosphere,thick asthenosphere with relatively low S-wave velocity in upper mantle,which indicates the high temperature volcano system at least extends to asthenosphere. There exist distinct low velocity layers in the crust of the volcano area.Beneath WQD station near to the caldera,the low velocity layer at 8km depth is 20km thick with the lowest S-wave velocity about 2.2km/s.Beneath the EDO station located 50km north of Tianchi caldera,no obvious low velocity layer was detected in the crust.The average crustal V_P/V_S near the caldera is higher than those obtained in surrounding area.In the volcanic region,the thickness of crustal low velocity layer is greater and the lowest velocity is more obvious with the distance shorter to the caldera.It indicates the existence of the high temperature material or magma reservoir in the crust near the Tianchi caldera. The receiver functions and inversion result from different back azimuths at CBS permanent seismic station shows that the thickness of near surface low velocity layer and Moho depth change with directions. The near surface low velocity layer is obviously thicker in south direction.The Moho depth shows slightly rising in the direction of the caldera located.We consider that the special near surface velocity structure is the main cause of relatively lower prominent frequency of volcanic earthquake waveform recorded by CBS station.The slightly rising of Moho depth beneath Tianchi caldera indicates there is a material exchanging channel between magma reservoir and upper mantle.

By analyzing seismic data recorded by temporary seismic network deployed at Changbaishan Tianchi volcanic region in the summers of 2002,2003 and 2005,we find one type of events whose spectra appear to be special.The station-averaged spectrum of each event consists of a series of evenly-spaced narrow peaks,the amplitudes of peaks change gradually with the frequency,the shape of spectrum is quite similar to harmonic signal in time domain.We called such event as harmonic-spectral event.During the three summers,38 harmonic-spectral events were detected,and most of them occurred in seismic swarms.Analyses show that the harmonic spectral characteristics of these events are most likely associated with seismic sources,which might be caused by interaction with crack walls and pressure disturbances within magmatic or hydrothermal fluids filled inside of cracks at special excitation conditions,during the rock rupture processes.We suggest that the apparent increasing of seismicity and emergence of harmonic events in Changbaishan Tianchi volcanic region are associated with deep magmatic intrusion activities.

Although volcano erupting has not been recorded in Chinese continent during recent years,there are 15 Holocene volcanoes in Chinese continent,and some disturbance signals were reported from some volcanoes in China.Strong hydrothermal activities occurred in Tengchong volcano,some hydrothermal explosions even took place since 1993.Seismic activity in Changbaishan Tianchi volcano has been getting strong remarkably since July of 2002,accompanied with significant surface uplift and geochemical anomalies.Estimation on volcanic threat level is the base of classification of active volcanoes.Based on the results of volcano monitoring and investigation in recent years,the volcanic threat levels and classification of active volcanoes in China continent are discussed in this paper.Based on the volcanic activity levels of the other countries,the volcano threat in China can be rated into 7 levels as safety,attention,stand by,alarm,threat,hazard and disaster,corresponding to the activity stages of dormancy,late-dormancy,disturbance,unrest,being critical,irreversible and violent,respectively.Based on the dangerous levels of volcanoes,the activity of volcanoes in Chinese continent can be classified into 4 kinds:(1)those in the active phase,such as Changbaishan volcano,which is in the process of disturbance;(2)those with some evidences of activity,such as Tengchong volcano,which stays in the late-dormancy stage,but has the potential threat of eruption;(3)those with some possibility of potential eruption,including Wudalianchi,Jingpohu,Haikou volcanoes,where geophysical and geochemical observations all fall in the background category;and(4)those whose volcanism is unclear yet at present,such as Aer Shan,ErKe Shan,XiaoGulihe,Wulanhada,Keluo,Turphan,west Tianshan,Ashi,and Kekexili volcanoes.

This study focuses on a NNE-striking, normal-right-lateral-strike-slip fault, the Fenhe Fault in Taiyuan City. It is one of the faults from the NNE-trending fault system developed within the Shanxi faulted basin zone, buried beneath the ground surface. In this paper, we demonstrate the high-resolution seismic reflection data for a depth range of several hundred meters across the Fenhe Fault. In combination with the relevant borehole logs, these data provide useful constraints on the accurate position, geometry and deformation rate of the fault, as well as the kinematics of recent fault motion. Two high-resolution seismic reflection profiles across the Fenhe Fault in Taiyuan City clearly demonstrate two continuous, strong-reflection interfaces(at two-way time of 85～110ms for the upper one and of 180～200ms for the lower one), and three reflection layers separated by these two interfaces. In addition, they reveal the near-surface location, geometry and activities of this oblique-normal right-lateral fault. Although the westward dipping fault at shallow level cannot be ruled out, we consider the eastward dipping high angle fault, which is located between the canal and the western embankment of the Fenhe River, to be the west branch of the Fenhe Fault. Based on the geological and drilling data, three seismic reflection layers are inferred to represent the Holocene-upper Pleistocene, middle-lower Pleistocene and Neogene strata, respectively. A total vertical separation of ～10m has occurred on the fault since the beginning of Quaternary, terminating at ～70m below the ground surface. In the upper ～190m section of the borehole log near the two seismic profiles, two closely spaced, subvertical strike-slip faults can be recognized. The eastern branch fault is more active than the western one. It lies beneath the eastern embankment of the Fenhe River, dipping to the west and cutting into the upper layer of Holocene-late Pleistocene strata with a maximum vertical offset of ～8m. Another borehole log across the northern segment of the Fenhe Fault reveals an eastward dipping subvertical, right-lateral strike-slip oblique normal fault, which has cut into the upper layer of Holocene-late Pleistocene strata with a maximum vertical offset of ～6m. The afore-mentioned data provide a minimum average Pleistocene-Holocene vertical slip rate of ～0.06～0.08mm/a and a maximum average recurrence interval of 5.0～6.7ka for the Fenhe Fault, providing that the vertical offset caused by large earthquake is 0.4m per event(similar to the offset caused by the Tangshan earthquake). This is a long-recurrence interval and a low vertical-slip rate probably similar to or more than those of the Tangshan No.5 Fault. If the 0.08mm/a vertical-slip rate characterized the history of faulting at seismic profiling site, then the currently active strand of the east branch of the Fenhe Fault had been active since middle Pleistocene(～150ka), and before that time the west branch of the Fenhe Fault was active but probably had stopped moving at that time. Epicentral distribution of earthquakes clearly demonstrates the seismogenic structures of the Shanxi faulted basin zone. Within these NNE-NS-trending basins, some large historical earthquakes have occurred on the NNE-NS-trending faults. Although within the Taiyuan basin and on the Fenhe Fault occurred only some moderate earthquakes, their structures and activities are similar to those of the large earthquakes. It can be concluded, therefore, that the seismic risk of the Fenhe Fault is an objective reality. As the fault just lies beneath the urban population center of the Taiyuan City, a large earthquake(M_S 7.0～7.5)on it may cause enormous damages. An insight into the activity of the fault is needed for the earthquake prediction and hazard reduction of the Taiyuan City.

In this paper, field observation on earthquake surface rupture zone and high-resolution seismic reflection profiling of faults in combination with related geological data have led to some new insights about shallow Quaternary geology and underground fault structure in the 1976 Tangshan earthquake area. On this basis, we analyze prospecting results of deep structure and then study the relationship between deep and shallow structures in the 1976 Tangshan earthquake area, as well discuss tectonic setting and seismogenic model for this earthquake. Tanshan fault zone is a complex structure zone associated with faults and folds in this area. It is mainly developed on southeastern flank of an anticline and northwest flank of the Kaiping syncline, and consists of a series of NNE-NE-trending parallel faults. This fault zone can be divided into southern and northern segments and/or bifurcated into eastern and western branches, controlling seismic activity and geological motion in this area. The eastern branch of southern segment of this fault zone is Tangshan Fault, which strikes to N30°E and consists of two parallel faults. The eastern fault dips to WNW at an angle of 70°～80° and represents the main fault with reverse-strike-slipping. The 1976 Tangshan M 7.8 earthquake produced four surface rupture zones. The main surface rupture zone is ～8km long and generally strikes to N30°E. It consists of many secondary right-lateral en echelon ground fissure zones on both southern and northern segments, distributed along Tangshan Fault. This surface rupture zone underwent horizontal and vertical dislocations, with horizontal separation of 1.53m in maximum and vertical offset of 0.2～0.7m. The western side of this zone has been uplifted and the eastern side subsided. The high-resolution seismic reflection profiling revealed the following relations between Tangshan Fault and earthquake surface rupture zone:(1)their positions are consistent;(2)their geometric attitudes are consistent, dips to west at angle of 70°～80°;(3)their movements are of reverse-strike-slip faulting, while strong-reflection interfaces show a compressive bending and strike-slip motion characters. These facts indicate that there exists a high-angle reverse strike-slip fault, i.e. the NNE-trending and WNW-dipping Tangshan Fault, the sudden motion of which produced the 1976 Tangshan M 7.8 earthquake. The high-resolution seismic reflection profiling also revealed a SE-dipping reverse fault with a dip angle of about 70°. It occurs on the western steep limb of the Kaiping syncline east of the Tangshan Fault, and may be a shallow bending-slip or bending-moment fault, on which a ground fissure zone produced by the 1976 Tangshan earthquake. Analysis on deep seismic sounding result indicates that the relation between shallow and deep structures is close in spatial location, geometric structure and movement nature. There exist W-dipping slope of the Moho discontinuity, crustal "anticline", horizontal detachment and propagation fault in the middle crust, high-angle reverse-strike-slip fault and fold structure in the upper crust. They form a structural pattern of multi-order and multi-step composite fault-propagation fold, and controll the tectonic deformation and seismic activity in this area. The 1976 Tangshan earthquake is a result of stick-slipping along Tangshan Fault zone caused by the detachment and fault-propagation in the middle crust and by the stress concentration on the base of upper crust. The cumulative vertical offset has reached 15m along the Tangshan Fault since the late Pleistocene. The vertical offset of ground is 0.3m caused by the Tangshan earthquake and 0.5m obtained by inversion of seismic wave data. If creepslip is not taken into account, the recurrence interval of earthquakes on the Tangshan Fault is 2.4～4.0ka, which is comparable with the result of regional paleo-seismological research.