The proposed 2000-meter-deep auxiliary shaft at the Xiling mine, Sanshan Island, Shandong Province, is an ultra-deep shaft construction project. Revealing the characteristics of the in-situ stress field in the shaft construction area is one of the necessary prerequisites for the design and construction of the shaft. We measured the in-situ stress in the deep shaft by hydraulic fracturing method to a depth of 1899.00 m and inverted the 2017.56-meter-deep in-situ stress field in the shaft construction area by numerical simulation. The results show that the maximum horizontal principal stress (S_H) ranges from 23.16 to 70.86 MPa, and the minimum horizontal principal stress (S_h) from 15.24 to 47.06 MPa in the depth range from 357.76 to 1899.00 m in the borehole tested by hydraulic fracturing; the principal stress increases nearly linearly with depth, and the measured maximum horizontal principal stress directions in the measured boreholes are NW 55.5°, NW 60.4°, and NW 58.4°, respectively. Horizontal stress mainly dominates the stress field in the shaft engineering area, the vertical stress (S_v) below 1200.00 m is the intermediate stress, and the average value of the ratio of S_H to S_v is 1.53. The in-situ stress field distribution pattern in the well-construction area with depth and stratigraphic changes is obtained by inversion analysis of FLAC 3D software. The inversion results are basically consistent with the measured values. It provides the fundamental scientific basis for shaft wall design and engineering risk assessment of shaft projects.

In order to obtain the in-situ stress field characteristics and analyze tectonic stability in the Motuo key area of the eastern Himalayan syntaxis, the in-situ stress measurement of one in-situ stress hole and 11 test sections of the Xirang section of the Motuo fault zone were carried out by the hydraulic fracturing method. The results show that the maximum and minimum horizontal principal stress values (S_H, S_h) in the test section of 61.43−121.34 m are 3.05−14.50 MPa and 2.16−9.87 MPa, respectively, and the vertical principal stress values (S_v) are 1.63−3.31 MPa, namely, S_H>S_h>S_v. The in-situ stress field at the measuring point is dominated by horizontal compression, and all of them belong to the in-situ stress state of reverse fault. The principal stress values gradually increase with the increase of depth, and the dominant direction of the maximum principal stress is NEE. In the whole range of in-situ stress depth, the lateral pressure coefficients (K_av) are 1.39−4.38, the maximum horizontal stress coefficients (K_Hv) are greater than 1, and the ratio increases with the increase of depth. The regional stress field of this key area is dominated by horizontal stress and it is highly directional. The horizontal stress coefficients (K_Hh) of all test sections are 1.23−1.66, which are similar to the calculation results of in-situ stress characteristic parameters in Linzhi−Tongmai section. The horizontal tectonic stress of the shallow level at 98 m is relatively small, and the stress accumulation level is low. The friction coefficient required to maintain fault stability is smaller than the critical friction coefficient of actual fault, and the tectonic environment is relatively stable. When the depth exceeds 98 m, the friction coefficient required to maintain fault stability is close to the critical friction coefficient value of the actual fault due to the dominant role of horizontal tectonic stress, and there is a small risk of fault instability slip. The superposition of the Coulomb stress change in the sinistral strike-slip direction and the thrust direction caused by the strong regional earthquakes on the fault plane of the Motuo fault zone in the study area are all negative numbers, which inhibits fault slip and does not increase the risk of fault activity in the study area.

Accurate in-situ crustal stress data are essential for excavation support design and long-term stability analysis of underground projects. We tested the main shaft of the Shaling Gold Mine for crustal stress using hydraulic fracturing technology, and the crustal stress state of 20 measurement points was obtained. The Brazilian test, uniaxial compression test, and acoustic emission test of the cores were conducted indoors to obtain the rock’s spatial inhomogeneity and strength distribution. We analyzed the relationship between the inhomogeneity of the rock and the hydraulic fracturing results. The analysis results show that the magnitude of the principal stress increases nearly linearly with the measurement depth, with the maximum horizontal principal stress value ranging from 20.78 to 45.2 MPa and the minimum principal stress value from 14.94 to 35.33 MPa. The average direction of the maximum horizontal principal stress is NW 65°. The inhomogeneity of each layer of the cores varies, and the inhomogeneity coefficient of the metagabbro is from 0.1 to 0.3. The number of acoustic emission signals under each intensity of the rock is basically the same, and the dispersion of the rock is small. The non-homogeneity coefficient of granite is up to 1.0, dominated by the acoustic emission signals generated by the intense-phase rupture at the late loading stage. The non-homogeneity of the rock affects the direction of expansion of the hydraulic fracture, and the angle

between the expansion direction and the maximum horizontal principal stress affects the measurement results of the horizontal maximum and minimum principal stresses and has a more significant effect on the horizontal minimum principal stress. The relationship between hydraulic fracture measurements and rock properties was analyzed, which is helpful for accurately detecting the distribution of stress fields in inhomogeneous strata.

Based on the strain data of three in-situ stress monitoring stations in different sections of the 1605 Qiongshan 7½ earthquake area, we studied the dynamic variations of in-situ stress and extracted the sudden stress changes recorded by them. We analyzed the in-situ stress variations and tectonic activity between March 2016 and May 2018 to discuss the geomorphological evolution trends and subsidence mechanisms at the Dongzhai Port. The results show that the study area was generally subjected to the NW-compressive stress field, which makes the tensile stress field dominate in the Yanfeng and Dazhipo areas in the hanging wall of the Maniao-Puqian fault（MPF） and the Puqian-qinglan fault（PQF), while the Jinshan area in the footwall of the faults by compressive stress. The MPF fault and the PQF fault have been constantly engaged in non-seismic activities to adjust the local stress field under the regional stress field, among which the MPF Fault had several activities in March–July 2016, October 2017, and April 2018, and the PQF fault had two activities in October 2017. The energy of the MPF activities is more intensive than that of the PQF. The variation trend of the stress field indicates a gradual upward trend in the east bounded by the MPF fault (F_13-1) and possible continued subsidence in Dongzhaigang in the west. The fault activity trend implies that the subsidence rate in  Yanfeng, the northern part of Dongzhaigang, bounded by the MPF fault (F_2-2), should be greater than that in the southern Sanjiang area. In addition, the volume strain monitoring data also reveals traces of magmatic activity in the lower part of the N–S seismic zone of Hainan Island. The comprehensive study concluded that the Dongzhaigang subsidence is mainly controlled by the positive fault activity of the MPF and PQF faults due to the upwelling of deep magma and influenced by the Holocene sea level change and the properties of soft soil depositional strata leading to soft soil flow slip, sand liquefaction, and seawater erosion. We innovatively apply the borehole strain observation technology to explore the evolution law and trend of typical earthquake subsidence landforms in coastal zones, which has essential academic value in the fields of in-situ stress monitoring and tectonic geomorphology research, and the results also have significant application value for mangrove protection and urban planning and construction in Dongzhaigang area.

Under the action of the Earth’s inner dynamics, the lithosphere shapes different types of the Earth’s surface, and the crustal stress state and its dynamic change law are captured by the comprehensive observation technology of drilling crustal activity. It is an important way for human beings to understand the internal dynamic process of the Earth and study the mechanism of inner dynamic geological hazards. The contribution of developed countries such as Japan, the USA, and the IODP International Cooperative Research Program to the development of integrated borehole crustal observation technology is summarized in this paper. The paper also systematically reviews the development history and present situation of borehole strain observation technology and borehole strain observation instrument in China. Especially since the 13th Five-Year, under the background of the national strategy of deep-sea exploration, the China Geological Survey Bureau (CGS), the China Earthquake Administration, and other systems have successively carried out research and development of the integrated geophysics observation system in wells, and have been put into use in integrated land and sea observation stations. The Institute of Geomechanics has successfully developed an integrated geophysics observation system for crustal activity using the key techniques of system integration. The system has a variety of strain, tilt, seismic, geomagnetic, geothermal, pore pressure, other sensors, and 16 components capable of observing crustal deformation, stress, strain, tilt, earthquake, and their induced geodynamic changes in the lithosphere, such as geotemperature, hydrology, geoelectricity, geomagnetism, etc. It has been put into use in Shandan (installed depth 253 m) and Pingwu (WFSD-4, 1600 m) observatories in Gansu and Sichuan provinces and has achieved initial results. It is a milestone for our comprehensive crustal activity observation technology to break through the 3000-meter-deep well in the future. It can provide vital information for geodynamics research, safe exploitation of deep mineral and geothermal resources, and prediction of internal dynamic geological hazards. At the same time, based on the national strategy of deep-sea exploration in the 14th Five-year, the future development direction of integrated observation system of deep-well crustal activity is pointed out.