Study on the pattern and mode of vertical crustal deformation during the seismogenic process of intraplate strong earthquakes

In this paper,the data of vertical crustal deformations associated with the Xingtai,Haicheng,Tangshan and Datong strong earthquakes in North China Block have been processed,analyzed and studied.The result shows that the seismogenic processes of strong earthquakes are accompanied by an evolution of crustal deformation as follows: ① The area of crustal deformation anomaly should be large in radius and extensive in range. ② There are both the 'field' and 'source' of crustal deformation anomaly,with the 'source' existing inside the'field' but differing from the 'field' distinctly. ③ The evolution process includes a number of steps.Firstly, movements in the 'field region transform from the normal state to an anomalous state to start the formation of field precursors.Secondly,movements in the 'source' region become outstandingly remarkable. Thirdly,anomalies in the 'field' region that surrounds the 'source' become increasingly intense.Fourthly,the 'source' region enters a state of immobilized-movement.Finally,an earthquake occurs. ④ There are usually one or more areas where the anomaly field forms a special pattern,mostly a four-quadrant distribution.The area which is'activated' first but becomes 'immobilized' afterwards often coincides with the source area.⑤ The appearance of an obviously immobilized area inside the obviously activated area is a precursory feature which suggests that an earthquake is impending.⑥ The longer the duration of immobilized-movement,the higher the magnitude of earthquake would be.The above might be the basic mode of vertical crustal deformation during the seismogenic process of strong intraplate earthquakes.

Three-dimensional crustal deformation before and after the Wenchuan earthquake in Guanzhong and adjacent regions

The recent plethora of GPS observations compensates for the 20-year-old lack in vertical displacement data for the Guanzhong region. The 2001—2007 three-dimensional（3D） crustal deformation data suggest regional movement with a horizontal velocity of 3—7 mm/a,predominantly from SSE in the west to SE in the east, and vertical inherited movement with velocity of -7 mm/a to 4 mm/a. After the Wenchuan earthquake, the GPS data suggest that the effect of the earthquake on the regional deformation is greater in the west than the east.The horizontal displacement increased during 2007—2008; however, the reverse was observed in 2008—2009. The vertical displacement in the western part of the region increased in 2008 and has been gradually returning to normal since 2009; however, in the eastern part,the effect of the earthquake remains.

Temporal variation of gravity field prior to the Ludian Ms6.5 and Kangding Ms6.3 earthquakes

Using mobile gravity data from the central area of Sichuan and Yunnan Provinces, the relationship between gravity variation and earthquakes was studied based on the Ludian Ms6.5 earthquake that occurred on August 3rd, 2014, and the Kangding Ms6.3 earthquake that occurred on November 22 nd, 2014; the mechanism of gravity variation was also explored. The results are as follows：（1） Prior to both earthquakes, gravity variation exhibited similar characteristics as those observed before both the Tangshan and Wenchuan earthquakes, in which typical precursor anomalies were positive gravity variation near the epicenter and the occurrence of a high-gravity-gradient zone across the epicenter prior to the earthquake.（2） A relatively accurate prediction of the occurrence locations of the two earthquakes was made by the Gravity Network Center of China（GNCC） based on these precursor anomalies. In the gravity study report on the 2014 earthquake trends submitted at the end of 2013, the Daofu-Shimian section at the junction of the Xianshuihe and Longmenshan fault zones was noted as an earthquake-risk region with a predicted magnitude of 6.5, which covered the epicenter of the Kangding Ms6.3 earthquake. In another report on earthquake trends in southwestern China submitted in mid-2014, the Lianfeng, Zhaotong fault zone was also classified as an earthquake-risk region with a magnitude of 6.0, and the central area of this region basically overlapped with the epicenter of the Ludian Ms6.5 earthquake.（3） The gravity variation characteristics are reasonably consistent with crustal movements, and deep material migration is likely the primary cause of gravity variation.

Effects of the February 12,2014 M_S 7.3 Yutian Earthquake on Seismicity of the Northeastern Edge of Qinghai-Tibetan Block

First of all,using the GPS velocity field from campaign GPS measurements implemented by CMONC( C hina Crustal Movement Observation) a nd TEONC( C hina Tectonic Environment Observation Networks) u p to 2013, w e analyzed the background of regional crustal horizontal movement and deformation before the M S7. 3 Yutian,Xinjiang earthquake on February 12,2014. Then,by comparing this to the vertical movement from leveling measurements,we studied the crustal movement deformation and the state of strain accumulation on the northeastern edge of Qinghai-Tibetan block.Finally,we investigated the possible effects on the earthquake activity of the northeastern edge of Tibet from the M S7. 3 Yutian earthquake. The result indicates that,the M S7. 3Yutian earthquake occurred against the background of strong tectonic movement and intensive intracontinental crustal differential movement on the edges of tectonic blocks in western China,and also that it happened in the period of the strong tectonic stress field in Qinghai-Tibetan block and its edges. The sinistral strike-slip and stress transfer of the Yutian M S7. 3 earthquake may accelerate the rupture of fault segments with high strain accumulation at the northeastern edge of Qinghai-Tibetan block( especially in Qilian Mountain fault zone,and border area of Gansu,Qinghai and Sichuan provinces on the south of western Qinling).