In 2018, a short-period seismic network was set up in Eryuan area of Yunnan Province to carry out continuous field observation of the sub-instability process of the earthquake. The relevant data of the Yangbi M_S6.4 earthquake sequence are mainly from the waveforms recorded by this network, combined with some other stations from Yunnan regional seismic network. The Yangbi earthquake sequence shows that the events in this area began to occur intensively on May 18. A total of 2 000 earthquakes with M>0.1 were recorded from May 18 to 23, including 770 foreshocks.

Seismicity analysis shows that two clusters of foreshocks occurred successively in the adjacent area of the main earthquake in the northwest segment of the rupture strip within 3 days, then in the subsequent impending period(within 1 hour before the main shock)about 60 events spread symmetrically from the center of the fracture zone to the ends. The spatial distribution of foreshocks in different periods shows the spatial migration of local fractures and accelerated expansion prior to the main shock. The spreading speed is about 5km/d from foreshock clustering process to 96km/d in impending earthquake period. The epicenter of the main shock is located at the edge of the cluster foreshocks and the northwest end of the final rupture zone. Subsequent aftershocks extend southeastward to the whole fracture zone in about half an hour, and the final fracture zone is more than 20 kilometers long, showing unilateral propagation of the rupture. Since 2018, b-value in the Yangbi area has been stable(0.9~1.1)for the past three years. After March this year, the b-value abnormally decreased to 0.6 before the main shock, reflecting that there was a significant process of continuous increase of local stress before the Yangbi earthquake.

The identification of short-term precursors and somehow definite information is one of the focus problems in earthquake prediction research. On the basis of the experimental results, Ma Jin proposed the theory of seismic meta-instability stage based on the characteristics of the load stress after the peak value from rock experiments and the corresponding change of related physical field, and considered that the degree of fault activity synergy was a sign to determine the stress state of the fault. When the fault activity changes from the expansion and increase of the stress releasing points in the early stage of meta-instability to the connection between the released segments at the late stage of meta-instability, that is, the quasi dynamic instability stage, the stress release on the fault will accelerate, and the acceleration mechanism is the strong interactions between the fault segments. In the context that the macroscopic stress state cannot be known directly, the original intention of the “meta-instability” test area is to try to capture the characteristic signal of the meta-instability stage described by the experimental phenomenon through the deformation and seismicity of the actual faults during the earthquake preparation process. It is clear that in this stage, the fault will continue to expand in the pre-slip zone theoretically, and it will enter into the quasi dynamic fracture expansion before the impending earthquake. This theory is obviously embodied in the foreshocks of this earthquake, forming the phenomenon of rapid migration of small earthquakes as mentioned above. From the current understanding of the meta-instability, it can be seen that the seismogenic fault is in the state of overall stress release at this stage, rather than the continuous increase of stress. Therefore, the decrease of b value before the earthquake shows that local faults have been activated and entered the final stage of nucleation process. The quasi dynamic spreading phenomenon before this kind of moderate-strong mainshock displayed by small earthquake activity can be identified as the precursor of a kind of earthquakes.

Recent studies have confirmed that the bedrock temperature changes when the crustal stress changes, and the information of dynamic change in crustal stress can be obtained through the observation of bedrock temperature. Moreover, there are abundant fluids in the shallow crust, and the deformation of the crust will inevitably lead to the migration of fluids, which will change the bedrock temperature. The temperature change of bedrock is equivalent to the secondary fluid thermal effect caused by crustal stress change and may be an another indirect sensitive index of crustal stress dynamic change. The bedrock temperature data of Xianshuihe fault zones show that the variation of groundwater flow rate after the Kangding M_S6.4 earthquake is consistent with the zoning characteristics of co-seismic volumetric strain in the strike-slip earthquake, indicating that the variation of near-field fluid migration characteristics is probably related to the variation of co-seismic static stress change. Moreover, the response form of bedrock temperature to the dynamic change of crustal stress and its secondary fluid effect is not consistent, as the former shows step-rise characteristics, while the latter shows exponential variation. The observation of bedrock temperature itself can obtain the dynamic change information of crustal stress and the information of shallow crustal fluid migration. Compared with crustal stress change, the variation range of fluid secondary heat effect caused by stress change may be significantly magnified(approximately an order of magnitude), which is more conducive to capturing signals, and thus may even obtain precursory fluid change information.
On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi, which happened in the bedrock temperature observation network. In particular, the Xike’er observation station is about 1.3 kilometers away from the epicenter, providing an opportunity to analyze bedrock temperature changes before and after the earthquake. The results showed that: 1)Obvious changes in bedrock temperature were found before and during the M_S6.4 earthquake. The appearance of co-seismic response means that these changes before the earthquake are related to the earthquake and may be precursory signals. 2)In terms of time, the bedrock temperature before the Jiashi earthquake first changed abnormally on the stable background, and the change reached the peak, and then fell back. When the earthquake was impending, there was a significant acceleration of the change, and the earthquake occurred after some time. The acceleration characteristics of change impending earthquake may be related to the meta-instability process of earthquakes. 3)Spatially, changes in temperature before the earthquake occurred in or near the seismogenic fault, and no obvious abnormal information was observed at the measurement points far away from the seismogenic fault, indicating that short-term and impending precursors are more likely the “near field” information; From the perspective of depth, the change in temperature before the earthquake was observed only at the local depth range. This implies that there is obvious uncertainty in the depth in precursor observation. Upon this, the ideal situation should be to carry out multi-depth joint observation, so as not to miss important precursor information. 4)Combining with the Kangding M_S6.3 earthquake on November 22, 2014, a comparative analysis is made. Similar to the Jiashi earthquake, the temperature at measurement points located in or nearby seismogenic fault of Kangding M_S6.3 earthquake shows significant changes. This means that change in the temperature before the Jiashi M_S6.4 earthquake is not an isolated case, and is a representative of universal phenomenon that occurs before strong earthquake. In a word, the change of bedrock temperature before and after the earthquake shows that the precursor information has the characteristics of near field, structural correlation and sensitive to stress change.

The coda wave propagation path has received extensive attention as it is more sensitive to small changes in the medium than the direct wave. During the process of loading, the wave velocity, medium or source changes may cause the coda wave to change. The physical mechanism of change in the ultrasonic coda wave varies during different deformation stages. Meanwhile, there exist local damages in the rock sample during the deformation, and it will be accompanied by acoustic emission. Combining the ultrasonic coda wave and acoustic emission is beneficial to characterize the coda wave characteristics and damage degree of the sample at different deformation stages. In this paper, three kinds of rocks, including granodiorite, marble and sandstone with the sizes of 50mm×50mm×150mm, are used to carry out observations of ultrasonic coda wave and acoustic emission during the whole process of loading so as to study characteristics of the coda wave at different deformation stages. The major results are given below: 1)There is a good correspondence between the coda wave variation and the acoustic emission evolution process. When the acoustic emission frequency increases, the coda wave changes accordingly. In particular, the coda wave changes in the early stages of increased acoustic emission frequency, which indicates that the early damage information of rock can be obtained by analysis of the coda wave. 2)The physical mechanism of the coda wave change is different in different deformation stages. At the initial stage of loading, there are obvious scatterer changes in the coda wave change; then, in the linear elastic deformation stage, the wave velocity change is dominating; in the late-stage of loading, the scatterer change increases and coexists with the wave velocity change, the scatterer change effect is related with the rock micro-fracture degree, the rock will locally be damaged before rupturing, and the role of the scatterer will be enhanced. 3)With the increase of loading, the amplitude of increase of the wave velocity generally decreases gradually, which is basically consistent with the understanding obtained through the direct wave. The interference of acoustic emission can be eliminated because of the Kaiser effect when analyzing the coda wave. The consistency of the wave velocity change and stress loading and unloading is further verified. 4)The micro-fracture generated during rock deformation will change the physical mechanism of the coda wave change, and the scatterer effect will be significantly enhanced. At the same time, the acoustic emission waveform will cause interference to the ultrasonic coda wave. This means that attention needs to be paid when analyzing rock damage using only coda wave data. In short, the ultrasonic coda wave and acoustic emission can reflect the damage inside the rock, and the change mechanism of the coda wave in different deformation stages is different. The joint observation of the two can play a mutual verification role, which is conducive to improving the reliability of the observation results.

The Fen-Wei rift is composed of a series of Cenozoic graben basins, which extends in an S-shape and strikes mainly NNE. Two distinct types of basins are defined in the Fen-Wei rift. The NEE-striking basins(or basin system) are bounded by active faults of mainly normal slip while the NNE-striking basins are characterized by their dextral strike-slip boundary faults. The adjacent NEE-striking basins(or basin systems) are linked by the arrangement of NNE-striking basins and horsts that is called the linking zone in this study. The segmentation of the Fen-Wei rift shows that the geometry and the activity of different rift segments are varied. The southern and northern rift segments strike NEE and are characterized by tensile movement while the central rift segment strikes NNE with transtensional motion. Previous field surveys show that the ages of the Cenozoic basins in the Fen-Wei rift are old in the southern rift segment, medium in the northern rift segment, and young in the central rift segment. The sizes of linking zones are large in the central rift segment, medium in the northern rift segment, and small in the southern rift segment. In addition, the east tip of Xinding Basin propagates towards NEE along the northern rift segment and the west tip of the basin grows towards NNE, while the shape of Linfen Basin is almost antisymmetric with respect to the Xinding Basin. However, the previous laboratory or numerical simulations cannot explain these features because they didn't pay enough attention to the control of the rift segmentation on the evolution of NEE-striking basins and their linking zones. In this study, based on the previous field studies, we study the fracture process of a clay layer under the segmented dextral transtension of the basement. The spatiotemporal evolution of the deformation field of the clay layer is quantitatively analyzed via a digital image correlation method. The experiment reproduced the main architecture of the Fen-Wei rift. The results show that:(1) The chronological order of basin initiation and the different sizes of linking zones in deferent rift segments are caused by the different obliquity angles(the angle between the rift trend and the displacement direction between the opposite sides of the rift) among the southern, northern and central rift segments.(2) The interaction between adjacent NEE-striking basins leads to the formation of NNE-striking linking zones.(3) The interaction between adjacent rift segments may cause the special distribution of Xinding and Linfen Basins. Thus, we propose that the differences of the Fen-Wei rift segments are mainly controlled by the different obliquity angles. The lack of considering the influences of pre-exiting structures leads to the limited simulation of the details within the southern and northern segments of the Fen-Wei rift. Further studies may improve the model if this is taken into account.