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According to the steady state of fault and energy balance, we provided a new idea to observe the precursors for a stressed fault. The meta-instability (or sub-instability) state of a fault is defined as the transition phase from peak stress to critical stress of fast instability (earthquake generation) during a full period of slow loading and fast unloading. The accumulative deformation energy begins to release in this stage. Identifying its deformation before fast instability would be beneficial to obtain premonitory information, and to evaluate the seismic risks of tectonic regions. In this study, we emphasized to analyze deformation process of the meta-instable stage with stain tensor data from a straight precut fault in granite at a slow loading rate, and observed the tempo-spatial features during the full deformation process of the fault. Two types of tectonic zones and instabilities occur on the stick-slip fault. The low- and high-value segments in the volume strain component appear along the fault strike with a load increment. The former first weakens and then becomes initial energy release segments; the latter forms strong stress-interlocking areas and finally turns into the initial region of fast instability. And there are two stages in the entire instable process of the fault: the initial stage is associated with the release of the low volume strain segments, which means fault pre-slips, slow earthquakes or weak earthquakes. The second one characterizes a strong earthquake through the release of high volume strain parts. The rupture acceleration in the first stage promotes the generation of the second. Moreover, fault instability contains two types of strain adjustments along the fault: the front-like strain change along the transition segments from low- to high- strain portions with volume strain release, and the compressive strain pulse of fault instability after the volume strain release extends to a certain range with loading increment. In laboratory experiments, the front-type strain occurs about 12 seconds before fast fault instability; the compressive pulse initiates within less than 0.1 second, and then the fault turns quickly into a dynamic strain adjustment, which appears quasi-synchronously between different measurement points, and, finally, an earthquake is generated.

Tectonophysical experiments show that the evolution of the Fen-Wei Rift is controlled by oblique rifting. A key characteristic of the model in our study is that the western and eastern borders of the transfer zone between the adjacent NEE-striking extensional basins tend to form right-lateral strike-slip faults with slight normal slip as a result of the interaction between the adjacent NEE-striking extensional basins under oblique rifting. The current deformation of the Fen-Wei Rift can be clarified by testing this predicted deformation characteristic. Our analysis of the relocation and focal mechanism solutions of the 1989 M 6.1 Datong-Yanggao earthquake swarm, which was the largest earthquake that occurred in the Fen-Wei Rift in the last 200 years, suggests that the transfer zone between the Yangyuan and Hunyuan basins is bounded by the NNE-striking right-lateral strike-slip faults with slight normal slip at its eastern and western edges. This consistency between the model and the current tectonic activity in the study area indicates that oblique rifting still plays an important role in the current deformation of the northern Fen-Wei Rift.