## 4 Discussion  
The depth distribution of shallow-source earthquakes in the crust is controlled by the strength of fault zones and the stability of sliding friction [18]. Destructive earthquakes generally occur in the regions where the lithospheric strength of the crust is the highest, i.e., the brittle-ductile transition zone [15, 16, 50, 51]. The brittle-ductile transition zone typically lies between depths of 10 and 15 km in the crust, where the temperature ranges from approximately 250 \({}^{\circ}\)C to 450 \({}^{\circ}\)C. This zone is a semi-brittle deformation domain, where quartz can undergo plastic deformation, while feldspar primarily deforms through brittle  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
**Sample** & **SiO\({}_{2}\)** & **TiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **FeO** & **MnO** & **MgO** & **CaO** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **P\({}_{2}\)O\({}_{8}\)** & **LOI** & **Total** \\ \hline AF19-43D1 & 56.67 & 0.87 & 17.16 & 3.63 & 5.22 & 0.13 & 3.48 & 6.30 & 3.52 & 1.23 & 0.21 & 0.70 & 99.12 \\ \hline AF19-43D2 & 49.92 & 0.94 & 18.87 & 3.63 & 6.12 & 0.20 & 4.70 & 9.17 & 4.70 & 0.63 & 0.25 & 0.94 & 99.02 \\ \hline AF19-43D & 57.63 & 0.78 & 16.49 & 7.97 & - & 0.12 & 3.31 & 6.52 & 3.71 & 1.06 & 0.20 & 1.55 & 99.34 \\ \hline AF19-44A & 50.62 & 2.17 & 15.72 & 11.69 & - & 0.15 & 4.55 & 7.14 & 3.50 & 1.51 & 0.32 & 2.26 & 99.64 \\ \hline \end{tabular}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Sample** & **SiO\({}_{2}\)** & **TiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **FeO** & **MnO** & **MgO** & **CaO** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **P\({}_{2}\)O\({}_{8}\)** & **LOI** & **Total** \\ \hline AF19-43D1 & 56.67 & 0.87 & 17.16 & 3.63 & 5.22 & 0.13 & 3.48 & 6.30 & 3.52 & 1.23 & 0.21 & 0.70 & 99.12 \\ \hline AF19-43D2 & 49.92 & 0.94 & 18.87 & 3.63 & 6.12 & 0.20 & 4.70 & 9.17 & 4.70 &fracture. The granite in the brittle-ductile transition zone exhibits unstable sliding [52], and its strength is controlled by temperature, pressure, fluids, and other conditions [11; 12; 13; 14].  
The coseismic processes triggered by unstable sliding and high-speed frictional heat generation may be the main mechanisms for the formation of pseudotachylyte. During the coseismic rupture phase, due to the sharp increase in strain rate, the depth of the brittle-ductile transition zone in the fault zone will extend deeper [1]. In the interseismic period, the ductile shear zone, which is in a plastic deformation state, can undergo brittle failure during the coseismic process, forming cataclasites and pseudotachylytes. During the post-seismic relaxation phase, the strain rate gradually decreases, returning to the background values of the normal region. The depth of the brittle-ductile transition zone will gradually become shallower, and at this time, semi-brittle non-steady-state flow becomes the main deformation feature of the fault. This is accompanied by hydrolysis of feldspar and static recrystallization, dynamic recrystallization, and other plastic deformation processes of quartz and other minerals, while the fractures generated by the earthquake begin to heal [17; 18; 53].  
This paper analyzes the microstructures of granite geniss, mylonite, fractured rock, and pseudotachylyte, providing geological evidence for the deformation processes during each stage of the earthquake cycle.