## 1 Introduction  
Reservoir-induced seismicity is an interesting area of research because there are still many challenges in figuring out how seismicity is triggered and how long it lasts. Fractures divide the rock mass into discontinuity and may control mechanistic behavior under certain conditions, which is one of the main reasons for inducing seismicity and slope sliding. In North Dakota, earthquakes are induced by the interaction of hydraulic fractures with large faults in the region [1]. Induced earthquakes are particularly pronounced in Oklahoma and Kansas, where studies have shown that the underground storage of salt water is the primary cause [2]. A bimodal distribution of fluid injection-induced microseisins was observed in 2012 at the Newberry Volcano Enhanced Geothermal (EGS) Demonstration Project in Oregon, USA. EGS presents challenges for enhancing permeability and reducing instability [3]. Especially near the water reservoir, the seepage field would change significantly due to impoundment. Critically stressed fractures are expected to be unstable since the original balance has been broken when considering the coupling influence between the liquid phase and the solid phase. Therefore, the slight disturbance of water pressure can crucially alter the state of a fracture. Particularly, fluid overpressure inside the fracture takes an obvious part in reactivating a fracture and releasing energy. Evidence shows that fluid overpressure is a significant factor in the nucleation and propagation of ruptures [4]. The maximum fluid overpressure could be predicted and controlled using tectonic stress and fracture orientation [5]. Changes in the stress state are accompanied by fluid redistribution and are ensured by the critical interdependence of differential stress and sustainable overpressure [6]. Fluid overpressure is commonly regarded as the reason for weak subduction plate boundaries [7]. The increase of fluid pressure first causes an accelerated seismic creep and fault opening; with the further increase of fluid pressure, the friction rate increases, which is conducive to seismic creep [8]. Besides, overpressure can reduce the effective overburden stress and the available stress, and can lead to under-consolidated sediment sequences, resulting in the unstable sliding of the slope and the induction of earthquake shaking [9]. Therefore, fluid overpressure is comprehensively discussed, since it is a hydrodynamic phenomenon. The fluid expansion mechanism and disequilibrium compaction mechanism are two categories used to explain the generation of overpressure [10]. Experimental analysis shows that pore pressure can be predicted based on elastic parameters [11]. The Mohr-Coulomb criterion can judge when the fractures fail, but cannot predict the mechanical behavior after the fractures break. Therefore, the energy budget is carried out to describe the input energy and output energy. The input energy equals the output energy, according to the Law of conservation of energy. Thus, the input energy, including the hydraulic energy and mechanic energy, is supposed to produce the same value of output energy, composed of friction energy, radiated energy, deformation energy and energy loss. This budget provides support to verify the role of fluid overpressure in inducing fracture sliding. As for the water reservoir, the alteration of the underground water level is usually delayed after the modification of the reservoir impounding, indicating that the overpressure of the fracture seepage might be caused by both mechanisms. This study concentrates on the fluid expansion mechanism. However, overpressure is usually described qualitatively in macro-geology. There are a few micro descriptions that can explain this mechanism explicitly. To reveal the mechanism of activating a stressed fracture, a shearing test, based on HM coupling, is adopted to concentrate on how the overpressure activates the sliding and discusses the relationship between the input hydraulic energy and output seismic energy.