This study reveals the important role of hydromechanical responses of large hydraulic fractures in the cause and mechanisms of induced seismicity associated with the M_w 5.5 earthquake at the Pohang geothermal site, South Korea. Fractures intersecting two stimulation wells, PX-1 and PX-2, were identified through a comprehensive analysis of various data produced from drilling and hydraulic stimulations, including injection flow-pressure and seismicity data. Coupled hydromechanical numerical modeling of five stimulations performed in the PX-1 and PX-2 wells, showed that injection-induced hydraulic jacking and fracture shearing induce immediate stress transfer that plays a significant role in understanding seismic response at the M_w 5.5 fault. The friction coefficient times fluid pressure change by the PX-2 stimulations accounts for only 20% of the total CFS change at the location of the M_w 5.5 earthquake. The absence of seismic events at the M_w 5.5 fault by the PX-1 stimulations is attributed to the transfer of reduced shear stress at the PX-1 fracture due to shear slip. Kaiser effects in the seismic response observed during the PX-1 stimulation is explained by the irreversible nature of shear slip of the PX-1 fracture for the first time. In addition, post-injection seismicity, observed during the PX-2 stimulations, is attributed to a low permeability of the PX-2 fracture. The current modeling study suggests that coupled hydromechanical analysis with appropriate consideration of fractures is a powerful approach in understanding the mechanism of injection-induced seismicity in fractured geological media.

本研究揭示了大型水力裂缝的流体力学响应在与韩国浦项地热站点_{M w 5.5 地震相关的诱发地震活动的原因和机制中的重要作用。}通过对钻井和水力增产产生的各种数据（包括注入流压力和地震活动数据）的综合分析，确定了与 PX-1 和 PX-2 两口增产井相交的裂缝。_{在 PX-1 和 PX-2 井中进行的五次增产的耦合流体力学数值模拟表明，注入引起的水力顶进和裂缝剪切会引起即时应力转移，这在理解 M w}处的地震响应方面起着重要作用5.5 故障。PX-2 刺激引起的摩擦系数乘以流体压力变化仅占 M _w 5.5 地震位置总 CFS 变化的 20%。在 M _{w没有地震事件}PX-1 增产的 5.5 断层归因于由于剪切滑移而在 PX-1 裂缝处传递减小的剪切应力。在 PX-1 增产期间观察到的地震响应中的 Kaiser 效应首次解释为 PX-1 裂缝剪切滑移的不可逆性质。此外，在 PX-2 增产期间观察到的注入后地震活动归因于 PX-2 裂缝的低渗透率。目前的建模研究表明，适当考虑裂缝的耦合流体力学分析是理解裂缝地质介质中注入诱发地震活动机制的有力方法。