Underground structures are less sensitive to seismic shaking compared to surface structures. Several case histories reported damages to underground structures during major earthquake events. The damages are mainly related to the earthquake duration, seismic magnitude, and distance from the epicentre, ground behaviour, the depth, and the properties of the underground structure. The concepts of seismic design and modelling seismicity on tunnels excavated in rock masses were reviewed. Bhasin and others (2006) conducted numerical experiments using Phase 2 version 5 which is a 2D elastic-plastic finite element stress analysis program for underground or surface excavations in rock or soil. They studied the seismic behaviour of rock support in circular lined tunnels excavated in weak and competent rock. Their research was verified and updated by using Phase 2 version 6. The verification included more representative factors in the analyses of the results. The software suitability in terms of numerical and theoretical procedures was investigated and found to be suited to the seismic simulations. The maximum axial force in the lining for tunnels in weak rock was found to increase 15% to 44% by Bhasin and others in 2006.In this thesis however it was updated to 19% to 38% after including the effect of rock support interaction. Numerical experiments were conducted to study the effects of seismicity on circular tunnels in rock masses with joints. The models were configured in a similar way to the earlier models by Bhasin and others in 2006. The effect of single joints and their orientation was simulated and studied in addition to two cases studies that include multiple joints. It was concluded that the competent rock deform along the joints. The maximum axial force in the lining occurs at the intersections between the joint and the tunnel lining. Neither the seismicity nor the orientation of the joint had a pronounced effect on tunnels in competent rocks with joints. Weak rock on the other hand may deform regardless of the locations of the joints and is affected clearly by seismicity. The maximum axial force in the tunnel lining does not occur necessarily at the intersections between the joints and the tunnel lining. Weak rocks were found to be less affected by seismic loads when they contain joints. The results were found to be in line with the Norwegian rock index system (Q system) guidelines and the earlier results by Bhasin and others in 2006.