10:45 AM - 12:15 PM
[SCG45-P46] Effect of the upper limit of the porosity on variation of behavior of slow and fast earthquakes
Keywords:slow earthquake, fast earthquake, BK model, phase transition, porosity
Transition from slow to fast earthquakes is a controversial problem. We have investigated the condition for the transition using the 1D Burridge-Knopoff model and the interaction among the heat, fluid pressure, and porosity (Suzuki and Matsukawa, in prep.). If the thermal (pore-generation) effect is dominant for the fluid pressure change, the fast (slow) earthquake occurs. For the case of a single block model, the transition from slow to fast earthquake after repetition of slow earthquake has been analytically understood as the first order phase transition, and the transition is controlled by a function derived based on the energy balance before and after the slippage of a block. The analytical treatment is done by neglecting the porosity change in the model parameters such as the bulk modulus of the medium, referred to as constant parameter assumption. If the change in these parameters due to the porosity change is negligible, the analytical treatment works well. To treat this porosity-change effect on the parameters, the porosity evolution law should be taken into account explicitly. The porosity monotonically increases with increasing the slip to the upper limit. Investigating the dependence of model parameters on the upper limit for the porosity is important step to understand the slow-fast earthquake behavior both for the models of a single block and of multiple blocks.
We consider multiple block model. We first assume the large upper limit. In this case, the analytical treatment is not effective given for a single block model and constant parameter assumption. By numerical treatment, we found that the slow-to-fast transition occurs, and several blocks slowly slip after the fast earthquake, which is interpreted as slow earthquakes. The slip and slip velocity of these earthquakes are negligibly smaller than those of fast earthquakes. These slow earthquakes occur in many regions on the substrate. Therefore, this behavior can be interpreted as a tremor, which is considered to consist of sequential slow earthquakes like low frequency earthquakes. This behavior also suggests that after the transition, another fast earthquake will not occur. When the upper limit of the porosity is reduced, the tremors are harder to occur because the pore generation effect on fluid-pressure change becomes smaller than that of the thermal effect. In this case, the fast earthquakes occur several times. Moreover, we found that slow earthquakes preceding the fast earthquakes also occur. Both the slow and fast earthquakes occur, but their prediction is impossible. When the upper limit of the porosity is negligibly small, the pore generation effect is almost negligible, and only the fast earthquakes repeat. The upper limit for the porosity plays an important role for the slow-fast earthquake behavior.
We consider multiple block model. We first assume the large upper limit. In this case, the analytical treatment is not effective given for a single block model and constant parameter assumption. By numerical treatment, we found that the slow-to-fast transition occurs, and several blocks slowly slip after the fast earthquake, which is interpreted as slow earthquakes. The slip and slip velocity of these earthquakes are negligibly smaller than those of fast earthquakes. These slow earthquakes occur in many regions on the substrate. Therefore, this behavior can be interpreted as a tremor, which is considered to consist of sequential slow earthquakes like low frequency earthquakes. This behavior also suggests that after the transition, another fast earthquake will not occur. When the upper limit of the porosity is reduced, the tremors are harder to occur because the pore generation effect on fluid-pressure change becomes smaller than that of the thermal effect. In this case, the fast earthquakes occur several times. Moreover, we found that slow earthquakes preceding the fast earthquakes also occur. Both the slow and fast earthquakes occur, but their prediction is impossible. When the upper limit of the porosity is negligibly small, the pore generation effect is almost negligible, and only the fast earthquakes repeat. The upper limit for the porosity plays an important role for the slow-fast earthquake behavior.