抄録
Earthquakes are complex phenomena. The notion of self-organized criticality (SOC) is recently used to explain various kinds of seismological relations expressed as power-laws, such as the size-distribution, the fractal spatio-temporal distribution and the decay of aftershocks. It is often claimed that earthquakes are not predictable if they are critical phenomena It is true that they are not predictable if the earth is at exact criticality. The notion of SOC. however, is not static but is dynamic. The crust gently approaches criticality and breaks down to generate a large earthquake. There are a variety of SOC models of earthquakes. In all of them, most of small and intermediate earthquakes occur while the system is at subcritical state approaching the exact criticality. Large earthquakes mostly occur near the criticality or at the supercritical state. Therefore, large earthquakes differ from small and intermediate ones even in the statistically scale-free SOC models. Large earthquakes in nature also are expected to be unusual events occurring at critical or supercritical states at which physical properties are extemely abnormal. Different SOC models exhibit different precursor phenomena before large events, as observed precursors differ from event to event. There cannot be universal methods to predict every large earthquake, but individual large earthquakes can be expected to have some precursors. It may be careless to reject observed precursors on a reason that they were not always observed before large earthquakes.
