We don't know it will happen. The release of seismic stress can happen in many ways and we don't know enough to know anything for sure. A series of 7 or 8 quake

We don't know it will happen. The release of seismic stress can happen in many ways and we don't know enough to know anything for sure. A series of 7 or 8 quakes over a decade could relieve identified stresses to the point where a 9+ is put off for another 100+ years. After that even if statistically you can expect a major event "at some point" in the future, the damage from that event decreases with time as our building methods improve. Even today buildings are being built (in Tokyo) going up designed to withstand a 9.0 quake.