Warning systems only evolve through the actual experience of disasters. According to a piece I translated for an NHK publication about disaster reporting in Japan, the Japanese tsunami reporting system has evolved through two major diasters:
In 1983, the Sea of Japan Chubu earthquake claimed 104 lives, of which 100 were the result of the tsunami that followed the offshore quake. At the time, it took 19 minutes to issue a tsunami warning, by which time it was too late.
Again, in 1993, the Hokkaido Nanseki-oki earthquake caused a 20 meter (65 ft.) tsunami that took a toll of 230 lives. By this time, NHK and the Meteorological Agency had cut the time between the quake and the warning to 7 minutes, but the tsunami struck the island of Okushiri, which was closest to the epicenter, 5 minutes after the quake.
At present, they have reduced the time to 3 minutes.
Tsunamis travel 500 km/h (310 mph) in the open ocean, and in shallower costal areas at 165 km/h (100 mph), meaning this is the speed they are traveling when they hit.
(Information taken from Disaster Reporting and the Public Nature of Broadcasting published by NHK Broadcasting Culture Research Institute; 2004.)
Obviously, with time frames like this, preventive measures are difficult. It requires not only an excellent reporting system, but an effective mechanism for immediate evacuation and an educated public. Considering the time differentials involved, this is next to impossible in most areas.