A tsunami warning is an alert, usually issued by a National Tsunami Warning Centre (NTWC), to indicate that a tsunami threat is expected and imminent. A tsunami warning may be issued for different levels of tsunami threat. For example, a low-level threat is one characterized by small sea level changes and strong ocean currents, and the tsunami is only a hazard at beaches, in harbours, and for recreational ocean activities. During a major threat, high amplitude waves along with powerful currents can be expected and could cause significant inundation and complete destruction of most near-shore structures. Dangerous waves may continue for several hours after arrival of the initial wave. Different levels of warning should trigger different types of response by emergency officials and by the at-risk public. Appropriate public safety action taken when there is a major threat includes the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters if there is time. Warnings may be updated, adjusted geographically, downgraded, or cancelled. To provide the earliest alert, initial warnings are normally based only on seismic information. Threat levels may be given different names by different countries depending upon their language and the standard nomenclature they use for other hazards such as weather events. In Japan, there are 66 coastal forecast regions, and warnings are issued specifically for each. There are three levels of threat based on the tsunami height forecast (major tsunami warning, tsunami warning, and advisory)