Tsunami Warning Centres issue four basic types of messages: (1) information bulletins when a large earthquake has occurred but there is little or no tsunami threat; (2) local, regional, or basin-wide watch, advisory, or warning bulletins when there is an imminent tsunami threat; (3) cancellation bulletins when destructive tsunami waves are gone; and (4) tsunami communication test messages to regularly exercise the system. Tsunami Messages should contain useful emergency official decision-making information, namely the tsunami's urgency, its severity, its certainty, and the area it will affect. To provide the earliest alert, initial warnings are based only on the faster arriving seismic information, specifically earthquake location, magnitude, and depth. Tsunami messages are updated regularly, or as needed, or cancelled when the threat is gone. Tsunami Messages are structured in a consistent manner, and should contain the following: Message Header (Message Number, Issuing Centre, Issue Time), Message Type and Affected Area; Authority Statement; Earthquake Parameters; Tsunami Wave Measurements (as they become available); Evaluation Statement or Assessment of Threat (may include advice on appropriate response actions, certainty; Estimated Arrival Times,Wave Forecasts ); and Next Message Schedule