North-Eastern Atlantic, Mediterranean and connected seas

Tsunami Information Centre

Tsunami Warning

A tsunami warning is an alert, usually issued by a National Tsunami Warning Centre (NTWC), to indicate that a tsunami hazard 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 NEAMTWS, the level of threat (established trough the Decision Matrix) for a given country or region is defined in terms of its distance to the earthquake source .When a country is in a “Watch” or “Advisory” status, the ETAs for its forecast points that meet the criteria will be listed in the tsunami alerts issued by the TWPs.

Decision matrix for the Mediterranean

Depth

Location

Mw

Tsunami potential

Type
of tsunami message

< 100

Offshore or close to the coast

(< 40 km inland)

5.5–6.0

Weak potential for a local destructive tsunami

Information Message

Local tsunami

Advisory

6.0–6.5

Potential for a destructive local tsunami

Regional Tsunami Advisory

Local Tsunami Watch

Offshore or close to the coast
(< 100 km inland)

6.5–7.0

Potential for a destructive regional tsunami

Regional and Local Tsunami Watch

Basin-wide Tsunami Advisory

> 7.0

Potential for a destructive basin-wide tsunami

Basin-wide Tsunami Watch

> 100

Offshore of inland
(< 100 km)

> 5.5

No tsunami potential

Information Message

 

Decision matrix for the NE Atlantic

Depth

Location

Mw

Tsunami potential

Type of tsunami message

< 100

Offshore or close to the coast

(< 40 km inland)

5.5–6.0

Weak potential of local tsunami

Regional Tsunami Advisory

6.0–6.5

Potential of destructive local tsunami < 100 km

Regional Tsunami

Watch

Basin-wide Tsunami Advisory

Offshore or close to the coast
(< 100 km inland)

6.5–7.0

Potential of destructive regional tsunami
< 400 km

Basin-wide Tsunami Watch

> 7.0

Potential of destructive tsunami in the whole basin >400 km

Basin-wide Tsunami Watch

Inland (> 40 km and ≤ 100 km)

5.5 – 7.0

Weak potential of local tsunami

Information Message

> 100

Offshore of inland
(< 100 km)

> 5.5

Nil

Information Message