What Is a Near-Field and a Far-Field Tsunami?
A tsunami generated close to the shoreline is known as a “near-field” tsunami; a tsunami generated by a source far from the point of impact is referred to as a “far-field” tsunami. In general, it takes a large earthquake (magnitude >7.0) to generate a damaging tsunami in the near-field and it takes a great earthquake (magnitude >8.0) to generate a damaging tsunami in the far-field. Near-field tsunamis pose a greater risk for coastal communities because the first waves can move on shore in minutes. Far-field tsunamis will not reach the coast for hours, and allow time to issue warning and evacuation notices. In some intermediate cases, a tsunami could hit the coast less than an hour after an event at a moderate distance from the coast. In this latter case, the earthquake may not be felt strongly, so warnings and evacuation notices will be essential for an effective response. The implications for detection and disaster warning and response are different for near-field and far-field tsunamis, and each scenario is considered in depth as part of this report.