피해야 할 때: 지진 후 쓰나미 경보

Natural Warning Signs: Strong Shaking Near the Coast

The most important principle in coastal TsunamiA series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can travel across entire ocean basins at jet speed (700+ km/h). preparedness is this: if you feel strong, prolonged shaking while near the ocean, treat it as a tsunami warning and begin moving to high ground immediately — before any official warning is issued, before you see any wave, and without waiting to verify anything. This is not overcaution; it is the behavioral protocol that prevents the largest class of preventable tsunami deaths.

Near-field tsunamis — those generated by a fault rupture close to the coast — can arrive at shore within five to fifteen minutes of the generating earthquake. The Pacific Tsunami Warning Center needs time to detect, analyze, and issue a warning; in near-field scenarios, this process takes five to twenty minutes. In the Tōhoku earthquake of 2011, the initial warning significantly underestimated the tsunami height, leading some communities to return to low-lying areas after the first wave. Earthquake Early Warning (EEW)A system that detects an earthquake and sends alerts to people and systems before strong shaking arrives. Can provide seconds to tens of seconds of warning, enough to take protective action. systems are more reliable for distant tsunamis, where warning times of hours allow for complete orderly evacuation.

Strong shaking means a duration of 20 seconds or more and intensity that makes it difficult to stand — roughly Modified Mercalli Intensity VII or above. A magnitude 7.5 or greater earthquake anywhere in the Ring of FireA horseshoe-shaped zone around the Pacific Ocean where about 90% of the world's earthquakes occur. It spans 40,000 km and includes 452 volcanoes. can generate a TsunamiA series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can travel across entire ocean basins at jet speed (700+ km/h)., and a magnitude 8.0 or greater on a Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs. fault is virtually guaranteed to do so if the fault rupture is beneath the ocean floor. The Cascadia Subduction Zone, which runs from Northern California to British Columbia, is capable of generating a magnitude 9.0 earthquake with tsunamis reaching 100 feet or more on the Pacific Northwest coast.

Official Tsunami Warning Systems

The Pacific Tsunami Warning Center (PTWC) and the National Tsunami Warning Center (NTWC) are the two U.S. federal agencies responsible for issuing tsunami warnings for the Pacific and Atlantic/Caribbean/Gulf of Mexico basins respectively. These centers operate 24 hours a day, seven days a week, and issue alerts at four levels: information statements, tsunami watches, tsunami advisories, and tsunami warnings. A tsunami warning — the highest level — indicates that a TsunamiA series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can travel across entire ocean basins at jet speed (700+ km/h). with the potential to cause inundation has been confirmed or is highly probable, and that immediate evacuation of low-lying coastal areas is required.

Warnings are disseminated through multiple channels: the NOAA weather radio system (which any Earthquake Emergency KitA pre-assembled collection of supplies for surviving the aftermath of an earthquake, typically including water (1 gallon/person/day for 3 days), food, first aid, flashlight, and radio. radio receiver can access), the Emergency Alert System on television and radio, Wireless Emergency Alerts (WEA) sent to all compatible cell phones in the threatened area, and sirens in coastal communities that have installed physical warning infrastructure. In many Pacific Northwest communities, WEA tsunami warnings have been incorporated into the ShakeAlert Los Angeles and ShakeAlertThe US earthquake early warning system operated by USGS and university partners. Covers the West Coast (California, Oregon, Washington) and sends alerts through Wireless Emergency Alerts. early warning system protocol, linking the detection of a qualifying seismic event directly to a coastal evacuation alert without human intermediation.

Know the specific alert protocols for your coastal jurisdiction before visiting or residing near the ocean. Some communities use a single long siren tone for all emergencies; others have distinct tsunami-specific patterns. Verify the meaning of sirens in each coastal community you visit, and do not rely on the assumption that all coastal communities use the same signaling convention. When in doubt, a siren near the coast after strong shaking means one thing: move to high ground immediately.

Evacuation Zones and Route Planning

Tsunami Evacuation ZoneA designated area at risk of tsunami inundation with marked evacuation routes to higher ground. Evacuation should begin immediately after feeling strong coastal shaking. maps for coastal communities are prepared by state and local emergency management agencies using NOAA tsunami inundation modeling and local topographic data. These maps are publicly available through state emergency management websites and are typically posted at entrances to coastal parks, on beach access signs, and in hotel rooms. Download the digital map for any coastal area you plan to visit more than a day in advance; cellular data service may not be available during and immediately after a major earthquake.

Your evacuation route should be identified and driven before you need it. The fastest route to high ground is not always the most obvious one, and road networks in coastal communities frequently have bottlenecks that become severely congested during an evacuation. Identify your route and a backup route in case the primary is blocked by traffic, debris, or a collapsed bridge. Note the distances: if the nearest safe elevation is two miles away and you are on foot, you have approximately 20 to 30 minutes of walking time. Compare this against the expected wave arrival time for your specific location using the Tsunami Risk Estimator.

For residential areas with large populations and limited road networks, Tsunami Evacuation ZoneA designated area at risk of tsunami inundation with marked evacuation routes to higher ground. Evacuation should begin immediately after feeling strong coastal shaking. planning authorities model expected evacuation times under various scenarios. These analyses are sometimes published as part of community hazard assessments and can reveal whether the road network of a specific community is adequate to evacuate the population before wave arrival. If you live in a community where this analysis indicates insufficient road capacity, vertical evacuation structures become a critical part of your personal plan.

How Fast Must You Move? Wave Arrival Times

Wave arrival times vary dramatically depending on the distance from the earthquake epicenter to the shoreline and the local bathymetry (underwater topography) of the approach. For the Cascadia Subduction Zone — the most significant near-field TsunamiA series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can travel across entire ocean basins at jet speed (700+ km/h). threat for Pacific Northwest communities — modeled wave arrival times range from 10 minutes at some outer coast locations in Washington and Oregon to 30 minutes in Puget Sound. For distant-source tsunamis from Alaska or Japan, arrival times range from 4 to 9 hours in Hawaii and 3 to 4 hours in California, allowing time for orderly official evacuation.

The Secondary Earthquake HazardsHazards triggered by earthquake shaking rather than the shaking itself — including tsunamis, landslides, liquefaction, fires, dam failures, and chemical releases. Often cause more damage than shaking. of tsunami inundation include debris-laden water moving at 10 to 35 miles per hour, which can deliver lethal blunt force even at depths that appear manageable from a standing-wave perspective. A one-foot depth of fast-moving water can knock a person off their feet; two feet of fast-moving debris-laden water is typically lethal. Swimming ability is irrelevant in a debris-laden tsunami surge; reaching high ground before any water arrives is the only reliable protection.

Use the Tsunami Risk Estimator to input your coastal location and the characteristics of a hypothetical generating earthquake, and receive an estimated wave arrival time and inundation zone boundary. This tool is designed for planning purposes and uses NOAA modeling databases; it is not a real-time warning system and must not be used as a substitute for official warnings during an actual event.

Vertical Evacuation: When You Can't Reach High Ground

In some low-lying coastal communities — particularly islands, peninsulas, and flat deltaic areas — sufficient elevation to escape tsunami inundation may not be accessible within the available warning time even for fit adults on foot. For these communities, vertical evacuation structures — purpose-built or designated existing structures designed to provide refuge above inundation levels — are a critical component of the emergency management system.

Vertical evacuation structures are typically reinforced concrete buildings of four or more stories, designed to withstand both earthquake shaking and the hydrodynamic loading of a tsunami wave and debris impact. FEMA has published design guidelines (FEMA P-646) for tsunami vertical evacuation refuge structures, and several Pacific Northwest communities have built or designated structures using these standards. Know whether your community has designated vertical evacuation structures and locate them on your route planning map.

A designated vertical evacuation structure is not necessarily a formal purpose-built refuge; in some communities, existing reinforced concrete buildings have been assessed and designated as informal vertical evacuation options. These designations are based on structural adequacy and are communicated through local emergency management agencies. The key characteristics are: reinforced concrete or steel construction (not wood frame), at least three stories above expected inundation, accessible upper floors, and a location that allows evacuation route access before wave arrival.

Using the Tsunami Risk Estimator to Assess Danger

The Tsunami Risk Estimator tool allows coastal travelers and residents to input their specific location and the parameters of a hypothetical or actual seismic event to receive a personalized inundation risk estimate and evacuation guidance. Input your GPS coordinates or address, the estimated magnitude and location of the generating earthquake, and your current elevation. The tool cross-references NOAA inundation modeling data, Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs. geometry, and local coastal topography to estimate the probability of inundation, expected water height, and estimated wave arrival time window.

Use this tool proactively when planning visits to coastal areas: input the address of your hotel or campsite and the parameters of the largest plausible TsunamiA series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can travel across entire ocean basins at jet speed (700+ km/h). for that coast to understand your baseline risk. Compare the estimated wave arrival time against the time required to walk to the nearest evacuation zone boundary at a comfortable pace — if the ratio is less than 2:1, your location warrants a more aggressive personal evacuation plan. Share the results with all members of your travel party and identify the specific route and assembly point together before arriving at the destination.

The Earthquake Early Warning (EEW)A system that detects an earthquake and sends alerts to people and systems before strong shaking arrives. Can provide seconds to tens of seconds of warning, enough to take protective action. infrastructure for distant-source tsunamis has improved dramatically since the 2004 Indian Ocean tsunami, with NOAA's Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy network providing real-time sea surface measurements that confirm whether a tsunami has been generated within 10 to 15 minutes of a qualifying earthquake. This improvement means that for distant-source tsunamis, official warnings now arrive well before wave impact in most monitored coastal areas. Near-field scenarios — particularly Cascadia for the Pacific Northwest — remain the primary planning gap, underscoring the critical importance of the "feel strong shaking, move immediately" behavioral protocol.