Tsunami warnings, explained
A tsunami board answers two questions: is there an official tsunami alert right now, and did a recent earthquake have the size and depth to cause one? This page checks official US alerts and USGS earthquakes; for any real threat, follow tsunami.gov and your local authorities.
How does tsunami early warning work?
Tsunami early warning is a multi-step process: 1) Seismograph detection — seismic networks detect and locate a large submarine earthquake (typically M7.0+) within minutes. 2) Initial alert — warning centers issue a preliminary advisory based on earthquake magnitude and location alone. 3) DART confirmation — DART buoy pressure sensors detect the actual tsunami wave passing underneath, confirming whether a significant wave was generated. 4) Refined forecast — numerical models calculate wave heights and arrival times for coastal communities. 5) Public warning — sirens, cell alerts, and emergency broadcasts are issued. The DART network is critical because many large earthquakes do not generate destructive tsunamis — DART confirmation reduces false alarms that erode public trust.
On this board, the status panel and the Active Warnings tile count tsunami warnings, watches and advisories from the US National Weather Service alerts API (api.weather.gov), checked every 5 minutes. That feed covers US coasts and territories only, so elsewhere follow your national warning centre.
What are DART buoys?
DART (Deep-ocean Assessment and Reporting of Tsunamis) stations are NOAA’s deep-ocean tsunami detectors, moored in water typically 1,000–6,000 metres deep. A bottom pressure recorder on the seafloor measures the weight of the water column above it precisely enough to detect a tsunami of about a centimetre in the open ocean. It sends its readings acoustically to a surface buoy, which relays them by satellite to the tsunami warning centres. In standard mode a station reports every 15 minutes; when it detects a possible tsunami, or a warning centre triggers it, it switches to event mode with 15-second readings at first, then one-minute values.
This board places 41 DART station IDs on the map at approximate positions and looks each one up in NDBC’s latest_obs.txt file. That file holds weather and wave observations, not DART water-column heights, so most DART squares show “no data” and their colours are not a tsunami measurement. For real DART readings, open the NDBC station page link in a station’s detail panel.
Why is the warning board usually showing no active warnings?
Because tsunamis big enough to cross an ocean are rare. Large undersea earthquakes happen every year, but most do not move the seafloor enough, or in the right direction, to make a significant wave, and most warnings that are issued are local or regional. Recent ocean-wide alerts followed the January 2022 Hunga Tonga eruption and the July 2025 M8.8 earthquake off Kamchatka. Two things on this board are worth knowing: the Active Warnings tile only reads US National Weather Service alerts, so it stays at zero for warnings issued by other countries, and the Days Since Warning counter counts from a fixed date (15 January 2022); it is not a live value. An empty board is normal and does not mean the data failed to load.
How fast does a tsunami travel?
Tsunami propagation speed depends directly on ocean depth via the shallow-water wave formula: v = √(g × d), where g is gravitational acceleration (9.8 m/s²) and d is water depth. In the deep Pacific (average depth ~4,300m), this gives speeds of approximately 740 km/h — comparable to a commercial jet aircraft. In shallower water (200m depth), speed drops to around 160 km/h. Near shore (10m depth), the wave slows to just 36 km/h but simultaneously amplifies in height through the "shoaling" effect. The simplified 500 km/h used in this simulator represents a rough average for trans-oceanic propagation. Real NOAA models calculate propagation on a fine grid incorporating full bathymetry.
What triggers a tsunami?
The most common tsunami trigger is a submarine earthquake — specifically a megathrust earthquake where one tectonic plate is suddenly thrust upward or downward, displacing the overlying water column. Generally, M7.0+ earthquakes at depths shallower than 100km along subduction zones pose the highest risk. Other triggers include: submarine landslides (which can generate locally devastating waves even from smaller earthquakes — the 1998 Papua New Guinea tsunami was triggered by a landslide); volcanic eruptions (the 2022 Hunga Tonga eruption generated a tsunami detected worldwide); and very rarely, asteroid or meteor impacts (no confirmed examples in recorded history, but modeled as catastrophic). The location of the seafloor displacement matters greatly — vertical fault motion is far more tsunamigenic than horizontal strike-slip motion like along the San Andreas Fault.
This board flags every earthquake of M6.5 or more and shallower than 100 km in the USGS feeds as a possible trigger, whether it struck under the sea or on land. Its “tsunami potential” label is a rough heuristic from magnitude and depth only; it knows nothing about the fault type.
How accurate is the travel-time simulator?
The simulator is an educational approximation, not a forecast. It assumes a flat 500 km/h and measures the straight great-circle distance from the point you click, so it ignores land in the way and does not check whether a city can be reached by sea. Rings mark 1, 2, 4 and 8 hours, and the table lists the 10 nearest of 29 reference cities with distance, arrival time (UTC), time from now and a label: IMMINENT under 1 hour, CRITICAL under 2, WARNING under 4, WATCH under 8, otherwise Distant. Real travel-time models used by NOAA and the Pacific Tsunami Warning Center (PTWC) account for the changing ocean depth along the path (deep ocean is faster than 500 km/h, shallow water much slower), refraction around submarine ridges and seamounts, reflection from coastlines and dispersion, so real arrival times can differ from this estimate by hours on long paths. For any real emergency, follow guidance from tsunami.gov, the Pacific Tsunami Warning Center, and local civil defense authorities.
Which third-party services does the page call?
Your browser requests public data only: earthquake feeds from earthquake.usgs.gov, active alerts from api.weather.gov, and NDBC’s observation file through the public api.allorigins.win proxy, because NDBC does not allow direct browser requests. Map tiles come from Esri. The page sends no personal data with these requests and needs no account.
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