What is a tsunami?
A tsunami is a series of ocean waves generated by sudden displacements in the sea floor, landslides, volcanic eruptions, glacier calvings, meteorite impacts, or other disturbances. Tsunamis can cause massive destruction when they strike populated coastal regions. A tsunami has an extremely long wavelength and period, allowing it to travel across entire oceans with little energy loss. When a tsunami enters shallow water near land, its speed decreases but its height increases dramatically as its massive amount of kinetic energy is converted to potential energy. The crest of the tsunami wave can reach over 100 feet in height before crashing ashore with devastating force.
How are tsunamis monitored?
Since tsunamis can originate thousands of miles away before striking coastal communities, early detection is critical for issuing timely warnings. An international system of deep ocean buoys, coastal sea level gauges, and seismic sensors allows organizations to monitor for tsunamis across the world’s oceans and issue alerts to potentially affected regions.
The Deep-ocean Assessment and Reporting of Tsunamis (DART) system consists of around 60 buoys strategically positioned across the Pacific and Atlantic oceans. These buoys use pressure sensors to detect subtle changes in the height of the water column indicative of a passing tsunami wave. Data is transmitted via satellite to warning centers within minutes of a potential tsunami being detected.
In addition to the deep ocean buoys, a network of over 100 coastal tide gauges and dozens of seismic sensors provide further data to help confirm a tsunami threat and refine models used to predict the intensity and arrival time of the waves. With data from multiple systems, warnings can be issued within minutes of significant seismic events with tsunamigenic potential.
Who issues tsunami warnings?
In the United States, tsunami warnings are coordinated by the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service. The NOAA Center for Tsunami Research operates the U.S. DART buoy network and provides tsunami detection and forecasting services. Other nations have their own tsunami monitoring and warning centers that coordinate regionally and share data globally through the Intergovernmental Oceanographic Commission of UNESCO.
When a potential tsunami is detected, the warning centers will issue alerts via multiple communication channels including the Emergency Alert System, Wireless Emergency Alerts, NOAA Weather Radio All Hazards, email, SMS text messages, social media, NOAA websites, and direct outreach to emergency managers and public safety officials. These messages are targeted at areas with sufficient time to take protective actions based on the tsunami’s projected arrival time and height.
How much warning time is there for tsunamis?
The amount of warning time provided by a tsunami alert depends on the distance from its point of origin. Locations close to the source of large tsunamigenic earthquakes may only receive minutes of notice that a tsunami is imminent. But thanks to the long wavelength and period of tsunami waves, more distant regions can receive hours of warning time.
For example, when the 2011 Tohoku earthquake in Japan generated a destructive Pacific-wide tsunami, Hawaii had over 10 hours of warning before the tsunami arrived. But in some parts of Japan closest to the offshore quake, people had less than 15 minutes of notice. Effective warnings require an extensive monitoring network as well as education on how to immediately get to higher ground on receipt of a tsunami alert.
Here are some examples of typical tsunami warning times:
| Location of tsunami origin | Warning time |
|---|---|
| Local earthquake near coast | 5 to 15 minutes |
| Regional earthquake in Pacific basin | 1 to 3 hours |
| Distant earthquake across ocean | Over 5 hours |
However, the first signs of an incoming tsunami may be noticed before an official alert is issued. A rapidly receding ocean can indicate a tsunami’s approach as the trough preceding the initial wave crest pulls water away from shore. This phenomenon is nature’s version of an early warning and a sign for immediate evacuation before damaging waves arrive just minutes later.
How accurate are tsunami warnings?
The sophistication of tsunami monitoring and modeling technology has increased warning accuracy and reduced false alarms. However, uncertainties still remain in precisely forecasting tsunami arrival times and heights, especially soon after the generating event. Warnings are based on theoretical models that can underestimate or overestimate a tsunami’s impacts.
Overwarning is generally preferred to underwarning for public safety, so initial alerts may be expanded for broader areas or extended timeframes as more data becomes available. With an abundance of caution, evacuations may be ordered for zones that ultimately experience little inundation. Similarly, all warnings are canceled after the threat passes, which may be precautionary based on models before confirmation that measured sea levels have returned to normal.
According to NOAA, over 85% of tsunami advisories and warnings issued by the U.S. Tsunami Warning Centers have been followed by actual tsunami waves. Continually incorporating real-time data from increasingly sophisticated monitoring networks and updating propagation models allows forecast accuracy to improve throughout an event. While not perfect, current systems provide enough predictive skill to enable appropriate evacuations that save many lives.
What should you do when a tsunami warning is issued?
The most important action when any tsunami warning is issued for your area is to immediately evacuate from the coast and get to higher ground or inland. Do not wait until you see signs of a tsunami or receive further information. A tsunami can arrive within minutes and grow to be over 100 feet tall, making it impossible to outrun once in sight. Follow designated evacuation routes displayed on tsunami warning signs where available. If not, head as far away and uphill from the coast as quickly as possible.
When a tsunami advisory is issued due to a distant seismic event, evacuation is generally not needed but staying away from beaches and marinas and being prepared to evacuate if it is upgraded to a warning is recommended.
Other tips when a tsunami warning is issued:
– Take your preparedness kit and other essential items with you if time allows
– Move inland, to higher ground, or up to the highest floor of a sturdy building
– Avoid bridges, docks, and piers that may be impacted by waves
– Listen to the radio and follow instructions from emergency officials
– Do not attempt to drive or outrun a tsunami once waves arrive
– Stay away until the all clear is given by officials after confirmation of wave heights
Proper education on how to react to tsunami alerts and warnings, along with well-established evacuation routes and signage, are key to saving lives when these rare but devastating events occur. Do not depend on receiving an official notification before evacuating if you notice a sudden draining of ocean waters or feel strong shaking from a local earthquake.
Conclusion
Tsunami warnings provide critical lead time to evacuate vulnerable coastal populations before destructive waves strike shore. Thanks to improvements in deep ocean buoys, tide gauges, seismic sensors, and modeling systems, tsunami warnings are becoming increasingly accurate and providing enough notification to enable evacuations and save lives. While warning times are highly variable based on distance from the tsunami’s origin, establishing robust monitoring networks, warning protocols, and public education campaigns helps ensure communities are prepared to react immediately when alerts are issued and reduce tsunami hazards. However, natural signs of receding ocean waters or prolonged shaking provide nature’s version of early warning to seek higher ground as soon as possible before an official alert is received. Continuing to enhance tsunami preparedness globally through monitoring technology, warning coordination, and public awareness can help reduce risk from these rare but potentially catastrophic events.