Sóng thần (được phát âm là soo-ná-mees), cũng được gọi là sóng biển địa chấn (gọi nhầm thành "sóng thủy triều"), là một loạt các đợt sóng lớn được tạo ra bởi sự hỗn loạn dưới nước như là động đất, lở đất, núi lửa phun trào, , hoặc thiên thạch. Sóng thần có thể di chuyển hàng trăm dặm mỗi giờ trong đại dương mở và va vào đất liền với các đợt sóng cao từ 100 foot trở lên. Từ khu vực nơi sóng thần bắt đầu, các đợt sóng đi ra ngoài theo tất cả các hướng. Ngay khi sóng đến gần bờ, chúng dâng lên cao. Địa hình bờ biển và đáy đại dương sẽ ảnh hưởng đến quy mô sóng. Có thể có nhiều đợt sóng và đợt sóng kế tiếp có thể lớn hơn đợt sóng trước đó. Đó là nguyên nhân tại sao sóng thần nhỏ ở bờ biển nào đó có thể là sóng khổng lồ ở cách xa vài dặm. Tất cả sóng thần đều có khả năng nguy hiểm, ngay cả khi chúng có thể không gây thiệt hại cho mọi bờ biển mà chúng đánh vào. Sóng thần có thể đánh vào bất cứ nơi nào dọc theo hầu hết bờ biển của Hoa Kỳ. Trận sóng thần có tính hủy diệt nhất nhất đã từng xảy ra dọc theo bờ biển tiểu bang California, Oregon, Washington, Alaska và Hawaii. Dịch chuyển do động đất gây ra ở đáy đại dương phần lớn thường tạo ra sóng thần. Nếu một trận động đất hoặc lở đất lớn xảy ra ở gần bờ thì đợt sóng đầu tiên trong loạt sóng có thể đến bờ biển trong vài phút, thậm chí trước khi có cảnh báo. Các khu vực có nguy cơ cao hơn nếu chúng thấp hơn mực nước biển 25 foot và nằm trong một dặm bờ biển. Chết đuối là nguyên nhân gây ra tử vong phổ biến nhất liên quan đến sóng thần. Các đợt sóng trong sóng thần và nước rút luôn hủy diệt các cấu trúc trong khu vực bị ảnh hưởng bởi các đợt sóng lên xuống. Các mối nguy hiểm khác bao gồm ngập lụt, nhiễm bẩn nước uống và hỏa hoạn từ đường dẫn ga hoặc thùng chứa vỡ.
Before a Tsunami
The following are things you can do to protect yourself, your family and your property from the effects of a tsunami:
- To begin preparing, you should build an emergency kit and make a family communications plan.
- Talk to everyone in your household about what to do if a tsunami occurs. Create and practice an evacuation plan for your family. Familiarity may save your life. Be able to follow your escape route at night and during inclement weather. You should be able to reach your safe location on foot within 15 minutes. Practicing your plan makes the appropriate response more of a reaction, requiring less thinking during an actual emergency.
- If the school evacuation plan requires you to pick your children up from school or from another location. Be aware telephone lines during a tsunami watch or warning may be overloaded and routes to and from schools may be jammed.
- Knowing your community's warning systems and disaster plans, including evacuation routes.
- Know the height of your street above sea level and the distance of your street from the coast or other high-risk waters. Evacuation orders may be based on these numbers.
- If you are a tourist, familiarize yourself with local tsunami evacuation protocols. You may be able to safely evacuate to the third floor and higher in reinforced concrete hotel structures.
- If an earthquake occurs and you are in a coastal area, turn on your radio to learn if there is a tsunami warning.
Know the Terms
Familiarize yourself with these terms to help identify a tsunami hazard:
A tsunami warning is issued when a tsunami with the potential to generate widespread inundation is imminent or expected. Warnings alert the public that dangerous coastal flooding accompanied by powerful currents is possible and may continue for several hours after initial arrival. Warnings alert emergency management officials to take action for the entire tsunami hazard zone. Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or canceled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.
A tsunami advisory is issued when a tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent or expected. The threat may continue for several hours after initial arrival, but significant inundation is not expected for areas under an advisory. Appropriate actions to be taken by local officials may include closing beaches, evacuating harbors and marinas, and the repositioning of ships to deep waters when there is time to safely do so. Advisories are normally updated to continue the advisory, expand/contract affected areas, upgrade to a warning, or cancel the advisory.
A tsunami watch is issued to alert emergency management officials and the public of an event which may later impact the watch area. The watch area may be upgraded to a warning or advisory - or canceled - based on updated information and analysis. Therefore, emergency management officials and the public should prepare to take action. Watches are normally issued based on seismic information without confirmation that a destructive tsunami is underway.
A tsunami information statement is issued to inform emergency management officials and the public that an earthquake has occurred, or that a tsunami warning, watch or advisory has been issued for another section of the ocean. In most cases, information statements are issued to indicate there is no threat of a destructive tsunami and to prevent unnecessary evacuations as the earthquake may have been felt in coastal areas. An information statement may, in appropriate situations, caution about the possibility of destructive local tsunamis. Information statements may be re-issued with additional information, though normally these messages are not updated. However, a watch, advisory or warning may be issued for the area, if necessary, after analysis and/or updated information becomes available.
During a Tsunami
- Follow the evacuation order issued by authorities and evacuate immediately. Take your animals with you.
- Move inland to higher ground immediately. Pick areas 100 feet (30 meters) above sea level or go as far as 2 miles (3 kilometers) inland, away from the coastline. If you cannot get this high or far, go as high or far as you can. Every foot inland or upward may make a difference.
- Stay away from the beach. Never go down to the beach to watch a tsunami come in. If you can see the wave you are too close to escape it. CAUTION - If there is noticeable recession in water away from the shoreline this is nature's tsunami warning and it should be heeded. You should move away immediately.
- Save yourself - not your possessions.
- Remember to help your neighbors who may require special assistance - infants, elderly people, and individuals with access or functional needs.
After a Tsunami
- Return home only after local officials tell you it is safe. A tsunami is a series of waves that may continue for hours. Do not assume that after one wave the danger is over. The next wave may be larger than the first one.
- Go to a designated public shelter if you have been told to evacuate or you feel it is unsafe to remain in your home. Text SHELTER + your ZIP code to 43362 (4FEMA) to find the nearest shelter in your area (example: shelter 12345).
- Avoid disaster areas. Your presence might interfere with emergency response operations and put you at further risk from the residual effects of floods.
- Stay away from debris in the water; it may pose a safety hazard to people or pets.
- Check yourself for injuries and get first aid as needed before helping injured or trapped persons.
- If someone needs to be rescued, call professionals with the right equipment to help. Many people have been killed or injured trying to rescue others.
- Help people who require special assistance—infants, elderly people, those without transportation, people with access and functional needs and large families who may need additional help in an emergency situation.
- Continue using a NOAA Weather Radio or tuning to a Coast Guard station or a local radio or television station for the latest updates.
- Stay out of any building that has water around it. Tsunami water can cause floors to crack or walls to collapse.
- Use caution when re-entering buildings or homes. Tsunami-driven floodwater may have damaged buildings where you least expect it. Carefully watch every step you take.
- To avoid injury, wear protective clothing and be cautious when cleaning up.
Find additional information on how to plan and prepare for a tsunami and learn about available resources by visiting the following websites:
- NOAA Tsunami program
- Federal Emergency Management Agency
- American Red Cross
- USGA Pacific Coastal & Marine Science Center
Listen to Local Officials
Learn about the emergency plans that have been established in your area by your state and local government. In any emergency, always listen to the instructions given by local emergency management officials.