The word tsunami is composed of two kanji: tsu meaning harbor and nami meaning wave. Tsunamis are long period waves generated by an underwater earthquake or volcanic activity. Tsunamis can occur thousands of miles from their origin as the waves move at speeds up to 430 mph. The typical wavelength of tsunamis in deep water is in excess of 125 miles with a wave height of only a foot and a half.
The last major tsunami to impact the Okinawa Islands occurred in 1771 on the Yaeyama Islands (including Miyako and Ishigaki Island). The wave that hit Ishigaki was approximately 250 feet high and pushed seawater 1 mile inland - on all sides. The death toll was around 12,000 people amongst the impacted islands. The ensuing malaria epidemics and destruction of agriculture decimated the island population to a third of its original size.
On March 11th, 2011, a 9.0 earthquake created a tsunami wave that devastated miles of Japan's coastline. The waves from the tsunami reached 40 feet high in parts of the Iwate Prefecture, and traveled nearly 6 miles inland upon impact in Sendai. Over 15,000 were confirmed killed and more than 340,000 displaced. Located within the "Ring of Fire", Okinawa is susceptible to earthquakes, and therefore vulnerable to tsunamis.
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Identify the fastest route to high ground from your home.
Take note of Tsunami Evacuation Route signs in your neighborhood
Tsunami warning sirens are now tested on Marine Corps bases across Okinawa every Friday at noon, make sure you and your family members know what they sound like! If you live out in town, check with your landlord for your city/ town's testing schedule.
Understand the different tsunami alerts:
TSUNAMI ADVISORY: An earthquake has occurred, which might generate a tsunami.
TSUNAMI WATCH: A tsunami was or may have been generated, but is at least two hours away. Prepare for possible evacuation if the watch is upgraded to a warning.
TSUNAMI WARNING: A tsunami was or may have been generated, which could cause damage. People in the warned area should evacuate the area.
Since 1771, the main island of Okinawa has only been hit by minor tsunamis without much effect, but this does not mean it will not happen in future, so you should always be ready.