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Tsunamis

by Terry Sovil

Tsumami is a word that originated in Japan from the late 19th century meaning tsu 'harbor' + nami 'wave' (harbor wave). It is a series of ocean waves caused by the same shift in tectonic plates that we learned about in previous articles on Earthquakes and Volcanoes. A sudden displacement in the ground from an earthquake, landslide or volcano can trigger a tsunami

Diagram Tsunami Wave Formation

Mariners know the safest place to be in a tsunami is out on the open ocean over very deep water. In water greater than 600 feet / 183 meters in depth, a tsunami wave may be only a few inches / centimeters high and rarely over 3 feet / 1 meter high. It will pass under a boat very quickly because the wave crests come spread over time and traveling at speeds of 500 miles / 805 km per hour!

Tsunamis travel approximately 475 mph / in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph as the wave grows in height. If a tsunami originated near the central Aleutian Islands (Alaska) it could reach Hawaii in about 4 hours and California in about 6. If it originated in Portugal it could reach North Carolina in about 8.5 hours.

Depending on the depths of water near shore it may come ashore gently or as a fast moving wall of water several meters high. As a tsunami moves into shallow water the wave height can increase over 10 times. The height of the wave will vary along a coast. The wave can be amplified by shoreline and sea floor features. A large one may flood low-lying coastal land over a mile from the coast.

Diagram Tsunami Formation


Like volcanoes and earthquakes a tsunami can't be prevented but monitoring points around the world make us aware of a potential volcano or earthquake and we can be made aware of a possible tsunami following a natural disaster. In the USA NOAA (National Oceanic and Atmospheric Administration) has primary duties for warnings, observation and research. Networks of monitoring can quickly sound an alarm for a possible tsunami.

Tsunamis are NOT tidal waves. Tidal waves are caused by the forces of the moon, sun, and planets upon the tides, as well as the wind as it moves over the water. With typical waves, water flows in circles, but with a tsunami, water flows straight. This is why tsunamis cause so much damage!

Wind generated waves typically come in periods or time between crests between 5 and 15 seconds. Tsunami periods are normally from 5 to 60 minutes. Wind generated waves break as they shoal and lose energy offshore. They are more surface oriented. Tsunamis are more like flooding waves, a column of water, so a 20 foot / 6 M wave is a 20 foot / 6 M rise in sea level.

Normally, a tsunami appears as a rapidly advancing or receding tide. In some cases, a wall of water or series of breaking waves may form. There are a few signs like hard ground shaking for 20+ seconds near the coast or a sudden sea level withdrawal. A sudden recession of water, a "draw down", should be a good reason to move away from the beach! Tsunamis may be accompanied by loud, booming noises.

Statistically there are about 2 tsunamis per year throughout the world. These are usually a "slip-strike" or horizontal motion earthquake that causes the tsunami and 10-15% of the damage. These inflict damage near the source, or origin, of the event (volcano or earthquake). About every 15 years there is a destructive ocean-wide tsunami.

OK. So what about Manzanillo, Colima and Mexico's Pacific Coast? Historical and recorded data indicate the Pacific coast of Mexico has had destructive tsunamis over at least the past 500 years. Also affected by hurricanes, and the huge 1985 Mexico earthquake which generated tsunamis that affected Ixtapa-Zihuatanejo and Michoacan.

The following link is to GeoScienceWorld, Seismological Research Letters, Updated Tsunami Catalog for the Jalisco-Colima Coast, Mexico, Using Data from Historical Archives:

Research Letters

The charts and maps are from Geoscienceworld.org. This group extensively researched online catalogs, newspapers, incidents recorded to create a new look at the activity near us over the years. A visit to their site is a great look at the history. As we noted before in the Earthquakes article, our area is a complex setting of tectonic plates. They crafted a revision of tsunamigenic earthquakes over the years to document our area. This includes the occurrence of 21 events; 2 earthquakes documented for the first time (in 1563 and 1816) for the Jalisco-Colima coast, and 8 large or medium intensity tsunamis also first reported (1816, 1818, 1900 [two tsunamis], 1911, 1933 [two tsunamis], and 1941) through documentary evidence.

Diagram Plate and Years of Possible Tsunamis
Tectonic setting, rupture areas plotted for tsunamigenic earthquakes from 1932 to 2016. A letter behind a number (1932a, 1932b) indicates more than one in that year.


Activity Close to Manzanillo Colima Mexico
Tsunamis triggered by local earthquakes on the Jalisco-Colima coast. The list has been shortened to our local area.
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