Oceanic Trenches: Tsunami & Earthquake Incubators

            History’s three most violent earthquakes struck in or near one of the world’s deep oceanic trenches, and all three of those events also produced major tsunamis.

Oceanic trenches can be nearly 7 miles (11km) deep and usually stretch in an arc for thousands of miles.  These elongated seafloor depressions are found where tectonic plates converge.  There are more than 20 oceanic trenches creasing the world’s seafloors.  18 of those are located in the Pacific Ocean on the Pacific Ring of Fire.

            The 27,000 ft (8400 m) deep Peru-Chile Trench was the site of the largest earthquake ever recorded, a magnitude 9.5 quake that struck at 2:11 pm on May 22, 1960. The epicenter was 100 miles (160 km) off the Chilean coast where the Nazca oceanic plate is subducting (sliding under) the continental South American Plate.  The focal depth of the earthquake was 22 miles (35 km), making it a relatively shallow and therefore especially destructive event.

            The deep ocean floor fault line where these two plates meet ruptured on a 500 mile (800 km) front.  In less than 4 minutes, 500 miles of the Nazca oceanic plate thrust under the South American Plate.  Along that entire line, the seafloor subsided 5 feet (1.5 m) and in some areas the continental plate uplifted 10 ft (3 m).  The sudden seafloor deformation that triggered this deep water earthquake displaced millions of tons of ocean and launched a powerful tsunami that sent waves towering 80 ft. (25 m) crashing into ports and villages on the coast of Chile, and waves up to 30 ft. high causing destruction across the entire Pacific Basin, including Hawaii and Japan.  It is estimated that 6,000 people died in the earthquake and tsunami.

            The world’s second most powerful earthquake, at magnitude 9.2, struck in the 25,000 ft. (7,600m) deep Aleutian Trench at 5:36 pm on Good Friday, March 27, 1964.  In what was termed a mega thrust event, the Pacific Plate rammed 30 ft (9 m) under the North American Plate along a 750 mile (1250 km) arc in less than 5 minutes, uplifting the seafloor on one side of the arc by up to 35 feet, and dropping it by as much as 10 ft. on the other side of the arc.  The epicenter of the earthquake was located a few miles north in Prince William Sound at a focal depth of 16 miles (25 km).  The relatively shallow epicenter produced a highly destructive earthquake, and the deep seafloor disturbance produced a tsunami with waves up to 70 ft (21 m) high that caused devastation along the Alaskan coast, and casualties as far away as California.  Property damage was estimated at over 2 billion in today’s dollars.  Deaths were limited to 130, mainly due to Alaska’s sparse population.                       

             The world’s third largest earthquake and history’s deadliest tsunami started in the 24,000 ft (7,400 m) deep Java Trench in Indonesia’s Sunda Strait, at 7:58 am on Dec. 26, 2004.  This magnitude 9.1 mega thrust earthquake followed a sudden undersea fault line rupture where the India Plate subducts the Burma Plate (part of the continental Eurasian Plate).  1,000 miles (1600 km) of fault line ruptured in two phases lasting 8 minutes, during which the India Plate slid 50 ft (15 m) under the Burma Plate.  The epicenter was fairly shallow at a focal depth of 19 miles (30 km) located only 100 mi (160 km) off the coast of Sumatra.  The earthquake caused widespread damage and casualties.  The tsunami caused by the seafloor disturbance in deep water produced waves up to 50 ft. high, catching coastal populations unaware, and killing more than 200,000 people in 11 nations bordering the Indian Ocean.          

            Common features of these 3 mega earthquake/tsunami events include (1) location in a deep oceanic trench where tectonic plates converge, (2) sudden release of stored-up mechanical stress, instant fault line rupturing, and plate subduction on a massive front, and (3) shallow epicenter close to land.  And it should be noted that all three trenches are on the Pacific Ring of Fire.

            Since there are more than 20 deep oceanic trenches around the world, their fault lines storing up thousands, even millions, of years of stress, it is a given that there will be many more large magnitude mega thrust earthquakes and major tsunamis in the future.  We just don’t know where or when.