Water And Oceanography

Understanding Underwater Earthquakes

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In the middle of the afternoon on March 11, 2011, one of the walls in a deep ocean trench just off the northeastern shore of Honshu, Japan, suddenly snapped. A piece of the sea floor about 186 miles long by 93 miles wide, per the US Geological Survey, suddenly shot up some 100-130 feet. This magnitude 9 earthquake displaced water, which then moved outward from the epicenter in great waves.

Nature devastated Japan’s nearby coastal regions twice that day: first by the powerful shaking and then by the huge tsunami that followed.

The 2011 Tohoku earthquake was one of the largest earthquakes ever recorded. In fact, eight of the world’s top 10 most powerful earthquakes have happened underwater and near coasts. That’s scary, but there’s no need to cancel your vacation plans for the beach. A little understanding of plate tectonics goes a long way in helping to understand why underwater earthquakes happen, which ones are dangerous and where the worst ones are most likely to happen.

♦ Why underwater earthquakes happen

All earthquakes happen when two blocks of the earth suddenly slip and move past each other. The point where they slip is called the hypocenter. The epicenter is the spot at the earth’s surface directly over the hypocenter. The surface that the two blocks move along is called the fault.

One of the blocks may move down relative to the other one (a normal fault), up (a reverse fault), or sideways (a strike-slip or transform fault). A thrust fault is a reverse fault at 45°. The Tohoku quake, like many of the others in the top 10, happened along a special kind of thrust fault, called a megathrust, where two great plates of the earth’s outer surface, or lithosphere, meet.

Geoscientists have found, that when two tectonic plates meet, the collision can raise mountain ranges, if they both have continents on them. One plate edge will subduct underneath the other if sea floor is involved, which is made of basalt and is much more dense than the silica-based rocks of continents.

The deep Japan Trench where the Tohoku megathrust quake happened in March 2011 is a subduction zone. There, the Pacific plate is meeting a continental plate or plates to the west. When such huge pieces of rock meet, friction and other forces make them stick. Nothing happens for a long time, as enormous pressures slowly build: then something big and life-changing for nearby people suddenly occurs.

Plate tectonics is the ultimate reason why underwater earthquakes happen.

♦ The most dangerous underwater quakes

The 2011 Tohoku quake, at magnitude 9, was slightly less strong than the M 9.1 Sumatra underwater quake in December 2004 that triggered massive tsunami waves, killing over 200,000 people and displacing almost 2 million more in 14 countries around the Indian Ocean. Both of these recent killer earthquakes were surpassed by Alaska’s M 9.2 in Prince William Sound on March 28, 1964. The largest earthquake ever recorded, at magnitude 9.5, happened in Chile in May 1960, just inland from a major offshore subduction zone.

Seismologists have set up magnitude scales as a way to measure earthquakes in comparison to one another. The Richter scale is a math formula developed in the 1930s that indicates the magnitude, or relative size of the earthquake, based on a logarithm. Today, earthquake magnitude usually refers to the seismic moment, or energy released by the earthquake. While it's more difficult to calculate, this moment magnitude is more precise for powerful earthquakes. However, all magnitude scales give roughly the same numerical value. The great earthquakes are generally around M 9, while lesser but still major quakes often run in the M 7 range.

Our modern society is built on communications, and even these lesser earthquakes can create havoc on land. In 1929, a M 7.2 earthquake, some 160 miles off the southern coast of Newfoundland, broke 12 transatlantic telegraph cables and caused tsunami waves up to 23 feet high that killed a total of 28 people along the Burin Peninsula. This tsunami was detected as far south as the Carolinas and also crossed the Atlantic, where it was recorded in Portugal.

More recently, communications, financial networks and Internet connections across East Asia were disrupted for hours because a pair of quakes, each around M 7, struck just 8 minutes apart off Taiwan’s coast, causing underwater landslides that broke several fiber optic cables in multiple places.

No one is quite sure of the fault movement for the 1929 submarine earthquake in the Grand Banks, but there is no subduction zone there and it probably wasn’t a megathrust event. Taiwanese geoscientists believe that the 2006 Pingtung earthquakes happened along normal and strike-slip faults. Submarine earthquakes also happen along transform faults, like those underneath and off the California coast near Cape Mendocino. In April 1992, a series of three shocks ranging from M 7.2 to M 6.5 happened there, and while the initial rupture was on land, the breaking fault line extended into the sea and caused a tsunami.

♦ Underwater earthquake danger zones

On July 6, 2011, the rocks in one wall of the great Kermadec ocean trench northeast of Australia broke, unleashing the energy of a M 7.6 earthquake. The Pacific Tsunami Warning Center issued warnings, but authorities in Australia and New Zealand reassured worried citizens that no major tsunami would hit them, and none did.

The Kermadec quake happened along a normal fault line, where one block of rock suddenly drops in relation to another, rather than at a 45-degree megathrust fault, where a piece of the upper rock suddenly springs upward. While this sudden motion in the Kermadec Trench wall still generated a tsunami, smaller than those in Japan and Sumatra but big enough to cross the Eastern Pacific, the fault was aligned in such a way that people living relatively close to the epicenter had very little to worry about.

Nonetheless, maps of the world’s largest earthquakes in modern times show that all have been located around the Pacific “Ring of Fire” and were megathrust quakes near subduction zones except for the 10th largest quake, in Tibet where two continental plates (India and Asia) are colliding. Plate tectonics also played a major role in the 1992 Cape Mendocino quakes. These happened in a complicated tectonic region where the San Andreas fault (a transform fault) goes offshore and the large Pacific and North American plates jostle together along with the much smaller Gorda Plate.

Nearby is the Cascadia subduction zone, where the last big megathrust earthquake, with a magnitude of 9, happened in January 1700 (the date and intensity can be determined because of written records of its tsunami, which killed a few people along the coast of Japan when the waves arrived there one cold winter night).

Many huge tsunamis have also happened in the Mediterranean Sea over the last 4000 years of recorded history. This region, where the African, Eurasian and other tectonic plates collide, has its own complicated geological history. Many volcanoes threaten the area, but earthquakes have caused most of its tsunamis.

♦ Learning from disaster

In May 2011, the Japanese closed the Hamaoka nuclear power plant near Tokyo, which sits close to the expected epicenter of a future underwater earthquake, already given the name of the Tokai quake, that could be as bad or worse than the 2011 Tohoku event.

We humans can learn from our mistakes.

This brief overview only touches upon some of the highlights of what scientists and emergency planners know about about underwater earthquakes today. If you use Google earth, an excellent way to learn more about the ocean floor and its tectonic activity is to load two layers from the Earth Galley - the USGS real-time earthquakes file and National Geographic’s Ocean Atlas - and start exploring; Japan would be a good place to start, as there are still many aftershocks ongoing there after the Tohoku quake. One online resource out of the many good ones on the Web, made available by reliable experts, is Plates on the Move, hosted by the American Museum of Natural History.

Certainly, if you live in a high-risk area or are planning to visit one, consult local sources for the best information on current hazards, emergency refuges, evacuation routes and so forth. Then let it go, and enjoy a wonderful beach experience, reassured that you now know more about what’s happening and can handle whatever Nature throws at you.

More about this author: B. J. Deming

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