The 8.8 magnitude earthquake that struck off the coast of Chile on February 27^th was the fifth largest earthquake recorded since 1900.  The earthquake and associated tsunami took the lives of more than 800 people, devastated port cities and fishing villages along the Chilean coast, and destroyed buildings, roadways, and bridges in Santiago and other inland areas.

                The earthquake resulted from a rupture of an undersea fault line 8 km (5 miles) offshore, where the oceanic Nazca Plate collides with and subducts, or slides under, the continental South American Plate.  At 03:34 local time, the stored-up tension on 400 km (250 mi) of fault line released, and the Nazca Plate suddenly thrust under the South American Plate, producing a fault line shift of 2 meters (6 ft) to the west plus significant uplifting of the South American plate.

The epicenter was close to the surface (35 km or 21 mi) under the seafloor, and located 115 km (71 miles) north northeast of Concepcion, Chile’s second largest city with a population of 600,000.  Much of the loss of life and destruction of property occurred in Concepcion.

                The tsunami associated with the earthquake reached 2.6 meters (8 ft. 6 in.) in Valparaiso and other Chilean coastal areas, destroying the port facilities at Concepcion and wiping out a number of fishing villages and vacation resorts.

Tsunami warnings were issued throughout the Pacific  Basin for a major event of up to 2.4 meters (8 ft.), but the waves reached only 2 to 3 feet (less than a meter) outside the immediate Chilean coastline.

                Usually, an 8.8 magnitude undersea earthquake can be expected to produce a much larger and stronger tsunami.  The 9.0 magnitude 2004 Indonesian earthquake spawned a 100-ft. tsunami that took over 200,000 lives in the Indian Ocean Basin.  The recent 8.0 magnitude earthquake near Samoa created a 40-ft. tsunami that caused loss of life and property damage in Tonga, Samoa, and American Samoa.

                Why did an 8.8 magnitude submarine earthquake generate such a small and weak tsunami?  The general answer is: the seismologists are still trying to figure it out.  Several possibilities have been suggested.  One is ocean depth at the epicenter.  Since the epicenter was only 8 km (5 mi) off the shoreline, it is possible it was on the edge of the Chile Trench in relatively shallow water, not in the deepest part which goes down to 8,000 meters (26,000 ft) further offshore.  The 2004 magnitude 9.0 Indonesian earthquake epicenter was located in the 7700 meter (25,000 ft) deep Sunda Trench.  The Samoa earthquake epicenter was deep in the Tonga Trench, which averages 6,000 meters (20,000 ft) in depth.  The deeper the ocean is at the epicenter, the deeper the water column, the greater the water volume, and the longer the wave amplitude, all of which contribute to the power and height of the tsunami.  However, this theory is only one of several put forth.  The scientific community will most likely agree on an underlying cause after further research and analysis.     

            Aftershocks occur in the same rupture zone area and at the same depth as the original quake.  The rupture zone area includes the original fault line and associated faults that intersect or run parallel to the main fault within a few kilometers of the epicenter.  The main shock seems to destabilize the fault structures in the immediate and nearby areas, resulting in small fault line slippages of as little as a few centimeters on both the main fault and associated minor faults.

            A study conducted by a Cal Tech research team of the 1992 Landers earthquake in California indicates that minor afterslip occurred as far away as 20 kilometers (12 miles) from the main shock epicenter.  Only 8% of the aftershocks occurred within 0.5 kilometers of the main shock epicenter, and 23% within 1.5 km, which means that 69% of the aftershock sources were outside the 1.5 km radius.  However, the closer the aftershock location is to that of the main shock, the stronger it is.  In the Landers study, 60% of the aftershock strength, or energy release, took place within 1.5 km of the main shock location.

            How much plate movement takes place in the main shock and in the aftershocks?  Aftershock fault slippage is usually only a small fraction of that of the main shock slippage.  In the Landers study, it ranged from 1% to 1/10%.  Using that scale, if the main shock fault slippage is 5 meters, for example, the aftershock slippage may be as little as few centimeters.  But plate slippage of even a few centimeters may produce a sudden energy release that can cause heavy localized shaking, especially when the epicenter is near the surface.

            The January 12, 2010 Haiti earthquake had a main shock magnitude of 7.0.  The strongest aftershock registered 6.1.  The energy release of the main quake was 10 times stronger than the 6.1 aftershock, but because the aftershock occurred at a shallow depth and probably close to the original epicenter, the ground shaking was still quite violent.

            Haiti’s January 12 earthquake is turning out to be one of modern history’s most costly natural disasters in terms of lives lost and property destroyed.  As of this writing, the death toll estimate is 150,000, but it is believed that many more than that may still be discovered.  Aid from the United States and many other countries around the world has been flowing in and helping to provide food, water, medical care, and temporary shelter to millions of Haiti’s traumatized people.

            The earthquake’s epicenter was 10 miles west of Port au Prince at a depth of 5 miles.  The epicenter’s shallowness and proximity to the densely populated city of 2,000,000 are the elements that made the quake so catastrophic.

             The Haiti earthquake occurred on the Enrequillo-Plantain Garden fault line, a transform, or strike-slip, fault that runs for 1,000 miles from Jamaica to the middle of the island of Hispaniola, which is shared by Haiti and the Dominican Republic.  This fault line running east-west marks the boundary between the Caribbean plate and the North American Plate.

A fault is called strike-slip when two tectonic plates meet and move horizontally in opposite directions.  In this case, the North American Plate is moving west and the Caribbean Plate is moving east at the rate of approximately one inch a year.  However, plate movement does not occur at a steady rate.  The friction of one plate pushing against the other can cause the plates to stick, and arrest plate movement.  When this happens, stress on the fault line builds up.  The last major earthquake on this fault line occurred in 1760, so pressure has been building for 250 years.  The fault stress level finally exceeded the strain threshold and a brief episode of violent slippage occurred, resulting in the magnitude 7.0 quake.

          Strike-slip fault earthquakes do not usually produce tsunamis, and none was evident in this case.  However, to the east of this fault line, the Lesser Antilles Volcanic Arc lies on the subduction zone that separates the Caribbean Plate and Atlantic Plate.  This volcanic chain that runs from the Virgin Islands on the north to Grenada on the south includes such famous volcanoes as Mt. Pele and Montserrat.  A fault line rupture along this subduction zone, where the Atlantic Plate is sliding under the Caribbean Plate, would be much more likely to produce a tsunami than the fault slippage that produced the Haiti earthquake.

In some cases, a local earthquake or undersea landslide close to shore can initiate a tsunami that strikes almost without warning.  In 1998, a 7.0 magnitude undersea earthquake near Papua, New Guinea, triggered a massive submarine landslide that started a 50-ft. tsunami close to shore.  The wave hit the shoreline within minutes and wiped out several villages along the New Guinea coast, stripping the land almost bare.  2200 people died.  In September, 2009 a 7.0 undersea quake and fault line rupture 125 miles from Samoa triggered a tsunami that hit Samoa and American Samoa without warning 15 minutes later.  Waves over 40 ft. high took more than 200 lives and caused extensive damage to property. 

Tsunami warning systems are effective when the undersea quake that starts the tsunami occurs in deep water several hundred miles from the nearest populated area.  In the Pacific Ocean, a quake will be picked up by seismometers, pressure sensors, and tidal gauges at the reporting stations of the Pacific Tsunami Warning System operated by 26 nations bordering the Pacific Basin.  The collected data registers on the instruments at the Pacific Tsunami Warning Center in Ewa Beach, Hawaii.  If readings indicate the disturbance may have started a tsunami, warnings are issued immediately to the areas in danger with approximate arrival time of the first wave.

As part of the international tsunami warning network, the United States has recently completed its own U.S. Tsunami Warning System that takes in the Pacific Tsunami Warning System, the West Coast & Alaska Tsunami Warning System, and the Atlantic Tsunami Warning System.  The U.S. system is composed of 39 DART (Deep Ocean Assessment and Reporting of Tsunami) and DART II stations.  Five stations are located in the Atlantic, Caribbean, and Gulf of Mexico, and the remaining 34 in the Pacific.  The DART system is made up of a pressure sensor resting on the ocean bottom that transmits continuous data by acoustic telemetry (sound waves) to a surface buoy anchored near the pressure sensor.  The buoy is equipped with a satellite link that relays the real time information to tsunami warning centers around the world.  Certain fluctuations in ocean bottom pressure can indicate the presence of a tsunami.

Shield volcanoes fueled by hotspots are different from the stratovolcanoes of the Pacific Rim. The magma pouring into shield volcanoes is mainly basalt, lighter and less viscous than the silica-based magma fueling the Pacific Rim volcanoes produced by the collision of tectonic plates.

The hotspot theory might well be demonstrated by the way in which the Hawaiian Islands were formed. The theory suggests that there is an area in the mid-Pacific hundreds of miles below the ocean floor in the earth’s mantle called The Hawaiian Hotspot. The hotspot size is estimated at 50 miles in diameter. The temperature in the hotspot is higher than that of the surrounding mantle. This intense heat melts the material from the lower part of the overriding tectonic plate and converts it into magma. The hotspot magma is lighter than the material in the rest of the mantle and rises to the earth’s crust in a narrow stream called a mantle plume. The mantle plume brings magma up to the ocean floor, and a volcanic cone begins to build. Over time, repeated eruptions produce more and more lava that continue to build the cone higher and higher, until it finally emerges above the surface of the ocean as an island many thousands of years later.

The tectonic plate on which the Hawaiian Islands sit is called the Pacific Plate. The Pacific Plate moves in a northwesterly direction at about 1 inch per year. Moving at that rate, it takes several million years for the island to pass over and clear the stationary Hawaiian Hotspot. Island building progresses during this long period of time, and large islands with tall volcanic mountains are created, as is the case with the island of Maui with its 10,000 ft. Haleakala volcano, and the Big Island of Hawaii with its chain of volcanoes in 13,700 ft. Mauna Kea, 13,600 ft. Mauna Loa, and currently active Kilauea.

Once the moving plate carries an island away from the hotspot plume, the volcanoes on that island go dormant and start to erode, and the plume proceeds to build the next island. Niihau and Kauai, the easternmost islands of the Hawaiian chain, were the first to be built up from the hotspot activity. The Pacific Plate continued its northwesterly movement over the hotspot and the islands of Oahu, Molokai, Lanai, and Maui were created in succession. The last island over the hotspot is the still volcanically active Big Island of Hawaii.

Just a few miles off the Big Island, a new island is starting to build. It is called Loihi, and has pushed up two miles from the ocean floor, but is still a mile beneath the surface of the ocean. It is estimated that Loihi will appear above the ocean surface in 220,000 years.

One source suggests there are 50 active hotspots throughout the world. Some, such as the Hawaiian Hotspot, lie under oceanic plates, and others under continental plates. Among the volcanic islands created by oceanic hotspots are Iceland, the Azores, the Galapagos, Samoa, the Marquesas Islands, the Society Islands, and Reunion.

The best example of continental plate hotspot activity is Yellowstone National Park. The heart of the park is an active caldera that powers hot springs, fumaroles, mud pots, and geysers such as Old Faithful. The thermal energy derives from a large hotspot underlying the caldera. This same hotspot has produced many other caldera and lava bed areas throughout the western states, as the North American Plate moves southwesterly. The Yellowstone caldera was created by a massive volcanic eruption 600,000 years ago.

In recent years, a number of scientists have challenged the idea of a stationary hotspot and of a mantle plume being the source of hotspot magma. Several other explanations have been put forth. Since hotspots are located deep in the earth’s mantle, no one has ever seen one. Their existence and the way they work can only be assumed.

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