Every once in a while planet Earth puts on a show of raw power for its human inhabitants, like earthquakes - as occurred recently in Chile. Of great interest was the remarkable relocation of the major city, Conception, which moved westward (into the ocean) by satellite measurement, a length of over ten feet, nearby cities of capital Santiago and also Valparaiso each intriguing about one foot. It was if a giant hand had stretched the land along an roughly directly westward line (slightly downward) for ten feet at Conception, while adjacent ground moved only a tenth that length - indicating valuable "shearing" of the land mass both north and south of Conception.
What causes quakes, by tectonic plate theory, is that the ground along either side of a fracture or crack in the ground, called a "fault", moves to comfort a local build-up of stress and strain in a worldwide cycle of strain buildup and strain release - the tectonic plates continent-size. Earthquakes are not randomly distributed over the Earth's surface, tending to concentrate in narrow zones where volcanoes and mountain ranges are also located. The thought of plate tectonics has revolutionized thinking in Earth's sciences in just the last decade, going beyond the initial thought of continental drift, proposed a century ago, and sea-floor spreading. Plate tectonics tells us that the Earth's rigid outer shell (lithosphere) is broken into a mosaic of oceanic and continental plates, which slide over the plastic aesthenosphere, the uppermost layer of the Earth's mantle. The lithosphere covers the entire Earth along with the ocean plates, intriguing sea-floor spreading and mid-ocean ridges, a theory of narrow submarine cracks which are found at the center of major oceans. The ocean floor is being continuously pulled apart along these mid-ocean ridges, with hot volcanic material rising from the Earth's mantle to fill the gap, continuously forming new oceanic crust.
Earthquake & Tsunami In Japan
We now know that there are seven major crustal plates, subdivided into a amount of smaller plates. A plate ordinarily encompasses an entire continent, such as the North American, South American and African plates. The plates are in constant motion, interacting along their margins, where major geological processes take place, such as formation of mountain ranges, valleys, earthquakes, and volcanoes. Techtonic plates are about fifty miles thick, and in constant motion relative to each other, the movement rates varying from 10 to 130 millimeters per year. As we learn more and more about the major plates, we find that many involved and intricate movements take place. We learn, too, that most of the geological activity - mountains, rift valleys, volcanoes, earthquakes, faulting - is due to dissimilar types of interaction at the plate boundaries.
One of the keys to plate tectonics was the discovery that the Earth's magnetic field has reversed its polarity 170 times in the last 80 million years. As molten basaltic material squeezes up into the mid-ocean cracks and solidifies, it becomes magnetized agreeing to the Earth's magnetic field polarity. If the field reverses its polarity, the strip of new material is magnetized oppositely, and as the oceanic floor continues to spread, the new strips of rock are carried away on either side. Using these magnetic strips as evidence of movement, it has come to be clear that the Earth's surface consists of a mosaic of crustal plates continually jostling each other. With a continual influx of new volcanic material at the ocean ridges, old crust becomes destroyed where plates collide, and earthquakes occur - also, as happens in Chile, one plate is forced under the other (subduction - downward movement), sometimes to be consumed deep in the mantle.
In the case of the Chilean earthquakes, the Nazca plate intriguing under the the South American plate is what caused the major quake back in 1960, as well as the recent one and others in between. The Nazca plate continues to dive down below the continent, this constant slow movement (with some occasional rapid shifts foremost to big jolts) is what creates frequent earthquakes throughout that region. The most superior earthquake in recent times was the 1960 Chile earthquake; measured by the amount of power released by the movement of the ground - 9.5 on the occasion Magnitude (Mw) scale. The epicenter (the point on the earth's surface directly above the focus point) was 36 miles below the ocean floor, about 100 miles west of the coast of Chile, in the Pacific Ocean. Two very large quakes happened in Chile: on March 3, 1985, and on July 30, 1995, both with a magnitude of about 8. Chilean earthquakes are neither rare nor small, seeming to occur about every 25 to 100 years - and will apparently continue as long as the Nazca plate continues subducting, crashing into and diving under the South American plate: the same geologic event that, over time, has built up the Andean mountain range.
It is estimated that about 140 earthquakes of magnitude 6 or greater will occur each year, roughly all within the valuable areas, composing about 10 percent of the Earth's surface. The interiors of the plates themselves are largely free of large earthquakes, although there have been famous exceptions: the 1811-1812 earthquakes at New Madrid, Missouri and the 1886 earthquake at Charleston, South Carolina.
Chile's 8.8 Earthquake - City of view Moves Over Ten Feet - Seismology 101
