Earthquakes Today

Earthquakes

Earthquakes happen when vibrations are caused by the movement of rock along a fault, a fracture that exists in Earth’s crust.  As the tectonic plates push against, pull away from, grind past or dive under one another, fault zones are created.  Sometimes tension builds up along a fault, and further movement can cause the release of energy in the form of seismic waves, or vibrations in Earth’s crust.  Those vibrations ripple violently through the crust, causing an earthquake.

Faults in Earth’s crust can take different forms, depending on the kind of tectonic stress involved, the strength of the rock, the presence of ground-water along the fault place, and the area of contact between the plates.  Movement along faults can be fast or slow.  Abrupt movement causes earthquakes; movement so slow as to be imperceptible is called fault creep.

A severe earthquake can produce underground movements-forward and back up and down, side to side and wavelike ripples.  Seismographs around the world sense at least a million earthquake movements a year.  People barely perceive most of these.

Like volcanic eruption, most earthquakes happen along the edges of tectonic plates.  California’s San Andreas Fault, for example, is a zone where the slow sideways movement of slabs has pushed rock formations some 350 miles from their sources. 

Major earthquakes tend to produce dangerous side effects such as landslides and tsunamis, adding greatly to the destruction and casualties.

Methods for measuring earthquakes

Until recently, scientist measuring earthquakes mostly used the Richter scale, developed by U.S. seismologists Charles F.Richter and Beno Gutenberg in the 1930s and 1940s.

In their logarithmic scale of earthquake magnitude, each number represents ab intensity ten times greater than the previous one.  No earthquake has exceeded a value of 9.5, which occurred in Chile on May 22, 1960.

The Richter scale measures only magnitude.  Other scales categorize earthquakes by other criteria.  The moment magnitude scale is based on the seismic moment: the area of rock displaced the rigidity of that rock, and the average distance of displacement.

The Mercalli intensity scale (named for Giuseppe Mercalli, the Italian scientist who originated it) uses Roman numerals to rate an earthquake by its effects on the surroundings.  During an earthquake rated I, people feel no Earth movement.  During a V, almost everyone feels movement, trees might shake, and liquid might spill.  During a X, most buildings and foundations are destroyed dams break, and cracks in the ground show.