Earth Science: EarthQuakes
The Earth is constantly shifting below our feet. The magnitude of this movement is usually too weak to be sensed by humans, yet various factors can result in highly destructive and intense earthquakes. In areas where this occurs frequently, safety precautions are of vital importance.
Causes
The constant motion within the layers of the Earth affects the surface, sometimes in the form of sudden events such as earthquakes. In highly populated regions, this can result in a devastating natural disaster.
The Earth’s lithosphere is very slowly but constantly moving; when lithospheric plates under the surface of the Earth reform, subduct, or slide, an earthquake can occur. These changes are the cause of 90 percent of all earthquakes. Different types of faults, or cracks, in the tectonic plates are under extreme tension and cause tremors when they slip. The two types of faults are dip-slip faults and strike-slip faults: the plates of dip-slip faults move up and down, while in strike-slip faults they move horizontally (left or right) across the fault plane.
The point of origin of an earthquake is called the hypocenter. Typically, the hypocenter of an earthquake occurs at a depth of less than 37 miles (60 km) below the surface of the Earth. Occurrences at core depths between 190 and 430 miles (300 and 700 km) are rare. The epicenter lies on the Earth’s surface, directly above the hypocenter. The highest intensity surface effects are seen at the epicenter.
Seismic waves
Energy generated from the restructuring of the Earth’s layers travels in the form of seismic waves. Seismic waves can be caused by shifts in faults and most often correlate with earthquakes.
The two main types of seismic waves are “body waves” and “surface waves.” Body waves form in the interior of the Earth and travel in a circular motion in all directions, including through the planet’s core. Surface waves are slower and travel along just below the surface.
Additional causes
Not all earthquakes are caused by tectonic changes, some are caused by volcanic eruptions. The ascent of magma through the volcano and the shifting of molten rock in the magma chamber beneath the Earth’s crust both cause tremors.
Hundreds of smaller earthquakes will usually occur prior to an eruption. Humans can also be the direct cause of earthquakes, for example by collapsing mines and tunnels or by conducting underground atomic bomb tests.
THE EPICENTER AND THE DISTRIBUTION OF WAVES
The fastest seismic waves are body waves, which include “P” or primary waves and “S” or secondary waves. P waves travel through rocks, liquids, and gases with a speed of 4-9 miles (6-14 km) per second S waves can only be transmitted through solids and are about half as fast as P waves.
Because the speed of the waves is known, the location of the exact epicenter of the earthquake can be determined using the time interval between S and P waves as they reach three separate seismological stations.
Types of surface waves include Love and Rayleigh waves, which are named after two British scientists Love and Rayleigh waves are even slower than S waves, and can cause significant damage Most of the shaking or tremors felt by humans during an earthquake are Rayleigh waves.