Stress in Earth’s Crust 1

  An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves.  Almost all earthquakes occur at plate boundaries.  When the stresses are greater than the internal strength of the rocks, the rocks snap. Although they return to their original shape, the stresses cause the rocks to move to a new position. This movement releases the energy that was stored in the rocks, which creates an earthquake. Earthquake 2

 3 Earthquake epicenters: Earthquake epicenters can be used to outline the edges of the lithospheric plates. Most earthquakes occur around the Pacific Ocean basins and in the Mediterranean-Asiatic belt.

  (Time 1) Elastic rebound theory. Stresses build on both sides of a fault, causing the rocks to deform plastically (Time 2). When the stresses become too great, the rocks return to their original shape but they move (Time 3). This motion releases the energy that creates an earthquake. Earthquake 2: 4

  The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.  Earthquakes are measured using observations from seismometers. The seismic activity: 5

  The point where the rock ruptures is usually below the Earth’s surface. The point of rupture is called the earthquake’s focus (hypocenter ).  Just above the focus on the land surface, is the earthquake’s epicenter. It is the epicenter of an earthquake that is reported by scientists and the media. Focus-epicenter 6

  There are two types of body waves – primary waves (P- waves) and secondary waves (S- waves).  They are named primary waves because they are the first waves to reach a seismometer. S-waves are slower and so are the second waves to reach a seismometer.  Surface waves travel along the ground outward from an earthquake’s epicenter.  There are two types of surface waves. Love waves move side-to-side much like a snake. Rayleigh waves move in rolls, like ocean swells. 7 Body waves and surface waves:

 Types of waves: 8

  The top figure shows how body waves, including P- waves and S-waves, movethrough a grid.  The bottom figure shows how surface waves move.  The two types of surface waves are Love waves and Rayleigh waves. 9 Types of waves 2 :

 Types of seismograms: 10

  The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe.  The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter magnitude scale.  These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth Magnitude: 11

  The Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the energy released by an earthquake.  The Richter scale is a base-10 logarithmic scale, which defines magnitude as the logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude. Richter Magnitude scale: 12

  n 1935, the seismologists Charles Francis Richter and Beno Gutenberg, of the California Institute of Technology, developed the (future) Richter magnitude scale, specifically for measuring earthquakes in a given area of study in California, as recorded and measured with the Wood-Anderson torsion seismograph. Richter Magnitude scale 2: 13

  Originally, Richter reported mathematical values to the nearest quarter of a unit, but the values later were reported with one decimal place; the local magnitude scale compared the magnitudes of different earthquakes.  Richter derived his earthquake-magnitude scale from the apparent magnitude scale used to measure the brightness of stars. Richter Magnitude scale 3: 14

 Richter Magnitude scale 4: 15

  The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake.  It measures the effects of an earthquake, and is distinct from the moment magnitude M_w usually reported for an earthquake (sometimes misreported as the Richter magnitude), which is a measure of the energy released.  The intensity of an earthquake is not totally determined by its magnitude. It is not based on first physical principles, but is, instead, empirically based on observed effects. Mercalli scale: 16

  The Mercalli scale (revised and correct in ) quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction).  In 1902 the ten-degree Mercalli scale was expanded to twelve degrees by Italian physicist Adolfo Cancani. It was later completely re-written by the German geophysicist August Heinrich Sieberg and became known as the Mercalli-Cancani-Sieberg (MCS) scale. Mercalli scale 2: 17

 Mercalli Scale 3: 18

 Mercalli scale vs Richter scale: 19

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