What Are Earthquakes??

An earthquake, according to the Global Seismology Research Group, British Geological Survey, is "the sudden release of strain energy in the earth's crust resulting in waves of shaking that radiate outwards from the earthquake source."
Great earthquakes begin as a murmer, swelling gradually into a loud roar, culminating with a principal convulsion.


Most earthquakes occur along fault lines, or the sides of a fracture that has formed in a very large piece of rock.  Sometimes the sides just easily rub together, and sometimes they rupture together suddenly, releasing energy in the form of an earthquake.  In this case, the sides of the fault slip in opposite directions, causing the rocks along the edge to no longer line up.  Faults are classified by their geometry and direction of the slip:


Fault Line

Normal Fault

Reverse Fault

Thrust Fault

Left-Lateral Normal Fault

Left-Lateral Strike-Slip Fault

Left-Lateral Reverse Fault

Source: Bolt, 1993

The Theory of Plate Tectonics

The basic theory states that the lithosphere is made up of seven large and somewhat stable slabs of solid rock called plates.  The plates extend to a depth of about 100 miles.  Each plate moves horizontally, sliding along the softer rock underneath.  At the plates edges, the geological structure of the earth us affected by the forces between the plates.  The epicenters of earthquakes usually lie along these edges, called faults.  The theory of plate tectonics is based of four assumptions:

  • Seafloor spreading generates new plate material, and new oceanic lithosphere is formed along active mid-oceanic ridges.
  • New oceanic lithosphere forms part of a moving plate which may or may not have continental material.
  • The growth of lithosphere is constantly balanced by its consumption becuase the Earth's surface is constant.
  • Motion between plates occurs almost completely along their boundaries because they can transmit stresse across great distances without buckling.

  • Measuring Earthquakes

    Charles Richter developed the Richter Magnitude Scale in the 1930s to use in Southern California.  An earthquake's magnitude is the measure of its size, determined by the logarithm of the amplitude or maximum displacement of the earthquake's signal seen on a seismogram.

    Types of Vibrations

    In great earthquakes, normal vibrations occur first, and are followed by transverse ones.
    Normal- to and from the origin or coincident with the direction of propagation
    Transverse- at right angles to that direction
    Thousands of individual waves or tremors occur during earthuakes that are imparted to the earth's mass.  They have different degrees of wave length and amplitude.  The waves move with an enormous amount of velocity, approximately 3 miles/second.

    Relive the Earthquake
    Prepare for the Next One