Transcript Earthquakes
Earthquakes earthquakes • Earthquakes are natural vibrations of the ground caused by movement along gigantic fractures in Earth’s crust or by volcanic eruptions. • Most earthquakes occur when rocks fracture, or break, deep within the earth. • Fractures form from stress , the forces per unit acting on a material, exceeds the strength of the rocks involved. Stress • There are 3 types of stress: • Compression: is stress that decreases the volume of a material. • Tension: is stress that pulls a material apart. • Shear: is stress that causes a material to twist • Strain: this is the deformation of materials due to stress • Ductile deformation: when stress exceeds the strain a material can withstand, ductile deformation occurs. Meaning the material becomes deformed or can no longer resume its former shape. • Elastic strain: this allows a material to undergo strain and stress and return to its former shape. (think of a rubber band) Faults • A fault is defined as a fracture or system of fractures in Earth’s crust that occurs when stress is applied too quickly or stress is too great. There are 3 types of Faults • Horizontal Compression or Reverse Fault • Horizontal Shear or strike – slip fault • Horizontal tension or normal fault. Normal Fault Reverse Fault Strike Slip Fault • Reverse Fault – form as a result of horizontal compression. Compressional force results in horizontal shortening of the crust involved. • Normal Faults – are fractures caused by horizontal tension. Movement along the fault is partially horizontal and vertical, the movement occurs in such a way as to extend the crust. • Strike slip – are fractures caused by horizontal shear. This movement is mostly horizontal. (the San Andrea is a strike slip fault) Seismic Waves • Every earthquake generates 3 types of waves • Primary waves (P-waves), secondary waves (S-Waves) and Surface waves What do the waves do? • P –waves: squeeze and pull rocks in the same direction along which the waves are traveling. • S-Waves: cause rocks to move at right angles in relations to the direction of the waves. • Both P and S waves move through the Earth’s interior and are called body waves • Surface waves - move in two directions as they pass through the rock. They move in an up and down motion similar to an ocean wave. This wave also causes rocks to move from side to side. Surface waves travel along the Earth’s surface. P waves arrive first, then S waves, then surface waves The Focus and Epicenter of an Earthquake • The point within Earth where faulting begins is the focus • The point directly above the focus on the surface is the epicenter A seismogram is the record produced by a seismometer. Seismometer or Seismographs record earthquake events by detecting the vibrations ~80% of all earthquakes occur in the circum-Pacific belt • Magnitude – Richter scale measures total amount of energy released by an earthquake; independent of intensity – Measures from 1 to 10 – Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an openended logarithmic scale • The Richter Scale was developed by Charles Richter in 1935.