Focal Mechanisms Explained

How do scientists know which direction the fault moved deep underground?

When an earthquake occurs, seismologists create graphics of focal mechanisms, informally referred to as beach balls,to show the faulting motions that produce the earthquake. They use the patterns of compressions and dilatations received by seismometers. Simply put, the focal mechanisms are based on the direction of the first arriving P wave. This is a difficult concept for many, but this animation walks viewer through steps to understand how scientists know what kind of fault motion occurred deep underground. The different faulting mechanisms for each focal-mechanism end member includes: strike-slip fault, normal fault, and thrust fault


Keypoints:

  • P-wave arrivals to many stations give information about an earthquake
  • Data yields what kind of fault occurred deep below ground
  • Graphics depict normal, reverse, and strike-slip faults and their focal mechanisms


Level: Intermediate

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