What are Seismographs?

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Strong motion accelerograph in Invercargill.

A seismograph is an instrument for measuring earthquake (seismic) waves. They are held in a very solid position, either on the bedrock or on a concrete base. The seismometer itself consists of a frame and a mass that can move relative to it. When the ground shakes, the frame vibrates also, but the mass tends not to move, due to inertia. The difference in movement between the frame and the mass is amplified and recorded electronically.

A network of seismometers is used to calculate the magnitude and source of an earthquake in three dimensions

Seismographs are used to determine:

  • Magnitude: the size of the earthquake
  • Depth: how deep the earthquake was
  • Location: where the earthquake occurred