Earthquakes
, Seismic Waves
• An earthquake is a vibration in the rocks of the crust and upper
mantle caused by a sudden dislocation of the rocks along a fault, but
sometimes from an explosion such as a nuclear bomb.
• The elastic strain energy released is transmitted through the rocks as
earthquake or seismic waves.
• Particles of rock vibrate, transmitting energy from one particle to the
next, away from the source or focus of the earthquake.
, Seismic Waves
• A seismometer detects and records ground motion. It is made of two
parts. One part is attached to a large inertial mass and does not
vibrate with earth movements, while the other is allowed to move
freely with the vibrations. The relative movement between the two is
recorded as a seismogram.
• Nowadays, seismometers measure and record ground motion
digitally. They are based on the relative movement between a coil of
wire and a magnet, which induces a current which is converted into a
digital recording. Modern seismometers can measure movements
smaller than one nm (one millionth of a millimetre). The data from
seismometers can be collected automatically and analysed by remote
computers.
• The seismometer combined with the seismogram is called a
seismograph. The terms seismograph and seismometer are often
used interchangeably.
• A seismic station will normally have an array of seismometers
arranged to pick up vibrations in the vertical and two principle
horizontal directions.
, Seismic Waves
• An earthquake is a vibration in the rocks of the crust and upper
mantle caused by a sudden dislocation of the rocks along a fault, but
sometimes from an explosion such as a nuclear bomb.
• The elastic strain energy released is transmitted through the rocks as
earthquake or seismic waves.
• Particles of rock vibrate, transmitting energy from one particle to the
next, away from the source or focus of the earthquake.
, Seismic Waves
• A seismometer detects and records ground motion. It is made of two
parts. One part is attached to a large inertial mass and does not
vibrate with earth movements, while the other is allowed to move
freely with the vibrations. The relative movement between the two is
recorded as a seismogram.
• Nowadays, seismometers measure and record ground motion
digitally. They are based on the relative movement between a coil of
wire and a magnet, which induces a current which is converted into a
digital recording. Modern seismometers can measure movements
smaller than one nm (one millionth of a millimetre). The data from
seismometers can be collected automatically and analysed by remote
computers.
• The seismometer combined with the seismogram is called a
seismograph. The terms seismograph and seismometer are often
used interchangeably.
• A seismic station will normally have an array of seismometers
arranged to pick up vibrations in the vertical and two principle
horizontal directions.
