Types of vibrations considered to study earthquakes

Types of vibrations According to actuating force

• Free vibrations: If the external force is withdrawn after certain displacement of the system, then the system vibrates on its own due to elastic properties. Such type of vibrations is called free vibrations. Frequency of such vibrations is known Natural Frequency. For example oscillation of a simple pendulum

• Forced vibrations: If a system is subjected to an exciting force (repeating type of force) then the resulting type of vibration is called forced vibrations  

Types of vibrations According to energy dissipation

• Undamped vibrations: If no energy is lost or dissipated in friction or other resisting element, such vibration is known as undamped vibration e.g. simple pendulum, Undamped free vibrations

• Transient vibrations: Theoretically, the free undamped vibrations may continue indefinitely as there is no damping. but due to air resistance the amplitude decays continuously and the vibrations vanishes. Such vibrations are called transient vibrations.

• Damped vibrations: If a damper is provided in vibrating system, the energy of the system gets dissipated in friction. there is reduction in amplitude after each cycle It is known as damped vibration, e.g. a motor cycle moving on rough road with shok absorb.

Types of vibrations According to behavior of vibrating system

• Linear vibrations: If basic component i.e. spring, mass and damper behave linearly, the vibration is known as linear vibration e.g., oscillating spring and mass system along its axis

• Non-linear vibrations: If any body of the  basic component of the vibrating system behave non-linearly, it is known as non- linear vibration e.g., motion of spring and mass system in transverse direction.

Types of vibrations According to motion of system w r t axis

• Longitudinal vibrations: When a body vibrates along its axis the vibration is known as longitudinal vibration e.g. oscillation of spring and mass system along its axis.

• Transverse vibrations: A simple pendulum oscillates perpendicular to its axis. This is transverse vibration.

• Torsional vibrations: When body rotates about its own axis, the vibration is known as torsional vibration.

Types of vibrations According to nature of actuating force at a given time

• Deterministic vibrations: If the value of excitation force acting on a vibrating system at any given time is known, it is known as deterministic vibration. 

• Random vibrations: If the valve of exciting force acting on a vibrating body at any time is non-deterministic (random) the resulting vibration is known as random vibrations e.g. a body, is subjected to random vibration.