GEOL 106 - Earthquakes and Resonance

Frequency

• Frequency of waves refers to the number of waves that pass through a fixed point in 1 second 

Period

• The amount of time it takes one wave to pass through the point 

Resonance

 The tendency of a system to oscillate with greater amplitude at some frequencies than at others

• Resonant frequency of any system is the frequency at which the max amplitude oscillation occurs 

• Resonance systems can be used to generate vibrations of a specific frequency to create music 

• Resonance is affected by mass, length, and tension

• Short period (low amplitude, high frequency) 

• Long period (high amplitude, low frequency

Resonance: Buildings 

• All buildings have a natural period or resonance, which is a number of seconds it take for the building to naturally vibrate back and forth 

  • Ground has specific resonance frequencies. Hard bedrock has higher frequencies than softer sediments

    • If the period of ground motion matches a natural resonance of a building, it will undergo the largest oscillations possible and suffer the greatest damage 

Small Vs Tall Buildings

• Small buildings, 1-2 stories, resonate naturally at much less than one second periods, that one second period will effect buildings about 10 stories 

• Small buildings on hard rock and large buildings on soft sediments will suffer more damage and ground motion effects from earthquakes then small buildings on soft surfaces, and tall buildings on hard rock

• By determining the resonant frequency of the ground beneath the building site, the building design can be modified so that a resonance disaster does not occur