Factors Influencing Motion at a Site

The main factors which influence the ground motion characteristics at a particular site are:


The local seismicity determines what types of source mechanism and earthquake magnitude are possible at a site. Factors examined to determine seismicity level include:


Source Mechanism

The motions at a site are to a large extent determined by the characteristics of the source mechanism which include:

  • Tectonic features
    - potential estimated from local fault lengths

  • Historic events
    - frequency of occurence
    - fault sections where no events have occurred historically
    - available strong motion records





  • Type of fault
    - relative horizontal and vertical movement
  • Rupture process
    - continuous and regular
    - multiple and irregular
  • Directivity effects
    - Doppler effect

  • Near-source effects
    - pulses and "fling"

More on Near-source Effects

Fault Parallel Direction

The fault parallel displacement offset corresponds to velocity and acceleration pulses with the same duration as the rupture.


Fault Normal Direction

High shear stresses at the tip of the rupture cause motion perpendicular to the direction of rupture.

Travel Path Effects

As distance from mechanism increases:

  • Intensity is reduced
  • Predominant period of motion lengthens
  • Apparent duration increases



  • Wave propagation
    - P waves -> compression, travel very fast
    - S waves -> shear, longer periods

    - Surface waves -> Love and Rayleigh waves

  • Reflection
  • Dispersion
  • Attenuation
  • Focusing

Local Soil Conditions


Soil acts like a dynamic oscillator that greatly affects the motions experienced by the buildings on top of it.

Softer, deeper, or weaker soil will have a longer predominant frequency content.

Soil Structure Interaction

Inertial Effects

The mass of the structure influences the overall response of the soil structure system. This affects stiff, heavy buildings on soft soil.

Kinematic effects

The flexibility of the soil influences the dynamic response of structures. This affects tall, light buildings on soft soil.