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Most earthquakes originate from the regions at the junctions of the plates that make up the earth’s crust. These plates are constantly shifting, creating stresses and distortions. Where the stress is greater than the strength of the crust, there will be sudden slippage that releases energy and causes seismic waves on the earth’s surface. The place where the energy is released is called the focus of the earthquake, and the point on the earth’s surface directly above the focus is called the epicentre.

The amount of energy released at the epicentre is typically measured using the Richter Magnitude Scale (M). A Magnitude M4 earthquake can be distinctly felt near the epicentre, M5 and M6 are moderate size earthquakes that may cause considerable damage, and M7 and M8 are often accompanied by wide-spread damage to buildings and other structures and can trigger landslides and permanent ground displacements.

If a building is located directly over a fault, it can be damaged by failure of the ground below the foundation. Unstable slopes and weak soil deposits can lead to foundation failure and damage or collapse of the building. However, most damage is a result of ground motions from the seismic waves radiating from the earthquake epicentre.

When the ground motion is strong enough, it moves buildings. The earthquake moves the foundation but inertial forces try to keep the upper storeys of the building in their original positions. If ground displacement were to occur very gradually, the building would ride along easily, however, earthquakes involve rapidly accelerating ground forces. The forces generated in the earthquake depend on the weight of a building and how quickly the ground accelerates. Since inertial forces are greater when objects are heavier, earthquake forces are higher in heavier buildings. Likewise, higher ground accelerations create more stress in the structure. Earthquakes will affect buildings differently depending on the characteristics of the ground motion and characteristics of the building structure.

The type of seismic ground motion at a building site is dependent on a number of factors:
-Distance of the building from the earthquake’s epicentre,
-Magnitude of the earthquake,
-Depth of the earthquake’s focus, and
-Soil conditions at the building site.

The way that a building responds to an earthquake depends on the size of the building and its stiffness characteristics. Earthquakes that have high peak ground accelerations pose the greatest challenge to wood-frame buildings. Measured ground accelerations, g, are recorded as a fraction of the acceleration due to gravity (10 m/s² or 32 ft/s² ) and are greatest at the epicentre. Although the Richter scale is often used to characterize an earthquake, peak ground acceleration at a given location is a better indicator of the potential damage of earthquakes on wood-frame buildings.

[The figures for acceleration due to gravity should be 10 metres per second2 or 32 feet per second2.]

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