An earthquake's strength is measured using a magnitude scale, commonly the Richter scale or the moment magnitude scale. This scale is logarithmic, meaning each whole number increase represents a tenfold increase in the measured amplitude of seismic waves and about 31 times more energy release. Earthquakes range from minor (magnitude less than 3.5, usually not felt) to great (magnitude 8.0 and above, capable of causing serious damage over large areas). Here is a typical breakdown of earthquake strength and effects by magnitude:

• Less than 3.5: Generally not felt, only recorded on seismographs.
• 3.5 to 5.4: Often felt but rarely causes damage.
• 5.5 to 6.0: Slight damage to well-designed buildings; more damage to poorly constructed ones.
• 6.1 to 6.9: Can cause damage in populated areas up to about 100 km.
• 7.0 to 7.9: Major earthquake, causing serious damage over large areas.
• 8.0 or higher: Great earthquakes that can cause severe destruction and loss of life over hundreds of kilometers.

The strength also depends on factors like the earthquake's depth and local ground conditions, affecting the intensity of shaking experienced. The intensity, different from magnitude, reflects the effects and experiences of shaking at specific locations. In summary, an earthquake's strength can vary substantially, from barely noticeable tremors to catastrophic events causing widespread destruction.