Earthquake Magnitude Scales

The severity of an earthquake is generally proportional to the amount of seismic energy it releases. Seismologists use a Magnitude scale to express this energy release. Here are the typical effects of earthquakes in various magnitude ranges.

Magnitude Effects
Less than 3.5  Recorded on local seismographs, but generally not felt.
3.5 - 5.4 Often felt, but rarely cause damage.
Under 6.0 At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
6.1 - 6.9 Can cause damage to poorly constructed buildings and other structures in areas up to about 100 kilometers across where people live.
7.0 - 7.9 "Major" earthquake. Can cause serious damage over larger areas.
8.0 - 8.9 "Great" earthquake. Can cause serious damage and loss of life in areas several hundred kilometers across.
9 Rare great earthquake. Can cause major damage over a large region over 1000 km across.

While each earthquake releases a unique amount of energy, the magnitude values reported by different seismological observatories for an event may vary. Depending on the size, nature, and location of an earthquake, seismologists may use several different methods and even different magnitude scales to estimate magnitude. The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data. It may be several days before different organizations come to a consensus on what is the best overall magnitude estimate.

Although each earthquake has a unique Magnitude, its effects will vary greatly according to distance, ground conditions, construction standards, and other factors. Seismologists use a different Intensity Scale to express the variable effects of an earthquake.