Earthquake Intensity

Mars and Earth
Interior Models

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Earthquakes

Measuring Intensity

Richter Magnitude Scale
In 1935 Charles F. Richter developed the Richter Magnitude Scale to mathematically rate how much force an earthquake releases based on seismograph readings. The scale takes into account the distance between various seismographs, the density of the ground and other factors to give a number for reference. Each increase in whole number on the scale is ten times the measured amplitude (strength) of the previous and corresponds to about 31 times more energy released by the quake.

Uncounted numbers of quakes less than 2.0 happen each year with several thousand over 4.5 on the scale. The highest on the scale and most damaging have had magnitudes of 8.0 to 8.9. Quakes in this range occur about once per year. The simplified version below gives an idea of how the scale works.

Magnitude	Richter Number	Relative Intensity
Low	Less than 2.0	Barely felt, minimal or no damage.
Moderate	2.0 to 4.4	Noticable, damage to buildings, sidewalks, weak structures.
Strong	4.5 to 7.0	Heavy shaking, falling walls and bridges.
Very strong	Greater than 7.0	Massive shaking, ground moving, landslides, great damage to structures.

Very simple version of the Richter Magnitude Scale

Modified Mercalli Intensity Scale
Earthquake intensity has been measured by various scales over the years but the one currently in use in the US is the Modified Mercalli Intensity Scale. Developed in 1931 by Harry Wood and Frank Neumann, this scale has 12 increasing levels of intensity ranging from "Not felt except by a few" to "Felt by all, damage slight" all the way up to "Few, if any masonry structures remain standing". The scale takes into account how people "felt" the quake and the observed structural damage.

For more information go the the websites below. These sites open in a new window, close window to return to trip.

USGS Earthquake Information
USGS Geohazards Information
USGS Western Region Earthquake Information
USGS National Earthquake Information Center
U of Washington: Surfing the Internet for Earthquake Data

Geology Terms

Here are some basic terms used on this page. Find more geology terms in the Glossary.

Basalt: Volcanic rock caused by partial melting of the Earth's crust.
Epicenter: The spot on the surface of the Earth directly above the area where an earthquake took place.
Fault: A split or fracture in the Earth's crust where two blocks of crust have slipped, slid or pushed against each other.
Focus (Earthquake): The area inside the Earth where an earthquake happened. Also known as the Hypocenter.
Hypocenter: The area inside the Earth where an earthquake happened. Also known as the Focus.
Intensity: The observed effects that an earthquake shaking the ground has on people, buildings, man-made structures and natural features.
Magnitude: The seismic (earthquake) energy recorded on a seismograph.
Mercalli Intensity Scale, Modified: Earthquake intensity measured on a scale that has 12 increasing levels. The scale takes into account how people "felt" the quake and the observed structural damage.
Richter Magnitude Scale: A scale that mathematically rates how much force an earthquake releases based on seismograph readings.

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