Probe International
May 1, 2008

The Richter scale provides an objective way of measuring and comparing the size of earthquakes using a mathematical device.

According to the United States Geological Survey (USGS):

http://earthquake.usgs.gov/learning/topics/richter.php [PDF]

• The Richter scale was created in 1935 by Charles F Richter.

On the Richter scale, magnitude (M) is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3.

Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

• M 2 or less are called micro earthquakes, not commonly felt by people.
• M 4.5 are strong enough to be recorded by sensitive seismographs around the world.
• M 8 or greater are great earthquakes with an incidence rate of about one per year globally.
• The Richter scale has no upper limit.

The technical explanation

Seismic waves, which are recorded by seismographs, are vibrations from earthquakes that travel through the earth. A zigzag trace shows varying ground oscillations (or fluctuations) beneath the seismograph which eventually produce data regarding the time, location and magnitude of the earthquake. According to the Nevada Seismological Laboratory website http://www.seismo.unr.edu a magnitude scale expresses seismic energy released from an earthquake.

The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenters of the earthquakes.

Important note

The Richter scale is not appropriate for expressing damage, as the effects of an earthquake in a densely populated area will be much different than a similar earthquake in an unpopulated zone or beneath the ocean. (See Scales of Seismic Intensity)

Scales of Seismic Intensity offer a subjective way of measuring and rating the effects or severity of an earthquake.

According to the Nevada Seismological Laboratory website: www.seismo.unr.edu/

• There can be many different intensity values for the same earthquake.
• Rating seismic intensity does not require instrumentation or measurements.
• Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.

Intensities typically increase close to the epicenter of an earthquake; however, due to factors including soil makeup, intensity can be greater further away.

Intensity scales can be helpful in classifying historical earthquakes in a region (for which technical data does not exist) to better understand the seismicity of the region and to attempt to predict future occurrences.

According to the USGS: http://earthquake.usgs.gov/learn/topics/mag_vs_int.php [PDF]

• Seismic intensity is determined by effects felt by people, human structures and the natural environment

The website Geology.about.com compares four scales of seismic intensity: 1) the Modified Mercalli scale from the US; 2) the Rossi-Forel scale, the first widely used intensity scale devised in the late 1800s; 3) the Omori scale, used throughout Japan,and 4) the European Macroseismic Scale, the current standard in Europe. Many other nations use local versions of these scales for purposes of emergency planning and quake-resistant construction guidelines.

The Mercalli Intensity Scale (US Standard).

I. Not felt except by a very few under especially favourable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Source: http://earthquake.usgs.gov/learning/topics/mercalli.php [PDF]

Prepared by Probe International, May 2008