The Richter Scale: Obsolete?

Misconception 2Geology 101, Fall 2012

Richter scale

• Charles F. Richter at Caltech developed the Richter scale to describe earthquake strength in 1934.

• The deflection (maximum movement) of the pen on the seismograph was used to determine magnitude.

Logarithmic scale for earthquakes

• Richter realized that earthquakes varied widely in strength, and to keep the numbers small, he used a logarithmic scale.

• For instance, if the seismograph pen moved 1 mm, that was a magnitude 3 quake; if the pen moved 10 times as much (1 cm), that was a magnitude 4 quake and so forth.

Shortcomings of the Richter scale

• Richter realized that distance reduces earthquake intensity, so for distant earthquakes, he developed a nomograph to account for distance reducing the pen movement.

• Worse, southern California, where Richter worked, had shallow earthquakes that had energy characteristics (slightly) different than many quakes around the world because different rocks transmit quake energy differently.

Moment magnitude (M) scale

• Hiroo Kanamori and others at Caltech in the 1970s developed a logarithmic scale to describe earthquake energy output that depended on knowing factors such as rock strength, area and amount of fault movement.

• This is considered to be a much better scale to determine earthquake energy output.

References• McCalpin, James P. Appendix 1: Earthquake Magnitude Scales. In James P. McCalpin

(ed.), Volume 95: Paleoseismology. 2nd edition. Retrieved from http://www.elsevierdirect.com/companions/9780123735768/casestudies/01~Appendix_1.pdf

• First slide image: Hargrove, Brantley (August 7, 2012). Connection Between Quakes and Underground Frack-water Disposal Looks Solid. In Dallas Observer. Retrieved September 30, 2012, from http://blogs.dallasobserver.com/unfairpark/2012/08/connection_between_quakes_and.php

• Second slide image: South Dakota Geological Survey (n.d.).Richter Scale Graphic Representation. In Earthquakes in South Dakota(1872-2012). Retrieved September 30, 2012, from http://www.sdgs.usd.edu/publications/maps/earthquakes/rscale.htm

• Third slide image: US Geological Survey (July 12, 2012). Earthquake Glossary – Richter Scale. In Earthquakes. Retrieved September 30, 2012, from http://earthquake.usgs.gov/learn/glossary/?term=Richter%20scale

• Fourth slide image: No author (n.d.). 11.8 How Are Earthquakes Measured? In Lecture 11 – Deformation of Rocks. Retrieved September 30, 2012, from http://gomyclass.com/geology10/files/lecture11/html/web_data/file75contents.htm