## Saturday, May 19, 2012

### Earthquake Magnitude

 2012, Magnitude 8.6 Off the west coast of northern Sumatra
Seismologists express earthquake size in units of magnitude. Magnitude is obtained from instruments called seismometers that measure motions of the ground.

The Moment Magnitude Scale is commonly used to measure the size of earthquakes.

Using the Moment Magnitude Scale, earthquake size is given in terms of the energy released based on the seismic moment of the earthquake. The seismic moment is equal to the rigidity of the Earth times the average amount of slip on the geologic fault and the size of the area that slipped.

The Moment Magnitude Scale was developed in the 1970s to replace the Richter Scale.

Shaking Amplitude

Magnitude is expressed on a base-10 logarithmic scale. An earthquake that measures 5.0 has a shaking amplitude 10 times larger than one that measures 4.0. Each whole number increase in magnitude represents a tenfold increase in measured shaking amplitude.

Energy Released

Each whole number increase in magnitude corresponds to an increase of 32 times the amount of energy released.
We learn geology the morning after the earthquake
~ Ralph Waldo Emerson
How Much Bigger?

The USGS How Much Bigger Calculator is an interactive tool that shows difference in the intensity and in energy released between two earthquake magnitudes.

Recent Large Earthquakes

The largest recorded earthquakes for each year over the past decade are shown in the table below:

Year Date Magnitude Human
Fatalities
Location
2012 04/11 8.6 0 Off the west coast of northern Sumatra
2011 03/11 9.0 20896 Near the East Coast of Honshu, Japan
2010 02/27 8.8 507 Offshore Maule, Chile
2009 09/29 8.1 192 Samoa Islands region
2008 05/12 7.9 87587 Eastern Sichuan, China
2007 09/12 8.5 25 Southern Sumatera, Indonesia
2006 11/15 8.3 0 Kuril Islands
2005 03/28 8.6 1313 Northern Sumatra, Indonesia
2004 12/26 9.1 227898 Off West Coast of Northern Sumatra
2003 09/25 8.3 0 Hokkaido, Japan Region

Human fatalities from seismic events depend on the magnitude and depth of the event and the proximity of the epicenter to concentrations of human population. Off-shore Earthquakes, volcanic eruptions and other underwater disturbances can cause a series of waves, or tsunamis, that can be destructive to on-shore populations.