## Earthquakes

Most earthquakes result from plate motions.  While we talk about motion being a few 10s of mm/year, rarely is the motion such a slow creeping.  Generally the bounding faults and locked, and the slow steady application of force until the plates overcome friction and the built up energy is released.

Magnitude is the most common measure of an earthquake's size. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like.  The magnitude takes the motion observed at a recording station (amplitude of the earth's motion), and adjusts for the distance to the focus (computed from the time differential between the arrival of the P waves and the S waves) to estimate the motion at the epicenter.  The Richter scale is an outdated method that is no longer used that measured the largest wiggle on the recording, but other magnitude scales measure different parts of the earthquake.

Earthquake size, as measured by the Richter Scale is a well known, but not well understood, concept. The idea of a logarithmic earthquake magnitude scale was first developed by Charles Richter in the 1930's for measuring the size of earthquakes occurring in southern California using relatively high-frequency data from nearby seismograph stations. This magnitude scale was referred to as ML, with the L standing for local. This is what was to eventually become known as the Richter magnitude.

As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges, typical for southern California where Richter worked. In order to take advantage of the growing number of globally distributed seismograph stations, new magnitude scales that are an extension of Richter's original idea were developed. These include body wave magnitude, Mb and surface wave magnitude, Ms. Each is valid for a particular frequency range and type of seismic signal. In its range of validity each is equivalent to the Richter magnitude.

Because of the limitations of all three magnitude scales, ML, mb, and Ms, a new, more uniformly applicable extension of the magnitude scale, known as moment magnitude, or Mw, was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. New techniques that take advantage of modern telecommunications have recently been implemented, allowing reporting agencies to obtain rapid estimates of moment magnitude for significant earthquakes.

A change of 1 in earthquake magnitude corresponds with 10 times more ground motion, and 32 times more energy released.

An earthquake of negative magnitude is a very small earthquake that is not felt by humans.  Negative magnitudes are measured where we have dense grids to monitor activity, such as in Iceland.

### Frequency of Occurrence of Earthquakes since 1900 (from USGS)

Magnitude Average Annually
8 and higher 1
7 - 7.9 15
6 - 6.9 134
5 - 5.9 1319
4 - 4.9 13,000
(estimated)
3 - 3.9 130,000
(estimated)
2 - 2.9 1,300,000
(estimated)

Intensity is a measure of the shaking and damage caused by the earthquake, and this value changes from location to location.  It uses Roman numerals, and is usually called the Mercalli scale.

Abbreviated description of the levels of Modified Mercalli intensity.

Intensity Shaking Description/Damage
I Not felt Not felt except by a very few under especially favorable conditions.
II Weak Felt only by a few persons at rest, especially on upper floors of buildings.
III Weak 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 Light 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 Moderate Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.
VI Strong Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII Very strong 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 Severe 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 Violent 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 Extreme Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

Abridged from The Severity of an Earthquake, a U. S. Geological Survey General Interest Publication. U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913

Maximum ground motion for selected earthquakes

 Year Location Magnitude Max motion Source 1906 San Francisco 7.8 8.5 m 1952 Kamchatka 9 > 10 m Walter and Amelung, 2007, Geology 1960 Chile 9.5 40 m Walter and Amelung, 2007, Geology 1964 Alaska 9.2 ~30 m Walter and Amelung, 2007, Geology 2004-2005 Sumatra-Andaman 9.3, 8.7 15 m Walter and Amelung, 2007, Geology April 2011 Japan 9 50 m horizontal, 7 m vertical April 2012 eastern Indian Ocean 9 21.3 m (largest strike slip ever) 2015 Nepal 7.8 to 8.1 7 m south, 1 m up

 Distribution by depth of 44,619 earthquakes from the CMT catalog.  Very few earthquakes are deeper than 200 km, and they occur in a limited number of subduction zones. Only 1431 are shallower than 10 km.

Last revision 11/12/2016