Wave Amplitude, Fault Size, Amount of Slip
There are a number of ways to measure the magnitude of an earthquake. Most scales are based on the amplitude of seismic waves recorded on seismometers. These scales account for the distance between the earthquake and the recording seismometer so that the calculated magnitude should be about the same no matter where it is measured. Another scale is based on the physical size of the earthquake fault and the amount of slip that occurred. Then there are also measures of earthquake shaking intensity. The intensity from one earthquake varies greatly from place to place.
What’s the difference between magnitude and intensity? This 8 minute video uses the analogy of a lightbulb to explain the how an earthquake can have different intensities at different places.
What Controls the Shaking You Feel?
A few of these scales are described in more detail below.
The Richter Scale
The first widely-used method, the Richter scale, was developed by Charles F. Richter in 1934. It used a formula based on the amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer. That scale was specific to California earthquakes and crust; other scales, based on wave amplitudes and total earthquake duration, were developed for use in other situations and they were designed to be consistent with Richter’s scale.
The Moment Magnitude Scale
Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally. The moment magnitude scale is based on the total moment release of the earthquake. Moment is a product of the distance a fault moved and the force required to move it. It is derived from modeling recordings of the earthquake at multiple stations. Moment magnitude estimates are about the same as Richter magnitudes for small to large earthquakes. But only the moment magnitude scale is capable of measuring M8 (read "magnitude 8") and greater events accurately.
Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and about 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are much too small to be felt by most people.
Magnitude scales can be used to describe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit. The largest recorded earthquake occurred along the subduction zone in Chile in 1960. It was a magnitude 9.5 but larger earthquakes may be possible.
Fortunately, large earthquakes are much less common than small ones. Here's a table describing the magnitudes of earthquakes, their effects, and the estimated number of those earthquakes that occur each year.
The Mercalli Scale
Another way to measure the strength of an earthquake is to use the observations of the people who experienced the earthquake, and the amount of damage that occurred, to estimate its intensity. The Mercalli scale was designed to do just that The original scale was invented by Giuseppe Mercalli in 1902 and was modified by Harry Wood and Frank Neumann in 1931 to become what is now known as the Modified Mercalli Intensity Scale. To help distinguish it from magnitude scales, the MMI scale uses roman numerals.
Although the Mercalli scale does not use scientific equipment to measure seismic waves, it has been very useful for understanding the damage caused by large earthquakes. It has also been used extensively to investigate earthquakes that occurred before there were seismometers.
Some factors that affect the amount of damage that occurs are:
- the size (magnitude) of the earthquake
- the distance from the epicenter,
- the depth of the earthquake,
- the building (or other structure) design,
- and the type of surface material (rock or dirt) the buildings rest on.
Different building designs hold up differently in an earthquake and the farther you are from the earthquake, the less damage you'll usually see. Whether a building is built on solid rock or sand makes a big difference in how much damage it sustains. Solid rock usually shakes less than sand, so a building built on top of solid rock shouldn't be as damaged as it might if it was sitting on a sandy lot.