California's infamous San Andreas Fault divides two tectonic plates. To the west is the Pacific Plate, and to the east is the North American Plate. The fault runs approximately from the Salton Sea to the south, northward toward the San Francisco Bay Area, and then it roughly follows the coast until terminating offshore at the northern end of the state. This photo is from a location approximately 100 miles (60 km) north of Los Angeles.
The MMS replaced the Richter scale
Where I live in Southern California, we get small or moderate earthquakes frequently, and occasionally a large one that does damage. The largest I've been in is magnitude 7.3, the Landers Quake in the High Desert in 1992. The most destructive I've been in was a 6.7, the Northridge Quake in January 1994, which did billions of dollars of damage and moved my bed about six inches from the wall early in the morning.
The Moment Magnitude Scale (MMS) for measuring earthquakes is commonly used around the world, and many people are at least somewhat familiar with it. Most know for example that level 8.0 or higher is what a truly massive earthquake registers, although major damage can occur at lower levels as well, such as 5.0 or 6.0. But what exactly do these numbers mean?
The scale was developed in the 1970s to replace the old Richter scale, which always had weaknesses in being unable to reliably measure certain types of earthquakes, most notably very large ones. The seismographs, with paper rolling past a needle, which shake and draw lines on the paper during quakes, could never reliably measure above about magnitude 7.0, and are best used for moderate earthquakes between 3.0 and 7.0.
Seismometers measure motion of the ground, including any seismic waves produced by earthquakes, volcanic eruptions, or other sources.
Measuring the amount of energy released
The logarithmic Richter scale, first used in 1935, measures how much energy was released overall in the earthquake. For every two points upward, such as from 5.0 to 7.0, there is an increase of 1,000 times the amount of energy released. For a gap of one point, such as from 5.0 to 6.0, there is an increase of about 31.6 times the amount of energy released.
For each tenth of a point, such as from 5.0 to 5.1, the amount of energy released is about 40% more. An increase of two-tenths of a point, such as from 5.0 to 5.2, is an approximate doubling of the amount of energy released.
This volcanic eruption of Mount Redoubt in Alaska in 1990 looks massive, but was smaller in size than the eruption of Mount St. Helens in the state of Washington in 1980. Large volcanic eruptions release sufficient energy to register on the Richter scale with numbers similar to those of large earthquakes.
Examples of Richter scale measurements
The largest earthquake ever recorded measured 9.5 or 9.6 on the Richter scale. It occurred in Chile, South America in 1960. The biggest one I've been in only had about 0.05% as much energy released. The magnitude 7.3 that I was in made the road move in waves in front of me (my family was on a highway at the time right near the epicenter).
The nuclear weapon dropped on Hiroshima, Japan during World War II released energy equivalent to magnitude 6.0. The most powerful nuclear weapon ever detonated released enough energy to measure 8.3 on the Richter scale.
The meteor that exploded over Chelyabinsk, Russia in February 2013 and shattered thousands of windows released energy equivalent to magnitude 7.0. The comet that smashed into the Earth 65 million years ago, ending the reign of the dinosaurs and some other major groups of animals, caused an explosion that would have registered about 12.5 to 13.0 on the Richter scale.
The violent volcanic eruption of Mount St. Helens in 1980 in Washington, USA was powerful enough to measure magnitude 7.8 . The largest volcanic eruption of the past 10,000 years was Mount Tambora in Indonesia in 1815. It's estimated that this massive eruption released enough energy to measure 9.1 to 9.2 on the Richter scale.
Introduction of the Moment Magnitude Scale
The Richter scale as mentioned best measures earthquakes within a certain range, 3.0 to 7.0, and it also works best at a specific distance from where the earthquake originated. The Moment Magnitude Scale however gives a more accurate picture of what happened by taking other factors into consideration.
Earthquakes may occur along a shorter or longer section of a fault. Sometimes they occur where no fault is present. The cause may be something else besides tectonic plates moving – such as a landslide, a bomb, a volcanic eruption, or a meteor impact. Sometimes they occur deep in the Earth or closer to the surface. The length of time over which the quake occurs can be from seconds to minutes.
What the MMS measures is overall seismic movement, which depends upon the rigidity of the Earth where the earthquake occurs, the amount of slip along a fault, and the size of the area that slips. Once the formula for making these calculations was sufficiently refined, it was made to give numbers very close to Richter scale measurements for the same earthquakes.
However, there are often discrepancies when the same quakes are measured by the two different scales. Sometimes the numbers come out the same, and sometimes they differ because they come up with their final magnitude ratings in different ways. Sometimes one scale will give a higher number than the other for the same earthquake, such as 6.5 versus 6.2.
The United States Geological Survey (USGS) uses the Moment Magnitude Scale. It is being further studied for possible additional refinements to enhance the accuracy even further.