Measuring Magnitude of an Earthquake

which are primary waves, and S waves, which are secondary waves. P waves go through anything, while S waves only go through rock as these are shearing-type waves.

P waves represent mild elevations above the baseline noise. The baseline squiggles you see all the time are called microseisms. P waves are still slight but they are fast and indicate a possible earthquake. Above these are the S waves, which are the largest ones you will see. No S waves can still mean that an earthquake happened but that it happened too far away on the other side of the planet. P waves can travel through the earth's liquid parts but S waves cannot. This is how you know when an earthquake was nearby or not. S waves come after P waves.

Surface waves are their own kind of wave. These only travel through rock and have a slow wavelength as a result. There are two kinds of surface waves: Raleigh waves and Love waves. Love waves move side to side across the wave's direction. Raleigh waves are elliptical. They are slower than Love waves but spread out wide. You'll see these last a long time after an earthquake.

MEASURING MAGNITUDE OF AN EARTHQUAKE

There is only one magnitude for any given earthquake. It is not location-dependent. The Moment Magnitude Scale has been in use since 1970 all over the world. This is different from the intensity, which as mentioned, is location-dependent. Geologists use the Modified Mercalli Scale to measure local earthquake shaking activity.

Magnitude scales for earthquakes are not linear; they are logarithmic. One point increase equals ten times the strength. The Richter scale is no longer in use as it was only valid in Southern California and does not put into play all of the seismographs in the world.

The MMS or moment magnitude scale is used to measure rock movement on a fault line. It is excellent for measuring the magnitude of large earthquakes. There are differences among the different magnitudes and what they mean. Within the MMS, there are classes of earthquakes called earthquake magnitude classes.

These are simple to remember.

• Minor quakes – 3 or more but up to 3.9

• Light quakes – 4 to 4.9

• Moderate quakes - 5 to 5.9

• Strong quakes – 6 to 6.9

• Major quakes – 7 to 7.9

• Great quakes – 8.0 or more

Intensity is different than magnitude and is based on what is seen on the ground after an earthquake. It looks at the environment and related things to say what the effect of the earthquake has been. It is entirely based on where you are on the planet near the quake.

Remember that the MMS is an international measure of magnitude; the MM is the Modified Mercalli Scale. This measures earthquake intensity. The original Mercalli Scale was developed in 1902 and modified 29 years later to say what the intensity is of any given quake. They use Roman numerals to depict the intensity rating.

An intensity of Roman numeral I is not felt by most people. Maybe a few people in tall buildings will notice the shaking. Roman numeral II is so weak that a few people feel it and some delicate things like chandeliers will sway. It is only at Roman numeral IV that car alarms go off and dishes are disturbed somewhat.

By the time you get to an intensity level of Roman Numeral VIII, you get partial building collapse, chimneys falling, monuments and walls cracked, and overturning of furniture. This would be a severe earthquake intensity. Violent and extreme earthquakes are Roman Numeral IX and X, respectively. The damage is severe to foundations, while wooden and masonry structures are destroyed. Railways can be bent.

Notice that intensity is based almost exclusively on damage to buildings, cars, and infrastructure. The milder intensities are based mainly on whether or not the earthquake is felt at all.