Seismic magnitude scales

Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. They help scientists and people understand how powerful an earthquake was. These scales are different from seismic intensity scales, which look at how strong the shaking felt in a specific place.

Magnitudes are usually figured out from measurements of an earthquake's seismic waves. These waves are recorded on a seismogram, which is like a drawing that shows the earthquake's activity. There are many different magnitude scales because earthquakes can vary a lot, and scientists need different ways to measure them depending on what they are studying.

Earthquake magnitude and ground-shaking intensity

The Earth's crust is pushed and pulled by forces from movement deep inside the Earth. When the crust breaks or slips, energy is released as waves that shake the ground.

We use magnitude to describe how strong an earthquake is. It tells us about the amount of energy released during the quake. Intensity describes how hard the ground shakes in a specific place. It depends on factors like the type of soil. Soft soil can make shaking feel stronger, even far from where the earthquake started. This is why some areas far from an earthquake's center can feel very strong shaking.

Magnitude scales

Earthquakes send out energy in the form of seismic waves. Scientists measure these waves using instruments called seismographs to determine the earthquake's magnitude, which tells us how strong the earthquake was.

Typical seismogram. The compressive P waves (following the red lines) – essentially sound passing through rock – are the fastest seismic waves, and arrive first, typically in about 10 seconds for an earthquake around 50 km away. The sideways-shaking S waves (following the green lines) arrive some seconds later, traveling a little over half the speed of the P waves; the delay is a direct indication of the distance to the quake. S waves may take an hour to reach a point 1000 km away. Both of these are body-waves, that pass directly through the earth's crust. Following the S waves are various kinds of surface-waves – Love waves and Rayleigh waves – that travel only at the earth's surface. Surface waves are smaller for deep earthquakes, which have less interaction with the surface. For shallow earthquakes – less than roughly 60 km deep – the surface waves are stronger, and may last several minutes; these carry most of the energy of the quake, and cause the most severe damage.

The Richter scale, developed in 1935, was the first way to measure earthquake magnitudes. It is a logarithmic scale, meaning each step up represents a ten-fold increase in wave amplitude and about a 32-fold increase in energy. However, the Richter scale has limitations and doesn't work well for very large or very deep earthquakes.

Since 2005, scientists have used more advanced methods to measure earthquake magnitudes, including body waves, surface waves, and the total energy released. These methods help provide a more accurate picture of an earthquake's strength. Different scales are used for different situations, such as local, regional, or global earthquakes.

Date	ISC #	Lat.	Long.	Depth	Damage	M_s	M_w	mb	M_e	Type of fault
06 July 1997	1035633	−30.06	−71.87	23 km	Barely felt	6.5	6.9	5.8	6.1	interplate-thrust
15 Oct. 1997	1047434	−30.93	−71.22	58 km	Extensive	6.8	7.1	6.8	7.5	intraslab-normal
Difference:						0.3	0.2	1.0	1.4

Earthquake magnitude and ground-shaking intensity

Magnitude scales

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