Earthquake Magnitude and Intensity: Measuring Seismic Power
When the Earth's crust is stressed by tectonic forces, the buildup of pressure can eventually lead to a rupture. This sudden release of energy sends vibrations through the ground, known as seismic waves, which cause the phenomenon we call an earthquake. To understand these events, scientists use two distinct measurement systems: magnitude, which describes the overall size or energy released, and intensity, which categorizes the severity of shaking felt at a specific location.
Because earthquakes vary in depth, distance, and wave characteristics, no single scale can capture every detail. Instead, seismologists rely on a variety of specialized scales to provide a complete picture of seismic activity.
Isoseismal map for the 1968 Illinois earthquake. The irregular distribution of shaking arises from variations of geology and ground conditions.
Key Facts
- Magnitude measures the total energy released at the source, while intensity measures the effects at a specific site.
- P waves (primary waves) are the fastest seismic waves and arrive first.
- S waves (secondary waves) travel slower than P waves and cause sideways shaking.
- Surface waves travel along the Earth's crust and often cause the most significant damage.
- Different scales, such as Richter or Moment Magnitude, are used depending on the earthquake's distance and depth.
Understanding Seismic Waves
To measure an earthquake, scientists must first understand the waves recorded on a seismogram (a graph of ground motion). These waves are generally divided into two categories: body waves and surface waves.
Body Waves
Body waves travel directly through the Earth's crust. The two main types are P waves (compressional waves) and S waves (shear waves). P waves are the fastest and act like sound passing through rock, while S waves arrive later and move the ground side-to-side. The time delay between these two waves helps scientists calculate how far away the earthquake occurred.
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.
Surface Waves
Surface waves, including Love waves and Rayleigh waves, travel only along the Earth's surface. While they carry less energy than body waves in deep earthquakes, they are much more destructive in shallow earthquakes (those less than 60 km deep) because they interact directly with the surface and can last for several minutes.
Major Magnitude Scales
Seismologists use different scales depending on the specific characteristics of the seismic event.
The Richter Scale (Local Magnitude, ML)
The Richter scale was originally defined based on the maximum horizontal displacement of a seismogram at a distance of 100 km. While famous, it has limitations; it is less effective for very distant earthquakes due to wave attenuation (the loss of energy as waves travel) and is less accurate for deep earthquakes where surface waves are smaller.
Body-Wave Magnitude (mB)
To overcome the limitations of the Richter scale for distant or large events, the body-wave magnitude (mB) was developed. This scale is based on P and S waves and does not "saturate" (reach a limit in measurement) until around magnitude 8. It is particularly useful for large events, though it is not sensitive to earthquakes smaller than approximately M 5.5.
Surface-Wave Magnitude (Ms)
The surface-wave magnitude (Ms) scale measures the amplitude of surface waves. This is particularly useful for measuring shallow or distant earthquakes that the original Richter scale could not accurately capture. It generally agrees with the Richter scale around magnitude 6 but can diverge significantly for larger events.
Moment Magnitude (Mw) and Energy Magnitude (Me)
The moment magnitude (Mw) scale is a modern standard used to describe the total energy released. Unlike some other scales, Mw and the energy magnitude (Me) are better at reflecting the actual physical energy and potential for damage, especially in complex fault scenarios.
Differences in the crust underlying North America east of the Rocky Mountains makes that area more sensitive to earthquakes. Shown here: the 1895 New Madrid earthquake, M ~6, was felt through most of the central U.S., while the 1994 Northridge quake, though almost ten times stronger at M 6.7, was felt only in southern California. From USGS Fact Sheet 017–03.
Comparison of Seismic Measurements
The following table illustrates how different scales can yield different results for the same earthquake, highlighting why choosing the correct scale is vital for accuracy.
| Date | Depth (km) | Damage Level | Ms | Mw | mb | Me | Fault Type |
|---|---|---|---|---|---|---|---|
| 6 July 1997 | 23 | Barely felt | 6.5 | 6.9 | 5.8 | 6.1 | Interplate-thrust |
| 15 Oct. 1997 | 58 | Extensive | 6.8 | 7.1 | 6.8 | 7.5 | Intraslab-normal |
Frequently Asked Questions
What is the difference between magnitude and intensity?
Magnitude measures the energy released at the source of the earthquake, whereas intensity measures the strength of the shaking and the observed effects at a specific location.
Why are there so many different magnitude scales?
Different scales are necessary because earthquakes vary in depth, distance, and the types of waves they produce. Some scales work better for shallow quakes, while others are better for deep or very distant ones.
Which seismic waves cause the most damage?
Surface waves (Love and Rayleigh waves) typically cause the most severe damage, especially in shallow earthquakes, because they travel along the Earth's surface and can last for several minutes.
Why did the October 1997 Chile earthquake cause more damage than the July one?
Even though their moment magnitudes (Mw) were similar, the October quake was deeper and occurred on a different type of fault, resulting in a much higher energy magnitude (Me) and more extensive damage.
What is an isoseismal map?
An isoseismal map is a visual representation showing the areas where a specific level of earthquake intensity was felt.