Earthquake Magnitude and Intensity

Magnitude and intensity are both related to the size of an earthquake, but they each measure different aspects. One is measured using seismometers; the others is felt. Let's use a lightbulb as an analogy.

Magnitude and intensity are different, yet related concepts. The size of an earthquake, or magnitude, is a single value, while seismic intensity, a measure of the strength of shaking at a specific location, varies depending on location. Three learning options provide hands-on activities to understand the differences between an earthquake’s magnitude and shaking intensity using different wattages of light bulbs, comparing factors affecting shaking intensity using USGS ShakeMaps and analyzing “Did You Feel It” personal accounts to create a shake map.

Magnitude (M) describes the size of an earthquake. The energy released in an earthquake relates to the difference in magnitude. Each step in magnitude represents a 32 times increase in the amount of energy released.

Breaking spaghetti illustrates this concept. If a Pasta Quake M5 earthquake is one spaghetti noodle, 32 noodles is a M6 earthquake! And 1024 noodles is a M7 earthquake - try to break that with your hands!

Understanding the magnitude change, thus the relative energy released from say, magnitude 7 to magnitude 8 can be challenging. Dr. Robert Butler (Univ. of Portland) uses spaghetti to illustrate the concept by breaking pasta to show how each step up in magnitude represents a huge jump in the size of the pasta bundles. Each step in magnitude is represented by 32 times more spaghetti noodles.

Why do seismologists change the magnitude of an earthquake? Obtaining an accurate preliminary magnitude can be difficult due not only to the complex processes that occur deep within the Earth, but because there are over a dozen techniques of for calculating the magnitude of an earthquake. 

There is often confusion surrounding the terminology of earthquakes. Large earthquakes are usually followed by hundreds and even thousands of smaller earthquakes, called aftershocks. 

The "moment magnitude" scale has replaced the Richter scale for large earthquakes. Scientists have developed far-more sensitive seismometers that, with faster computers, have enabled them to record & interpret a broader spectrum of seismic signals than was possible in the 1930's, when the Richter magnitude was developed. Find out what scientists learn from seismograms.

Seismic intensity is the shaking you experience during an earthquake. An earthquake's magnitude is a measure of the energy released and is the same for all locations. Unlike magnitude, the intensity of shaking you feel depends on your location.

In earthquake-prone regions, buildings can be designed resist being deformed, The way the building is constructed and your position in the building can have an impact on the shaking you feel. Learn more about construction as well as mitigation practices.

The rolling motion of a seismic wave may move at the same frequency of a building. The structure will then sway back and forth during the seismic vibration. A building's architectural design, height (how many stories), and construction materials affect resonance, or the seismic oscillation. Buildings engineered with earthquake shaking in mind will withstand resonance and serious damage.