Earthquakes: Magnitude and Intensity

The last days there have been many news about earthquakes at different locations. However, some of the news mix the concepts of magnitude and intensity. Magnitude and Intensity measure different characteristics of earthquakes.  Just some days ago an introduction about earthquakes was published. The present post will briefly explain the difference between an earthquake intensity and its magnitude.

Magnitude

Magnitude measures the energy released at the source of the earthquake and is determined from measurements on seismographs. It is a quantitative measure of the size of the earthquake at its source. The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake. Richter scale was proposed by Prof. Charles F. Richter. The most important thing to remember about Richter magnitude is that it is a logarithmic scale, meaning that an increase of one in magnitude corresponds to a factor of ten increase in the amplitude of ground motion. Mathematically, an earthquake of magnitude x results in seismic waves with amplitudes proportional to 10^x. Image 1 shows how to get the Richter magnitude of an earthquake by reading a seismogram.

Image 1. Method to get the Richter magnitude of an earthquake.

Image source: Department of geology Southern Illinois University

Intensity

Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment. The intensity of an earthquake at a particular locality indicates the violence of earth motion produced there by the earthquake. It is determined from reported effects of the tremor on human beings, furniture, buildings, geological structure, etc.  is variable over the area affected by the earthquake, with high intensities near the epicentre and lower values further away. This phenomenon is the result of seismic-wave attenuation, which is the reduction in wave amplitude and wave energy as they travel away from their source. In order to study the patterns of earthquake intensity during different earthquakes, a system has been devised to assign specific numbers to different levels of shaking. The Mercalli scale was developed in 1902 and modified in the 1930s. The Mercalli scale assigns a numerical value, from Roman numeral I to XII, to the intensity of seismic shaking at any one particular location. Lines of equal intensity (isoseismal lines) are not perfect circles. Image 2 shows the isoseismal of the 1947 Michigan earthquake.

Image 2. Isoseismal of the 1947 Michigan earthquake

Image source: Department of geology Southern Illinois University

A practical example with an everyday life situation may help to understand the concepts. Magnitude can be likened to the power of radio or television waves sent out from a broadcasting station. Intensity is how well you receive the signal, which can depend on your distance from the energy source, the local conditions, and the pathway the signal has to take to reach you.

References and Additional Reading

Department of geology Southern Illinois University

Earthquake hazards program

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