ArticlesWhen an earthquake makes the news, you usually hear a single number: a magnitude. Yet two people at different addresses can describe the same earthquake in completely different terms, one saying it barely rattled the dishes and the other saying it threw them off the shelves. Both can be right, because earthquake magnitude and earthquake intensity answer two different questions. Magnitude asks how big the earthquake was at its source. Intensity asks how strongly the ground shook in a particular place.
The short answer
The United States Geological Survey (USGS) puts the distinction plainly: magnitude is the size of the earthquake, and an earthquake has a single magnitude. The shaking it causes, by contrast, has many values that vary from place to place depending on distance, the type of surface material, and other factors [1]. Intensity is the name for that location-by-location shaking. New Zealand's GeoNet service makes the same point from a practical angle, noting that not all of the energy released will necessarily be felt at the surface, depending on the earthquake's depth, and that its intensity scale is a better indicator of effects on people and their surroundings [2].
A useful analogy is a light bulb. Its wattage is one fixed property of the bulb, much like magnitude. How bright a particular spot in the room looks depends on how far you stand from the bulb, what the walls are made of, and whether something blocks the light. That brightness is closer to intensity. The analogy is imperfect, since seismic waves behave differently from light, but it explains why one number can never tell you what your street experienced.
How magnitude came about
The idea of assigning an earthquake a size is less than a century old. According to the USGS, seismographs had existed since the 1890s, but it was in 1935 that Charles F. Richter of the California Institute of Technology introduced the concept of earthquake magnitude [1]. His definition originally applied only to California earthquakes within 600 kilometers of a particular type of seismograph. The basic idea was simple: if you know the distance from the instrument to the earthquake and you observe the maximum signal amplitude it recorded, you can rank the earthquake's inherent strength. Because the measurement is based on the logarithm of wave amplitude, the scale is logarithmic.
That logarithmic design has consequences that surprise many readers. The USGS explains that each whole-number increase in magnitude represents a tenfold increase in measured amplitude on a seismogram, and about 32 times more energy released [1]. A magnitude 7 earthquake therefore does not release a little more energy than a magnitude 6. It releases roughly 32 times as much. Two whole steps up, from 5 to 7, means about a thousand times the energy (32 multiplied by 32 is 1,024). This is arithmetic derived from the USGS figure rather than a separate claim from the source, but it shows why small differences in the headline number matter so much.
Why moment magnitude replaced the Richter scale
People still say Richter scale, but the USGS notes that in practice it is not commonly used anymore, except for small earthquakes recorded locally. Early magnitude scales based only on waveform amplitude were once thought to be equivalent, but for very large earthquakes some of them overestimate true size and some underestimate it [1]. Scientists therefore moved toward measures that describe the physical effects of the rupture rather than only the height of a wiggle on a recording.
The result is moment magnitude, abbreviated Mw. The USGS describes it as based on physical properties of the earthquake, derived from an analysis of all the recorded waveforms. First the seismic moment is computed as rigidity multiplied by area multiplied by slip. Rigidity is the strength of the rock along the fault, area is the portion of the fault that slipped, and slip is the distance the fault moved. That moment is then converted to a magnitude designed to be roughly equal to the Richter scale in the range where the two overlap [1].
The practical meaning is intuitive. A bigger patch of fault, stronger rock, or more movement all produce a larger magnitude. That is why great earthquakes are associated with long ruptures along major faults, while a small tremor may involve only a tiny fraction of a fault. Because the formula rests on the fault itself rather than on one instrument's reading, the USGS treats it as the more accurate measure for all but small, locally recorded earthquakes [1].
What intensity measures instead
Intensity is typically expressed in Roman numerals such as VI or X. The USGS says that traditionally it is a subjective measure drawn from human reports of felt shaking and damage. Data was once collected by postal questionnaire, then through web-based forms, and instrument readings at stations can also be used to estimate it [1]. In the United States the scale in use is the Modified Mercalli Intensity scale, though other countries use other scales [1].
GeoNet describes its New Zealand version in everyday language. At the low end, shaking is barely sensed by a very few people. In the middle, objects shift, glassware may break, and many people run outside. At the high end, the descriptions turn to damaged and destroyed weak buildings [2]. Notice that these descriptions are about observable effects: what people feel, what moves, what breaks. That is exactly why intensity is useful for emergency response and for judging how buildings performed, and why it cannot be reduced to one number for an entire earthquake.
A real comparison: Northridge and Nisqually
The USGS offers a vivid example of why magnitude and intensity can disagree. The magnitude 6.7 Northridge, California, earthquake produced more intense shaking over a wider area than the slightly larger magnitude 6.8 Nisqually, Washington, earthquake. The explanation given is depth: Northridge occurred closer to the surface, at roughly 3 to 11 miles, while the Nisqually hypocenter was deeper, at roughly 30 to 36 miles [1]. The shaking from a shallow source has less distance to travel before reaching buildings and people.
This is also why GeoNet emphasizes intensity in New Zealand, where earthquakes occur from near the surface to depths of more than 600 kilometers [2]. A deep event can have a large magnitude yet produce modest shaking on the surface, while a smaller shallow event directly beneath a town can be far more disruptive.
Local geology plays a role too. The USGS lists the type of surface geology beneath you and the direction in which the fault ruptured among the factors that shape intensity at a given site [1]. Two buildings a few blocks apart can experience different shaking because of the ground they stand on.
Reading earthquake reports more carefully
Once the two concepts are separate in your mind, news coverage becomes easier to interpret. A few habits help.
First, treat the magnitude as the earthquake's size and treat intensity as local impact. If a report gives a magnitude but nothing about shaking, it has told you about the source, not about your neighborhood. Second, look for depth. A shallow earthquake near a populated area generally deserves more concern than a deep one of the same magnitude, consistent with the Northridge and Nisqually comparison above. Third, remember that early magnitude estimates are revised as more data arrives. Agencies compute values from many recordings, so a first figure and a final figure can differ; this is a general feature of how measurements are refined rather than a claim from the sources cited here.
Fourth, remember the logarithmic scale when comparing events. Saying that a magnitude 8 is slightly bigger than a magnitude 7 understates it considerably. The USGS also notes that the many small and moderate earthquakes occurring frequently around the world release far less energy than a single great earthquake [1].
Finally, if you felt an earthquake, reporting it helps. Because intensity is built from human observations, felt reports are the raw material the USGS describes for building intensity maps [1]. Your experience at your address is data that magnitude alone cannot provide.
Where the sources differ in wording
It is worth flagging a small wording difference. GeoNet describes magnitude as the amount of energy released during an earthquake [2], while the USGS describes magnitude as the size of the earthquake and treats energy release as a related but separate way of measuring size. The USGS states that the energy magnitude and moment magnitude measure two different properties, so their values are not the same [1]. For everyday reading, the simple version is fine, but the more careful statement is that magnitude scales are different ways of quantifying size, and energy is one of several physical quantities connected to it.
Conclusion
Magnitude is the earthquake's single, source-based size. Intensity is the patchwork of shaking that people experience across a region. Keeping them apart explains why a moderate earthquake can be alarming near its center and unnoticed far away, why a deep large event may feel mild, and why a shallow moderate one can do damage. When you read the next report, ask two questions: how big was it, and how deep and how close was it to the place you care about?
Sources
[1] U.S. Geological Survey, Earthquake Magnitude, Energy Release, and Shaking Intensity - https://www.usgs.gov/programs/earthquake-hazards/earthquake-magnitude-energy-release-and-shaking-intensity
[2] GeoNet (New Zealand), Earthquake Intensity - https://www.geonet.org.nz/earthquake/intensity