How Earthquakes Are Measured: Magnitude vs Intensity

Magnitude: Measuring Energy at the Source

When scientists report that an earthquake had a MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. of 6.5, they are describing a single property of the earthquake itself — the total energy released at the source. Magnitude is calculated from measurements made by SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. instruments around the world, and it does not change depending on where you are. A magnitude 6.5 event in California is the same magnitude whether measured in Sacramento or Stockholm.

The most widely used magnitude scale today is the Moment Magnitude ScaleThe modern standard for measuring earthquake size (Mw), based on the seismic moment — the product of fault area, average slip, and rock rigidity. Accurate for all earthquake sizes. scale (abbreviated Mw), which measures the total "seismic moment" of an earthquake: the product of the fault area that ruptured, the average distance the fault slipped, and the rigidity of the rock. Scientists prefer it because it does not saturate at high values the way earlier scales did, making it reliable for earthquakes of all sizes. Use the Earthquake Energy Calculator to explore how different magnitudes translate to energy released and shaking radius.

Intensity: Measuring Effects at Your Location

Seismic IntensityA measure of the strength of shaking at a particular location, determined by observed effects on people, structures, and the natural environment. Decreases with distance from the epicenter. answers a completely different question: how hard did the ground shake at this particular spot? The same earthquake can produce very different intensities at different locations. At the EpicenterThe point on the Earth's surface directly above the hypocenter (focus) where an earthquake originates underground. Often reported as the earthquake's location in news reports., intensity may reach the highest levels on the scale. A hundred kilometres away, it might be much gentler. And in some distant city, sensitive people might barely feel it — or not feel it at all.

The standard measure of intensity in the United States is the Modified Modified Mercalli IntensityA 12-point scale (I-XII) that measures the observed effects of an earthquake at a specific location, from imperceptible (I) to total destruction (XII). Unlike magnitude, intensity varies by distance. scale (MMI), which uses Roman numerals from I to XII. MMI I means the quake was not felt at all; MMI V is felt by nearly everyone and may rattle windows; MMI VII causes significant damage to poorly built structures; MMI XII represents total destruction of virtually all buildings. Intensity data is collected by the USGS Did You Feel It? (DYFI)A USGS program that collects intensity reports from the public after earthquakes to create community-derived intensity maps. Allows anyone who felt an earthquake to submit a report. system, which aggregates thousands of reports from members of the public within minutes of any significant quake.

Why the Same Earthquake Has Different Intensities

Three main factors explain why intensity varies so widely. First, distance from the epicenter: shaking energy spreads out as waves travel, so intensity naturally decreases with distance. Second, Soil Amplification (Site Effect)The increase in shaking intensity caused by soft soil or sediment layers amplifying seismic waves. Structures built on soft soil can experience 2-10 times stronger shaking than those on bedrock.: soft sediments like bay mud, alluvial fans, and reclaimed land amplify shaking far more than hard bedrock. Certain areas of San Francisco built on former bay fill experienced intensities far higher than nearby bedrock neighbourhoods during the 1989 Loma Prieta earthquake, even though they were about the same distance from the epicenter. Third, building quality and local construction practices profoundly affect how much damage occurs at any intensity level.

Depth also matters. A shallow earthquake 10 kilometres deep will produce much more intense shaking at the surface directly above it than an identical-magnitude earthquake 100 kilometres down, because the energy has less distance to travel and spread out before reaching the surface.

The Logarithmic Nature of Earthquake Scales

One of the most commonly misunderstood aspects of earthquake measurement is that magnitude scales are logarithmic. Each whole-number step on the moment magnitude scale represents approximately 31.6 times more energy released. A magnitude 7.0 earthquake releases about 31.6 times more energy than a magnitude 6.0, and about 1,000 times more energy than a magnitude 5.0.

This means that while a magnitude 5.0 might feel significant to someone in its path, a magnitude 8.0 in the same region would release roughly one million times more energy. The Richter ScaleThe original logarithmic magnitude scale developed by Charles Richter in 1935 to measure local earthquake magnitude. Largely replaced by moment magnitude but still commonly referenced in media., which most people have heard of, was also logarithmic in this way, and its legacy continues to shape how the public thinks about earthquake sizes even though scientists now use Moment Magnitude ScaleThe modern standard for measuring earthquake size (Mw), based on the seismic moment — the product of fault area, average slip, and rock rigidity. Accurate for all earthquake sizes. for nearly all reporting.

How Scientists Calculate Magnitude and Intensity

Magnitude is calculated by analysing the SeismogramThe recorded output of a seismograph, showing ground motion as a function of time. Seismologists analyze seismograms to determine earthquake magnitude, depth, and location. — the record of ground motion recorded by a SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer.. Different magnitude formulas use different parts of the seismogram. Local magnitude (ML, the original Richter scale) uses the amplitude of the largest wave on a standard instrument. Surface wave magnitude uses the amplitude of surface waves at a period of about 20 seconds. Moment magnitude is derived from the long-period part of the seismogram and is increasingly calculated in near-real-time through automated systems.

Intensity is determined either by field surveys in the aftermath of a quake — where engineers and geologists inspect damage and interview residents — or increasingly through crowdsourced data from websites like USGS Did You Feel It?, which aggregates self-reported observations from tens of thousands of people. Modern ShakeMapA USGS product that displays the distribution of ground shaking intensity after an earthquake. Combines seismograph data, ground motion models, and 'Did You Feel It?' reports. products combine instrumental data with these reports and soil models to produce colour-coded maps of intensity within minutes.

Reading Earthquake Reports: A Practical Guide

When you see an earthquake reported in the news, here is how to interpret the key numbers. The magnitude tells you how large the event was at the source. A magnitude below 3.0 is usually imperceptible to humans. Magnitude 3–4 is felt by some people near the epicenter. Magnitude 5 can cause minor damage. Magnitude 6 is capable of significant damage. Magnitude 7 is a major earthquake. Magnitude 8 and above is a great earthquake that can affect entire countries.

The depth tells you how far below the surface the rupture began. Shallow earthquakes (less than 70 km) generally cause more surface shaking per unit of magnitude than deep ones. The intensity at your location depends on all these factors combined. If you felt an earthquake, reporting your experience through Did You Feel It? (DYFI)A USGS program that collects intensity reports from the public after earthquakes to create community-derived intensity maps. Allows anyone who felt an earthquake to submit a report. helps scientists refine intensity maps and better understand ground shaking in your area.