How Earthquakes Form and How Their Magnitude Is Measured

Earthquakes are a routine search topic worldwide, and interest spikes every time a notable tremor makes headlines. Two questions recur most often: what physically causes the ground to shake, and what the magnitude number reported afterward actually means. Both answers come from decades of seismology documented by agencies such as the U.S. Geological Survey (USGS) and research consortia like IRIS, the Incorporated Research Institutions for Seismology.

What causes an earthquake

According to the USGS, an earthquake is caused by a sudden slip on a fault. Earth's crust is divided into large tectonic plates that are constantly, if slowly, moving. Where plates meet, friction causes their edges to stick together even as the plates keep pushing. The USGS cites California's Pacific and North American plates as an example: they grind past each other at roughly two inches per year along the San Andreas Fault, a boundary that runs more than 650 miles long and over 10 miles deep.

While a fault's edges remain stuck, the plates keep moving, which bends and stresses the rock on either side. That stored energy builds until it finally exceeds the friction holding the fault together. When the fault "unsticks," the stuck rock springs back into a less strained position in a sudden slip, and the energy that had been stored radiates outward in seismic waves, described by the USGS as spreading "like ripples on a pond."

Not every fault behaves the same way over time. The USGS notes that in some areas faults move through near-constant creep, releasing small amounts of strain in frequent minor shocks. Where creep is not constant, strain can accumulate for decades or centuries before releasing all at once, producing a larger earthquake.

Focus, epicenter and seismic waves

The point below the surface where an earthquake begins is called the hypocenter, or focus; the point on the surface directly above it is the epicenter, according to the USGS. From the hypocenter, two main types of waves travel outward: P waves, which compress and stretch rock in the direction they travel and arrive first at monitoring stations, and S waves, which move the ground side to side and arrive after P waves. The USGS explains that the time gap between the arrival of these two wave types at different seismograph stations lets scientists calculate how far away an earthquake occurred.

How magnitude is measured today

Magnitude describes the size of the earthquake at its source; an earthquake has a single magnitude value, according to the USGS. This is different from intensity, which measures shaking severity at a specific location and can vary widely depending on distance from the epicenter, depth, and local ground conditions — IRIS compares magnitude to a lightbulb's wattage and intensity to the brightness felt at different distances from that bulb.

The scale most familiar to the public, the Richter scale, was developed by seismologist Charles F. Richter in 1934, according to Michigan Technological University. It calculated magnitude from the amplitude of the largest wave recorded on a specific seismometer type, adjusted for distance from the earthquake. The USGS states that this approach does not accurately measure very large earthquakes: some events get underestimated, others overestimated.

Seismologists now primarily use the moment magnitude scale (Mw), which the USGS and Michigan Tech describe as preferred because it works accurately across the full range of earthquake sizes and is applicable globally. Moment magnitude is derived from a physical quantity called seismic moment, calculated as rigidity of the rock multiplied by the area of the fault that slipped multiplied by the distance the fault moved. That moment value, estimated from seismogram data and geological measurements, is then converted into the magnitude number reported publicly.

Why each whole number matters so much

Magnitude scales are logarithmic. The USGS and Michigan Tech both state that each increase of one whole number corresponds to a tenfold increase in the ground-motion amplitude recorded by seismic instruments, and roughly a 32-fold increase in energy released. Michigan Tech gives illustrative energy comparisons: a magnitude 1 earthquake releases energy roughly equivalent to 6 ounces of TNT, while a magnitude 8 earthquake releases energy equivalent to about 6 million tons of TNT.

Magnitude versus intensity

Because magnitude is a single number describing the energy released at the source, it does not by itself describe how strongly a particular town or building shook. Shaking severity at specific locations is instead described using intensity scales such as the Modified Mercalli Intensity Scale, which the USGS says is expressed in Roman numerals and depends on factors including distance from the epicenter, earthquake depth, and local geology.