An earthquake must reach magnitude 8.0 or higher to be classified as “great.” This is the highest category in the standard system seismologists use to rank earthquakes by size, sitting above “major” (7.0–7.9), “strong” (6.0–6.9), and several smaller tiers. On average, the world experiences about one great earthquake per year.
How Earthquakes Are Classified by Size
Seismologists group earthquakes into named classes based on their magnitude. The standard classification runs from “minor” (3.0–3.9) up through “light,” “moderate,” “strong,” and “major,” with “great” reserved for anything at 8.0 or above. There is no upper boundary on the great category. A magnitude 8.0 and a magnitude 9.5 are both considered great, even though the 9.5 releases roughly 180 times more energy.
The threshold matters because magnitude 8.0 marks a dramatic shift in what an earthquake can do. Below it, damage tends to be regional. Above it, a single event can reshape coastlines, trigger ocean-crossing tsunamis, and cause destruction across hundreds of kilometers.
Why Moment Magnitude Is Used for Great Earthquakes
The original Richter scale, developed in the 1930s, works well for small and moderate earthquakes but runs into a problem called “saturation” at higher magnitudes. Seismographs used for the Richter scale measure seismic waves at specific frequencies, but extremely large earthquakes release most of their energy at lower frequencies than those instruments capture. The result is that a magnitude 8.5 and a magnitude 9.0 earthquake can look deceptively similar on the Richter scale, even though the larger one released several times more energy.
To solve this, seismologists Thomas Hanks and Hiroo Kanamori developed the moment magnitude scale in the 1970s. Instead of relying on a single wave measurement, moment magnitude accounts for multiple types of seismic waves across a wider range of frequencies. It also connects directly to the physical characteristics of the fault itself: how far the rock slipped, the size of the area that ruptured, and the strength of the surrounding material. The two scales are calibrated to roughly match up to about magnitude 7.0, but above that, moment magnitude gives a far more accurate picture. Every great earthquake you see reported today uses moment magnitude.
How Rare Are Great Earthquakes?
Based on records going back to about 1900, the world averages roughly 16 major-or-above earthquakes per year. Of those, about 15 fall in the magnitude 7.0–7.9 range. Only one, on average, reaches 8.0 or higher. Some years see two or three; other years see none.
Magnitude 9.0 events are far rarer still. Only five have been recorded since modern seismology began: Chile in 1960 (magnitude 9.5), Alaska in 1964 (9.2), Sumatra in 2004 (9.1), Japan in 2011 (9.1), and Kamchatka in 1952 (9.0). These represent the most extreme end of what Earth’s faults can produce.
What Makes a Great Earthquake So Destructive
The defining feature of a great earthquake is the sheer length of fault that breaks at once. For a fault to produce a magnitude 8.0 event, it needs to rupture across hundreds of kilometers. If the entire San Andreas Fault in California were to break end to end, roughly 1,400 kilometers, with about 10 meters of average slip, the USGS estimates it would produce a magnitude 8.47 earthquake. The 1960 Chilean earthquake ruptured a fault area of about 160,000 square kilometers, with rock on one side of the fault lurching an average of 24 meters past the other side. That fault plane stretched roughly 800 kilometers long and 200 kilometers wide.
This scale of rupture means the shaking lasts far longer than in smaller earthquakes. A magnitude 6.0 might shake intensely for 10 to 20 seconds. A great earthquake can produce strong ground motion for minutes. The 2011 Japan earthquake shook for approximately six minutes. That prolonged shaking is what causes buildings that might survive a brief jolt to progressively weaken and collapse.
Great earthquakes that occur beneath the ocean floor also displace enormous volumes of water, generating tsunamis that can cross entire ocean basins. The 2004 Sumatra earthquake sent waves that killed more than 280,000 people across South Asia and East Africa. The 1952 Kamchatka earthquake, thousands of kilometers from Hawaii, still caused over $1 million in tsunami damage there.
Is There a Maximum Possible Magnitude?
In theory, magnitude has no cap, but in practice, Earth’s faults have physical limits. A magnitude 10.0 earthquake would require roughly 14,000 kilometers of connected fault to rupture simultaneously, with an average slip of 30 meters. No known fault system on Earth is long enough and continuous enough to produce that. The South American subduction zone, one of the longest fault systems on the planet at about 6,400 kilometers, could theoretically produce a magnitude 9.86 if it ruptured end to end with 40 meters of slip. That’s close to the practical ceiling. The 1960 Chilean earthquake, at magnitude 9.5, remains the largest ever recorded and is likely near the upper bound of what current plate tectonics can generate.