Earthquakes are unique disasters because, unlike disasters directly related to weather and atmospheric conditions, such as tornadoes, hurricanes, and wildfires, earthquakes are entirely independent of the weather. This independence makes them almost impossible to predict, which is part of the reason they can be so dangerous.
The most powerful earthquake in the United States, as of 2025, was the 1964 Alaska earthquake, sometimes called the Good Friday earthquake because it occurred on Good Friday. More than 130 people died as the magnitude 9.2 quake rattled along more than 600 miles of the Aleutian Megathrust fault, causing more than $870 million (adjusted) in damages. Of those deaths, all but 15 were attributable to multiple tsunamis triggered by the quake in more than 20 countries, including one in Antarctica.
This article is the fourth in a series of charts providing information on tornadoes, hurricanes, wildfires, earthquakes, blizzards, and volcanoes.
Magnitude - How Big was the Earthquake?
Magnitude and intensity are two methods of measuring the impacts of an earthquake. The magnitude of an earthquake refers to the size of a quake at its source, sometimes referred to as the epicenter. This value is a single number and does not change based on location. An earthquake’s magnitude used to be measured by the Richter scale. However, according to the United States Geological Survey (USGS), this scale fell out of favor due to its limited applicability outside the area of immediate impact; now, the Richter scale measures the local magnitude (ML) and is only used to describe smaller earthquakes where it is often one of the only magnitudes that can be measured by a seismic instrument.
Currently, the USGS uses the moment magnitude (Mw) to measure the size of an earthquake. First, geologists calculate the seismic moment, which is calculated as a product of the rigidity, or strength of the rock, the area of the fault that moved, and how far it “slipped.” That seismic moment is then converted to a Richter-like scale, ranging from 1.0 to 10.0, for the final result. The magnitude may also be expressed in terms of the energy released by an earthquake, or the energy magnitude (Me). In the table below, the moment magnitude is listed next to the corresponding amount of energy released by an earthquake of that size. It is expressed in terms of the amount of trinitrotoluene (TNT) that would release the same amount of energy if detonated; for example, a magnitude 3.0 earthquake would release the same amount of energy as detonating 1,800 kg (almost 4,000 pounds) of TNT.
Intensity - How Hard did the Ground Shake?
The intensity of an earthquake, on the other hand, describes how the ground shakes during an earthquake. Unlike the (moment) magnitude of an earthquake, which is the same no matter where the quake is felt, the intensity of an earthquake varies based on multiple factors. The preferred measure of intensity, the Modified Mercalli (MM) scale, is based on specific descriptions of physical damage. These values, expressed as Roman numerals from I to X, are based on actual observations of the physical damages caused by an earthquake. Determining the MM rating of more powerful earthquakes—VIII and higher—may require the input of one or more structural engineers.
One of the key factors in the intensity of an earthquake is the depth of the epicenter. According to the USGS website, two earthquakes of roughly the same magnitude may have vastly different intensity values if one occurs deeper in the earth. The example given on that website compares the Northridge, CA earthquake (M6.7) to the Nisqually, WA earthquake (M6.8). Though the Nisqually earthquake had a slightly higher magnitude, and therefore released more energy, the Northridge quake was more intense because it occurred between three and eleven miles beneath the earth’s surface, compared to the 30 to 36 mile depth of the Nisqually event.