Disaster Intensity Scales

Disaster Scales for Earthquakes, Hurricanes, Tornados, and Volcanoes

The Volcanic Explosivity Index, the Richter Scale, the Saffir-Simpson Scale, and the Fujita Tornado Scale are used to warn communities of natural hazards.

Volcanic eruptions, earthquakes, hurricanes, and tornadoes are all forces of nature that have had negative impacts on communities all over the world. In an effort to raise awareness and mitigate the hazards associated with these natural processes, scientists have created natural hazard scales. The size and type of a volcanic eruption are classified using the Volcanic Explosivity Index; the magnitude of an earthquake is classified using the Richter Scale; the size of a hurricane is classified using the Saffir-Simpson Scale, and the size of a tornado is classified using the Fujita Tornado Scale.

Volcanic Explosivity Index

The Volcanic Explosivity Index, also referred to as VEI, was created by volcanologists Christopher Newhall and Stephen Self in 1982. In an article titled, “The Volcanic Explosivity Index (VEI) An Estimate of Explosive Magnitude for Historical Magnitude” published in the Journal of Geophysical Research, Newhall and Self introduced their scale to the world of volcanology as a way to classify the size of historical eruptions. Furthermore, the scale can be used to forecast the size of a volcano’s future eruptions based on the VEI magnitude of its historical eruptions.

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The VEI scale ranges from a classification of 0 to 8. A VEI 0 eruption is the smallest kind of eruption and is considered a non-explosive eruption, such as the passively erupting volcanoes of Hawaii. A VEI 8 eruption is the biggest kind of eruption and is considered a mega-colossal eruption, such as the eruption of Toba Volcano 75,000 years ago.

Earthquakes – Richter Scale

The Richter magnitude Scale, or local magnitude scale, was developed in 1935 by seismologist Charles Richter as a way to mathematically calculate the magnitude of an earthquake. The Richter Scale uses information from seismographs, such as the amplitude of seismic waves, to assign a magnitude value. These values go from 0 to 10 and are expressed as whole numbers and decimals – the smaller the number, the smaller the magnitude of the earthquake. For example, earthquakes with a magnitude of 2.0 or less are usually called microquakes and cannot be felt by people. Earthquakes with a magnitude of 4.5 or greater can be felt by people in the region and can usually be recorded by seismographs all over the world.

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Hurricanes – Saffir-Simpson Scale

The Saffir-Simpson Scale is used to assign an intensity to a hurricane, which is more often referred to as a “category”. Using the Saffir-Simpson Scale, meteorologists assign a category to a hurricane ranging from 1 to 5, with 5 being the most intense hurricane. This scale is used to give an estimate as to the potential wind speeds and storm surges that will accompany a hurricane. A category 1 hurricane usually has wind speeds of 74-95 miles per hour and storm surges of four to five feet above normal tide levels. A category 5 hurricane has wind speeds greater than 155 miles per hour and storm surges greater than 18 feet above normal tide levels.

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The Fujita Tornado Scale

The Fujita Tornado Damage Scale was created by meteorologist Theodore Fujita in an attempt to classify and size and intensity of tornadoes based on the damage tornadoes cause to man-made structures. In 1971, Fujita wrote a paper titled, “Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity” for the Satellite and Mesometeorology Research Project at the University of Chicago. In this paper, Fujita attempted to relate damaged structures to tornado wind speed. The scale values range from F0 to F5, with an F0 tornado causing light damage and an F5 tornado causing incredible damage.

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Natural Disasters of the 20th Century

The twentieth century saw some of the most violent earthquakes and destructive volcanic eruptions.

They can strike with little or no warning, and leave nothing but destruction in their wake. Throughout history, humans have always been at the mercy of Mother Nature and over the past century the ground has rumbled with terrifying ferocity, and dormant earthquakes have awakened with explosions loud enough to be heard hundreds of miles away. This list takes a closer look at some of the more devastating occurrences that have taken place between 1900 and 1999.

Earthquake – San Francisco, California. April 18, 1906 5:12 a.m.

At this point in its history, the city of San Francisco had a population of about 400,000 people. When the first jolt was felt around the Bay Area, it was followed by severe shaking some 25 seconds later which was also punctuated by violent shocks. Fires were burning everywhere after the quake subsided and lasted for three days, and were caused mostly by damaged electrical wiring, broken gas mains, and overturned stoves. 3,000 people died, 225,000 people were homeless and according to the NOAA (National Oceanic and Atmospheric Administration), the property damage totaled a whopping $400,000,000 (1906 dollars).

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Earthquake – Tangshan, China. July 28, 1976 3:42 a.m.

Officially called the worst earthquake to hit China in the 20th century, it measured 7.8 on the Richter scale and laid waste to Tangshan, which at that time had one million people. Although it lasted for approximately fifteen seconds, the resulting death toll was horrific. 248,000 perished, and more than 160,000 were injured. Most of the residents lived in one-story brick structures with solid concrete roofs which collapsed instantly, killing thousands. Almost all of them were sleeping and didn’t have time to take cover. The Chinese government refused foreign aid and barred any foreign journalists from entering Tangshan until 1983.

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Volcano – St. Pierre, Martinique. May 8, 1902 7:50 a.m.

On the northern side of this Caribbean island, Mt. Pelée had been rumbling ominously since April 23rd, and over the next few days, the residents began to panic and attempted to leave St. Pierre, a small but prosperous colonial city at the base of the volcano. In a cruel twist of fate, an election was scheduled for May 11th and Governor Louis Mouttet convinced the editor of the daily newspaper Les Colonies to assure residents that in fact there was nothing to worry about. When the volcano erupted, a huge black cloud consisting of superheated gas, ash, and rock headed down a v-shaped notch along the south side of the mountain at 100 miles per hour, in effect making St. Pierre a target on the end of a gun sight. The cloud advanced all the way to the harbor and destroyed at least twenty ships anchored offshore. 28,000 people perished from the searing temperatures and the ash from the pyroclastic flow (an avalanche of hot gas, pumice, and rock fragments). Only two people in St. Pierre were known to have survived this disaster.

Volcano – Skamania County, Washington State, U.S.A. May 18th, 1980 8:32 a.m.

On this pleasant Sunday morning, an earthquake measuring 5.1 on the Richter scale triggered the eruption of Mt. St. Helens, which before that day had been dormant for more than a century. Raging hot gases and small fragments of the mountain blasted in a northerly direction and swept along the ground at 300 miles per hour. Douglas fir trees, some of them 200 feet high, were stripped of their bark and snapped like toothpicks. Compared to other eruptions, the death toll was relatively light with 57 fatalities. However, thousands of elk, deer, and bears perished and the damage to highways, private homes, and railways was more than $1,000,000,000 U.S. Washington Governor Dixy Lee Ray flew over Mt. St. Helens on May 20th, and on her return stated, “I feel like I’ve just come back from the moon.”


  • U.S. Geological Survey
  • HistoryLink.org
  • National Geophysical Data Center
  • www.pbs.org

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