Earthquake Intensities

How bad a quake is on a scale from I to XII

seismic intensity map
Seismic intensity map of the 1811-1812 New Madrid, Missouri, earthquake series. U.S. Geological Survey image

The first measuring tool invented for earthquakes was the seismic intensity scale. This is a rough numerical scale to describe how severe an earthquake is in the place where you're standing—how bad it is "on a scale of 1 to 10." It's not hard to come up with a set of descriptions for intensity 1 ("I could barely feel it") and 10 ("Everything around me fell down!") and the gradations in between. A scale of this kind, when it's carefullly made and consistently applied, is useful even though it's based entirely on descriptions, not measurements.

Scales of earthquake magnitude (the total energy of a quake) came later, the result of many advances in seismometers and decades of data collection. While seismic magnitude is interesting, seismic intensity is more important: it's about the strong motions that actually affect people and buildings. Intensity maps are prized for practical things like city planning, building codes and emergency response.

To Mercalli and Beyond

Dozens of seismic intensity scales have been devised. The first to be widely used was made by Michele de Rossi and Francois Forel in 1883, and before seismographs were widespread the Rossi-Forel scale was the best scientific tool we had. It used roman numerals, from intensity I to X. In Japan, Fusakichi Omori developed a scale based on the types of structures there, such as stone lanterns and Buddhist temples. The seven-point Omori scale still underlies the Japanese Meteorological Agency's official seismic intensity scale.

Other scales came into use in many other countries.

In Italy, a ten-point intensity scale developed in 1902 by Giuseppi Mercalli was adapted by a succession of people. When H. O. Wood and Frank Neumann translated one version into English in 1931, they called it the Modified Mercalli scale. That has been the American standard ever since.

The Modified Mercalli scale consists of descriptions that range from the innocuous ("I. Not felt except by a very few") to the terrifying ("XII. Damage total . . . Objects thrown upward into the air"). It includes the behavior of people, the responses of houses and larger buildings, and natural phenomena. For instance, people's responses range from barely feeling ground motion at intensity I to everyone running outdoors at intensity VII, the same intensity at which chimneys begin to break. At intensity VIII, sand and mud are ejected from the ground and heavy furniture overturns.

Mapping Seismic Intensity

Turning human reports into consistent maps happens online today, but it used to be quite laborious. During the aftermath of a quake, scientists collected intensity reports as fast as they could. Postmasters in the United States sent the government a report every time a quake struck. Private citizens and local geologists did the same. If you're into earthquake preparedness, consider learning more about what quake investigators do by downloading their official field manual.

With these reports in hand, investigators of the U.S. Geological Survey then interviewed other expert witnesses, such as building engineers and inspectors, to help them map zones of equivalent intensity.

Eventually a contour map showing the intensity zones was finalized and published.

An intensity map can show some useful things. It can delineate the fault that caused the quake. It can also show areas of unusually strong shaking far from the fault. These areas of "bad ground" are important when it comes to zoning, for instance, or disaster planning or deciding where to route freeways and other infrastructure.

The State of the Art

In 1992 a European committee set out to refine the seismic intensity scale in the light of new knowledge. In particular, we have learned a great deal about how different kinds of buildings respond to shaking—in effect, we can treat them like amateur seismographs. In 1995 the European Macroseismic Scale (EMS) was widely adopted across Europe. It has 12 points, the same as the Mercalli scale, but it is much more detailed and precise.

It includes many pictures of damaged buildings, for instance.

Another advance was being able to assign harder numbers to intensities. The EMS includes specific values of ground acceleration for each intensity rank. (So does the latest Japanese scale.) The new scale cannot be taught in a single lab exercise, the way the Mercalli scale is taught in the United States. But those who master it will be the best in the world at extracting good data from the rubble and confusion of an earthquake's aftermath.

Forward into the Past

The study of earthquakes gets more sophisticated every year, and thanks to these advances the oldest research methods work better than ever. The nice machines and clean data make for good fundamental science. But one great practical benefit is that we can calibrate all kinds of earthquake damages against the seismograph. Now we can extract good data from human records where—and when—there are no seismometers. Intensities can be estimated for earthquakes all the way through history, using old records like diaries and newspapers.

Earth is a slow-moving place, and in many places the typical earthquake cycle takes centuries. We don't have centuries to wait, so deriving reliable information about the past is a valuable task. Take a look at what documentary evidence has told us about America's greatest earthquake, the 1811-1812 New Madrid shocks in the Missouri wilderness. Ancient human records are much better than nothing, and sometimes what we learn about past seismic events is almost as good as having seismographs there.