Modified Mercalli intensity scale

Earthquakes can be very different. Some shakes are tiny. Others are very big. One way to measure them is by how they feel.

This scale looks at the shaking at one spot. It asks how much things move. It also looks at what breaks.

At first, people might not feel a shake. They might just see a glass swing. Later, they might feel it like a heavy truck.

Big shakes can move heavy furniture. They can even break walls. Some shakes are so strong they break bridges.

This scale helps us learn about old shakes. It is a great way to study the Earth.

Earthquakes can be very different. Some release a lot of power. Others release very little. Scientists use two main ways to measure them. One way is magnitude. Magnitude measures the total strength of the quake. The other way is intensity. The Modified Mercalli intensity scale (MMI) measures intensity. Intensity is how much the ground shakes at one spot.

Intensity can change from place to place. It is usually strongest near the epicentre. The epicentre is the point on the surface above the quake. Shaking gets weaker as you move away. The depth of the quake also matters. Deep quakes spread their power over a large area. This can make the shaking feel weaker at the surface.

This scale uses what people feel and see. Small shakes might only make a glass swing. Stronger shakes can move heavy furniture. Very big shakes can break bridges or cause landslides.

Because it uses reports from people, it helps us study old quakes from long ago. This was before we had machines to measure them.

An earthquake can be measured in two very different ways. One way is magnitude, which tells us the total energy released. The other way is intensity, which tells us how much the ground shakes at one specific spot. The Modified Mercalli intensity scale, or MMI, is used to measure this intensity. It is helpful because shaking can change depending on where you stand. You might feel a strong shake near the epicentre, which is the point on the surface above the quake. However, the shaking usually gets weaker as you move further away.

How the shaking works depends on many different factors. The depth of the earthquake is very important for what people feel. A deep earthquake spreads its energy over a huge area, so the surface shaking might be less intense. Shallow earthquakes can cause much stronger shaking at the surface. The type of ground also matters a lot. Shaking can be made stronger in certain types of soil or sedimentary basins. This means the same earthquake could feel different in two nearby towns.

This scale has a long history of being improved by many people. An Italian scientist named Giuseppe Mercalli first made an intensity scale in 1883. He later published a second version in 1902 that was much more popular. In 1904, Adolfo Cancani suggested adding two more levels for very large disasters. Later, August Heinrich Sieberg added more details to these levels between 1912 and 1923. This version became known as the Mercalli–Cancani–Sieberg scale. It was used a lot in Europe for many years.

In 1931, Harry O. Wood and Frank Neumann translated the scale into English. They changed some of the descriptions to create the Modified Mercalli intensity scale. Later, Charles Francis Richter revised it in 1956. He called it the modified Mercalli scale of 1956 so people would not confuse it with his own Richter scale. Today, agencies like the United States Geological Survey use a version based on the work of Wood, Neumann, Stover, and Coffman. They use these descriptions to help understand how much damage might happen.

The scale uses simple observations to rank the shaking. Low levels describe things that people feel, like a swinging lamp or a rocking car. Higher levels describe damage to buildings and the land. For example, a level IV shake might make dishes or windows rattle. A level IX shake is violent and can shift buildings off their foundations. Very high levels, like XI or XII, can destroy bridges or cause the ground to crack. This way of measuring helps us study old earthquakes from before we had modern machines.

The Modified Mercalli intensity scale, often abbreviated as MMI, MCS, or MM, measures the effects of an earthquake at a specific location. It is important to distinguish this from seismic magnitude. Magnitude scales, such as the widely used moment magnitude scale, measure the inherent strength or total energy released by an earthquake. In contrast, the MMI scale measures the intensity of shaking experienced on the surface at any particular point. Because shaking can vary wildly from one street to the next, intensity provides a localized view of an earthquake's impact.

Several physical factors dictate how much shaking a person feels at the surface. First, the depth of the earthquake, known as the hypocentre, plays a major role. Deeper earthquakes spread their energy throughout a larger volume of the Earth. This means the energy reaching the surface is spread across a wider area but is often less intense. Shallow earthquakes interact more directly with the surface, often causing more violent shaking. Additionally, the local geology can amplify the effect. Shaking can be strengthened in sedimentary basins or in certain types of unconsolidated soils.

The scale is organized into different levels that describe human perception and structural damage. The lower degrees generally focus on how the earthquake is felt by people. For example, level II is described as weak and felt only by a few people at rest. Level IV is considered light, where dishes and windows might be disturbed. As the numbers increase, the scale shifts toward describing observed structural damage. Level VII is very strong, causing considerable damage to poorly built structures. Level IX is violent, capable of shifting buildings off their foundations and causing liquefaction, where soil behaves like a liquid.

At the highest levels, the scale describes extreme destruction. Level X involves the destruction of most masonry and frame structures. Level XI describes a state where few masonry structures remain standing and bridges are destroyed. Level XII represents total damage, where waves are seen on ground surfaces and objects are thrown into the air. Because the categories for "catastrophe" and "enormous catastrophe" are used so infrequently, the United States Geological Survey (USGS) often merges them into a single category called "Extreme," abbreviated as X+.

The history of the scale is a long process of scientific refinement. Italian volcanologist Giuseppe Mercalli formulated his first intensity scale in 1883. This version was an adaptation of the ten-degree Rossi–Forel scale. He published a second version in 1902 that expanded the descriptions of each degree. In 1904, Adolfo Cancani proposed adding two extra degrees for very strong events. August Heinrich Sieberg later augmented these descriptions between 1912 and 1923. He added peak ground acceleration indicators for each degree, creating the Mercalli–Cancani–Sieberg scale.

In 1931, Harry O. Wood and Frank Neumann translated the scale into English. They modified the descriptions and removed the acceleration criterion, creating the modified Mercalli intensity scale of 1931, or MM31. This version is sometimes called the Wood–Neumann scale. In 1956, Charles Francis Richter revised the scale for his textbook, *Elementary Seismology*. He called it the modified Mercalli scale of 1956 to avoid confusion with his own Richter scale. Today, the USGS uses a version that is nominally the MM31 but incorporates modifications by Carl Stover and Jerry Coffman.

Understanding the difference between magnitude and intensity is vital for seismic science. Magnitude is a single value representing the energy liberated by the earthquake. Intensity, however, is a variable value that depends on distance from the epicentre and local terrain. For instance, a magnitude 8.2 earthquake in Bolivia in 1994 was 631.3 km deep. Despite its massive energy, its maximum felt intensity was only VI. Conversely, a much smaller magnitude 2.2 earthquake in England in 1865 was only 1 km deep. This shallow event resulted in a much higher intensity of VIII.

Because the MMI scale is based on empirical observations rather than purely instrumental data, it is a powerful tool for historians. It allows scientists to estimate the magnitude and location of historical earthquakes that occurred before modern seismographs existed. By looking at the degree and extent of reported damage, researchers can compare old events to known local earthquakes. This makes the scale essential for seismic hazard assessment. It helps experts understand the potential risks to the built environment in different geological regions.