Mercalli intensity scale

Introduction

The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. Unlike the Richter scale, which quantifies the energy released by an earthquake, the Mercalli scale assesses the effects and damage caused by the seismic event. Developed by Italian volcanologist Giuseppe Mercalli in 1902, the scale has undergone several modifications and is now commonly referred to as the Modified Mercalli Intensity (MMI) scale. It is a qualitative scale that categorizes earthquake intensity into twelve levels, ranging from imperceptible shaking to catastrophic destruction.

Historical Development

The Mercalli intensity scale was initially developed to provide a more detailed assessment of earthquake effects than previous scales. Giuseppe Mercalli's original version was based on earlier scales, such as the Rossi-Forel scale, but introduced a more nuanced classification system. Over time, the scale was refined by various seismologists, including Adolfo Cancani and August Heinrich Sieberg, leading to the Modified Mercalli Intensity scale that is widely used today. The MMI scale was further adapted for use in the United States by Harry O. Wood and Frank Neumann in 1931.

Structure and Description

The Mercalli intensity scale is divided into twelve levels, each denoted by Roman numerals from I to XII. Each level describes the severity of shaking and the resulting damage:

I (Not felt): Earthquake is not felt except by a very few under especially favorable conditions.
II (Weak): Felt only by a few people at rest, especially on upper floors of buildings.
III (Weak): Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake.
IV (Light): Felt indoors by many, outdoors by few during the day. At night, some awakened.
V (Moderate): Felt by nearly everyone; many awakened. Some dishes, windows broken.
VI (Strong): Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster.
VII (Very strong): Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures.
VIII (Severe): Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse.
IX (Violent): Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb.
X (Extreme): Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations.
XI (Extreme): Few, if any (masonry) structures remain standing. Bridges destroyed.
XII (Extreme): Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Application and Usage

The Mercalli intensity scale is primarily used in the field of seismology to assess the impact of an earthquake on the built environment and human perception. It is particularly useful in areas where instrumental data is scarce or unavailable. The scale provides valuable information for earthquake engineering, urban planning, and emergency response planning. By evaluating the intensity of shaking and damage patterns, engineers and planners can design structures that are more resilient to seismic events.

Photograph of a damaged building after an earthquake, with debris scattered around.

Comparison with Other Scales

The Mercalli intensity scale differs from other seismic scales, such as the moment magnitude scale and the Richter scale, in its focus on qualitative assessments rather than quantitative measurements. While the Richter and moment magnitude scales measure the energy released by an earthquake, the Mercalli scale evaluates the effects on people, structures, and the Earth's surface. This makes it a complementary tool for understanding the full impact of an earthquake.

Limitations and Challenges

Despite its usefulness, the Mercalli intensity scale has several limitations. Its reliance on subjective observations can lead to inconsistencies in intensity ratings, especially in sparsely populated areas where fewer observations are available. Additionally, the scale does not account for variations in local geology, building practices, or population density, which can significantly influence the perceived intensity and damage. These limitations highlight the importance of using the Mercalli scale in conjunction with other seismic measurement tools.

Modern Adaptations and Future Directions

In recent years, advancements in seismic instrumentation and data analysis have led to the development of automated systems for estimating Mercalli intensities. These systems use data from seismographs and accelerometers to generate real-time intensity maps, providing valuable information for emergency responders and the public. Future research may focus on integrating these automated systems with geographic information systems (GIS) to enhance the accuracy and utility of intensity assessments.