Modified Mercalli Intensity scale

Introduction

The Modified Mercalli Intensity (MMI) scale is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake at a specific location. Unlike the Richter scale, which quantifies the energy released by an earthquake, the MMI scale assesses the effects and damage caused by the seismic event. It is a qualitative scale that ranges from I (not felt) to XII (total destruction), providing a detailed description of the earthquake's impact on people, buildings, and the Earth's surface.

Historical Background

The Modified Mercalli Intensity scale is an adaptation of the original Mercalli Intensity scale, developed by Italian volcanologist Giuseppe Mercalli in 1902. The original scale was based on earlier intensity scales, including the Rossi-Forel scale. Over time, the Mercalli scale was modified to better suit the needs of seismologists and engineers, leading to the development of the Modified Mercalli Intensity scale in the 1930s by American seismologists Harry O. Wood and Frank Neumann. The modifications aimed to provide a more comprehensive and standardized assessment of earthquake effects.

Scale Description

The MMI scale consists of twelve levels, each with specific criteria describing the observed effects of an earthquake. These levels are:

**I (Not felt):** The 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. Delicately suspended objects may swing.

**III (Weak):** Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly.

**IV (Light):** Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building.

**V (Moderate):** Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

**VI (Strong):** Felt by all; many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

**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; some chimneys broken.

**VIII (Severe):** Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

**IX (Violent):** Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

**X (Extreme):** Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

**XI (Extreme):** Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

**XII (Extreme):** Total damage – practically all works of construction are damaged or destroyed. The ground is distorted. Lines of sight and level are distorted. Objects thrown into the air.

Application and Usage

The MMI scale is widely used by seismologists, engineers, and emergency response teams to assess the impact of earthquakes. It provides valuable information for understanding the severity of an earthquake's effects in different areas, which is crucial for disaster response and recovery efforts. The scale is also used in the design and construction of earthquake-resistant structures, as it helps engineers understand the potential damage that different levels of shaking can cause.

Comparison with Other Intensity Scales

The Modified Mercalli Intensity scale is one of several intensity scales used to describe the effects of earthquakes. Other scales include the European Macroseismic Scale (EMS) and the Japan Meteorological Agency (JMA) seismic intensity scale. While these scales share similarities with the MMI scale, they have been adapted to suit regional needs and conditions. For example, the EMS scale includes additional criteria for assessing damage to buildings, while the JMA scale provides a more detailed description of the intensity of shaking.

Limitations and Challenges

While the MMI scale is a valuable tool for assessing earthquake intensity, it has certain limitations. The scale is subjective, as it relies on human observations and reports of earthquake effects. This can lead to variations in intensity assessments, particularly in areas with sparse populations or limited infrastructure. Additionally, the MMI scale does not provide a direct measure of the energy released by an earthquake, which is a critical factor in understanding the overall impact of a seismic event.

Advances in Seismic Intensity Measurement

Recent advances in technology have led to the development of more sophisticated methods for measuring seismic intensity. Instruments such as accelerometers and seismometers provide quantitative data on ground motion, which can be used to supplement and refine MMI assessments. These instruments are capable of detecting and recording the intensity of shaking with high precision, allowing for more accurate and consistent evaluations of earthquake effects.

Conclusion

The Modified Mercalli Intensity scale remains an essential tool for understanding the impact of earthquakes on people, buildings, and the environment. Despite its limitations, the scale provides valuable insights into the effects of seismic events and plays a crucial role in earthquake preparedness and response efforts. As technology continues to advance, the integration of quantitative data with traditional intensity assessments will enhance our ability to evaluate and mitigate the risks associated with earthquakes.