The Modified Mercalli Intensity (MMI) Scale quantifies an earthquake’s effects in a particular location. It focuses on observed impacts and human experiences, assessing how an earthquake is felt and the resulting damage to structures. Rather than measuring the energy released, the MMI Scale describes the perceived shaking and its consequences. This makes it a valuable tool for understanding the real-world impact of seismic activity on communities.
Measuring Earthquake Effects
The Modified Mercalli Intensity Scale is subjective and can vary significantly across an affected area. Factors such as distance from the earthquake’s epicenter, local geological conditions (like soil or rock type), and building construction quality all influence the experienced intensity. For instance, soft, unconsolidated sediments can amplify ground shaking, leading to higher intensity values compared to areas on bedrock.
Data for the MMI Scale is gathered through various means after an earthquake occurs. This includes collecting eyewitness accounts from the public, conducting detailed damage surveys of affected buildings, and observing changes in the landscape. Organizations often use online questionnaires, such as the “Did You Feel It?” system, to collect public responses about their experiences during an earthquake. These reports help seismologists and engineers map out the distribution of shaking intensity across a region, providing a comprehensive picture of the earthquake’s impact. The information gathered contributes to understanding seismic hazards and improving building codes.
The Mercalli Scale’s Levels
The Modified Mercalli Intensity Scale has twelve increasing levels, designated by Roman numerals I to XII, each describing observed effects. Level I indicates shaking not felt by most people, detectable only by very few individuals. Level II is felt by a few people at rest, typically indoors, and may cause delicately suspended objects to swing. At Level III, the earthquake is felt noticeably indoors by several people, often resembling a passing truck.
As intensity increases, Level IV is felt by many indoors and a few outdoors; some may be awakened at night. Dishes, windows, and doors can be disturbed, and standing motor cars may rock. Level V is felt by nearly everyone, with many awakened, and can cause some dishes and windows to break, unstable objects to overturn, and pendulum clocks to stop. At Level VI, the earthquake is felt by all, causing many to be frightened and run outdoors. Heavy furniture may be moved, and minor plaster falls or chimney damage can occur, indicating slight damage to structures.
Level VII causes people to run outdoors, with negligible damage to well-designed structures but slight to moderate damage in ordinary buildings, and considerable damage to poorly built ones, including some broken chimneys. Level VIII results in slight damage to specially designed structures but considerable damage to ordinary substantial buildings, potentially with partial collapse. Poorly built structures experience extensive damage, and phenomena like sand and mud ejection or changes in well water may be observed.
At Level IX, specially designed structures suffer considerable damage, and well-designed frame structures may be thrown out of plumb. Substantial buildings experience great damage with partial collapse, with buildings shifted off their foundations. The ground may crack conspicuously, and underground pipes can break. Level X leads to the destruction of most masonry and frame structures and their foundations, with the ground badly cracked. Rails may be bent, and large landslides can occur along river banks and steep slopes.
The most severe levels, Level XI and Level XII, describe catastrophic destruction. At Level XI, few, if any, masonry structures remain standing, bridges are destroyed, and broad fissures appear in the ground. Underground pipelines are rendered completely out of service. Level XII signifies total damage, where objects may be thrown upward into the air, waves are seen on ground surfaces, and lines of sight and level are distorted.
Mercalli vs. Magnitude
The Modified Mercalli Intensity Scale differs fundamentally from magnitude scales, such as the Moment Magnitude Scale. Magnitude quantifies the energy released at an earthquake’s source, providing a single, objective value for the entire event. A single earthquake, for example, has one magnitude measurement, regardless of where it is felt. The Moment Magnitude Scale is a more precise measure of the energy released than its predecessor, the Richter scale.
In contrast, the MMI Scale measures the subjective and variable intensity of shaking at a particular point on the Earth’s surface. An earthquake has only one magnitude but can produce many different Mercalli intensity values across the affected area. Intensity generally decreases with increasing distance from the earthquake’s epicenter. Both scales characterize earthquakes, but they serve different purposes: magnitude describes the earthquake’s size at its origin, while intensity describes its impact and perceived severity at various locations. Understanding this distinction is important for comprehending earthquake reports, as a large magnitude earthquake might cause varying intensities of shaking depending on local conditions and distance from the epicenter.