Thang Mercalli Được Chỉnh sửa: Đo lường những gì bạn cảm thấy

From Giuseppe Mercalli to the Modern MMI Scale

The story of earthquake intensity measurement begins in 19th-century Italy, where volcanic and seismic activity provided frequent natural experiments. Italian priest and geologist Giuseppe Mercalli developed an early intensity scale in 1883, later revised in 1902, that described the observable effects of earthquakes on people, objects, and structures. Unlike magnitude — a property of the earthquake itself — intensity describes what happened at a particular place.

Mercalli's scale was modified and expanded over the following decades by American seismologists Harry Wood and Frank Neumann, who published the Modified Mercalli Intensity (MMI) scale in 1931. This version, with further refinements in 1956, remains the standard intensity scale used in the United States today. Europe uses a similar but distinct scale called the European Macroseismic Scale (EMS-98). Both trace their lineage to Mercalli's original observational framework.

The 12 Levels: I Through XII Explained

The Modified Mercalli IntensityA 12-point scale (I-XII) that measures the observed effects of an earthquake at a specific location, from imperceptible (I) to total destruction (XII). Unlike magnitude, intensity varies by distance. scale runs from Roman numeral I to XII. At the low end, MMI I means the earthquake was not felt at all — only SeismographAn instrument that detects and records ground motion caused by seismic waves. Modern digital seismographs can detect movements smaller than a nanometer. instruments recorded it. MMI II is felt only by people who are resting, particularly on upper floors of tall buildings. MMI III is felt noticeably indoors, especially on upper floors, and may be mistaken for the vibration of a passing truck.

MMI IV–V marks the range where a quake is felt by most people and begins to cause minor disturbances: hanging objects swing, dishes rattle, liquids slosh in containers. MMI VI–VII is where light structural damage begins — plaster cracks, chimneys break, poorly constructed buildings sustain damage. MMI VIII–IX represents severe shaking that damages even well-built structures, causes partial collapses of weak buildings, and can produce Ground Rupture (Surface Faulting)Visible displacement of the ground surface along a fault during an earthquake. Structures built across a surface rupture zone can be torn apart regardless of their structural strength. and Earthquake-Triggered LandslideThe downslope movement of soil and rock triggered by earthquake shaking. Landslides can bury entire communities and may cause more casualties than the shaking itself.s. MMI X–XII describes catastrophic damage: well-built wood-frame structures thrown off foundations, bridges destroyed, and at XII, near-total destruction of all structures.

How Intensity Differs from Magnitude

The distinction between Seismic IntensityA measure of the strength of shaking at a particular location, determined by observed effects on people, structures, and the natural environment. Decreases with distance from the epicenter. and MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. cannot be overstated, because the two concepts are frequently confused in media reporting. MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. is a single number assigned to the earthquake at its source — it does not change based on where you stand. Seismic IntensityA measure of the strength of shaking at a particular location, determined by observed effects on people, structures, and the natural environment. Decreases with distance from the epicenter. varies across the affected region and is assigned to specific locations, not to the earthquake as a whole.

A single earthquake will produce many different intensity values across its affected area. A magnitude 6.5 earthquake occurring directly beneath a densely populated urban area might produce MMI VIII or IX at the epicentre, while a town 150 kilometres away experiences only MMI III. Another earthquake of identical magnitude occurring in a remote desert at depth might never produce MMI values above IV at any populated location. This is why earthquake fatality and damage statistics are driven far more by intensity than by magnitude.

Did You Feel It: Crowdsourcing Intensity Data

Traditional intensity mapping required weeks or months of field surveys by scientists interviewing survivors and inspecting damage. The Did You Feel It? (DYFI)A USGS program that collects intensity reports from the public after earthquakes to create community-derived intensity maps. Allows anyone who felt an earthquake to submit a report. system, developed by the USGS (United States Geological Survey)The primary US government agency responsible for monitoring earthquakes, operating the National Earthquake Information Center, and publishing real-time earthquake data worldwide. in 1999, revolutionised intensity mapping by crowdsourcing observations from the public. Within minutes of a significant earthquake, thousands of people visit the website to report what they experienced — whether they felt shaking, how strong it was, what objects moved or fell, and whether any damage occurred.

These self-reported data are aggregated using statistical algorithms that correct for reporting biases and extrapolate spatially, producing colour-coded intensity maps with unprecedented speed and geographic detail. By the time field geologists can mobilise, the Did You Feel It? (DYFI)A USGS program that collects intensity reports from the public after earthquakes to create community-derived intensity maps. Allows anyone who felt an earthquake to submit a report. database may already contain 100,000 or more observations. This data also feeds directly into ShakeMapA USGS product that displays the distribution of ground shaking intensity after an earthquake. Combines seismograph data, ground motion models, and 'Did You Feel It?' reports. products, which agencies use for emergency response planning.

Why Intensity Varies: Distance, Soil, and Building Type

Three factors dominate intensity variation. The most intuitive is distance: shaking energy spreads out as waves travel through the Earth, and intensity generally decreases with distance from the epicentre. The rate of decrease depends on regional geology — in the eastern United States, shaking propagates remarkably efficiently, and a moderate earthquake can be felt across ten times the area it would be in California.

Soil Amplification (Site Effect)The increase in shaking intensity caused by soft soil or sediment layers amplifying seismic waves. Structures built on soft soil can experience 2-10 times stronger shaking than those on bedrock. is often the most dramatic local factor. Soft, water-saturated sediments — river deltas, bay muds, reclaimed land, and thick alluvial deposits — can amplify shaking by factors of 10 or more compared to adjacent hard rock. This effect explains why certain neighbourhoods consistently suffer more damage than others in the same city. Building type and quality are the third critical factor. An unreinforced masonry building may collapse at MMI VII while a modern moment-frame structure nearby sustains no significant damage. The Seismic RetrofitStrengthening an existing building to improve its earthquake resistance. Common methods include adding steel bracing, reinforcing foundations, and bolting structures to foundations. of vulnerable buildings is therefore one of the most effective strategies for reducing intensity-driven casualties.

Using Intensity Maps for Emergency Response

Within minutes of a significant earthquake, the USGS (United States Geological Survey)The primary US government agency responsible for monitoring earthquakes, operating the National Earthquake Information Center, and publishing real-time earthquake data worldwide. publishes ShakeMapA USGS product that displays the distribution of ground shaking intensity after an earthquake. Combines seismograph data, ground motion models, and 'Did You Feel It?' reports. products that display estimated shaking intensity across the affected region. Emergency managers use these maps to rapidly identify which areas likely experienced the heaviest shaking and therefore require the most urgent response. The maps help prioritise deployment of Search and Rescue (SAR)Organized efforts to locate and extract survivors trapped in collapsed structures after an earthquake. The first 72 hours are the critical window for finding survivors alive. teams, guide media communications about affected areas, and trigger automated notifications to utilities and infrastructure operators.

Intensity maps also have long-term applications. Historical intensity data, compiled from old newspaper accounts, personal diaries, and church records, allow seismologists to reconstruct the shaking patterns of earthquakes that occurred long before seismographs existed. This historical record is an essential input into Seismic Hazard MapA map showing the probability of earthquake shaking exceeding specified levels over a given time period. Used by engineers, planners, and insurers to assess earthquake risk.s and probabilistic seismic hazard analyses that guide building codes and land use planning. Knowing that a particular valley experienced MMI VIII in an 1850 earthquake tells engineers something important about the ground conditions there, even without a single instrumental recording.