Understanding the Severity of Ground Shaking: A Comprehensive Overview of Earthquake Intensity Scales
PHIVOLCS Earthquake Intensity Scale | Photo Courtesy of PHIVOLCS-DOST on X
Earthquakes are among the most powerful and destructive natural phenomena on Earth. When an earthquake occurs, public information channels rapidly broadcast numbers to quantify its size. However, there is a frequent point of confusion between what an earthquake is and what an earthquake does at any given location. To clear up this distinction, seismologists rely on two different metrics: magnitude and intensity. While magnitude measures the absolute energy released at the seismic source, an earthquake intensity scale quantifies the severity of ground shaking, its human perception, and its physical impact on structures and the environment at a specific geographic point.
Understanding how intensity is measured, the historical development of these scales, and the variables that dictate local damage is essential for modern civil engineering, disaster response, and community preparedness.
Magnitude vs. Intensity: The Essential Distinction
To accurately interpret seismic data, one must distinguish energy scales from observational scales.
Magnitude Scales: Systems like the Moment Magnitude Scale (Mw) measure the physical dimensions of the fault rupture and the total energy released. An earthquake has only one magnitude.
Intensity Scales: Systems like the Modified Mercalli Intensity (MMI) scale evaluate local effects. An earthquake has many intensities, which typically decrease with distance from the epicenter.
A common analogy used by the U.S. Geological Survey (USGS) compares an earthquake to a lightbulb. The lightbulb has a fixed wattage (magnitude), which represents its power. The brightness experienced by a person reading a book in the room depends on how far they are from the bulb and whether any obstacles block the light (intensity).
Global Variation in Intensity Scales
Because architectural styles, construction materials, and regional geologies differ worldwide, various meteorological and geological agencies have optimized specific intensity scales for their home regions. Seismologists traditionally express these values in Roman numerals (e.g., MMI I to XII) to ensure they are never confused with decimal magnitude values.
The Modified Mercalli Intensity (MMI) Scale
Developed originally by Italian vulcanologist Giuseppe Mercalli and later updated by Harry Wood and Frank Neumann, the MMI scale is the standard across the Americas and the Western world. It grades shaking from I (Not Felt) to XII (Catastrophic Destruction) based on subjective human experiences and physical structural performance.
The Japan Meteorological Agency (JMA) Seismic Intensity Scale
Known domestically in Japan as Shindo (translated as "shaking degree"), the JMA scale takes a highly modern approach. Unlike Western scales that rely heavily on post-event human surveys, the JMA scale relies on instrumental data gathered from a dense network of digital strong-motion accelerometers across the country. It ranges from 0 to 7, with levels 5 and 6 split into "Lower" and "Upper" tiers, creating a total of ten distinct categories.
The PHIVOLCS Earthquake Intensity Scale (PEIS)
Developed by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the PEIS was specifically engineered to account for the unique geographical and socio-economic infrastructure conditions of the Philippine archipelago. Spanning from I (Scarcely Perceptible) to X (Completely Devastating), it provides detailed criteria for evaluating impacts on tropical vegetation, local water bodies, and common domestic building types like lightweight native housing and non-engineered concrete block structures.
The European Macroseismic Scale (EMS-98)
The EMS-98 is the standard framework utilized across European nations. It is highly analytical, incorporating strict classifications of building vulnerability classes (from A to F) and precise percentages of damage to better standardize observations across different countries with ancient masonry structures.
Detailed Breakdown: The Modified Mercalli Intensity Scale
The twelve degrees of the MMI scale demonstrate how seismic waves translate from imperceptible vibrations to landscape-altering disasters.
Intensity α f(Acceleration. Velocity, Local Geology)
Low Intensity: Human Perception (MMI I – MMI III)
MMI I (Not Felt): No structural or human perception. Recorded only by sensitive seismometers.
MMI II (Weak): Felt only by a small percentage of people, primarily those at rest or on the upper floors of high-rise buildings.
MMI III (Weak): Felt noticeably indoors. Hanging objects, such as ceiling lamps, swing gently. The vibration feels similar to a light truck passing by.
Moderate Intensity: Environmental Reactions (MMI IV – MMI VI)
MMI IV (Light): Felt indoors by many, outdoors by few. Dishes, windows, and doors rattle significantly. Walls emit creaking sounds.
MMI V (Moderate): Felt by nearly everyone. Many sleeping individuals are awakened. Liquid in cups may spill, and unstable or fragile items tip over.
MMI VI (Strong): Felt by all people, causing minor panic. Individuals walk unsteadily. Heavy furniture pieces shift from their positions, and plaster walls may suffer hairline cracks.
High Intensity: Structural Failure (MMI VII – MMI IX)
MMI VII (Very Strong): Standing upright becomes difficult. Drivers notice their vehicles veering. Considerable damage occurs in poorly built structures, whereas well-designed buildings experience negligible effects.
MMI VIII (Severe): Special damage to masonry buildings. Chimneys, elevated water towers, and monuments twist or topple. Heavy furniture completely overturns.
MMI IX (Violent): Panic is widespread. Well-designed frame structures are thrown out of plumb. Masonry buildings suffer partial collapse, and underground utility pipes snap due to shear stress.
Extreme Intensity: Landscape Transformation (MMI X – MMI XII)
MMI X (Extreme): Most masonry and wood-frame structures are destroyed along with their foundations. Riverbanks cave in, dams suffer damage, and extensive landslides occur.
MMI XI (Extreme): Very few structural masonry walls remain standing. Bridges collapse entirely. Broad fissures rip open in the ground, and underground pipelines are rendered completely unserviceable.
MMI XII (Catastrophic): Damage is absolute. The ground moves in visible waves, distorting lines of sight. Heavy objects are accelerated upward and thrown into the air.
The Japan Meteorological Agency (JMA) Seismic Intensity Scale
Japan's scale, known as Shindo, measures intensity across ten operational levels from 0 to 7 by splitting levels 5 and 6 into Lower and Upper categories. Unlike human-reported scales, Japan calculates Shindo mathematically using instrumental Seismic Intensity Meters that filter three-axis acceleration data to match human perception and structural vulnerability frequencies.
At the bottom of the scale, Level 0 is entirely imperceptible to humans and recorded only by instruments, while Level 1 is felt slightly by some quiet individuals indoors, and Level 2 causes hanging lamps to swing slightly as it is noticed by many people inside. As shaking reaches Level 3, it is felt by almost everyone indoors and causes dishes to rattle, progressing to Level 4 where widespread alarm occurs, hanging objects swing violently, and unstable ornaments tip over.
When the scale enters Level 5 Lower, shaking is strong enough to disrupt a person's balance, move unanchored furniture, and break window panes. Level 5 Upper makes walking difficult, causes unreinforced concrete-block walls to collapse, and topples vending machines. At the most severe tiers, Level 6 Lower makes it impossible to stand straight and heavily damages or collapses less earthquake-resistant buildings. Level 6 Upper forces people to crawl to move, triggers landslides, and destroys most unreinforced masonry. Finally, Level 7 represents catastrophic shaking that throws people through the air and severely damages or collapses even highly earthquake-resistant, modern structures.
The PHIVOLCS Earthquake Intensity Scale (PEIS)
Developed by the Philippine Institute of Volcanology and Seismology, the PEIS consists of 10 intensity levels optimized for the unique geography, tropical vegetation, and building materials of the Philippines. Intensity I is scarcely perceptible, felt only by occasional people at rest indoors while water in containers ripples gently, whereas Intensity II is slightly felt by few individuals and causes hanging objects to swing slightly. Intensity III is a weak shaking felt by many indoors that can induce dizziness and nausea, which elevates to Intensity IV where hanging objects swing moderately and dinner plates clack loudly.
Intensity V is strong enough to wake sleeping people, disrupt hanging objects, and cause loose items to fall. Intensity VI causes widespread fright, sending people running outdoors as heavy furniture moves and very old or poorly built structures suffer wall cracks.
At Intensity VII, the shaking becomes destructive, making it difficult for most people to stand, ringing big church bells, and toppling heavy furniture while failing poorly built structures. Intensity VIII is very destructive, making standing difficult even outdoors, cracking many well-built concrete block structures, and triggering landslides in hilly areas. The highest levels, Intensity IX and X, represent devastating and completely devastating forces; Intensity IX ruins most buildings, snaps utility lines, and opens ground fissures across plains, while Intensity X causes the absolute destruction of all man-made infrastructure, forces rivers to change course, and triggers massive mountain landslides that permanently alter the topography.
The European Macroseismic Scale (EMS-98)
The EMS-98 is the standard throughout Europe, operating on a 12-grade system that standardizes observations by combining human perception with precise building vulnerability classes and damage grades. Vulnerability classifications range from Class A, which includes highest-risk structures like earth-brick and unreinforced masonry, to Class F, which covers lowest-risk, highly engineered structures with base-isolation and steel-bracing.
The lower levels focus on human perception, where Level I is not felt at all, Level II is scarcely felt by very sensitive individuals at rest, Level III is weak and noticed by a few indoors, and Level IV is largely observed indoors, causing high-rise occupants to notice the shaking. Level V is strong enough that buildings tremble and masonry rattles, while Level VI becomes slightly damaging as all people feel it, many run outdoors, and Class A and B buildings suffer minor plaster cracks.
The upper half of the scale grades structural failure, where Level VII causes panic and heavy cracks or partial wall collapse in Class A structures, and Level VIII causes people to lose their balance while toppling monuments and inducing total wall collapse in Class A buildings. Level IX is destructive, throwing well-designed frame structures out of plumb and causing total structural collapse in Class A and heavy damage in Class B structures. The scale concludes with Level X, which completely destroys Class B and heavily damages Class C structures; Level XI, which devastates nearly all structures up to Class C; and Level XII, which is completely devastating, causing the absolute destruction of practically all engineering structures above and below ground.
Approximate Cross-Scale Global Comparison
To compare shaking severity across these different regional networks, seismologists look at the overlapping human experiences and structural impacts at different thresholds. Light shaking correlates to an MMI Level IV, a JMA Shindo 2, a PEIS Intensity IV, and an EMS-98 Level IV. Strong shaking matches up with an MMI Level VI, a Shindo 4, a PEIS Intensity VI, and an EMS-98 Level VI.
When the ground vibrations become severe, they register as an MMI Level VIII, a Shindo between 5 Upper and 6 Lower, a PEIS Intensity VIII, and an EMS-98 Level VIII. Violent or devastating forces map to an MMI Level IX, a Shindo 6 Upper, a PEIS Intensity IX, and an EMS-98 Level IX. At the highest extreme, catastrophic shaking registers across the world as an MMI Level XI to XII, a JMA Shindo 7, a PEIS Intensity X, and an EMS-98 Level XI to XII.
Determinants of Local Shaking Severity
Two locations equidistant from the same fault rupture line rarely experience identical intensity levels. Local shaking severity depends on three intersecting environmental variables:
1. Attenuation over Distance
Seismic energy dissipates as waves travel through the Earth's crust. Generally, intensity is highest near the epicenter and attenuates (weakens) progressively as distance increases.
2. Site Effects and Ground Amplification
The physical properties of the upper few hundred meters of soil alter seismic waves. Solid bedrock vibrates with low amplitudes. Conversely, soft soils, thick unlithified sediments, and artificially reclaimed coastal land mass amplify seismic waves. This phenomenon can cause severe structural shaking even at a vast distance from the earthquake core.
3. Engineering Quality and Building Resiliency
Intensity ratings look closely at structural performance. Unreinforced masonry walls possess low tensile strength and fail quickly under lateral seismic forces. In contrast, modern structures built with ductile reinforced concrete frames and base-isolation bearings bend and absorb kinetic energy, lowering the observed intensity score of that area.
Technical Context: Peak Ground Acceleration
To bridge qualitative observations with quantitative physics, modern engineers map intensity directly to Peak Ground Acceleration (PGA). PGA measures the maximum horizontal acceleration of the ground crust during shaking, expressed as a percentage of gravity (%g).
For instance, an intensity rating of MMI IV (Light Shaking) corresponds roughly to a PGA value of 1.4% to 3.9%g, whereas an extreme MMI X (Extreme Shaking) corresponds to a PGA value exceeding 124%g. These empirical mathematical calculations allow emergency personnel to generate automated structural impact estimates immediately following an instrumental alert.
References
European Seismological Commission. (1998). European Macroseismic Scale 1998 (EMS-98) (G. Grünthal, Ed.). Centre Européen de Géodynamique et de Séismologie.
Japan Meteorological Agency. (2019). Tables for JMA seismic intensity scale. jma.go.jp
Philippine Institute of Volcanology and Seismology. (2018). PHIVOLCS Earthquake Intensity Scale (PEIS). Department of Science and Technology. dost.gov.ph
U.S. Geological Survey. (n.d.). The Modified Mercalli Intensity Scale. U.S. Department of the Interior. usgs.gov
Wood, H. O., & Neumann, F. (1931). Modified Mercalli Intensity Scale of 1931. Bulletin of the Seismological Society of America, 21(4), 277–283. doi.org
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