Magnitude, Intensity, and Damage Expectations in Real Places

Why “Magnitude” and “Intensity” Get Confused

People often hear a single number after an earthquake and assume it directly predicts damage everywhere. That number is usually magnitude, which describes the size of the earthquake source. Damage, however, depends on how the shaking is experienced at a specific location. That location-based experience is intensity. A moderate-magnitude earthquake can cause severe damage in one town and little damage in another, while a large-magnitude earthquake can produce a wide range of outcomes depending on distance, building vulnerability, and ground conditions.

In practical hazard awareness, you use magnitude to anticipate the potential geographic reach of strong shaking and aftershocks, and you use intensity to anticipate what people and structures in a particular place are likely to experience. The key skill is translating “a magnitude happened” into “what does this mean for my neighborhood, my building stock, and my lifelines?”

Magnitude: A Measure of Earthquake Size at the Source

Magnitude is a single value intended to represent the overall size of an earthquake. Modern reporting typically uses moment magnitude (Mw). You do not need to compute Mw in the field, but you do need to interpret what it implies: larger magnitude generally means more energy released, longer rupture, longer shaking duration, and a larger area where shaking is felt.

What magnitude does (and does not) tell you

• Does tell you: relative size of the event; likely duration of strong shaking (larger events tend to shake longer); potential for widespread impacts; likelihood of significant aftershocks (larger mainshocks tend to have larger aftershocks).
• Does not tell you: how strong the shaking is at your exact location; whether your building will be damaged; whether a nearby slope will fail; whether liquefaction will occur; whether a particular bridge will be closed.

Magnitude scaling you can use operationally

Magnitude is logarithmic. A difference of 1.0 in magnitude corresponds to about 10 times greater wave amplitude on seismograms and roughly 32 times more energy release. Operationally, this means that moving from Mw 5 to Mw 6 is not “a little bigger”; it is a major jump in potential impact area and duration. Moving from Mw 6 to Mw 7 is another major jump.

Practical rule-of-thumb expectations (very approximate, varies by region and depth):

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• Mw 4–4.9: widely felt locally; minor damage possible in vulnerable buildings near the epicenter; short duration.
• Mw 5–5.9: capable of causing damage near the source, especially to unreinforced masonry and poorly anchored contents; can disrupt services locally.
• Mw 6–6.9: potentially damaging over a broad region; can produce serious damage in vulnerable building stock; longer duration; more likely to trigger landslides and liquefaction where conditions are right.
• Mw 7–7.9: major event; severe damage possible near the rupture; strong shaking over large areas; widespread lifeline disruption possible.
• Mw 8+: great earthquake; regional to continental-scale impacts; long duration; extensive cascading effects likely.

Intensity: A Measure of Shaking Effects in a Specific Place

Intensity describes the severity of shaking and its effects at a particular site. It is not a single number for the whole earthquake. Intensity varies from place to place because shaking decays with distance, is modified by local soils and topography, and interacts with the built environment.

Intensity can be expressed in descriptive scales such as the Modified Mercalli Intensity (MMI) scale (I to XII) or in instrumental terms such as peak ground acceleration (PGA) and peak ground velocity (PGV). For community resilience, MMI-style descriptions are useful for communicating expected effects, while PGA/PGV are useful for engineering and for understanding why certain damage patterns occur.

How to interpret MMI-style intensity levels

MMI is based on observed effects: what people felt, how objects moved, and what damage occurred. You can use it as a “field translation” between shaking and consequences. Approximate expectations:

• MMI IV–V: felt by many; dishes rattle; small objects shift; little to no structural damage.
• MMI VI: felt by all; some plaster cracks; items fall from shelves; minor damage in weak structures.
• MMI VII: difficult to stand; moderate damage in vulnerable buildings; chimneys crack; nonstructural damage becomes common.
• MMI VIII: heavy furniture moves; partial collapses in weak buildings; significant damage in unreinforced masonry; lifeline breaks become more likely.
• MMI IX+: severe to violent shaking; widespread damage; ground failures more common; major disruption.

Instrumental intensity: why PGA and PGV matter for damage

PGA relates to the maximum acceleration of the ground and often correlates with how strongly objects are thrown or how strongly short, stiff structures respond. PGV relates to the maximum velocity of ground motion and often correlates better with damage to buildings, especially mid-rise structures, because it reflects the energy in shaking at periods that align with building response.

In practice, you may see maps showing “shaking intensity” derived from instruments and models (often called ShakeMaps). These maps are a bridge between magnitude and local expectations: they show how intensity varies across real places.

From Magnitude to Intensity: The Main Controls in Real Places

1) Distance and depth

Shaking generally decreases with distance from the rupture. Depth matters: a deeper event can be felt over a wide area but may produce less extreme shaking at the surface near the epicenter than a shallow event of similar magnitude. For rapid expectations, ask: How far am I from the rupture? How shallow is it? A shallow Mw 6 close to town can be more damaging locally than a deeper Mw 7 farther away.

2) Rupture direction and directivity

Earthquakes rupture along a fault, and the direction of rupture can focus energy toward certain areas. Communities in the direction the rupture propagates may experience stronger, more damaging pulses than places at similar distances elsewhere. This is one reason damage can be asymmetric around the epicenter.

3) Local site conditions (soil, sediment thickness, and basin effects)

Soft soils can amplify shaking and lengthen its duration compared with nearby rock sites. Deep sedimentary basins can trap and reverberate waves, producing prolonged shaking that increases damage potential, especially for taller buildings. Two neighborhoods the same distance from the fault can experience different intensities if one sits on soft fill or river deposits and the other on bedrock.

4) Topographic effects

Ridges and hilltops can experience amplified shaking in some settings. Steep slopes also raise the likelihood of earthquake-triggered landslides, which can dominate damage outcomes even when building shaking damage is moderate.

5) Building vulnerability and construction practice

Intensity is partly “what the ground did” and partly “what the built environment did with it.” Vulnerable building types (for example, unreinforced masonry, soft-story apartments, poorly confined concrete, non-ductile frames, and older structures without seismic detailing) tend to show damage at lower shaking levels than well-designed ductile structures. Nonstructural components (ceilings, sprinklers, parapets, cladding, shelves) can fail at intensities that leave the main structure standing, yet still cause injuries and economic disruption.

6) Lifelines and cascading effects

Damage expectations must include lifelines: water, power, gas, communications, roads, bridges, ports, and hospitals. A community can experience limited building collapse but still face a major emergency due to water main breaks, power outages, and road closures. Lifeline fragility often depends on ground deformation (liquefaction, lateral spreading, landslides) as much as on shaking intensity alone.

Damage Expectations: Translating Shaking into What Fails First

To make intensity actionable, focus on what typically fails at different levels of shaking and in different building stocks. The same intensity can produce different damage patterns depending on vulnerability, but there are consistent “first failures” that help you anticipate needs.

Nonstructural damage (often starts early)

• Falling contents: books, TVs, monitors, kitchen items.
• Ceiling tiles and light fixtures in offices and schools.
• Sprinkler piping leaks leading to water damage.
• Parapets, chimneys, and façade elements shedding debris.

Nonstructural damage drives injuries and downtime. Even when buildings are safe to occupy structurally, nonstructural failures can close schools, clinics, and businesses.

Structural damage (depends strongly on vulnerability)

• Cracking in brittle materials (masonry, poorly detailed concrete) can begin at moderate intensities.
• Soft-story mechanisms in buildings with open ground floors can lead to partial collapse at higher intensities.
• Older bridges and unretrofitted columns may be vulnerable to shear failure.

Ground failure-driven damage (site-specific but high consequence)

• Liquefaction can tilt buildings, rupture buried utilities, and damage ports and riverfronts.
• Landslides can isolate communities, block rivers, and destroy roads and pipelines.
• Lateral spreading can tear apart pavements and embankments even when buildings nearby appear intact.

Step-by-Step: Rapidly Estimating What Your Place Should Expect After a Reported Magnitude

This workflow is designed for a community leader, facility manager, or informed resident who has access to basic earthquake information (magnitude, approximate location, depth) and can view a shaking map if available. It does not replace engineering evaluation; it helps prioritize immediate actions and situational awareness.

Step 1: Capture the event basics

• Magnitude (Mw) and whether it is a preliminary or updated value.
• Location relative to you (distance and direction).
• Depth (shallow events generally produce stronger near-source shaking).
• Any official shaking/intensity map if available.

Step 2: Convert “distance to source” into a first-pass intensity expectation

Without doing calculations, use a simple logic: closer + shallow + larger magnitude = higher intensity. If you have a ShakeMap/MMI map, use it directly for your neighborhood rather than guessing. If you do not, use observed effects (what people felt, what moved, what broke) to infer an approximate intensity level for your site.

Step 3: Identify your dominant vulnerability category

Make a quick inventory of what matters most in your area:

• Building stock: unreinforced masonry? older concrete? wood-frame? modern code-built?
• Critical facilities: hospitals, schools, emergency operations, water treatment, substations.
• Hazardous contents: chemicals, gas cylinders, lab equipment, server racks.
• Population vulnerability: elderly housing, daycares, high-occupancy venues.

Step 4: Overlay site conditions that can amplify or transform damage

• Soft soils, reclaimed land, river deltas, coastal plains: higher amplification and liquefaction potential.
• Steep slopes and road cuts: landslide potential.
• Basins and valleys: longer shaking duration possible.

If you do not know the local soil conditions, use proxies: proximity to rivers, bays, marshes, or known fill areas; neighborhoods with frequent flooding or high groundwater; areas with thick alluvium.

Step 5: Translate expected intensity into prioritized checks

Use intensity as a trigger for what to check first:

• MMI V–VI expected/observed: check for fallen items, minor cracks, sprinkler leaks; verify gas odors; check elevators; inspect parapets and chimneys from a safe distance.
• MMI VII expected/observed: restrict access to older masonry and buildings with visible cracking; check schools and assembly spaces for ceiling and lighting hazards; expect broken water lines; anticipate traffic signal outages.
• MMI VIII+ expected/observed: prioritize life safety and rapid building tagging by qualified personnel; expect road and bridge restrictions; prepare for extended utility outages; check for ground failure signs in susceptible zones.

Step 6: Anticipate aftershocks based on magnitude and initial damage

Larger mainshocks are followed by aftershocks that can be damaging, especially to already weakened structures. Operationally: if you observe structural cracking, treat aftershocks as a serious risk multiplier. Plan re-entry and inspections with aftershocks in mind (for example, do not send teams under damaged parapets without controls).

Worked Examples: Same Magnitude, Different Real-Place Outcomes

Example A: Mw 6.2 near two neighborhoods—rock hillside vs. river plain

Two neighborhoods are 15 km from the rupture. Neighborhood 1 sits on shallow bedrock on a hillside with modern wood-frame homes. Neighborhood 2 sits on a river plain with soft sediments, older masonry storefronts, and high groundwater.

• Expected intensity difference: Neighborhood 2 likely experiences higher intensity due to amplification and longer shaking; Neighborhood 1 may have sharper but less amplified shaking.
• Damage expectations: Neighborhood 1: mostly nonstructural damage (fallen contents, minor drywall cracks), localized slope issues on steep cuts. Neighborhood 2: higher rates of chimney/parapet damage, cracked masonry, broken water mains, possible liquefaction-related settlement and lateral spreading near the river.
• Action priorities: Neighborhood 2 should prioritize cordoning masonry façades, checking water/gas systems, and inspecting riverfront ground deformation; Neighborhood 1 should prioritize slope/road cut checks and securing contents.

Example B: Mw 5.5 directly under a town vs. Mw 6.5 farther away

Town X experiences a shallow Mw 5.5 almost beneath it. Town Y experiences a Mw 6.5 but 80 km away.

• Intensity reality: Town X may experience MMI VII locally due to proximity and shallow depth, producing moderate damage in vulnerable buildings. Town Y may experience MMI V–VI, widely felt but with limited structural damage.
• Messaging implication: Town X should not be reassured by “only 5.5,” and Town Y should not panic solely because “it was 6.5.” The correct question is: what intensity did we experience here?

Example C: Basin city vs. nearby foothill suburb during a large event

A large earthquake produces strong shaking across a region. The basin city experiences prolonged shaking due to basin effects; the foothill suburb experiences shorter duration but may face landslides.

• Damage pattern: Basin city: higher nonstructural damage in mid- and high-rise buildings, more elevator entrapments, more sprinkler leaks, higher downtime. Foothill suburb: road closures from slides, isolated neighborhoods, damaged hillside homes, disrupted water lines on steep terrain.
• Response planning: Basin city needs rapid building functionality assessments and interior hazard control; foothill suburb needs debris clearance routes, slope reconnaissance, and redundancy for access.

Using Intensity to Communicate Risk Without Overpromising

Effective public communication avoids two traps: (1) using magnitude as a damage predictor, and (2) giving a single damage expectation for an entire city. Instead, communicate in place-based terms: “Our area likely experienced MMI VI–VII; in soft-soil zones and older masonry districts, expect more damage; in newer construction on rock, expect mostly nonstructural issues.”

Practical communication template

• What happened: magnitude, approximate location, depth.
• What it means here: observed/estimated intensity range for specific neighborhoods.
• What to watch for: most likely hazards (falling debris, gas leaks, water breaks, slope failures).
• What to do now: prioritized checks and safety actions tailored to intensity and vulnerability.

Field-Ready Checklist: Estimating Damage Expectations Block by Block

This checklist helps you walk a neighborhood and connect observed intensity indicators to likely damage mechanisms.

1) Shaking indicators you can observe quickly

• Extent of items fallen from shelves across multiple buildings.
• Cracked plaster/drywall frequency.
• Chimney and parapet cracking or fallen bricks.
• Reports of difficulty standing or people being thrown off balance.

2) Building vulnerability flags

• Unreinforced masonry walls, especially with large storefront openings.
• Soft-story ground floors (parking or open commercial fronts).
• Heavy overhangs, weak connections, or visible deterioration.
• Unbraced water heaters, tall unanchored shelving, unsecured equipment.

3) Ground failure flags

• Sand boils, wet ground, new cracks with ejected sediment.
• Sidewalks and curbs offset or pulled apart near riverbanks.
• Fresh scarps, rockfall, or slumps on slopes and road cuts.
• Tilting fences, poles, or building steps indicating settlement.

4) Lifeline disruption indicators

• Loss of water pressure, discolored water, or pooling water in streets.
• Gas odors, hissing sounds, or meter/regulator damage.
• Power outages concentrated in certain corridors (possible substation or feeder issues).
• Bridge approach settlement, rail misalignment, or roadway buckling.

Practical Exercise: Build a “Place-Based Intensity-to-Damage” Table for Your Community

Create a simple table that links intensity levels to expected impacts for your specific building stock and site conditions. This becomes a reusable tool for drills and real events.

Step-by-step

• Step 1: Divide your community into 3–6 zones based on obvious site differences (river plain, hillside, downtown masonry district, modern suburb, industrial waterfront).
• Step 2: For each zone, list dominant building types and critical facilities.
• Step 3: For each zone, list likely ground failure concerns (liquefaction, landslides, settlement) and key lifelines.
• Step 4: Create rows for intensity bands (MMI V–VI, VII, VIII+). In each cell, write the top 3 expected impacts and the top 3 immediate checks.
• Step 5: Validate the table during exercises by walking the zones and noting where your assumptions about vulnerability or site conditions are wrong.

Example table structure you can copy:

Zone: Downtown masonry district (older brick, mixed-use) MMI V–VI: Impacts: fallen contents, minor cracks, some parapet looseness Checks: parapets, chimneys, gas odors, interior egress routes MMI VII: Impacts: masonry cracking, façade debris, broken water lines Checks: cordon sidewalks, rapid building screening, water system pressure MMI VIII+: Impacts: partial collapses in weakest buildings, major façade failures, widespread utility breaks Checks: life safety search, restrict access, coordinate utility shutoffs

Common Mistakes When Estimating Damage Expectations

Using magnitude as a neighborhood damage forecast

A single magnitude value cannot capture local amplification, directivity, or vulnerability. Always ask for or infer local intensity.

Ignoring duration

Two places can experience similar peak shaking but different durations. Longer shaking increases cumulative damage and nonstructural failures, especially in basins and during large events.

Assuming “no collapse” means “no problem”

Nonstructural damage, utility breaks, and ground failures can make buildings unusable and neighborhoods unsafe even without dramatic structural collapse.

Overlooking the most fragile elements

Parapets, chimneys, and unbraced contents often fail early and cause injuries. These are high-payoff targets for mitigation and for post-event cordons.

Putting It Together: A Practical Mental Model

Use this mental model during real events: magnitude sets the stage (how big, how widespread, how long), intensity tells the local experience (how hard it shook here), and damage expectations come from combining intensity with vulnerability and site effects. When you can name your local intensity and your local vulnerabilities, you can make faster, more accurate decisions about inspections, sheltering, route clearance, and public messaging.