Why Do Some Earthquakes Cause More Damage Than Others?
Quick Answer
Earthquake damage depends on far more than just magnitude. A magnitude 6.5 earthquake in one location might be catastrophic, while an identical-sized quake elsewhere causes minimal damage. The key factors include: earthquake depth (shallow quakes are more destructive), distance from populated areas, soil type (soft soils amplify shaking), building construction quality, population density, duration of shaking, time of day, and whether aftershocks follow. This is why scientists distinguish between magnitude (energy released) and intensity (damage experienced)—they often tell very different stories.
Magnitude vs. Intensity: Two Different Measurements
When an earthquake strikes, news reports focus on magnitude—the number that describes how much energy the earthquake released. As we explain in our article on magnitude scales, this is a single value calculated from seismograph readings.
However, magnitude alone tells us nothing about damage. That's where intensity comes in. Intensity measures the effects of an earthquake at specific locations, describing how strongly the ground shook and what damage occurred there. The same earthquake has one magnitude but many different intensities depending on where you measure.
Think of it like a light bulb. Magnitude is the bulb's wattage—a fixed property of the bulb itself. Intensity is the brightness you experience, which depends on how far you are from the bulb, whether there's dust in the air, and what's reflecting or absorbing the light. The bulb hasn't changed, but your experience of it varies dramatically.
🔍 Real-World Example
The 1994 Northridge Earthquake (California): Magnitude 6.7, caused 57 deaths and $20 billion in damage
The 2011 Christchurch Earthquake (New Zealand): Magnitude 6.3, caused 185 deaths and $40 billion in damage
The smaller earthquake caused more damage because it was shallower, struck directly under a major city during business hours, and lasted longer despite its lower magnitude.
The Key Factors That Determine Damage
Multiple factors combine to determine how destructive an earthquake will be. Let's examine each one.
1 Earthquake Depth
Depth might be the single most important factor after magnitude. Shallow earthquakes, occurring less than 70 kilometers deep, cause far more surface damage than deep earthquakes of the same magnitude.
Seismic waves lose energy as they travel through rock. An earthquake 10 kilometers deep delivers much stronger shaking to the surface than one 100 kilometers deep, even if they release identical energy. The waves from the deeper quake have to travel farther and weaken more before reaching buildings and people.
Very shallow earthquakes—occurring just a few kilometers below the surface—can be devastating even at moderate magnitudes. The 2011 Christchurch earthquake occurred at just 5 kilometers depth, concentrating intense shaking directly under the city despite being only magnitude 6.3.
2 Distance from Populated Areas
An earthquake's location relative to population centers dramatically affects casualty numbers and economic losses. As discussed in our guide to how far you can feel earthquakes, seismic waves weaken with distance.
A magnitude 7.5 earthquake in a remote wilderness causes no human casualties. The same earthquake under a major city could kill thousands. The 1906 San Francisco earthquake killed an estimated 3,000 people. In contrast, the 1964 Alaska earthquake—much more powerful at magnitude 9.2—killed only 131 people because most of the affected area was sparsely populated.
Even within affected regions, damage concentrates in areas closest to the epicenter. Buildings 50 kilometers away might experience moderate shaking while structures 5 kilometers away suffer catastrophic damage, all from the same earthquake.
3 Soil and Geology
The ground beneath buildings matters enormously. Solid bedrock transmits seismic waves efficiently but doesn't amplify them much. Soft soils, sediments, and fill material amplify seismic waves, sometimes doubling or tripling the shaking intensity.
During the 1989 Loma Prieta earthquake in California, the Marina District of San Francisco, built on landfill, experienced far more severe damage than nearby areas on bedrock, despite being farther from the epicenter. The soft fill amplified the shaking dramatically.
Liquefaction: When Solid Ground Turns Liquid
Liquefaction occurs when saturated, loose soil loses strength during shaking and behaves like liquid. Buildings can sink, tilt, or collapse even if structurally sound. Underground utilities rupture. Roads buckle and crack.
Liquefaction particularly affects areas with shallow groundwater and sandy or silty soils. Coastal regions, river deltas, and reclaimed land face high liquefaction risk. The 1964 Niigata earthquake in Japan famously demonstrated liquefaction, with entire apartment buildings toppling onto their sides despite remaining structurally intact.
4 Building Construction
Building quality and design perhaps determine damage levels more than any other human-controlled factor. Modern buildings designed to seismic codes in places like Japan or California can withstand strong shaking that would flatten older or poorly constructed structures.
Seismic Design Principles
Modern seismic design doesn't try to prevent all damage—that would be economically impractical. Instead, it aims to prevent collapse and protect lives. Buildings are designed to absorb seismic energy through controlled, repairable damage while maintaining structural integrity.
Key design features include flexible foundations that can move with the ground, reinforced concrete or steel frames that resist side-to-side motion from S-waves, and base isolators that decouple buildings from ground motion. Proper connections between walls, floors, and roofs prevent buildings from separating during shaking.
Unreinforced Masonry: The Deadliest Building Type
Unreinforced masonry buildings—made of brick or stone without steel reinforcement—pose the greatest hazard. These structures have killed more people in earthquakes than any other building type. They perform poorly in earthquakes because masonry is strong in compression but weak in tension. When shaken side to side, the walls separate and collapse.
Many older buildings, particularly those built before modern seismic codes, fall into this category. The 1988 Armenian earthquake killed over 25,000 people largely because of unreinforced masonry construction.
5 Population Density and Development
More people and infrastructure in the affected area means more potential casualties and higher economic losses. However, this factor interacts complexly with building quality.
A magnitude 7.0 earthquake in a densely populated area with modern construction might cause fewer casualties than a magnitude 6.0 in a densely populated area with poor construction. The 2010 Haiti earthquake, magnitude 7.0, killed over 100,000 people due to a combination of poor construction, high population density, and poverty. In contrast, the 2019 Ridgecrest earthquakes in California, including a magnitude 7.1, caused no deaths despite being larger, primarily because the affected area was sparsely populated.
Urban areas also face cascading failures. Damage to one system—such as water mains breaking—affects other systems like firefighting capability. The resulting fires after the 1906 San Francisco earthquake caused more destruction than the earthquake itself.
6 Duration of Shaking
People often don't realize that shaking duration varies dramatically between earthquakes. A magnitude 5.0 earthquake might shake for 5-10 seconds. A magnitude 7.0 might shake for 20-30 seconds. A magnitude 9.0 can shake for several minutes.
Longer shaking causes more damage because buildings experience more stress cycles, accumulating damage with each cycle. A building might survive 15 seconds of strong shaking but fail after 45 seconds of the same intensity shaking.
The 2011 Tohoku earthquake shook for approximately five minutes. Even well-designed buildings experienced significant damage simply because the shaking continued relentlessly. Each additional cycle of shaking weakened structures a bit more.
Duration also relates to earthquake magnitude and the fault rupture length. Larger earthquakes involve longer sections of fault rupturing, which takes more time and generates longer periods of shaking. Understanding what causes earthquakes helps explain why larger ruptures produce longer shaking.
7 Time of Day and Year
When an earthquake strikes dramatically affects casualties. An earthquake at 3:00 AM catches people sleeping in their homes. An earthquake at 3:00 PM finds people at work, school, or commuting. Buildings might collapse either way, but where people are located determines how many are affected.
The 1995 Kobe earthquake struck at 5:46 AM, when most people were home sleeping. The timing contributed to the high death toll of over 6,000. The 1989 Loma Prieta earthquake occurred at 5:04 PM during evening rush hour, yet casualties were relatively low partly because a baseball World Series game had kept many people home watching television instead of commuting on the collapsed sections of freeway.
Season matters too. Winter earthquakes in cold climates can make rescue and recovery more difficult. They also increase fire risk from damaged heating systems. Summer earthquakes might affect more people outdoors but provide better conditions for rescue operations and temporary shelter.
8 Aftershocks and Earthquake Sequences
As we discuss in our article on aftershocks, most significant earthquakes are followed by numerous smaller earthquakes. These aftershocks damage already-weakened structures and endanger rescue workers.
A building damaged in the mainshock might remain standing but become unsafe. Aftershocks can cause final collapse. The 2010-2011 Canterbury earthquake sequence in New Zealand demonstrated this effect. The initial magnitude 7.1 earthquake in September 2010 caused significant damage but no deaths. The magnitude 6.3 aftershock in February 2011 struck a city already weakened by months of aftershocks, causing 185 deaths and widespread destruction.
Aftershocks also complicate rescue efforts and recovery. Rescue workers must repeatedly evacuate damaged buildings when large aftershocks strike. People become exhausted and traumatized by ongoing shaking. Economic recovery stalls as aftershocks continue damaging repaired structures.
Frequency Matters: Resonance and Building Response
Buildings have natural frequencies at which they prefer to vibrate, similar to how a tuning fork vibrates at a specific pitch. When seismic waves match a building's natural frequency, the building vibrates more intensely—a phenomenon called resonance.
Taller buildings have lower natural frequencies and respond more to longer-period seismic waves. Shorter buildings have higher natural frequencies and respond more to shorter-period waves. This means the same earthquake can cause very different damage patterns to different buildings.
During the 1985 Mexico City earthquake, many mid-rise buildings (5-15 stories) collapsed while both shorter and taller buildings survived. The distant earthquake produced long-period waves that resonated specifically with mid-rise structures. The soft lake-bed soils underlying Mexico City further amplified these problematic frequencies.
🏢 Building Height and Earthquake Response
Short buildings (1-3 stories): Respond to high-frequency (short-period) waves, typically from nearby earthquakes
Mid-rise buildings (4-10 stories): Respond to moderate-frequency waves
High-rise buildings (15+ stories): Respond to low-frequency (long-period) waves, often from distant large earthquakes
This is why engineers must design buildings based on expected earthquake characteristics for their specific location and height.
Why Magnitude Alone Doesn't Tell the Story
Given all these factors, it becomes clear why asking "how big was the earthquake?" only tells part of the story. Two magnitude 6.5 earthquakes can produce vastly different outcomes.
Consider these contrasts:
A magnitude 6.5 earthquake at 100 kilometers depth under a remote mountain range: No casualties, minimal damage, perhaps not even felt at the surface.
A magnitude 6.5 earthquake at 5 kilometers depth under a densely populated city with poor building codes: Thousands of casualties, billions in damage, decades of recovery.
This is why scientists emphasize intensity measurements like the Modified Mercalli Intensity (MMI) scale alongside magnitude. While magnitude describes the earthquake itself, intensity describes what people actually experienced—the shaking strength, the damage observed, and the earthquake's impact.
It's also why determining what magnitude is dangerous requires considering all these contextual factors, not just the magnitude number itself.
Secondary Hazards Multiply Damage
Earthquakes rarely kill people directly through ground shaking alone. Instead, secondary hazards often prove most deadly.
Fire
Earthquakes break gas lines, electrical wiring, and other ignition sources while simultaneously rupturing water mains needed for firefighting. The 1906 San Francisco earthquake and subsequent fires destroyed 80% of the city. The 1923 Great Kanto earthquake in Japan killed an estimated 105,000 people, many in fires that spread uncontrollably through wooden buildings.
Tsunamis
Large underwater earthquakes displace massive volumes of water, generating tsunamis that can devastate coastal areas thousands of kilometers away. The 2011 Tohoku earthquake killed most victims through the tsunami rather than ground shaking. The 2004 Indian Ocean earthquake killed over 230,000 people, nearly all from the tsunami.
Landslides
Earthquake shaking can trigger massive landslides, particularly in mountainous terrain. The 1970 Ancash earthquake in Peru triggered a debris avalanche that buried the town of Yungay, killing 20,000 people in minutes. The landslide proved far more deadly than the earthquake shaking itself.
Dam Failures
Earthquakes can damage dams, leading to catastrophic flooding downstream. Even if a dam doesn't fail immediately, earthquake damage might weaken it, creating a time-delayed hazard.
Human Factors: Preparedness and Response
Human factors—before, during, and after an earthquake—significantly affect outcomes.
Before: Preparedness and Prevention
Communities with earthquake education, regular drills, and prepared emergency supplies fare better. Japan's extensive earthquake preparedness education has saved countless lives. Building retrofits to strengthen vulnerable structures prevent collapses. Land-use planning that prohibits development on liquefiable soils or steep slopes avoids predictable disasters.
During: Response and Behavior
How people respond during shaking matters. "Drop, Cover, and Hold On" saves lives. Running outside often proves more dangerous, as people are hit by falling debris. Understanding earthquake safety protocols reduces injuries.
After: Emergency Response
Rapid emergency response saves lives by rescuing trapped survivors, providing medical care, and preventing secondary disasters like fires from spreading. Countries with well-organized emergency services, trained search-and-rescue teams, and stockpiled supplies respond more effectively.
However, even the best emergency response can be overwhelmed if the disaster is large enough. The 2010 Haiti earthquake devastated the country's infrastructure, including hospitals, which made emergency response extraordinarily difficult.
The Bottom Line
Earthquake magnitude provides crucial information about the energy released, but it's only one piece of a complex puzzle. Damage depends on a combination of natural factors like depth, distance, and soil conditions, along with human factors like building quality, population density, and emergency preparedness.
This is why seismologists cringe when headlines focus solely on magnitude numbers. A magnitude 7.0 earthquake isn't automatically more damaging than a magnitude 6.5. Context determines outcomes. The earthquake that strikes 5 kilometers deep under a major city during business hours in winter, shaking soft soils and poorly constructed buildings for 45 seconds, with multiple large aftershocks following, will prove far more catastrophic than a larger earthquake occurring 50 kilometers deep in a remote area, even if the latter releases more total energy.
Understanding these factors helps explain why earthquake engineering focuses on managing the controllable variables: improving building construction, avoiding development in hazardous areas, preparing emergency response systems, and educating the public. We can't control earthquake magnitude, depth, or timing. But we can control how we build, where we build, and how prepared we are when earthquakes inevitably strike.
The most important takeaway is that earthquake damage is preventable. Proper construction, sensible land-use planning, and adequate preparedness can reduce casualties and damage by 90% or more, even for large earthquakes. The difference between disaster and resilience often comes down to human choices made years or decades before the ground starts shaking.
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