Earthquakes cause damage through a combination of violent ground shaking, ground failure, and secondary hazards like fires, tsunamis, and landslides. The shaking itself accounts for 60 to 75% of total economic losses and deaths, while secondary effects contribute the remaining 25 to 40%. Globally, earthquakes are responsible for over 25% of all economic disaster losses.
How Seismic Waves Damage Structures
Not all earthquake waves cause the same kind of destruction. The first waves to arrive, called P-waves, travel at roughly 15,000 miles per hour and cause buildings to vibrate up and down. These are followed by S-waves, which shake structures from side to side. S-waves are the most damaging because buildings are far more vulnerable to horizontal motion than vertical motion. Walls, columns, and joints that comfortably support weight pressing down on them can buckle or snap when pushed sideways.
The final waves to arrive, known as surface waves, roll along the ground at lower frequencies. These slower vibrations are especially dangerous for tall buildings. A building’s height determines its natural frequency of vibration, the rate at which it naturally sways back and forth. When seismic waves match that natural frequency, the building enters resonance and its swaying amplifies dramatically, sometimes to the point of collapse. Short buildings respond more to the high-frequency waves that arrive first, while skyscrapers and tall structures are more susceptible to the low-frequency surface waves that arrive last.
This is why earthquakes can level one building while leaving a neighboring structure of different height relatively intact. The ground itself also matters: soft sediments amplify lower-frequency waves compared to hard bedrock, which transmits higher frequencies. A tall building on soft soil faces a particularly dangerous combination.
Ground Failure and Liquefaction
Some of the most dramatic earthquake damage happens when the ground itself gives way. Soil liquefaction occurs when saturated, loosely packed soil loses its strength during shaking and behaves like a liquid. Buildings sink, tilt, or topple not because their walls failed, but because the ground beneath them could no longer support their weight.
Several conditions make liquefaction more likely: a shallow water table, sandy or silty soil with low density, and proximity to rivers, lakes, coastlines, or floodplains. The intensity and duration of shaking also play a direct role. Stronger, longer earthquakes are more likely to trigger liquefaction, even in soil that might survive a brief tremor. Areas built on filled land or former marshes are especially vulnerable, which is why some urban neighborhoods suffer disproportionate damage compared to nearby areas on solid rock.
Secondary Hazards: Fires, Tsunamis, and Landslides
The shaking stops, but the damage often continues. Roughly 30% of direct economic losses from earthquakes come from secondary effects, and that figure climbs to 38% when indirect costs are included. These secondary hazards sometimes rival or exceed the destruction caused by shaking alone.
Fires break out when gas lines rupture and electrical systems short-circuit. In dense urban areas, fires can spread rapidly through damaged buildings where water mains have also broken, leaving firefighters without water pressure. The 1906 San Francisco earthquake is a classic example: the fires that followed caused more destruction than the shaking itself.
Tsunamis are triggered when undersea earthquakes displace large volumes of water. The 2004 Indian Ocean and 2011 Tohoku earthquakes both produced catastrophic tsunamis responsible for the majority of deaths and a significant share of economic losses, with some events attributing over 10% of total losses to tsunami damage alone. Landslides, meanwhile, account for over 5% of global earthquake economic losses. That figure stays relatively low only because fewer people live in the mountainous terrain where earthquake-triggered landslides are most common.
Damage at Different Intensities
Scientists use the Modified Mercalli Intensity Scale to describe earthquake damage in practical, observable terms rather than abstract measurements. The scale runs from I to XII, and its descriptions paint a clear picture of how destruction escalates.
- Intensity IV (Moderate): Felt indoors by many people. Dishes, windows, and doors rattle. Walls make cracking sounds. Feels like a heavy truck striking the building.
- Intensity VI (Strong): Felt by everyone. Windows and dishes break, books fall off shelves, some plaster falls. People run outdoors and have difficulty walking steadily.
- Intensity VIII (Destructive): Considerable damage in ordinary buildings, with partial collapse. Chimneys, factory stacks, columns, and monuments fall. Heavy furniture is displaced.
- Intensity X (Disastrous): Most masonry and frame structures are destroyed along with their foundations. Even some well-built wooden buildings are destroyed. Railroad rails bend.
- Intensity XII (Catastrophic): Near-total destruction. The ground moves in visible waves. Objects are thrown into the air. Large rock masses shift position.
Building quality matters enormously at every level. At intensity VII, a well-designed building suffers negligible damage while a poorly built one sustains considerable damage. This gap only widens as intensity increases, which is why building codes save lives in earthquake-prone regions.
Injuries From Structural Collapse
The most common cause of earthquake deaths and injuries is building collapse. People trapped under rubble face crush injuries when heavy debris compresses their limbs or torso for extended periods. When someone has been pinned for more than four hours, the trapped muscle tissue begins breaking down and releasing toxic byproducts into the bloodstream. Once rescued and the pressure is released, those byproducts flood the body and can cause kidney failure and cardiac arrest. This makes the timing and method of rescue critical for survival.
Beyond crush injuries, earthquakes cause broken bones, lacerations from shattered glass, head trauma from falling debris, and respiratory problems from inhaling concrete dust and particulate matter. In the hours and days after a major quake, overwhelmed infrastructure (collapsed hospitals, blocked roads, disrupted water and power) compounds the health toll by delaying medical care and cutting off clean water supplies.
The Economic Scale of Earthquake Damage
Earthquake losses are staggering in absolute terms. Global disaster costs, which earthquakes contribute more than a quarter of, averaged $70 to $80 billion per year between 1970 and 2000. Between 2001 and 2020, that figure jumped to $180 to $200 billion annually. When cascading effects and ecosystem damage are factored in, total disaster costs now exceed $2.3 trillion per year worldwide.
The impact is not evenly distributed. Wealthier regions absorb larger dollar amounts but feel less economic pain. In 2023, North America recorded $69.57 billion in direct disaster losses, representing just 0.23% of its GDP. Micronesia, by contrast, lost $4.3 billion, a fraction of that amount, but it represented 46.1% of its subregional GDP. For smaller and lower-income nations, a single earthquake can set back economic development by years or decades.