Earthquakes can trigger tsunamis, landslides, fires, flooding, soil liquefaction, and even volcanic eruptions. These secondary disasters are far from minor footnotes. Looking at major earthquakes from 1900 onward, secondary effects account for roughly 40% of both fatalities and economic losses globally, rivaling the destruction caused by ground shaking itself.
Tsunamis
Tsunamis are the most devastating secondary disaster an earthquake can produce. They happen when a shallow earthquake beneath the ocean floor displaces a large volume of water, sending powerful waves radiating outward. The key requirement is vertical movement of the seafloor, which is why tsunamis are almost always generated by thrust faults at subduction zones rather than by strike-slip faults that move sideways.
Not every undersea earthquake creates a dangerous wave. The U.S. Geological Survey breaks it down by magnitude:
- Below 6.5: Very unlikely to trigger any tsunami.
- 6.5 to 7.5: Small sea level changes near the epicenter are possible, but destructive tsunamis are rare. When they do occur at this magnitude, it’s usually because the quake triggered an underwater landslide that displaced additional water.
- 7.6 to 7.8: Destructive tsunamis become possible near the epicenter, though damaging waves at great distances remain uncommon.
- 7.9 and above: Destructive local tsunamis are likely, and significant damage can extend across entire ocean basins.
The 2011 Tohoku earthquake off Japan’s coast, magnitude 9.1, generated a tsunami that destroyed over 92,000 buildings and partially destroyed another 78,000. Waves reached heights above 30 feet in some coastal areas, traveling miles inland. That single tsunami caused more structural damage than the earthquake’s ground shaking.
Landslides and Avalanches
Earthquake shaking destabilizes slopes that may have been marginally stable for years. Steep hillsides, cliffs, and mountain faces can collapse during or immediately after a quake, sending rock, soil, and debris downhill at high speed. In mountainous regions during winter, the same shaking can release snow avalanches.
Two factors determine whether a slope will fail. For shallow landslides or quakes with low-frequency shaking, the critical factor is peak ground acceleration: how hard the ground jerks sideways and up. For deeper, thicker landslide masses or high-frequency shaking, peak ground velocity matters more. In practical terms, this means a moderate earthquake close to a steep slope can be just as dangerous as a larger quake farther away, because the intensity of shaking at the surface is what counts.
Earthquake-triggered landslides are especially destructive in wet conditions, when soil is already heavy and saturated. They can bury entire villages, block rivers to create temporary lakes, and sever roads that rescue teams need to reach survivors.
Fires
Post-earthquake fires have historically caused as much destruction as the shaking itself. The 1906 San Francisco earthquake is the classic example: fires burned for three days and destroyed far more of the city than the ground shaking did. The same pattern played out in the 1995 Kobe earthquake in Japan, where hundreds of fires broke out within hours.
The ignition sources fall into two categories. External sources include ruptured gas pipelines, damaged electrical substations, and compromised industrial storage facilities. Internal sources, inside buildings, include overturned containers of flammable liquids, electrical short circuits from deformed wiring, sparking from mechanical stress on appliances, and broken internal gas lines. Chimneys cracking open near combustible materials are another common trigger.
What makes post-earthquake fires so dangerous is that they start at a time when firefighting capacity is at its lowest. Water mains break during the shaking, leaving hydrants dry. Roads are blocked by debris. Multiple fires ignite simultaneously across a wide area, overwhelming crews who would normally handle them one at a time. Countries with well-engineered automatic shutoff valves for gas and electricity have significantly fewer post-earthquake fires, which points to how preventable many of these ignitions are.
Soil Liquefaction
During strong shaking, certain types of ground can behave less like solid earth and more like a thick liquid. This process, called liquefaction, happens when loose, sandy soil that is saturated with groundwater loses its structural strength. The repeated shaking causes water pressure between soil grains to spike, and the ground temporarily loses its ability to support weight.
The results are dramatic. Buildings tilt and sink unevenly into the ground. Underground pipes and tanks float upward to the surface. Roads buckle and crack. Sand and water erupt in small fountains called sand boils. The damage is particularly insidious because structures that survived the shaking itself can be destroyed minutes later as the ground beneath them gives way.
Liquefaction requires a specific combination: loose granular soil (typically sand or silty sand), a high water table that keeps the soil saturated, and shaking intense enough to rearrange the grain structure. Coastal areas, river deltas, and reclaimed land are the most vulnerable. The degree of saturation in the soil is one of the most important variables. Partially saturated soils are significantly more resistant to liquefaction than fully saturated ones, which is why areas with deeper water tables fare better.
Flooding and Dam Failure
Earthquakes cause flooding through several different mechanisms. The most direct is dam failure. Shaking can crack earthen dams, cause concrete dams to shift on their foundations, or trigger liquefaction in the soil a dam sits on. If the dam holds but the surrounding slopes don’t, landslides falling into the reservoir can send water surging over the top. The U.S. Army Corps of Engineers requires that dams be evaluated for all of these scenarios: strong shaking, earthquake-induced landslides, liquefaction beneath or around the structure, and reservoir waves.
Seiches are another flood mechanism most people don’t think about. A seiche is a standing wave that forms in an enclosed or partially enclosed body of water, like a lake, reservoir, or even a swimming pool, when earthquake waves rock the basin back and forth. The water sloshes from one end to the other, sometimes for hours. In a reservoir behind a dam, seiches can send water over the top of the dam even if the dam itself is undamaged. In lakes, seiches can flood shoreline communities without warning.
Earthquakes can also cause flooding indirectly by triggering landslides that block rivers. The temporary lake behind the debris dam fills rapidly, and when the natural dam eventually fails, a catastrophic flood rushes downstream.
Volcanic Eruptions
Large earthquakes can sometimes push a volcano into eruption, but only if that volcano is already primed. According to the USGS, two conditions must be met: the volcano must contain enough molten rock that is ready to erupt, and pressure within the magma storage area must already be significant. If both conditions exist, a nearby earthquake above magnitude 6 can act as the final trigger.
The mechanism works something like shaking a sealed bottle of carbonated water. Dissolved gases in the magma come out of solution when the earthquake’s vibrations pass through, rapidly increasing pressure inside the volcanic system. That pressure boost can be enough to overcome the last bit of resistance holding the magma underground. A magnitude 7.7 earthquake in 1975 caused significant damage in Hawai’i Volcanoes National Park and was linked to volcanic unrest in the area.
It’s worth noting that the reverse relationship, volcanic activity causing earthquakes, is far more common. Most earthquakes near volcanoes are caused by magma movement rather than the other way around.
Triggered Earthquakes
One earthquake can trigger additional earthquakes, sometimes hundreds of miles from the original event. The 2011 Tohoku earthquake set off a cascade of smaller quakes detected across Japan’s dense seismic monitoring network. These triggered earthquakes occur because seismic waves from the mainshock alter stress conditions on distant faults that were already close to their breaking point.
These secondary quakes are typically smaller than the original, but they can still cause damage in areas that weren’t directly affected by the mainshock. They can also reactivate landslide-prone slopes or further weaken structures already stressed by the first event, compounding the overall disaster.
Why Secondary Disasters Are So Deadly
Only about 1% of damaging earthquakes account for roughly 93% of all earthquake fatalities worldwide. Within those deadliest events, secondary effects play a disproportionate role. The 40% figure for secondary-effect casualties and economic losses is an average. In individual disasters, the ratio can be far worse. The 2011 Tohoku earthquake killed the vast majority of its nearly 20,000 victims through the tsunami, not the shaking.
The compounding nature of these events is what makes them so dangerous. A single earthquake can trigger a landslide that blocks a river, creating a flood, while simultaneously rupturing gas lines that start fires in a city where the water mains are broken. Each secondary disaster unfolds on its own timeline, stretching emergency response thin and catching survivors off guard hours or even days after the initial shaking stops.