Post-earthquake fires, soil liquefaction, earthquake-triggered landslides, tsunami risk, and the mental health toll of prolonged aftershock sequences.
The popular mental model of an earthquake is of the ground shaking and things falling down. When shaking stops, the hazard is over. This model is dangerously incomplete.
Historical earthquake casualty records reveal that the shaking phase itself is responsible for a minority of total deaths and economic damage in many major events. The 1906 San Francisco earthquake killed far fewer people during the actual shaking than were killed and left homeless by the fires that burned for three days afterward. The 2011 Christchurch earthquake caused severe damage and casualties through liquefaction of the city's soil beneath buildings and roads. Earthquake-triggered landslides have buried entire villages.
Understanding these secondary hazards — what they are, how they develop, and how to respond to them — is as important as knowing what to do during the shaking itself.
Earthquakes rupture gas lines simultaneously across wide areas, and the same shaking that breaks the pipes also breaks or disrupts electrical connections that create ignition sources. The result is the simultaneous ignition of multiple fires across a neighbourhood or city at a time when:
In the 1906 San Francisco earthquake, fire burned 500 city blocks over three days and destroyed approximately 28,000 buildings. The earthquake itself caused perhaps 500 deaths directly; the fires caused the rest of the estimated 3,000 total deaths and the wholesale destruction of the city.
In the 1995 Kobe earthquake in Japan, fires broke out in approximately 200 locations simultaneously. Narrow streets blocked by collapsed buildings prevented fire trucks from reaching many of the fires, which burned unchecked.
Liquefaction occurs when saturated, loose, granular soil (typically sand) is subjected to the rapid cyclic stress of an earthquake. The soil grains lose contact with each other as pore water pressure increases, and the soil temporarily behaves like a dense liquid rather than a solid.
The effects are dramatic:
The 2011 Canterbury earthquake sequence in Christchurch, New Zealand, caused spectacular liquefaction across large parts of the city, damaging tens of thousands of homes and requiring the demolition and clearance of entire residential suburbs. Liquefaction caused approximately 50% of the total insured losses.
Liquefaction is most likely to occur where all of the following conditions are present:
High-risk areas are typically:
Checking your risk: The USGS provides liquefaction susceptibility maps for many US areas. State geological surveys (California, Oregon, Washington, etc.) produce detailed liquefaction hazard maps that can be searched by address. If you live in a potentially susceptible area, a geotechnical engineer can assess your specific property.
| Liquefaction Risk Factor | Low Risk | High Risk |
|---|---|---|
| Soil type | Hard clay, rock, dense sand | Loose sand, silt, fill material |
| Water table depth | >10 metres | <3 metres |
| Location | Upland, bedrock | River delta, reclaimed land, coast |
| Fill history | None or compacted engineered fill | Historic bay fill, uncontrolled fill |
| Recent development | Established site | On former water body or wetland |
Earthquake shaking destabilises slopes by:
A slope that is marginally stable under normal conditions can fail almost instantly during strong earthquake shaking. Unlike rainfall-triggered landslides, which develop over minutes or hours, earthquake-triggered slides can mobilise simultaneously across many slopes throughout an earthquake-affected area.
The 1964 Alaska earthquake triggered thousands of landslides across the state. The 2008 Sichuan earthquake in China triggered tens of thousands of landslides, killing approximately 20,000 people through direct slide impact and through dam formation that subsequently failed, releasing stored water downstream.
In the US, the Pacific Northwest, Alaska, Hawaii, and parts of the Appalachians all have significant earthquake-triggered landslide risk.
If you live or travel in mountainous or hilly terrain in an earthquake region:
Earthquakes can damage dam structures and their foundations. A major earthquake near a dam can trigger:
After a major earthquake, dam safety inspections are conducted by state and federal dam safety programmes. If you live downstream of a dam in an earthquake-affected area, monitor official communications for dam safety updates.
If you receive a dam failure warning or observe an obvious and sudden increase in downstream water levels after an earthquake, treat it as an immediate flood emergency: move to high ground immediately.
Not all earthquakes generate tsunamis, but submarine earthquakes (occurring under or near the ocean floor) with:
...can generate tsunami waves that travel across ocean basins and devastate coastlines.
Critical rule for coastal areas: Any strong earthquake felt on the coast should be treated as a potential tsunami trigger. Do not wait for an official warning. Move inland or to high ground immediately.
This is most critical for local source tsunamis — where the earthquake and the coast are close together. In these situations, the tsunami arrives in minutes — well before any official warning system can issue an alert and before most people can receive it.
| Tsunami Source Type | Distance | Expected Warning Time |
|---|---|---|
| Local (within 100km) | Nearby coast | Minutes — natural signs only |
| Regional (100–1000km) | Regional coast | 30 minutes to 2 hours |
| Distant (1000km+) | Remote coast | 2–24 hours — PTWC warning possible |
The Pacific Tsunami Warning Center (PTWC) and National Tsunami Warning Center (NTWC) monitor seismic activity and issue tsunami watches and warnings. In local tsunami situations, however, the natural warning (feeling strong shaking at the coast) may be the only warning you receive.
Most disasters have a beginning and an end. Earthquake aftershock sequences do not. For days, weeks, and months after a major earthquake, the ground continues to shake. Each tremor, even a minor one, triggers the fight-or-flight response. Sleep is disrupted. The sense of threat does not resolve.
Communities affected by major earthquakes report:
After the 2010–2011 Canterbury earthquake sequence in New Zealand (which included over 10,000 aftershocks over several years), mental health presentations increased significantly across the region, with anxiety and post-traumatic stress disorders the most common presentations.
⚠️ If you experience persistent sleep disturbance, inability to concentrate, severe anxiety, or other symptoms significantly affecting daily function for more than 2–3 weeks after an earthquake, speak with a healthcare provider. Post-traumatic stress is a medical condition with effective treatments.
| Situation | Action |
|---|---|
| Smell gas after earthquake | Exit immediately, do not use switches, call utility from outside |
| Small fire just starting | Use fire extinguisher if available and safe — otherwise evacuate |
| Multiple fires in neighbourhood | Alert neighbours, call fire services, evacuate if fire spreading |
| Ground sinking or tilting after quake | Move away from structure — liquefaction may be occurring |
| Sand boils or water erupting from ground | Move away — ground is actively liquefying |
| Near mountain slope after quake | Move away from slope base — earthquake landslide risk |
| Strong earthquake felt on coast | Move to high ground immediately — potential tsunami, do not wait |
| Downstream of dam after major quake | Monitor official communications; be ready to move to high ground |
| Aftershocks occurring frequently | Drop, cover, hold on each time; treat every tremor as potentially serious |
| Ongoing anxiety weeks after earthquake | Seek mental health support — this is a normal response with treatment options |
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