Aftershock Events: Understanding, Preparing for, and Staying Safe After Earthquakes

What Are Aftershocks?

Aftershocks are smaller earthquakes that occur in the same region following a larger mainshock. They result from the Earth’s crust readjusting to changes caused by the main earthquake.

Key Facts About Aftershocks

• Can continue for weeks, months, or even years after the main quake
• Typically less powerful than the mainshock (but can still be dangerous)
• Follow the “Omori’s Law” pattern – frequency decreases over time
• May cause additional damage to already weakened structures

The Science Behind Aftershocks

Why Do Aftershocks Happen?

1. Crustal Adjustment: The Earth’s crust settles into a new equilibrium
2. Stress Redistribution: Strain transfers to nearby faults
3. Fault Healing: The ruptured fault zone gradually strengthens

Aftershock Patterns

• Usually cluster near the mainshock’s epicenter
• Magnitude generally 1-2 units lower than mainshock
• Number of aftershocks decreases exponentially with time

Potential Dangers of Aftershocks

1. Structural Damage

• Buildings weakened by main quake may collapse
• Bridges and infrastructure sustain cumulative damage

2. Landslides & Tsunamis

• Can trigger additional landslides in unstable areas
• May generate localized tsunamis if underwater

3. Psychological Impact

• Prolonged stress and anxiety for survivors
• Disrupts recovery efforts and return to normalcy

How to Prepare for Aftershocks

Before Aftershocks Occur

✔ Secure heavy furniture and objects that could fall
✔ Identify safe spots in each room (under sturdy tables, against interior walls)
✔ Prepare emergency kits with food, water, and first aid supplies
✔ Know how to shut off gas, water, and electricity

During an Aftershock

➤ Drop, Cover, and Hold On – Protect yourself immediately
➤ Stay indoors until shaking stops
➤ Avoid doorways (not safer than other areas)
➤ If outside, move to open areas away from buildings/trees

After an Aftershock

✓ Check for injuries and damage
✓ Be prepared for potential utility outages
✓ Listen to emergency broadcasts for updates
✓ Avoid damaged areas that could collapse

Notable Aftershock Events in History

Aftershock vs. Foreshock vs. Swarm

Aftershock: Follows main quake
Foreshock: Smaller quake before main event
Earthquake swarm: Series without clear mainshock

When to Expect the Worst Aftershocks

• First few days after mainshock are most critical
• Largest aftershock typically 1 magnitude unit smaller than mainshock
• 5% chance the “aftershock” could be larger (actually a new mainshock)

Staying Informed About Aftershocks

• USGS ShakeAlert system (U.S.)
• MyShake app (California)
• Japan Meteorological Agency warnings
• Local emergency management alerts

Special Considerations

• Construction crews: Extra caution working on damaged structures
• Emergency responders: Need to monitor ongoing risks
• Business owners: May need to delay reopening for safety

Conclusion: Living With Aftershocks

While aftershocks can’t be prevented, preparedness reduces risks. Understanding these follow-up quakes helps communities recover smarter and stay safer in earthquake-prone regions.

Remember: After any significant earthquake, expect aftershocks and maintain readiness for at least several weeks. Your preparedness could save lives when the next one hits.