Earthquake vs Aftershock: What's the Difference?

After a major earthquake strikes, you often hear about "aftershocks" continuing for days, weeks, or even months. But what exactly is an aftershock, and how is it different from the main earthquake?

What Is an Aftershock?

An aftershock is a smaller earthquake that occurs in the same region as a larger earthquake (called the mainshock). Aftershocks are caused by adjustments in the crust around the fault that slipped during the main earthquake.

Think of it like this: When a major earthquake occurs, the fault ruptures and releases stress. But this rupture creates new stresses in the surrounding rocks. As these rocks adjust to their new configuration, they produce aftershocks.

Key Differences

Magnitude

By definition, aftershocks are smaller than the mainshock. However, they can still be significant earthquakes in their own right. A M7.0 mainshock might produce M6.0 aftershocks, which are still strong enough to cause damage.

Typical aftershock pattern:

• Largest aftershock usually 1-2 magnitudes smaller than mainshock
• Most aftershocks are much smaller (M3-4)
• Number of aftershocks decreases over time

Timing

Aftershocks begin within minutes of the mainshock and can continue for months or even years. The frequency and size of aftershocks decrease over time following a predictable pattern described by Omori's Law.

Omori's Law states: The rate of aftershocks decreases roughly in proportion to the inverse of time since the mainshock. In simpler terms: If you have 100 aftershocks on day one, you might expect 50 on day two, 33 on day three, and so on.

Location

Aftershocks occur near the mainshock, typically within 1-2 fault lengths of the rupture. They cluster around the ends of the fault rupture and in areas where stress was transferred during the mainshock.

How Long Do Aftershocks Last?

Aftershock sequences follow a predictable pattern, but can continue for surprisingly long periods:

First Day

Hundreds to thousands of aftershocks possible, depending on mainshock magnitude. The largest aftershock typically occurs within the first day, but not always immediately after the mainshock.

First Week

Dozens of aftershocks per day. These can still be strong enough to be felt and cause additional damage to already-weakened structures.

First Month

Several aftershocks per day. The rate continues to decrease but significant aftershocks remain possible.

First Year

Occasional aftershocks, typically small. However, aftershocks above M5.0 can still occur months after the mainshock.

Years Later

Very rare aftershocks are still possible years after a major earthquake. Some seismologists believe aftershock sequences from very large earthquakes can continue for decades.

Example: The 1964 M9.2 Alaska earthquake produced aftershocks for over 50 years. A M5.2 aftershock occurred in 2018 - 54 years after the mainshock!

Are Aftershocks Dangerous?

Yes! Aftershocks can be extremely dangerous. Here's why:

1. Collapse Already-Damaged Buildings

Buildings weakened by the mainshock may appear structurally sound but can collapse during large aftershocks. This is why buildings are inspected and sometimes condemned after major earthquakes.

2. Trigger Landslides

Hillsides destabilized by the mainshock can give way during aftershocks, even if they survived the initial earthquake.

3. Hamper Rescue Efforts

Rescue workers searching through rubble face continuous danger from aftershocks. This slows rescue operations and puts first responders at risk.

4. Cause Psychological Trauma

The constant fear of the next aftershock causes severe psychological stress. Many earthquake survivors report that continuous aftershocks were more traumatic than the mainshock itself.

Famous Aftershock Sequences

2011 Tohoku Earthquake, Japan

The M9.1 mainshock was followed by an extraordinary aftershock sequence:

• Over 1,000 aftershocks above M4.0 in the first month
• Several aftershocks above M7.0 (major earthquakes in their own right)
• M7.3 aftershock occurred one month after mainshock
• Aftershocks continued for years
• A M7.4 aftershock occurred in 2022 - 11 years later!

2019 Ridgecrest Earthquakes, California

This sequence demonstrates the complexity of earthquake classification:

• July 4: M6.4 earthquake (thought to be mainshock)
• July 5: M7.1 earthquake (actual mainshock - made the M6.4 a foreshock)
• Thousands of aftershocks followed
• Aftershocks continued for over a year
• Largest aftershock was M5.5

1992 Landers Earthquake, California

The M7.3 mainshock produced:

• Aftershocks that continued for over a year
• Multiple M5.0+ aftershocks months later
• Triggered earthquakes as far as 800 km away
• Changed the stress on nearby faults

Foreshocks: The Opposite of Aftershocks

Sometimes small earthquakes occur BEFORE a large earthquake. These are called foreshocks. However, it's impossible to know if a small earthquake is a foreshock until after a larger earthquake occurs.

The problem:

• Only about 50% of mainshocks are preceded by foreshocks
• Most small earthquakes are NOT followed by larger ones
• We can't distinguish foreshocks from independent earthquakes in real-time

Example: In the 2019 Ridgecrest sequence, the M6.4 was initially called the mainshock. Only after the M7.1 occurred was it reclassified as a foreshock.

Triggered Earthquakes vs Aftershocks

Not all earthquakes following a mainshock are technically aftershocks. Some are "triggered earthquakes" that occur on different faults far from the mainshock.

Differences:

• Aftershocks: Same fault system, near mainshock location
• Triggered earthquakes: Different faults, can be hundreds of km away

The 1992 Landers earthquake triggered earthquakes as far away as Yellowstone National Park and geothermal areas in Nevada.

How Scientists Study Aftershocks

Aftershock sequences provide valuable information about:

• Fault geometry: Aftershock locations outline the fault plane
• Stress transfer: Where stress moved during the mainshock
• Future earthquake potential: Identifying loaded fault sections
• Building response: How structures perform during repeated shaking

Staying Safe During Aftershocks

Immediately After the Mainshock

• Expect aftershocks - they WILL occur
• Check for injuries and hazards
• Exit damaged buildings carefully
• Stay away from damaged structures
• Keep emergency supplies accessible

During Aftershocks

• Drop, Cover, Hold On (same as mainshock)
• If outdoors, stay in open areas
• If in vehicle, pull over safely
• Protect your head and neck

After Each Aftershock

• Check for new damage
• Watch for falling hazards
• Stay alert for more aftershocks
• Avoid entering damaged buildings

The Psychological Impact

Many earthquake survivors report that continuous aftershocks are more psychologically difficult than the mainshock itself. Each aftershock triggers:

• Fear that it might be another mainshock
• Anxiety about building safety
• Sleep disruption
• Constant hypervigilance
• PTSD symptoms

This is normal and expected. Seek support if aftershocks are causing severe distress.

The Bottom Line

Aftershocks are a normal and expected part of the earthquake process. Key points to remember:

• Aftershocks are smaller earthquakes following a larger mainshock
• They can continue for days, months, or even years
• Large aftershocks can still cause significant damage
• Frequency decreases over time but never reaches zero instantly
• Buildings weakened by mainshock are vulnerable to aftershock collapse
• Psychological impact can be severe

If you experience a major earthquake, expect aftershocks and take them seriously. Each aftershock poses real danger, especially to already-damaged structures.

Learn More

Prepare for earthquakes and aftershocks with our complete preparedness guide.