Two Magnitude-7+ Quakes Hit Days Apart — Are They Connected and What To Know

Two powerful earthquakes — a magnitude 7.0 in remote Alaska on Dec. 6 and a magnitude 7.6 in northern Japan on Dec. 8 — struck within days of each other, prompting injuries, evacuations and a flurry of aftershocks. While the timing raised concerns about a larger, related event, seismologists say the two quakes appear unrelated and that clustered large quakes can occur by chance.

What Happened

Alaska: A magnitude-7.0 quake struck a sparsely populated area near the Canadian border on Dec. 6. Because the region is remote, damage was limited.

Japan: On Dec. 8 a magnitude-7.6 tremor hit northern Japan, triggering tsunami warnings that were later lifted. Officials reported at least 30 injuries and tens of thousands of people evacuated their homes. Both events produced multiple aftershocks.

Are They Linked?

Short answer: almost certainly not. The Alaska and Japan quakes occurred on different plate boundaries thousands of miles apart. Aftershocks typically occur close to the mainshock, and there is no identified physical mechanism linking these two events.

“It’s not all the time, but it’s not that crazy,” said Brandon Schmandt, a seismology professor at Rice University, underscoring that occasional clustering of large quakes is expected by chance.

What Science Says About Timing and Risk

The U.S. Geological Survey (USGS) estimates an average of about 16 major earthquakes worldwide each year (roughly 15 of magnitude 7.0+ and one of magnitude 8.0+), based on records since about 1900. Caltech seismologist Lucy Jones notes the world averages roughly one magnitude-7.0 quake per month, and those events are randomly distributed in time. That randomness means you can see long quiet stretches or brief clusters.

After any earthquake there is a higher short-term likelihood of another nearby event: Jones estimates roughly a 5% chance that a nearby quake within a few days will be larger than the initial event. That elevated short-term risk is why authorities — such as those in Japan following the Dec. 8 quake — may urge heightened vigilance.

Long-Term Hazards

The USGS warns that much of the United States is at risk of damaging earthquake shaking within the next century and has identified hundreds of previously unrecognized faults. The greatest hazards are along active plate boundaries, especially the U.S. West Coast and southern Alaska.

The Cascadia Subduction Zone — which runs from Northern California through Oregon and Washington into Canada — is a particular concern. Agencies estimate about a 37% chance of a megathrust earthquake of magnitude 7.1 or greater in that zone within the next 50 years; a major event there could be felt across the Pacific Northwest.

What To Know About Magnitude Effects

For public context, Michigan Technological University summarizes typical effects by magnitude:

• Below 2.5: Generally not felt
• 2.5–5.4: Minor or no damage
• 5.5–6.0: Slight building damage
• 6.1–6.9: Serious damage
• 7.0–7.9: Major earthquake; serious damage possible
• 8.0+: Massive damage; can destroy communities

Preparedness Tips

Earthquake prediction remains unreliable, so preparedness is the best defense. Key steps include:

• Secure heavy furniture and objects that could fall.
• Have an emergency kit with water, food, medications and a flashlight.
• Know safe spots in each room (under sturdy tables, away from windows).
• Review evacuation and communication plans with family members.

Sources: U.S. Geological Survey, Caltech, Michigan Technological University, Oregon Department of Emergency Management.