What Today’s M6.0 Means in the Shadow of the 1964 Megathrust Giant
🌎 Today’s Big Shaker: The M6.0
Earlier today, Southcentral Alaska was rattled by a magnitude-6.0 intraslab earthquake beneath the region west of Willow/Wasilla. Deep, sharp, widely felt, but not unusual for Alaska’s seismic personality.
This quake struck within the subducting Pacific Plate, not along the boundary between plates. These deep intraslab events commonly produce long, rolling motion that people across Anchorage, Mat-Su, and the Kenai Peninsula recognize instantly.
No tsunami risk. No catastrophic structural damage. But certainly a wake-up call.
🕰️ How 1964 Fits In: The Megathrust Benchmark
The 1964 Good Friday Earthquake, magnitude 9.2, was a completely different type of event:
- Type: Megathrust (the Pacific Plate thrusting upward beneath the North American Plate)
- Magnitude: 9.2, second-largest earthquake ever recorded on Earth
- Rupture length: ~500–800 miles
- Vertical land movement: Up to 38 feet of uplift/subsidence
- Slip: Up to 60 feet on the fault
- Tsunami: Devastating, both locally and across the Pacific
Megathrust quakes only occur on the shallow interface of the subduction zone, the place where plates lock, accumulate strain, and then release it catastrophically.
By contrast, today’s M6.0 happened deeper inside the descending Pacific Plate, not on the megathrust interface.
📚 Sidebar: What Is a Graben? (1964 Anchorage Example)
A graben is a geological term for a block of the Earth’s crust that drops downward between two parallel faults.
In simpler terms:
- Picture the ground splitting along two long cracks.
- The middle block sinks, while the sides stay higher.
- That sunken block is the graben.
Why it happens:
Tension in the crust pulls the ground apart. As the bounding faults shift, the central block falls relative to the surrounding terrain.
In the 1964 Anchorage Fourth Avenue collapse:
During the 1964 Good Friday megathrust earthquake:
- The ground along Fourth Avenue was stretched and pulled apart.
- The central block (the graben) dropped about 11 feet.
- The surrounding ground remained higher, producing the now-iconic image of the street torn open and slanted, with buildings perched at dramatic angles.
This graben collapse remains one of the most visible and widely studied examples of ground deformation from a major earthquake in U.S. history.
🔍 Side-by-Side: Why Today’s 6.0 Is NOT Like 1964, But Still Matters
1. Different Fault, Different Physics
- 1964: A shallow megathrust rupture, capable of 9.0+ magnitudes and massive tsunamis.
- Today: A deep intraslab quake, typically magnitudes 5–7, with shaking but usually limited damage.
2. No Tsunami Potential Today
Megathrust = tsunami generator.
Intraslab = NO vertical seafloor displacement → no tsunami threat.
3. But Both Are Part of the Same System
Even though the two quake types behave differently, they both originate from the same subduction process:
- The Pacific Plate is continually descending beneath Alaska.
- Stress builds not only at the megathrust but within the plate itself.
- Alaska therefore produces both catastrophic megathrust events and frequent mid-6 magnitude intraslab quakes.
4. Today’s Quake Tells Us Alaska Is Still Very Much “On”
Seismic quiet is not something Alaskans get for long.
A 6.0 like today is a reminder of active tectonic loading, the region is doing exactly what a major subduction zone does:
- Release small and moderate quakes often
- Occasionally produce strong crustal events
- Rarely (but inevitably) deliver a megathrust giant like 1964
Today’s quake does not mean a megathrust is imminent. Science cannot forecast timing.
But it does confirm:
➡️ The subduction zone is active
➡️ Strain continues to accumulate
➡️ Alaska remains one of the world’s top seismic hotspots
🔮 What This Means for Alaska’s Future
Based on seismic science and hazard research:
• Expect continued mid-M5 to mid-M6 intraslab quakes
These are routine and will keep rattling Southcentral Alaska for decades.
• Expect occasional strong crustal faults to rupture
Crustal faults crossing Anchorage, Eagle River, and parts of Mat-Su have produced damaging quakes historically.
• A future megathrust earthquake is inevitable, but timing is unknowable
Subduction ensures Alaska will have another 8–9+ earthquake at some point.
The question is when, not if.
• Preparedness, not prediction, is the scientific recommendation
The Alaska Earthquake Center and USGS emphasize:
- Strengthening older buildings
- Ensuring utilities and pipelines have seismic protection
- Keeping emergency supplies updated
- Reporting damage and felt reports to improve seismic models
📌 Bottom Line for the Shakesgiving Special
Today’s M6.0 was deep, widespread, and vigorous, but geologically normal.
It belongs to the “everyday big” category Alaska experiences regularly.
The 1964 Good Friday megathrust quake remains the benchmark for catastrophic seismic hazard. A totally different beast.
However, both events are connected by the same tectonic machinery that ensures Alaska will always be one of the most earthquake-active regions on Earth.
