* completely contrived term by this author, not a scientific term or theory

The M9.2 Alaska Earthquake and Tsunami

Seward highway damage
The Seward Highway at the head of Turnagain Arm near Anchorage after the quake.

The M9.2 1964 Good Friday, Alaska earthquake ruptured a massive 800km long by 250km wide fault, with two areas that released the most energy – near Kodiak Island and beneath Prince William Sound. In many places the tsunami came ashore more quickly than expected from uplift along the deep subduction zone interface. For example, a large tsunami locally impacted the coastal town of Seward only 1.5-2 minutes after the start of shaking, and then 30 minutes after the earthquake another large tsunami inundated Seward and many other towns along the coast of Prince William Sound. Yet another tsunami later moved out across the ocean to affect distant coastlines as well. Sudden displacement of the seafloor must have occurred somewhere close to shore, but where and why?

Montague Island is the southernmost island in Prince William Sound, and it lies 140km inland from the subduction zone boundary – a perfect place to look for clues to what happened. Shortly after the earthquake, scientists discovered 9m of unexpected uplift along the Montague Island shoreline caused by slip on the Patton Bay Fault. This location was farther from the coast than the landslide, but closer than the subduction zone boundary far offshore near the oceanic trench. They concluded that the Patton Bay fault was the source of the uplift that produced the tsunami that reached the shore so quickly. Subsequent investigations identified local underwater landslides as the source of the first tsunami that arrived at Seward within just a few minutes after the earthquakes started. Later research clarified that the Patton Bay Fault was the first documented example of a "megathrust splay fault", or a fault that connects to the subduction boundary but provides a shortcut for sudden plate motion to reach the surface. Why was the fault motion and uplift occurring on these splay faults rather than the subduction boundary?