63.5‑Million‑Cubic‑Meter Rockslide Generates 481‑Meter Tsunami in Alaska

### Context At 5:26 a.m. local time, a steep rock wedge detached from a mountain above Tracy Arm fjord in southeastern Alaska. The wedge, estimated at least 63.5 million cubic meters in volume, fell into the deep water at the terminus of the South Sawyer Glacier. The glacier, retreating under a warming climate, had long buttressed the slope.

### Key Facts - The rockfall generated an initial breaking wave over 100 m high that raced across the narrow fjord at speeds above 70 m s⁻¹. - When the wave struck the opposite shoreline, it surged up the sheer rock face to a run‑up height of 481 m above sea level. - Aram Fathian of the University of Calgary, co‑author of a *Science* paper that reconstructed the event, called it the second‑highest tsunami ever recorded, surpassed only by the 530‑m Lituya Bay wave of 1958. - No injuries or fatalities were reported, largely because the event occurred before sunrise and the area is sparsely populated. - The *Science* study used high‑resolution satellite imagery, lidar‑derived topography, and eyewitness video to model the landslide volume, impact velocity, and resulting wave dynamics. Researchers calibrated the model by matching simulated wave heights to the observed 481‑m run‑up, confirming the rock volume estimate.

### What It Means Landslide‑generated tsunamis differ from earthquake‑driven waves by concentrating energy in a confined water body, producing extreme local run‑ups. Since 1925, 27 such events have exceeded 50 m in height; the Tracy Arm wave joins the short list of megatsunamis that dwarf typical coastal tsunamis, which rarely surpass a few tens of meters.

The retreat of the South Sawyer Glacier illustrates a feedback loop: warming reduces ice support, increasing slope instability, which can trigger massive rockfalls. While this incident caused no loss of life, similar failures in more populated fjords could pose severe risk.

### Looking Ahead Scientists will monitor glacier retreat rates across the Stikine Icefield and refine landslide‑tsunami models to improve early‑warning capabilities. Future research will focus on identifying other vulnerable fjords where a comparable rock volume could generate destructive megatsunamis.