A Landslide in Alaska Set Off a Tsunami. There May Be More to Come.

Jackie Caplan-Auerbach, a seismologist at Western Washington University and an author on the new study, was among the first to hear about the tsunami: Her neighbors, whose boat was anchored at sea some 50 miles from the landslide, texted her about a strange surge of water that had hit their vessel. Other firsthand accounts trickled in from Harbor Island, where camping kayakers said their gear had been carried away by the wave, and from a 150-passenger cruise ship, the National Geographic Venture, that was sitting just outside the fjord....

When Dr. Caplan-Auerbach dug deeper into the seismic data from the landslide, she noticed patterns of land movement similar to those that she had studied for decades, which sometimes preceded landslides on volcanic slopes.

These tremors were “probably tiny bits of slip on the base of the landslide, and it can do that only so much before it’s got to break apart and fall,” she said. Tiny coalescing fractures within the mountain eventually reach a crescendo, a threshold at which point the rock can no longer hold itself together, and the slope gives way.

It is not yet clear how many landslides display these seismic signals as precursors, but since there were no other cautionary signs, they provide a hope of early warning. If the signals are subtle — the slopes “whispering to us, not yelling,” as Dr. Caplan-Auerbach put it — it is possible the seismometer network in Alaska is spread too thin to typically pick them up.