Alaskan Cruise Ships Beware

If an unstable hillside above Tidal Inlet in Glacier Bay National Park suddenly collapses, it would generate a tsunami large enough to damage any ships cruising in its path.

Severe earthquakes in the region are common, and one could trigger catastrophic failure of the precarious slope.

Until the late 19th century, the Alaskan mountainside was supported by the one-km thick glaciers that filled most of the park. Old photos show that shortly after the ice retreated, the slope rising out of the ocean moved at least 25 metres, possibly in response to strong earthquakes felt in 1899 and 1900.

But the weak hillside of glacial till and bedrock has not sat still since then. A 2002 inspection of the fault discovered clues of thinly-vegetated scarps and disrupted drainage that reveal the steep ground recently shifted, sometimes suddenly. Global positioning system (GPS) measurements also record the 700-metre-high face creeping nearly four cm downhill between 2002 and 2004.

If the dangerous slope releases abruptly, it will fall directly into Tidal Inlet. As much as 10 million m³ of material could hit the water at 230 km an hour. Modelling shows this generates enough force to create 77 metre-high waves in the inlet.

Within four minutes, a tsunami would reach the nearby West Arm of Glacier Bay, a popular summer destination for travellers aboard cruise ships. For several hours the tsunami would fill the arm with a complex interplay of waves, some over 10 m high. Scientists predict that this scenario will likely sink or damage any cruise ships in the bay, and human fatalities are possible.