Liquefaction and the Dancing Red Giraffes

This post follows on others (here and here) I have recently shared regarding the study of and hazards associated with earthquakes in the Puget Sound region in Washington state.

Among the fascinating stories I had the privilege of hearing while visiting PNSN and the Seattle field office of the USGS was a firsthand account of liquefaction in action during the 2001 Nisqually earthquake from USGS geophysicist Bob Norris.

Liquefaction is the process whereby dry sandy or silt-rich soil becomes a water-saturated slurry and loses its mechanical strength.  It is a frequent side-effect of earthquake-induced ground shaking, and is a particular hazard throughout the Puget Sound region where structures are built on unlithified till and deltaic deposits.

"Big Steel Giraffes": Commercial shipping cranes at the Port of Seattle.  Harbor Island (center) is tucked between SoDo (foreground) and West Seattle (background).  (Image by Vmenkov via Wikimedia Commons)

On the morning of February 28, 2001, as Norris arrived to collect data from a seismometer located on Harbor Island in Seattle, his truck began rocking “from side to side.”  It took him several seconds to realize what was happening, and although there was no noise from the shaking, it was strong enough that he “thought I was going to get whiplash,” he said.

Harbor Island was constructed in the early 20th century from fill that was “not well engineered,” said Norris. Today, it hosts large commercial shipping cranes and other industrial infrastructure.

Near where the seismometer was housed, a geyser of water burst from the ground, gushing up faster than it could spread laterally.  It had evidently been pooling below ground because not long after the geyser began, the soil around it collapsed in a heap of “wet, slurpy sand.”

A large whirlpool of muddy water formed as the water continued flowing.  Norris said he feared the ground below his truck would give way.

A cloud of dust rose from the island as he waited out the strong shaking, and the large red cargo cranes that tower hundreds of feet overhead flexed “like jello.”

They looked like “big steel giraffes that were trying to dance,” Norris said.

The flow of water abated after several minutes, but, he said, light ground shaking that felt like a conveyor belt underfoot continued for some time.  Liquefaction on the island and in the area was widespread, said Norris, but it could have been worse but for the relatively dry conditions that preceded the earthquake.

Further Information

• A version of Norris’ account, as well as a brief history of Harbor Island, can be found at here.
• Search "liquefaction" on YouTube or Google to see footage, photos, and demos.