Giant quake sloshed fjords half a world away

The steep walls of Aurlandsfjord, an arm of the Sognefjord, seen near Flåm, Norway. These fjords and others experienced a seismically generated wave, or seiche, after the March 2011 Tohoku quake in Japan.

Credit: 

Copyright Tinglestan, Creative Commons Attribution 2.0 Generic

On the morning of March 11, 2011, Leif Hus and his wife Gry Melas Hus were having breakfast in their kitchen overlooking Sognefjord in Leikanger, Norway. It was low tide on a calm and windless day with near-freezing temperatures. As they stood, coffee cups in hand, looking out the window at the fjord, they saw an unusual wave roll in. The wave continued to rise, surging over the seawall into their backyard before receding back into the fjord. Then another wave surged in, and another. As the water rose, engulfing the ladder on their dock, Leif grabbed his cell phone and started filming.

Half an hour earlier, the plate boundary off the northeast coast of Japan had ruptured, producing the magnitude-9 Tohoku earthquake and triggering a massive Pacific Ocean tsunami. Half a world away, in Norway, the fjords were sloshing. What Leif Hus and others had seen was a rarely documented phenomenon — a seismically induced water wave known as a seiche. By the day’s end, reports of the unusual waves had come in from five other locations in Norway as well. 

None of the seiche waves reached much above the normal high tide level for the area and no damage was caused, likely because the seiche waves struck at low tide.

Seiches are standing waves established in lakes and other enclosed and semi-enclosed basins when the period of waves generated by usually distant earthquakes matches the period of the basin. The exact mechanism, however, is not well understood. Until now, seiches were thought to be caused by the arrival of seismic surface waves, which have components of both horizontal and vertical motion and travel only through Earth’s crust.

In a new study of the seiches observed in the fjords of western Norway after the Tohoku quake, Stein Bondevik, a geophysical engineer at Sogn og Fjordane University in Sogndal, Norway, and colleagues determined that the seiches began — not with the arrival of the surface waves, but during the passage of secondary body waves (S waves), which oscillate perpendicular to the direction of travel and move through Earth’s interior.

Bondevik’s team reconstructed the seiches by combining data from the nearest seismic station — on the island of Sula at the mouth of  Sognefjord, 80 to 130 kilometers west of the areas that experienced the seiches — with information gleaned from eyewitnesses, cell phone photographs and videos, and time-stamped footage from surveillance cameras.

By locating objects to use as measuring sticks — like the Hus family’s dock ladder and a ship (with a known mast height) bobbing in the harbor next to a fixed dock — Bondevik’s team calculated wave amplitudes of 1 to 1.5 meters and frequencies of 67 to 100 seconds, the authors reported in Geophysical Research Letters.

The timing and characteristics of the seiche waves better matched those of the horizontal S waves, which move in a snake-like motion from side to side, more so than the surface waves from the quake, indicating the S waves were likely the cause of the observed oscillations in the fjords.

“How the earthquake ground motions generate seiches is not simple to explain,” says Aggeliki Barberopoulou, a geophysicist at the Tsunami Research Center at the University of Southern California who was not involved in the new study. Until now, she says, “the scarcity of records has allowed [only] for speculation on the mechanism.”

In this study, the use of seismic records combined with reconstructed water records and numerical simulations “helped identify which part of the ground motions triggered the standing waves and constrained the duration of water activity,” Barberopoulou says, “which anecdotal records alone cannot reveal.”
 

Sara E. Pratt
Monday, December 2, 2013 - 07:00

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