Standing waves in Norway’s fjords caused by earthquakes

Ars Technica : About 30 minutes after the 2011 Tohoku earthquake , some Norwegian fjords began swelling and ebbing 1.5 m every minute, and continued to do so for several hours. The phenomenon is known as a seiche —a standing wave in an enclosed or partially enclosed liquid. Stein Bondevik of Sogn og Fjordane University College in Norway and his colleagues input local seismic readings into a model of fjord tides and storm surges and found that the result closely matched observations. They were able to determine that the seiches were primarily caused by S-waves —waves whose motion is perpendicular to the direction the wave is traveling and that pass through Earth’s interior from the quake’s epicenter. Surface waves, which arrived several minutes later, amplified the effect. Although S-waves cause less shaking than surface waves, the frequency of the S-waves generated by the Tohoku quake matched the resonant frequency of the waves crossing the fjords, which created the standing-wave effect. And the path of the S-waves followed the length of the fjords, which is why seiches were present in 2011, but not following the similar-strength 2010 Chilean earthquake .