I wonder if the shape of the fjords caused more complex wave interference that led to misinterpretation of the wave period?
Actually just thinking about if the rock underneath is moving in an S-wave would you then notice it at the shoreline even if the wave doesn't propagate through the water?
I edited my post a bit. I'm super tired and totally missed an important piece of information. He concluded it was horizontal S-waves that caused the seiche to happen, which actually makes much more sense. Horizontal S-Waves move in a s-curve like motion (side to side), which would most likely resonate in the narrow fjords.