November 1, 2012
Last weekend a large quake rocked the Pacific Coast of Canada. In my post about it I mentioned that not much footage had surfaced. There’s still rather little, largely–I suspect–because of the quake’s remote epicenter. Nonetheless it was a very large quake, so the distant, low-frequency waves swung chandeliers and played with people’s balance across western Canada. The videos people have posted so far are collected in the compilation below. In all but the last, the quiet, slow rocking indicates substantial distance from the source of the shaking (that is to say, ~the epicenter). The last video appears to be much nearer the epicenter given the relatively high frequency and violence of the shaking (skip to 3:00 minutes in the video if you’re eager to just that exciting part).
Earthquakes’ high-frequency energy gets dissipated quickly as they ripple outward, relative to the long-period, low-frequency waves. You observe this effect when a car drives by you with the bass thumpin’: you hear (or feel!) the deep resonant rhythm, but unless you’re inside the car, you can’t hear the high-pitched song going on above it, which gets absorbed by vibration of the car parts.
Slow rocking is a tell-tale sign that a big earthquake is going on and that you’re very far from its epicenter.