The above image combines a map from the USGS Earthquake Hazards Program with a formula from a Connexions module by Sunil Kumar Singh that teaches forced oscillation. The map was captured as a screenshot from the USGS website 2 days after the Sichuan Earthquake began, and as the large squares on the map indicate, the aftershocks were continuing.
The Connexions module text accompanying the formula explains:
The resonance is an interesting feature of oscillation. This phenomenon attracts interest as it makes possible to achieve extra-ordinary result (material failure of large structure) with small force! Resonance also explains why earthquake causes differentiating result to different structures – most devastating where resonance occurs! The condition for maximum amplitude is obtained by differentiating amplitude function with respect to applied frequency as [the illustrated formula sets out.]
Thomas L. Pratt, who teaches research geophysicists at the University of Washington, provides a webpage that explains frequencies, periods, and resonance in which he includes this simple explanation: “Resonance is when motion at a given frequency is amplified by waves of that same frequency. For example, when a child is being pushed on a swing, the swinging is increased by a push being applied at the right time (at the correct frequency) during each swing.”
At Science Fair Central you can follow instructions for a simple experiment with 2 paper circles and a piece of cardboard to show why earthquakes shake some buildings more than others.