The response of thin glassy films of polystyrene and poly(vinyl acetate) to a raster-scanned, sliding SFM tip was investigated. Several of the previously proposed mechanisms of the familiar scan-induced patterns are discussed. Increases in film volume and frictional response are quantified, and suggest that the observed tip-induced plastic deformation may relate to a second-order phase transition (glass-to-rubber) beneath the sliding tip. Analysis of the scan-induced patterns suggests a crazing mechanism for the observed plastic deformation. The susceptibility of the film to plastic deformation was examined as a function of scan geometry, applied load, and the gain of the feedback loop that maintains a constant applied load. An empirical quantity called the roughening susceptibility is defined and shown to be linear with respect to variations in the scan conditions. The roughening susceptibility is highly robust in quantifying the dependences on scan history and load. This finding will be further exploited in the second paper of the series, analyzing rate and temperature dependences and their relationship to the glass transition.