The page you found is a report on 'felt' reports that have come in which they use to build the shake maps. If its a recent quake you can easily find the waveforms but if its more than a week old they may not be around (or around as an image) - If you find the quake on the california maps, click on the individual quake and near the bottom of the page is usually a link to the waveform.

I think the site is to be trusted - it is run by the USGS - i trust their data.

Here is the link to a southern cal quake catalog: http://www.scecdc.scec.org/catalogs.html
Don can probably tell you a northern cal address. With all the servers being re-organized mine is probably out of date.

Canie