Where and how can seismic data for earthquakes and volcanoes be downloaded?
The easiest way to look at earthquake data that you would like in a data set you can work on (filter,plot,process,etc.) is three steps.


Get yourself a copy of Matlab
Go to the iris website and download a program called IrisFetch.m (see http://ds.iris.edu/ds/nodes/dmc/manuals/) . 
Next you need a list of events and a list of stations from which to download the event, both of these can be gained simultaneously with a program from iris called Jweed. Just download both lists into a cvs file and import to Matlab and run through IrisFetch. This requires a very basic understanding of programming in Matlab but its not hard to figure out at all. Later on you will learn that you don't need Jweed at all to do this but instead can do it all through IrisFetch. Jweed is popular and easy to use though.


If this is too complicated, you can use the web-based part of iris to look at specific earthquakes at specific stations. But I don't think you get to interact with the data this way.