just spotted this thread. You should have a look at the Public Seismic Network site, http://psn.quake.net/
which is dedicated to home brew seismometers. It's been going for decades and even has Dr Lehman posting from time to time. There's a wealth of information there on all aspects.
For local quakes you need to capture signals up to 10 Hz.
For tidal effects (microseisms), they are usually around 0.17 Hz
For international quakes (teleseismic), they're between 1 to 0.01 Hz, generally, for us amateurs.
For plotting, have a look at PGPLOT, though there may be something easier these days.
I use a mixture of soft and hardware filtering. For software filtering, you need to oversample by several factors (so you'll need a faster A/D) and I'm not sure if the raspberry pi is up to it. Perhaps best to let it do the acquisition and network the data to a 386 machine for the rest.
I've got a 16 bit national instruments pci a/d in a 386 machine. For each "sample" I sample at 3 different gains and select the one with the highest gain that hasn't saturated. This is a cheat to get it up to around 21 bits of resolution.
I post the results to http://www.iasmith.com/
Currently the sensor driving the lower graph is broken (a 4 Hz geophone) and I think a spider has gotten into my Lehman enclosure (top graph).