Where can I download historical sesimic data in CSV/Excel format for processing in R?
You can download seismic data in csv format at USGS, from past hour, past day, past week and past month.

If you wish to download ancient data you need to use the USGS API. There is a query option: format=csv.

This is an example of an URL that gives you seismic data in csv format from 2014-01-01 to 2014-01-02:

https://earthquake.usgs.gov/fdsnws/event/1/query?format=csv&amp;starttime=2014-01-01&amp;endtime=2014-01-02

User Kristjan shared a Python script to ask the API for all available data since 1900.