line chart of california seismic activity

You could try googling U.S. Geological Survey + Earthquakes + Southern California.

Or, you could check out this thread...Butrflynet keeps us pretty up to date on what's happening.

http://able2know.org/topic/126216-1

Here's a map of what's been going on recently:

This is a list of all quakes in the last 7 days that had a magnitude greater than 1.0

http://earthquake.usgs.gov/earthquakes/recenteqsus/Quakes/quakes_all.php

This is the same list but filters out quakes less than 3.0 in magnitude.

http://earthquake.usgs.gov/earthquakes/recenteqsus/Quakes/quakes_big.php

Input those data points into Excel and you'll have the line graph you seek.

For additional points on your homework project, you might want to include this animation map that pinpoints the same data points on a map of the LA area:

http://earthquake.usgs.gov/earthquakes/recenteqsanim/la/