Use Google analytics to alert on traffic spike source

Solution 1:

I would use the following Alert Conditions:

This applies to: Referral Path
Condition: Does not match exactly
Value: (not set)

Alert me when: Visits
Condition: % increases by more than
Value: 25
Compared to: Previous day

You could also change the 'applies to' condition to a regex to filter out any regular refers that fluctuate above the increase % you treat as a spike.

Solution 2:

I'd take the Google Analytics alert as an indicator you should take a look at your logs. Something like webalizer or awstats will give you the detail you want. Analytics is good, but it can't tell you everything your log files can.