I know that in Hawaii and here in Central Ca. there are sensors that are linked by radio. In Hollister Ca. you can monitor the frequency for a slight warning of earthquake. The problem is that there is a steady beeeep when normal, and it "warbles" with activity. Sometimes when I get woke up in the middle of the night I punch up that freq and check to see if it was an earthquake. IIRC it's a 160 MHZ freq, I can look it up for anyone that's interested.
Dave