Similar to how you can simply check the weather forecast to get a good sense of upcoming weather, is there a tool I can use online to alert me of an impending earthquake?
I am travelling in California right now and all these news reports on the TV are telling me there might be a bigger earthquake soon. I am staying here for a few more weeks and feel a bit paranoid thinking there might be an earthquake.
Is there a tool to get alerts for an incoming earthquake or possible earthquakes?
Earthquakes are notoriously hard to predict, for the US they are working on this an early warning system which predicted the recent earthquake. Giving about ten seconds warning. Which I think may be less than you were looking for ...
In general if you're in an earthquake zone there's always a (remote) chance of a sudden quake. There are minor earthquakes all the time, a sequence of minor earthquakes does not make the 'big one' more likely. Arguably, smaller quakes relieve pressure and could make big quakes less common, but they could also move things in such a way that it does cause a bigger quake. People have been studying this for years and the best we've got is ten seconds warning ...
There are a few things you can do -- first find out if you're in an earthquake zone, check the local government for wherever you are or here's a nice map of them. Second, know what to do in the case of an earthquake. Finally know that earthquakes are pretty scary and can be dangerous depending what you're doing (i.e. driving over a bridge) but places that are in earthquake zones are designed to handle even relatively big quakes. Quakes on the scale of the Japanese one a few years ago are incredibly rare and honestly not really worth worrying about (if it happens, it happens).
You can track recent earthquakes on various sites and, yes, there are apps for that -- here's the Red Cross one.
One thing you can watch for is earthquake caused tsunamis, (or any tsunamis). These are (mostly) predictable, often more dangerous than the earthquake, and getting a warning will give you time to take action. Here's the UNESCO page and there are apps for that as well but I've no experience of them so I don't want to recommend one at random.
There is a page at cwarn.org that looks like it covers a lot of the above.
I will say once again, earthquakes and other geological events are very hard to predict. Warnings often err on the side of caution, while some events still take everyone completely by surprise. It's one of those things you should know how to handle if it happens, and then try and not worry about it to much. I say this as someone that lives less than half a kilometer away from a major fault-line.
Quick Edit: The question was changed after I answered to be California specific, I'm not going to trim the answer just to that since I think it's generic enough to be applicable. The links I gave are still useful but for specific stuff consider the ca.gov earthquake pages and the Southern California Earthquake Center (SCEC) .
Edit 2 (three months later): I just wanted to edit this to add a link to a upcoming tool called QuakeAlert. It looks to have grown out of ShakeAlert which is linked in the other answer and offers personal and industrial early earthquake warning solutions (it's not clear from the site how much warning they can give).
According to the answers to Is earthquake prediction possible? on Earthsciense.SE, no technology currently exists to predict earthquakes with enough accuracy and advance warning to be very useful, nor are we anywhere close to being able to develop it.
There is an experimental system called ShakeAlert being developed at UC Berkeley that was able to give a warning 10 seconds in advance of the severe shaking of the Napa quake on August 24, 2014 (by detecting and analyzing the initial, less damaging waves of the quake that had already arrived). This system is not yet considered mature or reliable enough to make its alerts available to the public, and since it appears to be the cutting edge of research, this suggests that nothing of any usefulness is publicly available.
Hence, the answer to your title question is you can't.
Your best bet is preparedness: knowing how to respond should an earthquake start, having emergency supplies on hand, staying in seismically safe buildings and structures, etc.
There are various services that will send you alerts after an earthquake occurs, but that doesn't sound like what you want.
Basically, living with that risk is part of life in California and other seismically active areas, and like the locals you simply have to get used to it.
If you have a Twitter account, you could follow people from a few key points near you in California. During the recent earthquake in Napa, CA, people started tweeting about the earthquake right when it was going on. Earthquakes travel at the speed of sound, but tweets travel at 2/3 the speed of light. This means that about 100 km away from the epicenter, the tweets about the earthquake will actually reach you BEFORE the earthquake itself.
You won't get much warning (just a few seconds), but that's enough time to get away from glass, chandeliers, heavy cabinets and other stuff that can injure you.
SPECULATION: In theory, you can create an app that uses the accelerometer of your phone to monitor earthquakes as they happen, let them send it to a local server, and from there send out tweets and alerts. I can imagine that such an app is currently in development or even already in use.
In addition to using Twitter, the Federal government also has a special system based on celltowers that sends free messages to everyone in a certain area. http://www.ctia.org/your-wireless-life/consumer-tips/wireless-emergency-alerts explains how it works. You might have to enroll in it though, because your phone needs to be compatible. Contact your local mobile network provider to learn more.
