Tesla's earthquake machine did indeed work, although it didn't create real earthquakes quite like mother nature does. According to what I've heard, residents around Tesla's lab felt weird shaking from time to time, across different parts of the neighborhood. Since Tesla had a bitof a reputation as a "Mad Scientist", he was the first person the police came to question.

It worked on a principle called resonance.
To understand that, think of a block of wood attached to a spring. If you give it a push, it will start oscillating back and forth at a given frequency. That's called the oscillator's natural frequency. If you try to drive that spring with a motor at a random frequency, it won't do much. But if you drive it at that exact natural frequency, it will start to oscillate a whole lot.

Many things in the world have some sort of natural frequency. They may not be springs, but things like planks of wood or metal can wobble back and forth, as well. This is how Tesla's oscillator machine worked. He noticed that if he stuck a motor onto one of the supporting beams in his lab, he could make different parts of his lab wobble around by changing the frequency that the motor is operating at. One thing he didn't realize, though, was that these vibrations were transmitted to neighboring buildings, and if certain parts of those buildings had the right natural frequency, they would be driven to oscillate, as well.

However, I don't think this would be a very efficient way to mess with or attack people, so I don't think you would necessarily have to worry about protecting yourself from it. The vibrations would not travel very far, and at most, would only cause parts of your house to wiggle around a bit, perhaps knocking a knick-knack off a shelf or a picture off a wall. Since California, as well as many other places, have built their buildings to withstand strong earthquakes, it would be very, very unlikely that any damage could be caused. In addition, almost every household appliance, cars driving down the road, and many other things will vibrate at some frequency, so all buildings built recently should be made to "damp out" these vibrations very quickly.

However, if this ever were to happen, there is one thing you could do to stop the vibrations. The natural frequency I spoke of earlier depends on how much mass the vibrating object has. If you were to change the mass of the object, it would no longer respond to the driving motor very well, and would stop oscillating. So if someone were to cause part of your house, such as a wall, to vibrate using Tesla's method, adhering something to the wall to change the mass and shape of the wall would probably cause it to stop oscillating.

One of Nikola Tesla's many interests was in figuring out a way to transmit energy from one point on Earth to another.He knew that if he was able to build an apparatus that could create strong vibrations at a particular frequency, the vibrations would travel through the Earth and could be measured far away. Essentially, he would be able to cause an earthquake. The design of this oscillator is fairly straightforward: air (or steam) is forced inside the oscillator and then undergoes compression oscillations at frequencies which could be varied. Amazingly, the device Tesla constructed is quite smallseven inches long and weighing less than 2 lbs. Purportedly, while operating this device in 1898, Tesla's lab was destroyed by violent shaking.

The reason this device has the potential to work concerns the mechanical principle behind resonance frequencies. Think about a child playing on a swing. If you give the child a push at the proper time during the swinging cycle, even small additions of energy can generate a significant end result. Similarly, even large buildings or objects (even the Earth!) have characteristic resonance frequencies, whereby once the object begins to vibrate, the continuing addition of the input of energy at the proper frequency result in larger and larger vibrations.

In reality, it is very unlikely that Tesla's oscillator would cause an earthquake. In fact, recently on the Discovery Channel, a group of scientists working on the show Mythbusters attempted to test this idea exactly. They built a working oscillator and placed it on one end of a bridge and moved to the other end of the bridge. While this device was able to produce vibrations that could be felt hundreds of feet away, it was not nearly strong enough to produce earthquakes. In other words, there is no real reason to be worried about someone using this to create earthquakes or damage to buildings and other structures. Engineers who design large buildings and bridges understand very well the idea of resonance frequencies and make sure their structure would be sufficiently strong even if the building was exposed to its resonance frequency.