Straps
One of the best ways to secure your refrigerator in the event of an earthquake is to secure it to the wall using nylon earthquake straps. Installing these straps is fairly simple. One end of each strap attaches to a wall stud; the other end attaches to the refrigerator with a strong hook-and-loop fastener, which means you don't have to drill or put a screw into the refrigerator. The areas where you attach the fasteners must be very clean to get good adhesion, however.
Zee Clip
You can use an item called a zee clip to secure the bottom of your refrigerator. A zee clip is a little tab of metal with a hole on one side and a triangular slot on the other. Attach the end with the hole to the floor using a screw. The other side goes around a leg on the refrigerator. This won't prevent the refrigerator from tipping, but can keep it from moving along the floor and away from the walls during a tremor.
Angle Bracket
Angle brackets can be used to secure a variety of tall, heavy items in the home, including refrigerators. One end of the angle bracket attaches to a stud in the wall, like an earthquake strap, and the other attaches to the refrigerator. This method is much less visible than the strap method, and may appeal to you aesthetically, and prevents the refrigerator from tipping, just like the strap method. With this method it's necessary to attach a screw into the refrigerator, which is more complex than simply attaching a hook-and-loop fastener.
Securing Doors
During an earthquake refrigerators have a hazard in addition to the possibility of moving or tipping over. The doors can come open and strike you as they swing or as the fridge is projected across the room, and the open fridge can spill its contents onto the floor in the process. Refrigerator locks designed to prevent small children from opening the doors work well for preventing this. You can also purchase straps similar in design to an earthquake strap that attach with a hook-and-loop fastener to secure the door in place.