An important part of being prepared for a natural disaster is knowing where to find and how to operate your home's utilities. Water leaks can cause serious structural and mold damage to your home, and in extreme cases can be a source of drowning or electrocution.
There are two types of water shut-offs: inside the building, and water department shut-offs which cut supply of water to the entire building. It's unlikely you will have access to the water department shutoff, which is typically under the sidewalk in front of the building and requires a water key.
The internal water shutoff is generally located inside a garage, alley, or basement. Look for a red switch or a yellow wheel on the water pipe, near the front of the building. To check operation, turn the switch or wheel no more than 45 degrees, and immediately turn it back to starting position. The switch is on when it is parallel to the pipe, and off when it is perpendicular. The wheel turns clockwise to stop flow.
Make sure that you do not turn off water completely when testing operation: the water company needs to turn it back on in both emergency and non-emergency situations.
Water is arguably the most important resource to have in an emergency, for everything from hydration to hygiene to cooking. Make sure that your emergency kit contains a minimum of 1 gallon/person/day, for 3 days. Filtration systems, purification tablets and bleach are also good ways to make sure that you will have an uninterrupted water supply. Only turn off the building's water in case of an emergency, including:
A serious leak inside the building, or a broken water pipe Partial collapse or major structural damage, after an event like an earthquake You are evacuating, and believe the building may sustain further damage
Always, always be sure that the building is safe to enter before you turn off the water. If there is any doubt regarding structural integrity, electricity, or drowning hazards, evacuate for your emergency plan meeting point, and call 911 to report.
There may be less serious situations where quick action can prevent hazards like black mold or damage to the building and your belongings. If your water heater is leaking, you will find a red switch or yellow wheel on the pipes feeding in and out. Turning off the water at this level can reduce or prevent damage while you call a plumber.
You should also locate the controls to each major appliance that uses water, such as your dishwasher and washing machine. If there is a leak from one of these machines or their pipes, turning off their controls allows you to continue using water for all other purposes, but prevents damage from the appliance-level leak.
For those in earthquake-prone areas, it's especially important to make sure your water heater is secured to wall studs: one strap should be 1/3 from the top, and another 1/3 from the bottom of the heater. Inspect the area around the heater and remove any important objects that could be damaged were it to fall over during an earthquake.