Water is much, much more important than food, you can only last a couple of days without water, but weeks without food.
Collect water after a quake any way you can, if it’s raining, put out some buckets or pots and pans (or anything you can get your hands on) to collect it.
Drain the cistern, the water in there is safe to drink (unless you have some kind of ‘toilet block’ in there).
Drain your hot water tank, this is also safe to drink (just be careful you don’t scald or burn yourself).
Collect water from animal troughs, or water butts you use for gardening, it can all be purified as described below.
If the water has dirt or other debris in it, filter it through a t-shirt (or similar) first to take out the bulk of the ‘lumps’.
You can purify water using ordinary household bleach (make sure it isn’t ‘scented’ though). 2 drops of bleach to 1 litre of water, leave for 30 minutes or so and it will be safe to drink. There will be a slight smell of chlorine, similar to that in a swimming pool, but don’t be put off by this, it indicates the water is safe to drink. You can also use iodine (often found in first aid kits) in a similar way.
If you need to boil water to purify it, then you should do so for at least 20 minutes to make sure it isn’t contaminated by bacteria or other illness causing microbes. Leave your water on a rolling boil for the recommended time.
Of course, if you have water purification tablets, you should use these following the instructions given with them.
If you have room in your freezer, fill some bottles and store it there. Otherwise, store it in any way you can.