Living in California meant it was always a good idea to be prepared for fire or earthquake. My perspective: it's essential to have enough food for at least 3 days of activity, enough water for 24 hours, and a way to purify water thereafter. I favor Mainstay 3600 blocks as they’re easy to pack and easy to consume on the go. Water should be stored in strong impact-resistant canteens, not in crappy little supermarket bottles that will rupture easily.
A medkit that includes QuikClot, basic bandages & sutures, a needle for pneumothorax pressure-relief, a razor-sharp scalpel, scissors, sterilizing iodine, tweezers, gram-negative and gram-positive antibiotics, analgesics, a dental repair kit, and of course a tourniquet are all must-have items.
Gore-Tex jacket and pants are an excellent idea because if you get wet you will die. A sleeping bag inside a Gore-Tex shell, mated to a Thinsulate inflatable air mattress, will enable you to survive even if you can’t find shelter. You should also have an emergency bivvy-bag made of insulating material which you can use as an inner sleeve to your sleeping bag or on its own if the weather is more clement.
Have a pack of wet-wipes for personal hygiene and a small trowel so you can dig a pit for waste matter. Pack at least 3 changes of socks and 3 changes of shirt, plus a spare pare of pants, because you will want something dry to change into at night before attempting to sleep. In the morning change back into your sweat-soaked clothes from the day before. It will feel horrible but it’s worth it because you’ll be keeping your night-wear dry for the next evening’s use.
You’ll need a sharp strong hunting knife, ideally with a polymer handle. No need to carry around a heavy ax — you can place the blade of the knife against the surface you want to chop and strike the top of the blade with a rock. Not quite as nice as swinging an ax but the knife won’t make you wish you had something much lighter to carry.
Which brings us to the go-bag itself. It should not be a bag. Really. You’ll need both hands and carrying a bag will make you walk with an uneven stride. Get a military patrol pack, a British Army Bergen (my personal go-bag for the last 35 years), or a properly-fitting daypack from an outdoor store. Pack mosquito repellent in order to make life fractionally less miserable and if you’re in the USA pack the gel that counteracts the toxins in poison oak and poison ivy. If you get a breakout rash far from help, you can become infected within 24 hours and die of sepsis within 72 hours, which is Not Good.
If you have a reliable vehicle and ideally a 4x4 with good all-terrain tires, make sure you’ve got (i) at least 10 gallons of water ready to throw into the back, and great if you also have (ii) at least one 5-gallon jerrycan of gasoline, because who knows whether or not you’ll have much in your tank when the moment unexpectedly arises.
Finally, have a headtorch and lots (and I mean LOTS) of spare batteries. Have storm-proof matches. If you want luxury, pack a JetBoil and a spare cannister of gas. Do not ever, ever, take candles — they’re a massive fire hazard. Always have 100 meters of paracord and a roll of duct tape — you’ll be surprised how useful this can be.
If you live in bear country or have strange neighbors, a 9mm or .40 semi-automatic handgun (Glock, H&K, and SIG-SAUR are all superb and highly reliable) in a drop-holster with at least two extra magazines will provide some deterrence; my personal recommendation for more effective defense is (i) don’t be seen by wackos and stay away from bears, but if that fails then (ii) the DesertTech MDR in .308 with an EOtech holographic sight is a superb personal weapon that’s effortless to use, very reliable, and will slow down or stop outright most forms of threat. Again, carry at least two spare magazines.
But here’s the thing: if you can’t run 6 miles in under an hour, and if you can’t move reasonably quickly over rough terrain while carrying that go-bag, chances are it doesn’t matter what you have in it. You are the most important element in any emergency situation and if your idea of exercise is to waddle from sofa to fridge then basically you’re fucked. Being prepared doesn’t mean buying shiny kit; it means staying on top of your self-administration and staying in good physical condition. Always.
If you can’t manage that, don’t bother with a go-bag. Chances are, it won’t make much difference to the outcome you’ll experience when an emergency strikes.