Stupid question, but one I haven't had to have the knowledge of, since we don't have earthquakes.
Are all buildings in California (including apartments, stores, etc) built in such a way that they're more resistant (possibly not totally so, but at least more than would be in non-earthquake states) to damage by an earthquake? Are there reinforcements put in automatically when building a structure?
Yes, and the building codes are upgraded after every major earthquake, when they find some new weakness that caused building failure. They are also enforced firmly, as far as I've ever heard. A lot of the building and infrastructure damage here in California is to a) older buildings that have survived so far, but the bouncing uncouples important structural parts and things pancake, or b) something was built to withstand a lesser earthquake and 'this' one is bigger than that, or c) the earthquake is a close-to-the-surface quake and right under whatever just turned to kibble bits - because not much survives those. Thank heaven that kind is pretty rare around here. Starting in the late 60's and early '70's, they started getting really fancy with very tall buildings here - a lot of them have sub-basement rollers that allow the building to sway from side to side during a quake, so that they flex and don't break. I knew somebody in a 23rd floor apartment during a pretty hefty quake in 1971. She said the building whipped back and forth, which was frightening, and there was damage to their belongings because nothing was secured to be stable during that kind of motion, but no actual building damage. Museum putty was, I think, invented to hold small brittle things on a shelf during a quake. If the shelf was secured to not fall off the wall, and the item on the shelf could stay put, it would survive the earthquake. Museum putty does not, however, stabilize a picture on the wall against cat paws... A lot of the damage from quakes isn't actually caused by the shaking, it's from fires after the quake, when gas lines rupture, and water lines rupture so you can't put OUT the fires, and building and road collapse makes getting to the fires difficult. A lot of the destruction of San Francisco in 1906 wasn't from the actual quake, bad as it was, it was from the fires that swept the city following the quake, when the firefighting capacity, such as it was, was hampered by the destruction and obstruction. My family lost a LOT of property in that one.
Many countries have actually pretty good earthquake codes - Japan has excellent codes. They ENFORCE them. I don't know how good the building codes in China are, but after every major quake they have, I read about cheating and substandard building construction that causes building collapse and people's deaths. No enforcement of codes over there and in many other countries...
We were 75 miles away from the Northridge quake in Southern California in 1994. That was the first quake my children woke up for and were scared during. It was long.... We had no damage other than a solo wineglass that fell over on the kitchen counter and broke right there. We had video tapes fall off of the entertainment center in the living room, and the shelf in the girls bedroom, and those made a lot of noise (which is what woke up the children) and makeup fell off a shelf in my bathroom. The kids woke up and cried and called for me, so I went in (walking 20 odd feet during a hard earthquake is quite the trick, let me tell you!!) and I stood at the foot of the bunk bed and the child on the bottom bed hung on to my legs, and I hugged the two in the top bunk, and the littlest one complained that the Very Punkin had come and "Make it go away, Mommy!"
All that said - I'm 58. I've only been through 2.5 major quakes in my life - the Northridge quake in '94, and the Sylmar quake's in '71 and '72. I've been in many smaller ones (they're fun) and most of the small ones are hard to distinguish from a major truck going by - they both rattle the house. Big ones are not common at all - and I'll call 6 and up big enough to interrupt my day, and send me under a desk. I've been through several in the high 5's, and been lucky enough to be at least 30 miles from the epicenter every time, with no damage to my work place or my home.