are these things isolated? or do earthquakes have warm-ups for bigger ones? I dont know much about them
blocks of rock are always moving past each other. when the move freely at something like a cm or two a year, you don't notice it. but when it sticks, energy builds up. if it sticks for a long time lots of energy builds up. when it finally breaks all that energy is released at once and you feel an earthquake.
the Pacific plate is moving past the North American plate. where they touch we call the San Andreas fault. the problem for southern California is that there is a bend in the fault right at the San Gabriel mountains. Lots of rock has to work its way west to get around that bend to continue traveling north. that's why soCal has hundreds of little faults besides the really big San Andreas fault. An 8.5 is certainly possible on the San Andreas, but that's well outside the city. The smaller faults will never create anything that big. they break long before they can build up that much energy.
Earthquakes release energy. so after an earthquake there is less energy in the system, so on the one hand a big earthquake makes it just a little less likely that there will be an even bigger one in the very near future. on the other hand, the rock doesn't necessarily break all at once, so an earthquake could release say 10% of its energy and it could be a matter of days or months before the other 90% gets released as an even bigger earthquake.
An earthquake can occur at any time. Statistically, really big earthquakes occur on the southern part of the San Andreas every 150 years or so. We're well overdue. when it does break, a long stretch between Palmdale and Palm Springs will open up like a zipper. If it starts in Palmdale and rips south, most of the energy will be directed away from Los Angeles. But if it starts in Palm Springs and rips north, LA will receive the brunt of it.
(MS Geophysics USC 1985)
