At http://geology.com/press-release/san-andreas-earthquakes-frequency/ earthquakes have been a regular feature along the San Andreas fault system for a long time and a series of tremblors have been charted over the last 700 years. The September issue of Geology says that large ruptures have occurred on a stretch of the fault 100 miles NW of Los Angeles (where the research took place), at a rate of between 45 and 144 years apart. The last really big quake was in 1857, over 150 years ago. It should have happened earlier is basically what is being said. Charcoal samples dug out of trenches were formed during wildfires, washed into the basin by rain. The charcoal builds up over the centuries in layers that are fragmented during earthquakes – and this is how they produced the figures. Very clever.