"A Crack in the edge of the World : the great American earthquake of 1906" by Simon Winchester.
I've previously read Winchester's books "The Surgeon of Crowthorne" and "Krakatoa" so I was aware of the way the Author likes to explain all facets of how such a catastrophe could occur (even tracing back the continenal plate drifts to the Pangaea era). It's wordy and a bit heavy going in places but still a fascinating read.
I'm quite enjoying it - even though most of the first half of the book only lightly touches on what occured in 1906 (page 215 is where the actual disaster of San Francisco starts being mentioned in full), And I've learnt about the history of California; continental plate drift, the various pre-1906 earthquakes in the San Andreas fault zone; and even some back history of some of the famous geologists (who would have known that creator of the Richter magnitude test scale - Charles Richter - was an amourously-inclined vegetarian!).
Highly recommend the book to anyone wanting to know how earthquakes occur