Hi Canie,

Your right that "LA is built on runoff from the mountains via rivers (and the occasional landslide)", but the study states that "The San Gabriel Valley, east of downtown Los Angeles, and the San Fernando Valley, just north of downtown, sit on basins of sediment laid down by ancient seas and rivers."
I think it was a combination of both. In any case, if you look at the seismic history of the region, it appears to be shattering.

All the best,
Billion