A strong earthquake hit the Napa valley yesterday.

The US Geological Survey has provided a detailed summary of the quake and the possibilities of both strong aftershocks and the remote potential of an aftershock stronger than the quake (Magnitiude 6.0)
The earthquake occurred within a 70-km-wide (44 miles) set of major faults of the San Andreas Fault system that forms the boundary between the Pacific and North American tectonic plates. The persistent northwestward movement of the Pacific plate relative to North America primarily causes right-lateral slip across the major faults, but also causes deformation between the major faults. The ongoing complex deformation field is revealed by modern geodetic surveys and earthquake patterns as well as the regional geologic structure.