The early morning event centered about 6.7 miles (10.8 kilometers) underneath Northern California's wine country. There, like most locales along the Pacific rim, ocean crust and continental crust clash to create numerous faults and quakes.  (Related: "Massive Chile Earthquake May Not Be the 'Big One.'")

Felt from San Francisco to Sacramento, the quake was one of the largest to strike the region since 1989's magnitude 6.9 Loma Prieta quake. Early news reports on Twitter suggest that a local hospital has received at least 70 patients with injuries.

All earthquakes spring from faults deep underground, but it can take scientists some time to identify the particular type of fault-line activity behind a specific earthquake. That will likely be the case