Napa County experienced a 6.0 magnitude earthquake, which is estimated to have caused as much as $1 billion in property damage. Published on September 2, 2014 in Bay Area experiences strongest earthquake in 25 yearsFull resolution (500 × 334) Napa County experienced a 6.0 magnitude earthquake, which is estimated to have caused as much as $1 billion in property damage. Napa County experienced a 6.0 magnitude earthquake, which is estimated to have caused as much as $1 billion in property damage.