A magnitude-6.0 earthquake in California’s Napa Valley last month caused extensive damage, with estimates ranging from “at least $US100 million ($106.91 million)” to “at least $US1 billion ($1.07 billion)”.

A series of aftershocks, including a magnitude-3.9 quake last Tuesday, have added to the bill, including further damage to buildings and infrastructure that had already been assessed.

Damage to the region’s wineries has been assessed at $US38 million ($40.63 million) so far, but this is considered a preliminary estimate. Several rare and historic wine collections that were lost have been described as “priceless”.

California Governor Jerry Brown declared a state of emergency on the day of the quake, allowing state aid to flow almost immediately to the stricken region.

It is not just physical damage that has the region worried. Napa Valley, 70km north of San Francisco, attracts 3 million visitors a year, who spend $US1.4 billion ($1.5 billion).

The earthquake struck at 3.20am on Sunday August 24, injuring at least 200 people. There were reports of power failures, damaged roads and destroyed bridges, but no deaths.

The US Geological Survey says it was the strongest earthquake to hit the San Francisco Bay Area since the Loma Prieta magnitude-6.9 event in 1989.

It says there is a 5-10% chance a bigger quake will follow in the next week or so.

The Loma Prieta earthquake led to insured losses of $US1.8 billion ($1.9 billion). In 1994 a magnitude-6.7 event in Northbridge, California, caused insured losses of $US24 billion ($25.7 billion).

The US Insurance Information Institute says the two quakes, despite the difference in insured losses, prompted almost the same number of property and casualty claims: 198,920 and 199,478 respectively.

Institute President Robert Hartwig expects “far fewer claims” this time and that both insured and economic losses will be “materially lower”.

He says early estimates put insured losses in the Napa Valley below $US1 billion.