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The U.S. Geological Survey (USGS) recently published a paper about the chance of an earthquake that would include the Hayward and Rodgers Creek faults simultaneously. Earlier studies suggest the results could cost 6,000 lives and $8 billion in infrastructure and building damage. The effects, depending on the size of the earthquake, could be much worse.

The USGS analysis shows:

This scenario shows the ground shaking from a magnitude 7.2 earthquake involving rupture of both the Hayward and Rodgers Creek faults. In this scenario, rupture begins beneath San Pablo Bay and simultaneously proceeds towards the south on the Hayward fault and towards north on the Rodgers Creek fault.

Also:

This scenario earthquake ruptures 51 miles (83 km) of the Hayward fault from San Pablo Bay to Fremont and 39 miles (63 km) of the Rodgers Creek fault from San Pablo Bay to north of Santa Rosa. The average slip or offset across the Hayward fault is about 4 ft (1.2 m). For the Rodgers Creek fault it is about 3 ft (0.9 m).

The recent estimates of deaths and property damage are predicated on a magnitude 7.2 earthquake, so the results of the USGS scenario would be different.

According to an earlier study from the same organization:

The study found that all ten of the most likely forecast earthquakes would cause social and economic disruption equal to or greater than the 1989 magnitude 6.9 Loma Prieta earthquake when current building replacement/repair costs in the San Francisco Bay Area are utilized. The M6.9 Loma Prieta earthquake resulted in at least $6 billion in damage to buildings and infrastructure. This amount does not include the resulting economic losses related to this damage.

And:

A repeat of the 1906 magnitude 7.9 earthquake, the worst case scenario for the Bay Area, is estimated to result in about 5800 fatalities if it strikes during working hours. This estimate is comparable to the approximately 6000 deaths caused by the 1995 M6.9 Kobe earthquake that occurred in the afternoon directly beneath an urban area with a population of 1.52 million people. Most scenarios, however, have maximum projected fatalities on the order of several hundred, reflecting the success of earthquake-resistant design and construction practices in California, particularly in residences.

The recent study is about a “two fault” incident. Any huge earthquake in San Francisco would cause hundreds, if not thousands, of deaths and billions of dollars in property destruction.