The Californian quake caused hundreds of millions of dollars of damage (Image: Josh Edelson/AFP/Getty Images)

It was bad, and it might make things worse. The magnitude-6.0 earthquake that hit northern California in the early hours of 24 August piled stress on to the Hayward fault, which is due for a major rupture.

Centred on the Napa Valley, the quake – the largest in the area since 1989 – injured more than 200 people and caused hundreds of millions of dollars in damage. Seismologists with the US Geological Survey (USGS) quickly modelled its effect on other faults in the area. The results suggest that the northern part of the Hayward fault, which runs to the east of San Francisco Bay, ended up under greater stress.

The Hayward fault is overdue a major earthquake. Its last earthquake was in 1868, almost 150 years ago. The past five large earthquakes on the Hayward fault have been about 140 years apart.


Like a tyre

However, this week’s quake doesn’t mean the Hayward fault is now primed, says Ross Stein of the USGS. “We’re saying it gave it a kick. Whether or not that will be large enough to trigger something, we don’t know.”

The additional pressure on the Hayward fault is 0.5 bars, or about a fifth of the pressure in your car tyres, Stein says. That is probably not enough to trigger a large earthquake directly.

“It’s a fairly small increase in stress, but the fault is thought to be one that has a pretty high probability of rupturing in the next 30 years,” says USGS scientist Thomas Parsons.

The USGS estimates that there is a 1 in 3 chance of a magnitude-6.7 or larger quake on the Hayward fault by 2037. Such an event would be devastating to San Francisco, Berkeley and the 7 million people living in the Bay Area.

“It’s the interplay between these faults and sea level fluctuations that give us the special conditions that give us the vineyards in the Napa Valley,” Stein says. “We enjoy the fruits of the faults, we should expect the [downsides] too. This is the yin and yang of California life.”