Scientists have found the missing link between two faults that they say will most likely result in the next major earthquake to hit the San Francisco Bay area.
If the two rupture together, the resulting quake could be devastating to the region, which is home to more than seven million people.
The Hayward fault runs directly under densely populated areas to the east of San Francisco. The new study, published in Science Advances, discovered where this meets the Rodgers Creek fault, beneath San Pablo Bay just to the north of the city.
Until now, the relationship between the two faults has been a mystery.
“Having a continuous fault does certainly make it easier for an earthquake rupture coming from either the north or the south to continue straight through,” Roland Burgmann, a geophysicist at UC Berkeley who died not take part in the research, told the LA Times.
The previously unknown connection between the two faults was discovered by scanning the bay for the magnetic anomalies that accompany fault lines.
Historically, the Hayward fault ruptures every 140 years and the last major quake there was 148 years ago. The Rodgers Creek fault has historically been a less active fault line, but geologists say pressure has built up along it.
“The concerning thing with the Hayward and Rodgers Creek faults is that they’ve accumulated enough stress to be released in a major earthquake. They’re, in a sense, primed,” says Janet Watt, a geophysicist at the US Geological Survey who led the study.
However, the two combined faults run for nearly 190 kilometres and were they to rupture together, the study predicts it would result in a 7.4 magnitude quake – five times more powerful than the 1989 Loma Prieta quake.
The last time the Hayward fault ruptured was in 1868, resulting in a magnitude 6.8 earthquake that killed 30 people.
Bill Condie is a science journalist based in Adelaide, Australia.
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