Well, any earthquake could be followed by a larger event. These earthquakes could be foreshocks. We don’t know if that’s the case here. It could be. And it wouldn’t be a surprise if we had a magnitude 7.0 on the Hayward Fault. But these recent events don’t make us think that there’s a heightened hazard of a larger quake.
Are we due for a big earthquake on the Hayward Fault?
We can’t predict earthquakes. The reason we don’t use that “p” word is that there really haven’t been patterns that tell us an event is imminent. We do longer term forecasts. Like, we believe there’s a 30% chance of an earthquake on Hayward Fault that’s 6.7 or greater in the next 30 years. But not in a two to three week window as some of these misinformed reports have said.
How do we know that?
We have a 12 earthquake record going back several thousands years on the southern Hayward Fault in Fremont, and those earthquakes are about 160 years apart on average. You add 160 years to 1868, when the last major earthquake occurred on the Hayward Fault, and we’re certainly in the window.
The Hayward fault is the most likely fault in the Bay Area to produce a large earthquake. But if you look at the entire Bay Area, there’s a two in three chance or 63% chance of a magnitude 6.7 or greater somewhere in the Bay Area in the next 30 years.
What’s happening on the Hayward Fault?
It’s a strike-slip fault, which means the two sides are sliding past each other horizontally. There’s not a lot of vertical motion. And it’s what we call a right lateral strike-slip. So if you stand on one side and look across to the other side, that side will move to the right during an earthquake. Over the long-term, that’s happening at a rate of about nine millimeters per year.
Do small earthquakes like these release stress on the fault and decrease the risk of a big quake?
A lot of people ask that. These earthquakes are relatively small compared to a magnitude 7.0. The difference in energy released is huge. You’d need thousands of magnitude 3.5 to equal the amount of stress released by a 7.0. So they probably don’t do anything to release the hazard.