California is referenced as 'earthquake country', but some areas are affected more then others due to their placement by faults.
People may not realize that the state of California experiences around 500 to 700 earthquakes every week, with a handful of magnitudes being very small and causing no prominent damage.
A majority tend to be near known surface faults.
Yolo County is not one of those typical places, but on Monday they had a number of earthquakes that shook parts of the county.
"In the case of the Yolo quakes, the hypocenters were 9-10 miles deep and there was no indication at the surface of a fault being present." Don Drysdale, representative for the California Dept. of Conservation, said. "It could be that whatever fault mechanism is at work under Yolo County moves very infrequently and very slowly."
Also, although it does happen sometimes, the underlying mechanisms aren’t very well understood and there's a good chance of subsurface faults that scientists and the public are unaware of, said Drysdale.
Below is a map of the recent earthquakes in California during the past week and you can see where Yolo County (red circle) is located compared to other areas that are near an active surface fault:
Most of the quakes that happened in the past week were generally on or near faults and the ones that weren't, like Yolo County, though not uncommon, are very unusual.
Sacramento is another area that doesn't sit atop any known faults, but if there's a big enough quake on the Hayward Fault then the following affects could have an impact whether it's severe or not.
Unless a fault has broken through the surface, such as the San Andreas fault, or has caused a significant earthquake in modern times without breaking through to the surface, the 1994 Northridge quake being an example, then scientists may not know it exists, according to Drysdale.
Again, California is earthquake country, so what we do know is that there are still some unknowns about what's beneath.
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