Northern California may not be out of the woods yet. On Sunday they were slammed with a 6.0 magnitude earthquake 51 miles southwest of Sacramento.
But the fear has only begun, as a significant aftershock could follow Sunday’s shaker, and it might be really big.
According to the USGS the chance that an aftershock will hit the Napa Valley area in the next week is about 29%. That’s a dauntingly large number for people who are already trying to pick up the pieces from Sunday’s earthquake. The USGS describes such a shock as “strong and possibly damaging.” That percentage is revised down from as high as 54% on Sunday.
There are actually most likely going to be 30 to 70 small aftershocks during this time along with that 29% chance of a big one (magnitude 5 or larger). So no matter what, this is the beginning of an event, not an end. The USGS’ “rule of thumb” is that anything over a magnitude of 5 has the potential to cause damage.
Occasionally aftershocks do end up being bigger than the initial shock but in this specific case the USGS says there is only a 5-10% chance that will happen.
In basic terms, earthquakes happen when two pieces of the earth suddenly slip by one another at a fault or fault plane. There are actually three different categories of shocks within an event. The foreshocks, main shocks, and aftershocks. California’s 6.0 quake was the main shock.
As for this specific earthquake, the USGS tells us this:
Aftershocks tend to occur near the mainshock, but the exact geographic pattern of the aftershocks varies from earthquake to earthquake and is not predictable. The larger the mainshock, the larger the area of aftershocks. While there is no “hard” cutoff distance beyond which an earthquake is totally incapable of triggering an aftershock, the vast majority of aftershocks are located close to the mainshock. As a rule of thumb, a magnitude 6 mainshock may have aftershocks up to 10 to 20 miles away, while a magnitude 7 mainshock may have aftershocks as far as 30 to 50 miles away.
Aftershocks happen very close to the epicentre of the main shock. The power of earthquakes is associated with a release of energy and stress. In most basic terms, it is this release that causes aftershocks to happen in rocks near the epicentre or fault where the original earthquake happened.
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