After Sunday's powerful Bay Area earthquake, USGS estimates a nearly one-in-three chance of a strong aftershock in the next week

Monday, August 25, 2014, 1:31 PM - Even as residents of northern California clean up from the strongest earthquake to strike the area in a quarter-century, the US Geological Survey is keeping a sharp eye out for the possibility of powerful aftershocks.

Aftershocks are always a concern with every earthquake that shakes the ground, especially the more powerful ones. The 'mainshock' quake happens as the two faults rub against each other and overcome friction with a sudden massive jolt, while the aftershocks are smaller rattles as the faults continue to move in the wake of that jolt.

Consider it like pressing your fingertip against the top of your desk and then trying to push it along the surface. At first, friction resists until your finger suddenly moves. That's the mainshock. As your finger continues to move after that, it may move along smoothly or it may stop and start a few times, shuddering across the surface of the desk (usually depending on what the surface is made of, how dry your finger is, etc) before everything smooths out. Those shuddering stops and starts are like aftershocks.

While most aftershocks are significantly weaker than the mainshock, this isn't always the case. Sometimes, the initial mainshock is followed by another quake that's nearly as powerful, and can be just as damaging (or add significant damage to what's already happened). In some rare instances, an even more powerful quake can follow.

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Credit: US Geological Survey

In this case, after the magnitude 6.0 quake on Sunday morning, the USGS is predicting between 12 and 40 aftershocks of magnitude 3.0 to 5.0 over the next seven days, with the possibility of an even stronger aftershock during that time as well.

"At this time (one day after the mainshock)," their latest forecast (as of 6:20 am EDT) reads, "the probability of a strong and possibly damaging aftershock [greater than magnitude 5.0] in the next 7 days is approximately 29 per cent."

That probability is down from the 54 per cent chance issued by the USGS in their aftershock warning on Sunday, right after the mainshock struck.

"However," the statement adds, "there is a small chance (approximately 5 to 10 per cent) of an earthquake equal to or larger than this mainshock in the next 7 days."

Even with these probabilities in hand, the chances of having only weak and diminishing aftershocks following this quake far outweigh any other outcome, but Bay Area residents should be on guard for the next week. Also, aftershocks from a quake of this magnitude can rattle regions up to 15 to 30 kilometres away from the epicenter.

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