Recent quake in Japan might be aftershock from 2011

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Scientists are still gathering data from Monday's 6.9 earthquake off the coast of Japan, but some are already suggesting it may be an aftershock from the massive quake that struck in the same area in 2011.

That magnitude 9.0 temblor created a tsunami that led to a meltdown at the Fukushima Daiichi nuclear power plant and resulted in more than 15,000 deaths.

Fortunately, this recent quake wasn't nearly that destructive, with only a few injuries reported and no widespread damage.

Still, it may seem strange that a quake from five years ago could result in an aftershock today.

"It's not unusual to get a large, late aftershock like this one," said Ken Hudnut, science advisor for risk reduction at the US Geological Survey.

An aftershock is any earthquake triggered by a larger, earlier quake. As time goes on, the number of expected aftershocks drops until eventually the shaking reaches the base level of what was happening before the first quake struck.

Morgan Page, also with the USGS said the larger the initial quake, the more aftershocks it produces over a longer period. That's why 2011's mega-quake might still be producing today.

"With a magnitude 9, you have a huge number of aftershocks," she said.

Page crunched the numbers and agreed that Monday's quake falls within the expected range of aftershocks, but just barely. She said we're just at the point where the rate of rumbling is decaying back to background levels.

"So I wouldn't say it's certain it's an aftershock, but it could be," she said.

Part of the calculation is how aftershocks are defined, Hudnut said.

In some cases, a secondary quake is so damaging that it is considered it's own event. In fact, a large aftershock can charge a fault enough to trigger its own suite of smaller quakes.

He said that an aftershock can strike at a different location along the fault, bringing more severe damage to an area that may have been spared the first time around.

"Aftershocks in and of their own right can be very dangerous and very damaging," Hudnut said.

All of this is a good reminder that we in Southern California are overdue for major shaking on the San Andreas fault.

A recent analysis of the fault found that a magnitude 8.3 quake stretching from the Bay Area down to Los Angeles could damage as many as 3.5 million homes.

For those looking to prepare for such a disaster, KPCC has a useful list of things to do before the Big Ones strikes.

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