(Bloomberg Gadfly) – Tuesday's magnitude-7.4 earthquake offthe coast of Japan's Fukushima prefecture could hardly have come ata worse time for the nation's insurers.

April's Kumamoto tremor is alreadythe country's second-most costly on record. Some 329 billion yen($3 billion) in claims had been paid by late June, a figurelikely to rise further as more cases are resolved. Suchdisasters have a long tail of costs: Five years after the country'smost expensive temblor, Japan Earthquake Reinsurance Co. paid some5.25 billion yen of claims last fiscal year related to the2011 Tohoku earthquake.

Affected households limited

Along with the natural worry about loss of life and property,that's led to aftershocks in the market. Sompo Holdings Inc. fellas much as 2.2 percent in Tokyo morning trading, while TokioMarine Holdings Inc. dropped 0.6 percent. MS&AD Insurance Group Holdings Inc., which getsabout a quarter of its revenue from life insurance, was the onlygainer, with a 0.2 percent advance as of 10:30 a.m. local time.

Investors shouldn't worry overly about the impact. For onething, the quake struck so close to the Fukushima Daiichi nuclearreactor that the number of affected households is seriouslylimited. The main city exposed, according to the U.S. Geological Survey, is likely to be Namie,a town lying within the Fukushima exclusion zone that has beenevacuated since the 2011 disaster.

Only about 222,000 people live in areas where they'relikely to feel strong tremors, according to the U.S.Geological Survey. Contrast that to this April, which exposed theentire city of Kumamoto to violent shaking, with 4.6 million peopleliving in areas that suffered jolts rated at least as“strong.”

Meanwhile, the threat of a tsunami — which caused theoverwhelming number of fatalities in 2011 — has now “largelypassed,” according to the Pacific Tsunami Warning Center.

Japan's insurers in 'pretty good shape'

Japan's insurers are in pretty good shape. Despite the impact ofKumamoto, loss ratios — claims paid out as a percentage ofpremiums received — came out in the high fifties in the latestfilings by the nation's big three property and casualty insurers.That puts them in a stronger position than larger peers in SouthKorea, Canada, the U.S. and China.

Thanks to a spate of recent overseas takeovers, they're alsoless exposed to the domestic market, helping to limit the impact ofany Japan-specific disasters.

It's tempting when trouble strikes to worry that the insuranceindustry, as the first line of defense, will take the brunt of theblow. But Japan's earthquake-reinsurance plan has seen itsurvive bigger disasters than this one. The country's insurers arebuilt on strong foundations. They're not going to topple justyet.

Related: Disasters cost insurers $37B in 2015, according toSwiss Re

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