The earthquake in Japan caused insured losses of $12 billion to $25 billion, making it one of the costliest natural disasters in history for global insurers, according to risk modeling firm Eqecat.
Eqecat's Wednesday estimate — which includes the earthquake, the subsequent tsunami and a series of fires — is sharply lower than that of competitor AIR Worldwide, which has estimated losses of $15 billion to $35 billion, excluding the effects of the tsunami.
The discrepancy points to the difficulty in assessing the disaster last Friday, which scientists say was much more intense than they had ever modeled for in that part of Japan.
"This narrows the range and brings the top end down a bit (compared with AIR). It doesn't feel out of kilter with what we've been hearing," said Joanna Parsons, insurance analyst at Royal Bank of Scotland in London.
Insurance shares around the world have been under pressure for days as skittish investors sell on the possibility that some insurers or reinsurers may have to cut share buybacks or raise capital to offset losses.
S&P insurance shares were down 2.1 percent in afternoon trade, in line with the decline in the broader market.
The carriers considered most affected include Prudential, MetLife, AIG, ACE and Aflac, as well as top global reinsurers Swiss Re, Munich Re and Hannover Re.
Standard & Poor's said in a statement that most insurers were dealing with the disaster from a position of capital strength, so it did not expect widespread credit ratings actions to follow.
The biggest vulnerability lies with Japanese domestic property insurers, S&P said.
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