Zenith National Insurance Corp. has filed with the California Department of Insurance to reduce its worker’s compensation rates in California some time on or after January 1 of 2006. The rates will be an average of 13.1 percent lower than rates in effect since July 1 of this year. “Our rate decrease is in response to the favorable short-term trends in claim costs,” said Stanley Zax, Zenith’s chairman and president. “We continue to be optimistic that our business strategy will provide favorable results.”