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Latrodectus geometricus
Spider's leg span around 1 inch.
If you can or are able to take any other photos of this spider, you may be able to get a species. I'm not sure from this angle. I think it might be a Western Black Widow, Latrodectus hesperus, but Lactrodectus geometricus, the Brown Widow, a nonnative that is threatening to take over native Western Black Widows in Southern California, is another possibility.