Jerry
Do you have other pictures of this turtle? That doesn't look like a red-ear shell. It looks like a painted turtle. While painted turtles are not native to California, the western painted is an alien, introduced species. That shell sure doesn't look like a red-ear shell.
You may not have scared it away. Turtles often have "false starts" when getting ready to lay their eggs. The will start digging, then stop, maybe move to another spot and start digging again, or may even go back to the pond and try again the next day. Most will dig the nest in the afternoon to early evening.
Don