Yes, you should to prevent these types of items from tipping over during an earthquake. This could be covered under the Title 8 (Cal-OSHA) general duty clause – which covers everything. But, we found something a little better and more relevant to the issue. While the book is older and very difficult to find, it gives a nice measurement for determining which bookcases should be secured.
The book was published by California’s old OES (now Cal-EMA) and it was entitled “Identification and Reduction of Nonstructural Earthquake Hazards in California Schools by OES”. It’s a 1990 black and white printed book and is a real treat to read. Under Arnold Schwarzenegger the book was updated and can be found at http://w3.calema.ca.gov/Operational/OESHome.nsf/PDF/SB1122/$file/SB1122-as.pdf I would still suggest hunting down the 1990 version – it’s a great read.