The City of Petaluma has a website which discusses crime prevention in Petaluma neighborhoods. They say they will be adding a graphical display of neighborhood statistics. http://cityofpetaluma.net/police/crimeprevention.html
You can also try googling Petaluma Crime statistics to see what you come up with and contact the Sonoma County Sheriff's Department as well. As Realtors we are not allowed to provide this information to you. Be aware that some areas in Sonoma County fall under the county's jurisdiction if the homes are outside a particular city limits. Since so many residences here are rural that is often the case. Good luck with your search.