CMAS stands for "Commercial mobile alert system". Information related to CMAS is carried out by SIB12. If you check, Paging message in RRC spec. It has IE which shows the presence of CMAS. when UE decode the paging message, it comes to know the presence of CMAS and immediately it start to receive and decode SIB12 just after reading SIB1, which carry the information of scheduling of other SIBs.