em-cmdline-client -l entmgrhost -p 15000 -u Administrator -w STC
-s AlertService51x -n

Note: the order of the parameters is important.
Available methods and parameters:

-m deleteAlerts -Pfilter=<filter>
-m getAllAlerts
-m observeAlerts -Pfilter=<filter>
-m resolveAlerts -Pfilter=<filter>
-m resolveAllAlerts
-m deleteAllAlerts
-m observeAllAlerts
-m getAlertQueryFields
-m getAlerts -Pfilter=<filter>
-m resetAlerts -Pfilter=<filter>
-m resetAllAlerts