You can select multiple alerts in the alerts pane and perform the same action on all the selected alerts at one time.
Tip: Performing the same action on multiple alerts is a time-saving method for clearing, acknowledging, and marking alerts as seen.
To select adjacent alerts
Select the first alert, and then hold down the Shift key and select the last alert.
To select nonadjacent alerts
Select the first alert, and then hold down the Ctrl key and select each additional alert.
To select all alerts
Click Select All on the Edit menu.