Multi_manager, trigger the env_shake multiple times after each other...
Or create a 'loop', e.g. a multi_manager that triggers the env_shake and triggers itself (the Multithreaded flag must be set to allow looping).
Or, if you want it to be stoppable, let the multi_manager trigger the env_shake and a func_button. This func_button triggers the multi_manager again. So, it's a loop, but this one can be interrupted by changing the target of the button using a trigger_changetarget.