Use a repeating event then. Or if that's too regular for your liking you could try something like this.
every-time
-and
--has-time-elapsed
---When the bombardment starts
---
ExplosionTimeVariable(0) --<
---
NumberOfExplosionsSoFarVariable(0) ---Number of Explosions you want
-Explosion-effect
-- +
---lowest x value for the explosion location.
---rand
----0
----Some value
-- +
---lowest y value for the explosion location.
---rand
----0
----Some value
-- +
---lowest z value for the explosion location.
---rand
----0
----Some value
--All the other explosion effect settings
-modify-variable
--
ExplosionTimeVariable(0)--Rand
---0
---3
-modify-variable
--
NumberOfExplosionsSoFarVariable(0)-- +
---
NumberOfExplosionsSoFarVariable(0)---1
Okay. Now to explain it. The event above will make a set number of explosions occur in the bounding box you've specified using the X+somevalue settings at random intervals of about 0-3 seconds.
Any questions?