I'm not sure there's an actual "blow up ship" ability trigger, but you can always have the ability cast on self to do some obscene amount of damage to kill itself and then call a second buff that does damage to nearby targets. Volatile Nanites has the AE damage on death setup. You can either use DoDamage to make your ship take one time damage that kills it, or things like Disintegrate does damage-over-time so you can look at the keyword there.

Hope this helps a bit

One buff needs to call the other buff to chain effects together. There are lots of examples of this, just look at all the buff files that are named "Self"/"Spawner" and "Target". The ability calls the "self" buff, which does something and calls the "target" buff next. You can string a bunch of them together though it obviously gets a little more confusing each time

Ok, let's go through an easy one : AbilityDesignateTarget (Cielo cruiser's Designate Target ability) 

In the ability file we have:

buffInstantActionType "ApplyTargettedBuffToSelf"
instantActionTriggerType "AlwaysPerform"
buffType "BuffDesignateTargetCaster"
targetFilter
 numOwnerships 1
 ownership "Enemy"
 numObjects 3
 object "CapitalShip"
 object "Frigate"
 object "PlanetModule"
 numConstraints 0

Note the buffInstantActionType, it applies a buff that will need targets to itself and that buff is BuffDesignateTargetCaster. The targetFilter points to enemy ships, because the overall ability targets an enemy.

So we open BuffDesignateTargetCaster:

numInstantActions 2
instantAction
 buffInstantActionType "PlayPersistantAttachedEffect"
 instantActionTriggerType "OnDelay"
 delayTime 0.000000
 effectInfo
  effectAttachInfo
   attachType "Ability"
   abilityIndex 0
  smallEffectName "Buff_DesignateTargetChanneling"
  largeEffectName "Buff_DesignateTargetChanneling"
  soundID ""
instantAction
 buffInstantActionType "ApplyBuffToTargetNoFilterNoRange"
 instantActionTriggerType "OnDelay"
 delayTime 0.000000
 buffType "BuffDesignateTargetOnTarget"

So this buff does two things: first it plays a graphical effect on the casting ship (that's why the ability called this buff file first) and then it applies *another* buff to the target (which you remember from the ability was any hostile ship). The second buff is BuffDesignateTargetOnTarget:

numInstantActions 1
instantAction
 buffInstantActionType "PlayPersistantAttachedEffect"
 instantActionTriggerType "OnDelay"
 delayTime 0.000000
 effectInfo
  effectAttachInfo
   attachType "Above"
  smallEffectName "Buff_DesignateTargetOnTarget"
  largeEffectName "Buff_DesignateTargetOnTargetLarge"
  soundID ""
numPeriodicActions 0
numOverTimeActions 0
numEntityModifiers 1
entityModifier
 buffEntityModifierType "DamageAsDamageTarget"
 value
  Level:0 0.250000
  Level:1 0.000000
  Level:2 0.000000

.. which also plays an effect on the target and makes it take 25% more damage.

So this is the general method for doing it. Your overall ability has a specific goal. In this case, the ability is cast on an enemy ship. If your self-destruct-for-big-boom case, the ability cannot be cast on another ship, because it's the ship that has the ability that needs to go boom. So for you, you can just use "ApplyBuffToSelf" since the ability isn't targetted.

Buffs are simple things. They can only affect one thing at a time. You can't have a single buff that does something to your ship and something else to an enemy ship. That's why you need two. The first buff your ability will call will self-destruct the ship and call the second buff. The second buff's job will be to do damage to all enemy ships in a radius (ApplyBuffToTargetsInRadius) which will then need its own target filter.