The agent of an action is sometimes marked with tarafından. Does the referent of this NP have to be agentive (human/animate, intentional) or can the grammatical agent refer to inanimate things and circumstances also?

There were some other ways to mark the agent also but it seems that I forgot...-ce?

Hi Abla, maybe I can help. These come from some notes supplied by one of my Turkish teachers (Ani), and slightly adjusted by me, with basic explanations for other learners who might read this as well.

Eden öbeği (agent phrase)

In passive sentences, you can use forms of tarafından to express who or what performed the action.

examples

Kitabı yazdım (I wrote the book)

Kitap yazıldı (The book was written) This is a passive sentence.

Kitap benim tarafımdan yazıldı (The book was written by me) 

Kitap senin tarafından yazıldı (The book was written by you)

Kitap onun tarafından yazıldı (The book was written by him/her)

Kitap Ali tarafından yazıldı (The book was written by Ali)

Kitap öğrenci tarafından yazıldı (The book was written by the student)

Kitap bizim tarafımızdan yazıldı (The book was written by us)

Kitap sizin tarafınızdan yazıldı (The book was written by you)

Kitap onlar tarafından yazıldı (The book was written by them)

Note - bunun, şunun and onun (this and that forms) can also be used.

In those cases in which the agent is an institution (or similar, eg. companies, banks, political groups), it can be expressed by adding the ca suffix to the noun.

for example

Dernek dilekçeyi yazdı (The association wrote the petition)

Dilekçe dernek tarafından yazıldı

(The petition was written by the association)

Dilekçe dernekçe yazıldı. (same meaning as above)

Herkes bu konuyu biliyor (Everybody knows this topic)

Bu konu herkes tarafından biliniyor.(This topic is known by everybody)

Bu konu herkesçe biliniyor. (same meaning as in the line above)

I have searched the internet for examples of situations where the agent of the action is an event and found

Kadın büyük bir deprem tarafından korkutuldu.

The lady was frightened by the big earthquake.

But just because it is on the internet, it doesn´t necessarily guarantee correct usage. 

This is exactly what I ment.

Thank you for this reminder, Henry. Somehow I feel Turks don´t like this structure. You don´t see it all the time even though passive voice is in frequent use  -  but Turkish passive is partly impersonal: agent is not an issue in many sentences which are formally passive. So this might be part of the reason. Besides, it is difficult for a learner to "hear" where passive sounds natural and where active should be used instead.

Actually I could write here the sentence I was trying to translate -  what am I saving it for anyway? Is passive good here, what about the agents?

The disaster was made possible by a dry summer preceding the event, a fire-spreading storm rising on the night of the fire, and a lack of extinguishers. Felaket, olaydan önceki yağmursuz bir yaz, yangını yayan fırtınanın yangın gecesinde çıkması ve söndürücülerin az olması tarafından mümkün kılındı.

Yangın söndürücülerin eksikliği, yangın gecesi ortaya çıkıp yangını körükleyen fırtına ve hadise öncesinde  kuru bir yaz yaşanmış olması felaketin yaşanmasına sebep oldu.

´´ alll those negative factors made the disaster possible ´ sounding better way in Turkish.

´ all those negative factors  caused the disaster to happen ´

So, making the agent active, clear is more natural Turkish in your sentence.

Ohhh, a huge difference between the proper sentence and mine. We are experiencing a language barrier here.

Thank you, tunci.

In my opinion,  it is because English way of thinking causing this confusion, For example, in your example , "by" is perfectly fit in English whereas it does not in Turkish. It is the spirit of the language, therefore sometimes while passive sounds perfect in English, we choose active instead  in Turkish.

But in other examples it can match perfectly such as ;

The roof of the house was demolished by the severe storm.

Evin çatısı şiddetli rüzgar tarafından yerle bir edildi.

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If we adapt above sentence into your original sentence

The disaster [ demolishing the house of the roof] was made possible by the severe storm.

Felaket, şiddetli rüzgar tarafından mümkün kılındı. ----> sounds weird and confusing

To fix the sentence to make sense , we need to make it in active form.

Şiddetli rüzgar felakete  neden oldu.

The severe storm caused the disaster to happen.

Şiddetli rüzgar evin çatısının yerle bir olmasına neden oldu.

The severe storm caused the roof of the house demolished.

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I think , the problem is caused by the causative in the passive sentence.

The disaster was made possible by .........

and even if we remove the causative form from the sentence and keep the passive form "to be caused" ;

The disaster  was caused by ......................

Felaket................................................................tarafindan sebep [neden] olundu.

It still does not sound good in Turkish.

My conclusion would be ; Passive form of " to be caused"  [neden olunmak] has to be translated and understood  as its active form " to cause" [neden olmak] in Turkish.

This was a good lesson. The limits of a grammatical passive are not the same in Turkish and English (makes sense also) and often in Turkish active is a safer choice for transitive sentences. Thanks again for explaining it, tunci.