Fasting over a Sefer Torah That Fell

Hello, Rabbi
I was staying with my parents for the holidays.
I bought a seat in their synagogue for that purpose.
On Simchat Torah I prayed with them, and a Sefer Torah fell there from the Aron Kodesh.
The rabbi of the synagogue designated a fast day from alot hashachar until halachic midday for “members of the community and the regular worshippers.”
Must I participate in the fast even though I do not pray there regularly and am not a member of the community?
If so, is it preferable to fast until tzeit hakochavim?
Are there Selichot/prayers/customs for this fast?

Hello and blessings 

In terms of the synagogue rabbi’s enactment, you are not obligated to fast, since the enactment was explicitly for “members of the synagogue and regular worshippers.” 

 There are those who have the custom to fast a full day [until tzeit hakochavim] if they saw a Sefer Torah fall, and in that respect it would apply to you as well. 

There are no fixed Selichot or customs for this fast. One should add “Aneinu” in the Minchah prayer, in the blessing of Shome’a Tefillah. 

If there are ten people fasting the entire day, they read “Vayechal” at Minchah, as on any public fast. 

Magen Avraham, Orach Chaim, siman 44, se’if katan 5, citing the words of Mishpetei Shmuel. And in accordance with his words, the Mishnah Berurah wrote (siman 40, se’if katan 3). 

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