Prayer: If mild quakes are reported but not felt, is āyāt prayer still obligatory?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: If mild quakes are reported but not felt, is āyāt prayer still obligatory?

English:

Question #712:

The center of seismography reports the occurrence of several mild earthquakes in the area we live, mentioning their number, though we felt none of them. Is āyāt prayer obligatory for us in such a case?

Answer #712:

If it happens in a way that no one feels its occurrence and it is only understood by using an instrument, the āyāt prayer will not be obligatory.

-Imam Khamenei, Practical Laws of Islam, Importance and Conditions of Prayer