C11 6.7.9 Initialization p19 covers this (my emphasis)
The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject;151) all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.
Section 6.7.9 p10 states that
If an object that has static or thread storage duration is not initialized explicitly, then...if it has arithmetic type, it is initialized to (positive or unsigned) zero;