A still—Volcano—Life—
That flickered in the night—
When it was dark enough to do
Without erasing sight—
A quiet—Earthquake Style—
Too subtle to suspect
By natures this side Naples—
The North cannot detect
The Solemn—Torrid—Symbol—
The lips that never lie—
The volcanic energy (Dickinson’s inner self), described throughout the poem, is mentioned as being “too subtle to suspect”, a reference to her self-control and silence. In this way, the use of “natures” would refer to the contrasting qualities of the speaker; that is quiet but eruptive, which go unsuspected by others. This includes those closest to her.  Hence, “this side Naples”, would reference proximity to the volcano, as Naples is the Italian city closest to the volcano Vesuvius (Sandra M. Gilbert and Susan Gubar, The Norton Anthology of Literature by Women). 
 Vesuvius (picture from Swide, “The 10 best Volcano tours in the world”): 

A still—Volcano—Life 601