From what I gathered from the various text files 4zb4's description of Terra being for earth and Silva for nature is correct, with the two being merged at some point.
And Warheck is correct: if you look in define.dat and ivandef.h you'll see that TERRA and SILVA are actually both defined as the same god, meaning anything labeled TERRA in the scripts is actually attached to Silva.
#define VALPURUS 1
#define LEGIFER 2
#define ATAVUS 3
#define DULCIS 4
#define SEGES 5
#define SOPHOS 6
#define TERRA 7
#define SILVA 7
#define LORICATUS 8
#define MELLIS 9
#define CLEPTIA 10
#define NEFAS 11
#define SCABIES 12
#define INFUSCOR 13
#define CRUENTUS 14
#define MORTIFER 15
#define ATHEIST 16