I call Tremor's chest Richter scale.

Tremor's original bio is still in the N64's manual but placed accidentally under Rain. So if you take the bio from N64's Rain in the manual and place Tremor's name where you see Rain's you have what would have been Tremor's original bio when he was slated to be in MK Trilogy.

"Rain is an assassin working freelance for the Lin Kuei alongside Sub-Zero, but left the clan under mysterious circumstances. Years later, Rain is found working for Shao Kahn as an assassin in the Outworld. Born a human, he finds himself questioning his loyalty towards Kahn after his invasion."

Rain's bio in PlayStation, PC and Saturn's the same but different than the one above featured in N64's manual. In those versions he is from Kitana's home world of Edenia which is what we all know him to be to this very day. I also believe in Special Forces it is also mentioned that Tremor was once a Lin Kuei, but I cannot truly remember. There is no mention where his elemental powers come from but other media like Konquest revealed the Lin Kuei trained all of their deadliest of assassins in darker arts so maybe that is how. In Special Forces he had fireball and ground pound abilities. Tremor is human that somehow gained earth elemental abilities sometime in his life. Maybe in the future we'll learn how?

Edit: After checking Tremor's MKX ending basically hints at how he became an earth elemental:

Standing over Shinnok, Tremor reveled in his power. Much had changed since the Black Dragon's excursion to the Dream Realm.
Kano had sent Tremor's team there to retrieve a "psych-bomb," to be used in Kano's theft of Shinnok's Amulet.
Exposure to that realm had increased Tremor's power--and expanded his mind. He would evolve into an Earth elemental: a demi-god whose power would rival that of Raiden and Fujin.

I'm a Virgo, so his power/element is my (zodiac) sign. Would make sense if he was too.