If SEISMO is written as TEJSNO, then how would EARTHD be coded in that language?

Let us check the letter-to-letter shifts between the word $\text{SEISMO}$ and its code $\text{TEJSNO}$:
* $\text{S}\stackrel{+1}{\to }\text{T}$
* $\text{E}\stackrel{+0}{\to }\text{E}$
* $\text{I}\stackrel{+1}{\to }\text{J}$
* $\text{S}\stackrel{+0}{\to }\text{S}$
* $\text{M}\stackrel{+1}{\to }\text{N}$
* $\text{O}\stackrel{+0}{\to }\text{O}$

The coding pattern shifts alternating letters by $+1$ and $+0$. Applying this exact operation to $\text{EARTHD}$:

* $\text{E}\stackrel{+1}{\to }\text{F}$
* $\text{A}\stackrel{+0}{\to }\text{A}$
* $\text{R}\stackrel{+1}{\to }\text{S}$
* $\text{T}\stackrel{+0}{\to }\text{T}$
* $\text{H}\stackrel{+1}{\to }\text{I}$
* $\text{D}\stackrel{+0}{\to }\text{D}$

The final coded word is $\text{FASTID}$.