"What would be Matthew's goal in using a different word?"
Matthew 8:24 specifies that it was only the sea that became violent, so the "result" was a tempest, not an earthquake. The earth is not mentioned. Clearly, the Greek, "seismos" covers that result. Moreover, Verse 26 instructs us that the "causation" of that result was the wind, not an upheaval of the earth:
And he saith unto them, Why are ye fearful, O ye of little faith? Then he arose, and rebuked the winds and the sea; and there was a great calm.
In each of the other NT occasions that "seismos" was translated as "earthquake", the result, the causation, or both mandated the use of the word "earthquake", because the earth actually quaked in each of those cases.
Re: Matthew:27:54, verse 51 states:
And, behold, the veil of the temple was rent in twain from the top to the bottom; and the earth did quake, and the rocks rent;
Likewise, Matthew 28:2, Mark 13:8, Luke 21:11, Acts 16:26, and Revelation 6:12; 8:5; 11:13; and 16:18 clearly indicate an upheaval of the earth, not the sea.
Even in Revelation 11:19, though the temple of God was opened in heaven, it cannot be taken that there was a shaking of that temple, or of that heaven where that temple was located.
Although translators and commentators may vary in strictness and understanding, there does not appear to be a difficulty between the INSPIRED WRITERS PERSONAL CHOICE OF INSPIRED WORDS and the INSPIRED WORDS USED--ALL IS WELL.