It could be purely coincidental. Earthquakes happen all the time, even when we don't know it. But, on the other hand, the fact that a small tremor happened as you were envoking the elemnts could be a positive or negative sign.
If this happened whilst you were invoking the element of Earth, you most likely won't know that the spell was a success until you begins to see signs of a positive change.
Hope. One of the main reason why a spell works is because you believe , the more you believe the spell will work, the higher the chances of success. I have always said that there is no such thing as hope, if you believe, you will get things done.