If you're going to have the tremor tokens have a symbol on both sides, it can be the same symbol on both sides. When a tremor gets encountered you can remove the token to the cup and draw a Tremor Encounter Card. If the tremor reaches Arkham you can draw a Tremors in Arkham Card. If the Tremor Encounters cards are (in general) less bad than the Tremors in Arkham cards, players will be incentivized to go exploring them rather than passively waiting for them to be unleashed upon town.
For example: if you encounter some of the cultists while they are still underground, you may get beat up and you may rescue one of the current hostages (removing a Blight and possibly getting a reward for doing so). The equivalent card in the Tremors in Arkham deck simply involves someone getting kidnapped and a Blight going down.
-Frank