According to these maps, half of the Hazu Mountains were submerged for a time. Whether this means Tashalar was effected or not remains unclear, but it seems like the mountains are formed by continental plate convergence, sank when the plates diverged, and rose again open subsequent convergence. Given the proximity to the plate boundary, we can assume the people at least experienced some seismic activity, and at worst have evacuated the city as the relative sea levels rose. However, it’s likely that mages and druids employed magic to protect themselves from devastation.
Tomb of Annihilation, stylized
DM’s Guild supplement for Adventure’s League Season 7/Misc, Chultan Adventures
Just keep in mind that the Chultan Peninsula is not a welcoming place. It’s craggy mountains and dense jungles hide many lost cities and monstrous things, both living and undead. Tashalar is the one of the most inviting cities in the region, but few who live there venture south. My advice would be to borrow heavily from Tomb of Annihilation and the supplemental sources that detail the ecology of the Peninsula. This region is rich with ancient, primal forces.
Consider the Grung (poison dart frog people,) Kuo Toa, Yuan-ti, Dino-folk, Jungle-kobolds (and other variant humanoids,) as squamous-skinned, and giant insectoid creatures thrive in the oppressively-humid lands.
The jungle takes what it wants, and leaves many mysteries behind. Merchant Princes rewrite history with alterations and omissions. On the Chultan Peninsula, Survival is the only truth.