ocean trenches and diversity of species

kurious

Why is there such a diversity of species in ocean trenches given that earthquake-induced landslides kill many individuals.? The landslides can't be wiping out whole species, so a given species must be distributed all over the trench.Also, why do some holothurians flock together at certain times? Are they eating food, mating, or migrating? Do they know when an earthquake is coming and move on ahead of it?