The punishment of the people of Sayyidna Shu'aib (علیہ السلام) has been identified as earthquake (Rajfah, Zalzalah) while in other verses it appears as: فَأَخَذَهُمْ عَذَابُ يَوْمِ الظُّلَّةِ (26:189) which means that they were seized by the punishment of Yowm az-Zullah. Yowm az-Zullah means day of the shade. It refers to what happened to them when to come on them first was the shade of a deep and thick cloud under which they all assembled. Then, from this very cloud, stones or fire was rained down.
To accommodate and reconcile the sense of the two verses given above, Sayyidna ` Abdullah ibn ` Abbas ؓ has said: First of all, the people of Sayyidna Shu'aib (علیہ السلام) were subjected to such intense heat, as if the door of Hell has been thrown open towards them. This made them suffocate. Shade or water, nothing seemed to work. Driven by heat, they went into basements. They were more hot. With no choice left, they ran from the city into the nearby forest. There, Allah Ta` ala sent a thick cloud with cool breeze passing underneath. Out of their senses under the impact of heat, they all rushed for refuge under the shade of that cloud. At that time, the whole cloud became a cloud of fire raining on them and also came the earthquake which turned them to ashes. In this manner, the punishments of the earthquake and the shade had simultaneously converged on these people. (Al-Bahr Al Muhit)
Some commentators have said that it is also possible that the people of Sayyidna Shu'aib (علیہ السلام) were split in different groups or sections where some were hit by the earthquake while others were destroyed by the punishment of the shade.