What is typically the first indication of an earthquake's impact on structures?

The first indication of an earthquake's impact on structures primarily comes from intensity measures. Intensity measures refer to the quantitative assessment of the earthquake's effects on people, buildings, and the Earth's surface, which help gauge the level of damage and shaking experienced in specific areas. This is often expressed through various scales, such as the Modified Mercalli Intensity Scale, which categorizes the effects and provides a general sense of how structures are impacted by the shaking.

Seismic waves are generated at the earthquake's source and propagate through the Earth, but they are not the indication of damage on structures themselves; rather, they are the primary cause of the shaking. Tremors are a general term that may refer to the shaking sensation felt during an earthquake but do not specifically quantify or indicate structural impact. Aftershocks occur after the main earthquake and signify continued tectonic activity but do not represent the initial structural impact from the original event. Therefore, intensity measures serve as a vital tool for understanding and assessing the immediate effects of an earthquake on buildings and other structures.