Which measure is considered the oldest useful measure of an earthquake's strength based on observed effects?

The measure recognized as the oldest useful method for assessing an earthquake's strength, based on observed effects, is intensity. Intensity refers to the observed effects of an earthquake on people, structures, and the Earth's surface rather than a direct measurement of energy release, which is what magnitude quantifies. The concept of intensity traces back to the 19th century with various scales developed to describe the severity of shaking experienced in different locations and the damage observed.

This observation-based assessment relies on subjective reports and concrete evidence of damage, allowing for a comprehensive understanding of an earthquake’s local impacts. Other measures, such as magnitude (often represented by the Richter scale), quantify the energy released at the quake's source and were introduced later as technology advanced for measuring seismic waves. Seismic activity encompasses the general occurrence of earthquakes in a region, rather than individual measurement of strength. Therefore, intensity remains crucial for conveying the effects and consequences of seismic events from a societal and structural perspective.