Final answer:
An earthquake has only one magnitude, representing the energy released at the source, but multiple intensity values, which measure the effects of the earthquake at various locations. These intensities vary due to the decrease in amplitude of waves as they spread out and due to local conditions such as geology, construction, and wave interference patterns.
Step-by-step explanation:
An earthquake can have many intensity values because intensity measures the earthquake's impact as it varies with distance, local geology, and construction practices, not the energy released at the source. The magnitude of an earthquake, often measured on the Richter scale, is a single number that represents the total energy released at the source. This magnitude is a fixed value and does not change regardless of where you are relative to the quake's epicenter.
However, as earthquake waves spread out, their amplitude decreases, leading to varying degrees of shaking and damage at different locations. The intensity is essentially the power per unit area that the earthquake waves impart on the surface of the Earth. This is similar to how sound waves decrease in power as they move away from a source. Factors such as interference patterns of waves (constructive or destructive), resonance with structures, and the absorption characteristics of the local geology all contribute to the variable intensity experienced during an earthquake.
It is also possible for conditions such as constructive interference to occur where waves overlap, adding to their amplitude and causing greater destruction at points further from the epicenter. Destructive interference, on the other hand, can occur near the epicenter, sporadically sparing certain areas from damage.