What controls the shaking you feel during an earthquake?
We compare a light bulb to an earthquake to clarify the difference between magnitude and intensity, two concepts that are often used interchangably, but in error. Though both are related to a measured earthquake, intensity reflects what is felt during an earthquake at many locations, and the magnitude is a single measured value. Intensity was originally calculated by gathering personal, anecdotal reports during an earthquake. intensity can be estimated by instrumentally, but the definitive intensity is assigned either via Did You Feel It (DIFY), or via expert judgement, as was done historically. (An IRIS/USGS collaboration)
Earthquake shaking is controlled by:
- Magnitude: how big was the earthquake?
- Distance from the hypocenter: Intensity varies from place to place
- Local rock and soil conditions: Soft sediment shakes more than hard bedrock.