Magnitude vs intensity
Magnitude measures the size of an earthquake at its source — one number per event. Intensity measures the shaking at a particular place, and changes with distance, depth and ground type.
Circle area = energy released. Each step ≈ 32× more.
Comparable: Virginia, USA (2011)
Moment magnitude
Modern magnitude (Mw) is based on seismic moment: the fault’s area × how far it slipped × the stiffness of the rock. Each whole step means about 32 times more energy and 10 times more ground motion on a seismogram. An M8 releases roughly 1,000 times the energy of an M6.
Intensity — what people feel
The Modified Mercalli Intensity scale runs from I (not felt) to X+ (extreme), written in Roman numerals. It is built from observations — including reports like the ones you send from sisma — so one M6 can be intensity VIII nearby and III a few hundred kilometres away.
Why soft ground shakes more
Loose sediment and landfill amplify and prolong shaking compared with bedrock. Mexico City’s ancient lakebed made the 1985 M8.0 earthquake devastating 350 km from its epicentre.