One earthquake, many experiences
Magnitude estimates the earthquake’s size at its source. It is expressed as one number for the entire event. Intensity describes shaking effects at a particular location—what people felt and what damage occurred. One earthquake can therefore have a single magnitude but many intensity values.
The magnitude scale is logarithmic. An increase of one unit represents a large change rather than an equal step. Magnitude, however, does not automatically equal destruction. Depth, distance, soil, building quality and shaking duration all affect consequences. Two cities equally distant from the source can experience different motion.
What early warning actually does
Earthquake early warning does not predict an earthquake. Sensors detect an event that has already begun, rapidly estimate its location and size, and send a signal before the strongest waves reach more distant places. Locations close to the epicenter may receive little or no warning. Farther away, seconds can allow trains to slow, machinery to stop and people to take protective action.
What to do when an alert arrives
In most situations, drop, cover and hold on. Move away from windows and do not run toward stairs during shaking. Afterward, expect aftershocks, check immediate hazards and follow authorities. Near a coast, strong or prolonged shaking can be a natural tsunami warning; move toward high ground when local guidance calls for it.
How to read preliminary reports
Initial earthquake figures can change as additional instruments report. A revision does not mean the earthquake changed; it means the estimate improved. Consult USGS and local seismic agencies, note depth and location, and do not treat a viral post as the only source.