Magnitude is about energy, not damage
The moment magnitude scale (Mw) measures the total seismic energy released by an earthquake at its source. It replaced the older Richter scale because it works consistently across all earthquake sizes and distances. Each whole number increase represents roughly 32 times more energy release — so a magnitude 7.0 earthquake releases about 1,000 times more energy than a magnitude 5.0.
But magnitude alone does not tell you about damage. A magnitude 7.0 earthquake deep beneath the ocean floor may cause less surface damage than a magnitude 5.5 directly under a city. That is why depth and location context matter as much as the headline number.
Focal depth changes everything
Earthquakes are classified by the depth of their focus (hypocenter): shallow (0–70 km), intermediate (70–300 km), and deep (300–700 km). Shallow earthquakes tend to cause the most surface damage because seismic waves have less distance to travel and less rock to absorb energy before reaching buildings and infrastructure.
On PlanetSentry, earthquake markers from USGS include the focal depth alongside magnitude. A magnitude 6.0 at 10 km depth in a populated area is a genuinely dangerous event. The same magnitude at 500 km depth might barely be noticed on the surface. Reading both numbers together gives a far more accurate picture than magnitude alone.
- Shallow earthquakes (0–70 km): most damaging, often felt strongly at the surface
- Intermediate earthquakes (70–300 km): less surface impact but can affect large areas
- Deep earthquakes (300–700 km): usually minimal surface damage, rarely cause tsunamis
Modified Mercalli Intensity vs magnitude
While magnitude measures the earthquake at its source, the Modified Mercalli Intensity (MMI) scale measures what people actually experience at specific locations. It ranges from I (not felt) to XII (total destruction) and varies dramatically depending on distance from the epicenter, local soil conditions, building construction, and depth.
USGS publishes ShakeMap products that show estimated intensity across the affected region. These are far more useful than raw magnitude for understanding real-world impact. PlanetSentry displays USGS-sourced data that includes both the instrumental magnitude and, when available, community-reported intensity information through the Did You Feel It program.
How to read earthquake feeds intelligently
When monitoring earthquake activity, do not fixate on magnitude alone. Look at the complete picture: magnitude, depth, distance to populated areas, and whether the event triggered a tsunami warning. Check whether the earthquake is part of a swarm (many small quakes in one area) or an isolated event. Aftershock sequences are also important — a large mainshock often generates dozens of aftershocks over days or weeks.
PlanetSentry pulls earthquake data from USGS at regular intervals and presents it alongside other natural events on the globe. This context matters: an earthquake near an active volcano might signal magma movement, and an undersea earthquake might relate to a storm-surge or tsunami watch in the same region.