Reading a ShakeMap
A ShakeMap estimates how strongly the ground shook across a region after an earthquake. Learn what the colours, legend and parameters mean, and what a ShakeMap cannot tell you.
Key concepts
- What a ShakeMap estimates
- Intensity colours and the official legend
- PGA and PGV
- Versions, uncertainty and limits
What a ShakeMap is
A ShakeMap is a map produced by the USGS and its partner networks soon after a significant earthquake. It estimates ground shaking across the affected region, combining recordings from seismic instruments, reports from people who felt the earthquake, and models that predict how shaking decreases with distance and changes with local ground conditions.
Where there are many instruments and reports, the map relies mostly on measurements. Where data are sparse, it relies more on models, and the estimates are more uncertain. A ShakeMap is therefore a best estimate of shaking, not a direct photograph of it.
Reading the colours and the legend
The most familiar ShakeMap shows estimated shaking intensity on the Modified Mercalli Intensity scale. Each colour band corresponds to an intensity level, and the official legend lists the perceived shaking (from not felt to extreme) and the potential damage typically associated with each level. Always read the colours using the legend that comes with the map rather than assuming what a colour means.
Intensity is not the same as magnitude. A single earthquake has one preferred magnitude, while its ShakeMap shows many intensities. The highest intensity on the map is not the earthquake's magnitude and should never be reported as if it were.
Other parameters: PGA and PGV
ShakeMaps also estimate instrumental quantities. Peak ground acceleration (PGA) is the largest acceleration of the ground, usually given as a percentage of the acceleration of gravity (%g). Peak ground velocity (PGV) is the largest ground speed, given in centimetres per second. Engineers use these values because different kinds of structures respond to different aspects of shaking.
Each parameter has its own units and its own colour scale, so a colour on a PGA map does not mean the same thing as the same colour on an intensity map.
A worked example
Suppose a magnitude 5.5 earthquake occurs about 90 km deep beneath southern Alaska. Its ShakeMap might show the highest estimated intensity, around MMI IV (light shaking), in a band above the hypocentre, with lower values farther away. A town 150 km away might fall in the MMI II–III range, where shaking is weak and many people may not notice it.
To read the value for a particular town, find the town on the map, read its colour against the legend, and remember that a value in a sparsely instrumented area is an estimate. On EQRadar's Shaking Maps page you can search for a place: the estimate comes from the nearest cell of the official ShakeMap grid, and if the place lies outside the map, EQRadar says so rather than guessing.
Versions and limits
ShakeMaps are updated as new recordings and felt reports arrive, so the first version can change. The map's version number and review status tell you how mature it is. An automatic map may be revised; a reviewed map has been checked by staff but can still be updated.
A ShakeMap estimates shaking. It is not a damage assessment, and it cannot tell you whether a specific building was damaged, because damage also depends on how the building was designed, built and maintained. After a damaging earthquake, follow official information from local authorities.
Three takeaways
- A ShakeMap is an estimate of shaking that combines instrument recordings, felt reports and models, and it is more uncertain where data are sparse.
- Read colours with the map's own legend, and never report the highest intensity as the earthquake's magnitude.
- ShakeMaps are revised as data arrive and are not damage assessments for individual buildings.
Sources
- ShakeMap — U.S. Geological Survey
- ShakeMap documentation — U.S. Geological Survey
- The Modified Mercalli Intensity Scale — U.S. Geological Survey