Earthquake intensity maps show how strongly the ground shook at different places. They are not maps of magnitude, because magnitude estimates the overall size of the earthquake while intensity varies with distance, depth, rupture direction, local geology and buildings. Systems such as the USGS ShakeMap combine station measurements of acceleration and velocity, reported effects, earthquake location and ground-motion models. Where instruments are sparse, the map relies more heavily on prediction; where observations are dense, data constrain the pattern. Early maps are produced rapidly for emergency response and may change as more stations and reviewed information arrive. Reading them responsibly means checking the mapped quantity, version, uncertainty and whether intensity is observed or instrumentally estimated.
Key takeaways
- One earthquake produces one preferred magnitude but many local intensities.
- Seismic stations measure acceleration, velocity and waveforms.
- Ground-motion models fill gaps between observations.
- Local soil and rock can amplify or reduce shaking.
- Rapid maps are revised as data and source models improve.
- An intensity map is not a direct damage or casualty map.
Magnitude and intensity
Magnitude describes source size using seismic recordings or moment. Intensity describes shaking and effects at a location. The distinction is explained further in earthquake magnitude scales.
A city does not have a magnitude; it experiences a level of shaking from an event. Several towns can report different intensities for the same earthquake.
Instrumental measurements
Strong-motion instruments record ground acceleration. Seismometers also provide velocity and waveforms. Peak ground acceleration and peak ground velocity are common mapping measures.
Sensors have calibration, orientation and site metadata. A clipped or malfunctioning record is flagged rather than treated as extreme shaking.
Locating the source
The map begins with origin time, epicentre and depth from the process in earthquake location. Early source parameters may be revised.
For large earthquakes, a point source is inadequate. Finite-fault models represent the long rupture and improve predicted spatial patterns.
Ground-motion models
Models estimate expected shaking from magnitude, distance, fault type and regional attenuation. They provide values where instruments are absent.
The model is not neutral background truth. It has uncertainty and may perform differently in a region with limited calibration data.
Combining data and models
ShakeMap blends observations with predictions, weighting them according to uncertainty and distance. Close to a reliable station, the measurement strongly influences the map.
Far from stations, the product resembles the model. Users should consult uncertainty maps rather than assuming every colour is equally observed.
Converting motion to intensity
Empirical relationships convert acceleration and velocity into instrumental intensity scales approximating levels of felt shaking and damage.
USGS notes that estimated instrumental intensity is not identical to conventional Modified Mercalli intensity based on observed effects on people and structures.
Felt reports
Public questionnaires describe whether people felt motion and what objects or buildings did. Many reports can reveal patterns beyond the instrument network.
Reports are influenced by population density, internet access and building type. An empty rural area may mean no responses, not no shaking.
Site conditions
Soft sediment can amplify certain frequencies and extend shaking, while hard rock often behaves differently. Basin geometry can trap waves.
Site corrections use geological and geotechnical information. Local variation within a map cell can still be large, especially across artificial fill and valley edges.
Rupture direction and depth
A rupture can direct energy towards one region, producing stronger motion than equal distance alone predicts. Shallow events generally create stronger nearby shaking than deep events of similar magnitude.
These patterns arise from fault rupture mechanisms described in what causes earthquakes.
Rapid versions and revisions
An automatic map may appear within minutes. More stations, reviewed magnitudes and finite-fault models then update it. Version timestamps matter when comparing screenshots or reports.
Revision is expected scientific improvement. Emergency users receive a rapid estimate, while later researchers use the best-reviewed product.
Uncertainty
Uncertainty is larger where stations are sparse, source geometry is unknown or site conditions are complex. A smooth colour surface can hide this variation.
Good decisions consider a range of plausible shaking, especially near boundaries between response thresholds.
From shaking to damage
Damage also depends on building design, maintenance, height, frequency response and landslide or liquefaction hazards. ShakeMap supports loss tools but does not directly observe every building.
Emergency planners combine shaking with exposure and vulnerability. A high-intensity unpopulated area and a moderate-intensity dense city present different response needs.
Mapping and communication
The product is managed in a geographic information system with stations, faults and administrative boundaries. Legends must state units and scale.
Colour choices help rapid reading but can encourage false boundaries. Shaking changes continuously and contains local variation below map resolution.
Historical earthquakes
For events before dense instruments, researchers reconstruct intensity from archives, newspapers and engineering reports, then model likely motion.
Historical ShakeMap atlases make events comparable, but uncertainty is greater and socially uneven reporting can bias the pattern.
Modified Mercalli intensity
The Modified Mercalli scale uses Roman numerals to describe effects from not felt to extreme damage. It incorporates human perception, movement of objects and building performance. The same physical motion can produce different descriptions where construction differs.
Macroseismic investigators review reports rather than converting every anecdote mechanically. Dense consistent reports can reveal historical patterns where instruments did not exist.
Peak acceleration and velocity
Peak ground acceleration is relevant to rapid forces on structures, while peak ground velocity often relates well to damage over other frequency ranges. Neither captures the full waveform, duration or spectral content.
ShakeMap displays several products because users need different quantities. Engineers should use the measure and response spectrum appropriate to a structure rather than treating instrumental intensity as every aspect of motion.
Liquefaction and landslide layers
Strong shaking can trigger liquefaction in saturated loose sediment and landslides on susceptible slopes. These hazards require groundwater, geology and terrain information in addition to shaking.
A ShakeMap can feed secondary-hazard models, but the output should be labelled as a susceptibility or probability estimate. Damage from ground failure may occur where mapped shaking alone does not look most severe.
Emergency use and uncertainty
Responders use rapid maps to prioritise reconnaissance, hospitals, transport routes and utility checks. They also incorporate calls, remote sensing and local knowledge. A map guides deployment rather than replacing reports from the ground.
Where consequences are serious, teams consider the uncertainty range and possible isolated pockets of stronger motion. Rapid science supports action best when its limitations travel with the product.
A worked mapping example
A magnitude 6.2 earthquake occurs beside a sedimentary basin. Stations on rock record moderate acceleration, while instruments in the basin show longer, stronger motion. Early ShakeMap colours follow the point-source model; later data and a finite-fault solution extend the strong-shaking zone along the rupture.
Felt reports are dense in the city but sparse in farmland. Analysts do not treat report density as shaking density. The revised product combines the instruments, site corrections and model, with higher uncertainty where coverage remains poor.
Using intensity maps in education
Students can compare the magnitude, instrumental-intensity and felt-report maps for one event. They should identify what each layer measures and explain why boundaries differ. Overlaying geology tests whether soft sediment corresponds with amplification.
The exercise demonstrates that maps are arguments built from data and models. Asking which cells are observed and which are interpolated develops critical spatial literacy beyond memorising an intensity scale.
Maps for aftershocks
Aftershocks can produce damaging shaking in structures already weakened by the main event. Rapid maps help compare their motion with the earlier sequence, but a smaller magnitude does not guarantee harmless effects near the new hypocentre. Emergency managers update exclusion zones and inspections using observed shaking, building status and landslide conditions rather than applying one sequence-wide rule.
After a major sequence, archived versions show how knowledge changed from minutes to days. Researchers can evaluate which early assumptions mattered and improve future response. Preserving those versions is important: replacing the rapid map with the final one would hide the information emergency teams actually had when decisions were made. That record supports after-action review and safer rapid mapping in future earthquakes.
Conclusion
Earthquake intensity maps combine instruments, human observations and physical models to estimate the distribution of shaking. Their rapid production helps responders identify likely affected areas, while later revisions support recovery and research. The map is strongest near reliable data and more model-dependent elsewhere. It must be read with its version, uncertainty, site assumptions and mapped quantity. Magnitude tells how large the source was; intensity mapping explains how that energy was experienced across a landscape.
Frequently asked questions
What is a ShakeMap?
A near-real-time map of ground motion and shaking intensity produced from observations and models.
Is intensity the same as magnitude?
No. Magnitude estimates earthquake size; intensity varies by location.
Why do intensity maps change?
More station data and better source information arrive after the first rapid solution.
Can a map predict exact building damage?
No. Damage depends on construction and vulnerability as well as shaking.
What do blank areas mean?
They may lack observations; the map may still contain model estimates or insufficient coverage.
