Intensity is a measure of the severity of earthquake effects at a specific location, based on observed damage to structures, ground deformation, and human perception. It varies from place to place for a single earthquake, unlike magnitude, which is a single value for the event.
Intensity is a descriptive measure of the local effects of an earthquake at a given location. Unlike magnitude, which quantifies the earthquake's size at its source, intensity describes how strongly the earthquake was felt and how much damage it caused at a specific site. Intensity is typically assigned using macroseismic scales such as the Modified Mercalli Intensity (MMI) scale in the United States, the Medvedev-Sponheuer-Karnik (MSK) scale in Europe and parts of Asia, or the European Macroseismic Scale (EMS-98) in modern European practice.
Intensity is assigned by observation, not by instruments. Traditionally, intensity was estimated by trained observers who surveyed damage to buildings, ground failures, and human perception. Modern practice increasingly supplements this with instrumental intensity measures β such as peak ground acceleration or spectral acceleration β that correlate with observed intensities. ShakeMap, developed by the USGS, converts recorded ground motions into estimated intensity maps that are published within minutes of an earthquake, enabling rapid post-event response. Intensity is especially valuable for historical earthquakes, where no instrumental records exist but written accounts of damage can be used to estimate intensity and infer magnitude.
Intensity and magnitude are complementary, not interchangeable. A single earthquake has one magnitude but a field of intensities that typically decreases with distance from the epicenter, though local soil conditions and rupture directivity can create anomalous patterns. The intensity distribution β called an isoseismal map β provides insights into the rupture geometry, the attenuation characteristics of the region, and the vulnerability of the built environment. In modern earthquake engineering, intensity is used for post-earthquake damage assessment, for calibrating ground motion prediction equations against historical observations, and for communicating seismic risk to non-specialists. The ShakeMap and PAGER systems, both operated by USGS, are prime examples of intensity-based tools that bridge seismology, engineering, and emergency response.