Mercalli intensity scale

Mercalli intensity scale
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Name	Mercalli intensity scale
Developer	Giuseppe Mercalli; modified by Harry O. Wood and Frank Neumann
Introduced	1902
Type	intensity scale
Range	I–XII
Unit	Roman numerals

Mercalli intensity scale The Mercalli intensity scale is a seismic intensity scale that classifies the observed effects of an earthquake on people, structures, and the natural environment. Developed in the early 20th century, it has been applied in assessments of damage after events such as the 1906 San Francisco earthquake, Great Kantō earthquake, and Lisbon earthquake to provide descriptive gradations used alongside instrumental records from observatories like the United States Geological Survey and institutions such as the Italian National Institute of Geophysics and Volcanology. The scale remains widely cited in historical seismicity studies, emergency response reports by organizations including the Red Cross and planning documents from municipal authorities such as the City of Los Angeles.

Overview and history

Giuseppe Mercalli, an Italian vulcanologist and seismologist, proposed the original scale in 1902; later modifications by Adolfo Cancani and the American seismologists Harry O. Wood and Frank Neumann produced the Mercalli–Cancani–Sieberg and Modified Mercalli Intensity versions used in the United States. The scale’s evolution involved comparisons with instrumental work at observatories like the Royal Observatory, Greenwich and the California Institute of Technology seismological network, and with reports collected by newspapers such as the New York Times and scientific journals like Nature and Science. Historical adoption across countries—by agencies including the Instituto Geográfico Nacional (Spain), Geological Survey of Canada, and Japan Meteorological Agency—reflects interactions between national civil protection agencies (e.g., Protezione Civile), academic research at universities such as University of Tokyo and University of California, Berkeley, and international bodies like the International Seismological Centre.

Scale description and intensity levels

The scale uses Roman numerals I through XII to describe increasing severity from imperceptible to total destruction. Lower levels (I–III) correspond to observations by individuals and reports to municipal services such as the Metropolitan Police Service or local fire departments, while midrange levels (IV–VII) reference damage patterns in structures designed by firms and architects influenced by codes from organizations like the American Society of Civil Engineers and building inventories managed by planning authorities such as the New York City Department of Buildings. High intensities (VIII–XII) denote severe structural collapse and landscape changes noted in post-disaster assessments by teams from FEMA, UNICEF, and academic crews from Massachusetts Institute of Technology conducting rapid field surveys. Descriptors include effects on masonry, timber, bridges, canals, and ground rupture documented in case reports from agencies such as the U.S. Army Corps of Engineers.

Methods of assessment and data collection

Assessment combines eyewitness reports, damage surveys, and archival sources compiled by national agencies like the USGS and research centers including the Advanced National Seismic System. Field teams from universities such as Stanford University and nongovernmental organizations like Doctors Without Borders gather data on building damage, ground deformation, and human impacts. Historical intensity mapping uses newspapers such as the Los Angeles Times, photographs archived by institutions like the Library of Congress, and insurance records from companies including Lloyd's of London. Modern methods integrate macroseismic questionnaires distributed by municipal emergency management offices (for example, Los Angeles Emergency Management Department), social media aggregated by platforms and analyzed in collaboration with research units at Columbia University and ETH Zurich, and intensity estimation algorithms run on datasets from seismic networks maintained by the European-Mediterranean Seismological Centre.

Applications and limitations

Agencies including FEMA, national ministries such as the Ministry of the Interior (Italy), and urban planners at municipalities like San Francisco use Mercalli-based maps for hazard zoning, retrofit prioritization, and public communication. The descriptive nature aids historians at institutions such as the Smithsonian Institution and engineers at firms like ARUP in contextualizing structural performance during events like the 1989 Loma Prieta earthquake. Limitations include subjectivity of eyewitness reports, building-code variability across jurisdictions like California and Italy, and difficulties comparing intensities across heterogeneous urban fabrics noted by scholars at Cambridge University and Imperial College London. The scale does not substitute for instrumental magnitude measures used by observatories such as USGS or networks operated by Japanese Meteorological Agency when estimating seismic energy release.

Comparison with magnitude scales

Magnitude scales—such as the Richter magnitude scale, moment magnitude scale, and other instrumental measures developed by institutions like the Carnegie Institution for Science—quantify seismic energy, while Mercalli intensities describe local effects and damage recorded by agencies including NOAA and teams from University of Washington. A single earthquake can produce a wide range of Mercalli intensities across regions administered by different provinces (for example, zones in California or Sichuan), whereas a magnitude value reported by centers like the International Seismological Centre is a single scalar. Studies by research groups at ETH Zurich, Purdue University, and University of Tokyo have developed empirical relationships linking intensity distributions to magnitude and distance, but local site conditions influenced by geology cataloged by organizations such as the British Geological Survey remain a confounding factor.

Notable uses and case studies

Notable applications include intensity mapping for the 1906 San Francisco earthquake used in urban reconstruction overseen by city authorities and engineers from firms like Schenck & Williams; the Great Kanto earthquake studies informing building regulations at the Ministry of Construction (Japan); post-event surveys after the 1995 Kobe earthquake conducted by teams from Kobe University and international groups including WHO; and modern rapid intensity mapping after the 2011 Tōhoku earthquake and tsunami by collaborations between Tohoku University, the Japan Meteorological Agency, and international partners such as JICA. Historical seismicity research at institutions like the Seismological Society of America and archival reconstructions using materials from the British Library rely extensively on Mercalli-type descriptions to extend catalogs and inform risk assessments by insurers such as Munich Re.

Category:Seismology