Modified Mercalli Intensity scale

Modified Mercalli Intensity scale
Name	Modified Mercalli Intensity scale
Abbreviation	MMI
Type	intensity scale
Units	intensity
Developed	Giuseppe Mercalli; Adolfo Cancani; Harry O. Wood; Frank Neumann
First published	1902; 1931 (modified)
Countries	Italy; United States

Modified Mercalli Intensity scale is a seismic intensity scale used to describe the effects of earthquake shaking on people, structures, and the natural environment. The scale translates observed damage and human perception into a twelve‑degree qualitative classification, informing post‑event assessments by agencies such as the United States Geological Survey and planners in jurisdictions like California and Japan. Practitioners in seismic engineering, emergency management, and historical seismology rely on the scale alongside tools developed by organizations including the International Seismological Centre, Pacific Northwest Seismic Network, and United Nations Office for Disaster Risk Reduction.

Overview

The Modified Mercalli Intensity scale provides descriptive intensity levels from I to XII that characterize shaking felt at specific locations rather than the energy release measured by instruments such as the Richter magnitude scale or moment magnitude used by the Global Seismographic Network. Each intensity level correlates to typical effects on buildings, chimneys, bridges, and geological features observed in settings like San Francisco, Lisbon, and Naples. Emergency responders from entities like the Federal Emergency Management Agency, Tokyo Metropolitan Government, and Provincia di Roma use MMI reports along with instrumental data from networks such as the Incorporated Research Institutions for Seismology to prioritize inspections and relief. The scale complements historical inventories compiled by scholars at institutions like the Smithsonian Institution and University of California, Berkeley.

History and development

The origins trace to the 19th‑century work of Giuseppe Mercalli and earlier intensity schemas used in Italy and across Europe, later expanded by Adolfo Cancani and adapted internationally. In the United States, the scale was substantially revised by Harry O. Wood and Frank Neumann in the 1930s, producing a version widely adopted by agencies including the National Oceanic and Atmospheric Administration and the U.S. Geological Survey (USGS). Subsequent refinements incorporated observational standards promoted by research centers such as California Institute of Technology, Massachusetts Institute of Technology, and the University of Tokyo to improve consistency during events like the 1906 San Francisco earthquake, the 1755 Lisbon earthquake, and the 1964 Alaska earthquake. Historians and seismologists from institutions like Columbia University and Purdue University have analyzed archival damage reports to calibrate intensity assignments for pre‑instrumental earthquakes.

Scale definitions and intensity levels

The twelve divisions range from I (not felt) to XII (total destruction). Typical descriptors reference effects on structures from types common in Rome, Istanbul, and Los Angeles and on infrastructure such as highways and railways analogous to systems in France and Germany. For example, midrange levels describe damage to ordinary masonry familiar in historic districts of Athens and Mexico City, while higher levels describe catastrophic collapse seen in reports from Port-au-Prince and Sendai. Agencies such as the United States Geological Survey and research groups at Seismological Society of America provide guidelines linking qualitative descriptions to expected consequences for specific building types assessed by bodies like American Society of Civil Engineers and Japan Society of Civil Engineers.

Measurement and data collection

Data for MMI assignments derive from field surveys conducted by teams from organizations including the USGS, Japan Meteorological Agency, and regional centers like the Pacific Tsunami Warning Center. Citizen reports submitted through platforms hosted by European-Mediterranean Seismological Centre, ShakeMap, and mobile apps developed by universities such as University of Washington augment instrumental records from networks including the Global Seismographic Network and regional arrays maintained by Instituto Geofísico del Perú. Historical intensity estimation uses archival research in repositories at British Library, National Archives (UK), and municipal archives in cities like Naples and Lisbon. Remote sensing and post‑event imagery from satellites operated by agencies such as European Space Agency and NASA increasingly aid the mapping of high‑intensity zones.

Applications and mapping

MMI atlases and intensity maps inform seismic hazard assessment used by planners at municipal and national levels in places like California, Italy, and Japan. Tools such as ShakeMap integrate MMI‑derived observational data with instrumental ground‑motion models produced by researchers at California Institute of Technology and United States Geological Survey to produce rapid post‑event maps for first responders including FEMA and military units such as elements of the Japan Self‑Defense Forces during disasters. Historical intensity catalogs compiled by institutions like Lamont‑Doherty Earth Observatory and Seismological Society of America support retrospective studies that influence building codes established by organizations such as International Code Council and Japanese Building Center.

Comparison with other intensity and magnitude scales

MMI differs fundamentally from magnitude scales such as the Richter magnitude scale and moment magnitude scale because it is location‑specific and based on observed effects, whereas magnitude quantifies overall seismic energy released as used by networks like the Global Seismographic Network. Other intensity schemes, such as the European Macroseismic Scale (EMS‑98) developed by experts at European Seismological Commission and national scales used in Italy and Greece, offer alternative descriptive frameworks; researchers at University of Athens and Istituto Nazionale di Geofisica e Vulcanologia often cross‑calibrate these with MMI for transnational studies. Engineering response spectra and instrumental metrics produced by labs at Massachusetts Institute of Technology or ETH Zurich provide quantitative complements to MMI’s qualitative assessments.

Limitations and criticisms

Criticisms of the MMI include subjectivity in observational reports, variability introduced by building practices across locales such as Mexico City, Istanbul, and Los Angeles, and challenges in applying consistent assignments in rural areas like parts of Alaska or Nepal. Scholars from Royal Society‑affiliated groups and universities such as University of Cambridge and Stanford University have noted difficulties reconciling MMI with instrumental measures under conditions of deep sedimentary basins found in regions like the Gulf of Mexico and Po Basin. Proposals for improvement emphasize integration with instrumental ground‑motion parameters from arrays maintained by Incorporated Research Institutions for Seismology and enhanced crowdsourcing via platforms run by organizations such as European-Mediterranean Seismological Centre and USGS.

Category:Seismic scales