moment magnitude scale

moment magnitude scale
Name	Moment magnitude scale
Developer	Kārlis Batāris; Caltech; United States Geological Survey
First published	1979
Magnitude range	−1 to 10+
Unit	moment magnitude (Mw)
Related	Richter magnitude scale; seismic moment; moment tensor

Contents

moment magnitude scale The moment magnitude scale quantifies the size of earthquakes by measuring the seismic moment derived from fault slip. It is used by institutions such as the United States Geological Survey, California Institute of Technology, and the International Seismological Centre for global seismic catalogs and hazard assessments. The scale replaced older magnitude measures for large events in operational seismology conducted by agencies like National Oceanic and Atmospheric Administration and research groups at Massachusetts Institute of Technology.

Overview

The moment magnitude scale expresses earthquake size through a logarithmic measure tied to physical faulting parameters and is reported alongside seismic products from networks such as the Global Seismographic Network, Incorporated Research Institutions for Seismology, and regional observatories like Japan Meteorological Agency and Geoscience Australia. Seismologists at institutions including Scripps Institution of Oceanography, ETH Zurich, and University of Tokyo employ the scale in studies of plate boundaries like the San Andreas Fault, Ring of Fire, and Himalayan front. Emergency management agencies such as Federal Emergency Management Agency and Japan Self-Defense Forces use Mw values for post-event response and tsunami-warning decisions coordinated through organizations like the Pacific Tsunami Warning Center.

Moment magnitude (Mw) is defined from the seismic moment, M0, where M0 = μAD (shear modulus μ × fault area A × average slip D), a concept used in source inversion methods developed in laboratories at Caltech and Imperial College London. Calculation pipelines integrate waveform data from arrays operated by European-Mediterranean Seismological Centre, Canadian Hazard Information Service, and observatories such as Instituto Geográfico Nacional (Spain) and apply moment-tensor inversion algorithms refined by groups at University of California, Berkeley and Purdue University. The scale uses the relation Mw = (2/3) log10(M0) − 6.07 (with M0 in N·m), a formulation standardized in joint reports by International Union of Geodesy and Geophysics panels and implemented in software packages like those maintained by USGS and University of Washington.

Compared to the Richter magnitude scale developed at California Institute of Technology and the local magnitude ML used by regional networks such as British Geological Survey, Mw remains stable for large ruptures where Richter and surface-wave magnitudes saturate. Moment magnitude correlates with energy measures used in studies from Los Alamos National Laboratory and matches seismic-energy estimates cited in papers from American Geophysical Union journals. For tsunami potential, Mw is supplemented by tsunami-specific scales used by Pacific Tsunami Warning Center and regional centers like Météo-France in the Indian Ocean. Historical catalogs from agencies including Instituto Geofísico del Perú are often reanalyzed to convert ML or Ms values to Mw for consistent hazard modeling by groups at Columbia University and Tokyo Institute of Technology.

The concept of seismic moment traces to theoretical work in the mid-20th century by researchers affiliated with Massachusetts Institute of Technology and USGS laboratories; formal adoption of Mw as a preferred magnitude for large events accelerated after pioneering studies at Caltech and publications in Bulletin of the Seismological Society of America. International bodies such as International Seismological Centre and standards committees of International Association of Seismology and Physics of the Earth's Interior evaluated formulations leading to the 1979 formulation widely used today. Subsequent improvements in broadband instrumentation by initiatives like the Global Seismographic Network and data sharing through Incorporated Research Institutions for Seismology supported refinement of inversion techniques by teams at NOAA, University of Oxford, and Australian National University.

Mw is central to seismic hazard maps produced by agencies such as USGS, Geoscience Australia, and regional authorities like KTH Royal Institute of Technology collaborators; it informs engineering codes from bodies like International Organization for Standardization and post-event casualty modeling used by Red Cross affiliates. Limitations include reduced resolution for very slow-slip events studied at Jet Propulsion Laboratory and challenges in near-field rupture complexity requiring dense networks such as those operated by Ocean Networks Canada or temporary deployments by IRIS PASSCAL. For very small microseismicity monitored by institutions like Lawrence Berkeley National Laboratory, alternative measures such as local ML remain practical. Conversions between magnitude types require careful calibration as performed in reanalysis efforts by European Centre for Medium-Range Weather Forecasts-adjacent research groups.

High-Mw events cataloged by USGS and historical compilations from International Seismological Centre include megathrust ruptures on subduction interfaces like the 1960 Valdivia earthquake near Chile and the 2011 Tōhoku earthquake offshore of Japan. Large intraplate events reported by agencies such as Natural Resources Canada and studies at University of Alaska Fairbanks include the 1957 Andreanof Islands event near Alaska. Mw estimates for events such as the 2004 Indian Ocean earthquake were produced collaboratively by NOAA, Japan Meteorological Agency, and research teams at University of Southern California to support tsunami modeling by the Pacific Tsunami Warning Center and humanitarian response planning by United Nations Office for the Coordination of Humanitarian Affairs.