Moment magnitude scale

Moment magnitude scale. The moment magnitude scale (M_W) is a measure of an earthquake's size based on its seismic moment, representing the total energy released. Developed in 1979 by Thomas C. Hanks and Hiroo Kanamori of the California Institute of Technology, it was designed to address the saturation limitations of earlier scales like the Richter magnitude scale. It has become the standard scale used by seismological authorities worldwide, including the United States Geological Survey, for reporting the magnitudes of large earthquakes.

Definition and development

The moment magnitude scale is fundamentally defined by the seismic moment (M₀), a measure derived from the product of the area of the fault rupture, the average slip displacement, and the rigidity of the surrounding rocks. This physical basis provides a direct link to the earthquake's source parameters. The scale was developed by Thomas C. Hanks and Hiroo Kanamori as a successor to the surface-wave magnitude (M_S) scale, which was known to saturate for events larger than about magnitude 8. Their work, published in 1979, built upon foundational concepts from Keiiti Aki and was quickly adopted by major institutions like the United States Geological Survey and the Japan Meteorological Agency. The development was significantly influenced by analyses of major historical events, such as the 1960 Valdivia earthquake and the 1964 Alaska earthquake.

Comparison with other magnitude scales

Unlike the local Richter magnitude scale, developed by Charles Francis Richter for California earthquakes, the moment magnitude scale does not saturate and provides consistent estimates for earthquakes of all sizes, from very small to the largest possible. It also differs from body-wave magnitude (m_b) scales, which are based on shorter-period P-waves and saturate around magnitude 6.5. While the surface-wave magnitude (M_S) scale was useful for teleseismic events, it too saturates for great earthquakes. For public communication, moment magnitude values are often reported simply as "magnitude" by agencies like the United States Geological Survey, effectively replacing the Richter magnitude scale in modern seismology. Notable recalculations include the 1906 San Francisco earthquake and the 1960 Valdivia earthquake, whose magnitudes were revised upon the scale's adoption.

Calculation and measurement

The calculation begins with determining the seismic moment (M₀) in newton-meters, typically through the analysis of long-period data in seismograms from global networks like the Global Seismographic Network. The moment magnitude M_W is then calculated using the logarithmic formula M_W = (2/3) log₁₀ M₀ - 6.07, which is calibrated to align with earlier magnitude scales at moderate sizes. Measurements rely on advanced techniques such as waveform inversion and data from broadband seismometers. Key figures in refining these methodologies include seismologists like Adam M. Dziewonski and organizations like the Incorporated Research Institutions for Seismology. The process often involves analyzing events recorded by arrays such as the TERRAScope network.

Applications and use

The moment magnitude scale is the international standard for quantifying and comparing the size of all tectonic earthquakes. It is used by every major seismological agency, including the United States Geological Survey, the European-Mediterranean Seismological Centre, and Geoscience Australia, for rapid public reporting and scientific cataloging. Its consistent scaling is crucial for seismic hazard assessment, informing building codes in regions like the San Andreas Fault zone and Japan. The scale is also fundamental in studies of global seismicity, tsunami warning systems operated by the Pacific Tsunami Warning Center, and engineering analyses of historical events like the 2011 Tōhoku earthquake and tsunami and the 2004 Indian Ocean earthquake and tsunami.

Limitations and considerations

While the scale itself does not saturate, accurate measurement requires high-quality, long-period seismic data, which can be challenging for very small, very deep, or complex earthquakes. The calculation assumes a simple double-couple source model, which may not fully represent events involving significant landslide or volcanic processes. Furthermore, the reported single magnitude value does not convey important details about directivity, rupture velocity, or peak ground acceleration, which are critical for understanding shaking intensity. These parameters are separately analyzed in studies by organizations like the Southern California Earthquake Center. Public misunderstanding can also arise from the logarithmic nature of the scale, where a single unit increase represents a roughly 32-fold energy release difference.

Category:Earthquake scales Category:Geophysics Category:Seismology