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The formula above made it much easier to estimate the energy-based magnitude M w , but it changed the fundamental nature of the scale into a moment magnitude scale. USGS seismologist Thomas C. Hanks noted that Kanamori's M w scale was very similar to a relationship between M L and M 0 that was reported by Thatcher & Hanks (1973)
An earthquake's seismic moment can be estimated in various ways, which are the bases of the M wb, M wr, M wc, M ww, M wp, M i, and M wpd scales, all subtypes of the generic M w scale. See Moment magnitude scale § Subtypes for details. Seismic moment is considered the most objective measure of an earthquake's "size" in regard of total energy. [50]
The formula to calculate surface wave magnitude is: [3] = + (), where A is the maximum particle displacement in surface waves (vector sum of the two horizontal displacements) in μm, T is the corresponding period in s (usually 20 ± 2 seconds), Δ is the epicentral distance in °, and
The connection between seismic moment and a torque is natural in the body-force equivalent representation of seismic sources as a double-couple (a pair of force couples with opposite torques): the seismic moment is the torque of each of the two couples. Despite having the same dimensions as energy, seismic moment is not a measure of energy.
While the "Richter" and moment magnitude scales are similar, they do differ, particularly for earthquakes stronger than about M 6.5, or deeper than about 50 km. In general: where any source specifies magnitude on a scale other than "Richter", or uses "M" (or "m") with one or more other letters (such as ML, Mw, Ms, or mb), that can be relied on ...
The Modified Mercalli intensity scale (MM, MMI, or MCS) measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake. Magnitude scales measure the inherent force or strength of an earthquake – an event occurring at greater or lesser depth. (The "M w" scale is ...
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The concept of Earthquake Duration Magnitude – originally proposed by E. Bisztricsany [1] in 1958 using surface waves only – is based on the realization that on a recorded earthquake seismogram, the total length of the seismic wavetrain – sometimes referred to as the CODA – reflects its size. Thus larger earthquakes give longer ...