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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 ...
As seismic instruments (and accelerometers in particular) become more common, it becomes necessary to correlate expected damage with instrument-readings. The old Modified Mercalli intensity scale (MM), a relic of the pre-instrument days, remains useful in the sense that each intensity-level provides an observable difference in seismic damage.
The second, now known as the Mercalli intensity scale, had ten degrees, and elaborated the descriptions in the Rossi–Forel scale. [4] The Mercalli intensity scale is, in modified form, still used. [5] Unlike the Richter scale, which measures the energy released by an earthquake, the Mercalli intensity scale measures the effects of an
The Mercalli intensity scale uses personal reports and observations to measure earthquake intensity but PGA is measured by instruments, such as accelerographs. It can be correlated to macroseismic intensities on the Mercalli scale [3] but these correlations are associated with large uncertainty. [4] [5]
The JMA scale is expressed in levels of seismic intensity from 0 to 7 in a manner similar to that of the Mercalli intensity scale, which is not commonly used in Japan.The JMA uses seismic intensity meters to automatically calculate ground acceleration in real-time, reporting intensities based on measurements from observation points.
Seismic intensity scales categorize the intensity or severity of ground shaking (quaking) at a given location, such as resulting from an earthquake. They are distinguished from seismic magnitude scales , which measure the magnitude or overall strength of an earthquake, which may, or perhaps may not, cause perceptible shaking.
Instruments come in three main varieties: short period, long period and broadband. The short and long period measure velocity and are very sensitive, however they 'clip' the signal or go off-scale for ground motion that is strong enough to be felt by people. A 24-bit analog-to-digital conversion channel is commonplace.
The seismic intensity peaked at VIII (Severe) on the Modified Mercalli intensity scale at Adak and Umnak. [2] As the shock occurred before the World Wide Standardised Seismological Network was in operation, few instruments recorded the event, and its mechanism is not understood well as a result.