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CrustalTrudger t1_j7oybui wrote

Reply to comment by [deleted] in Do Little Earthquakes Prevent Big Earthquakes? by cpassmore79

To clarify, the media isn't using the Richter scale, the media is reporting what ever magnitude a given service (e.g., the USGS or GFZ Potsdam GEOFON, etc) reports and then calling it a "Richter" magnitude. That magnitude is typically a moment magnitude, but depending on the location and details, it might be one of several seismic magnitude scales, e.g., occasionally you'll see a body wave magnitude (mb) or a surface wave magnitude (Ms) reported for a particular earthquake. As to why calling everything a "Richter" magnitude has persisted, it's unclear. The Richter scale was the first, but it was always a local scale (i.e., it was only really calibrated to be used in one part of the world) and it hasn't effectively been used for >50 years.

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[deleted] t1_j7oyhcv wrote

Thanks for the insight. But what makes a scale locally bound? What makes it non applicable in other parts when discussing seismic activities?

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CrustalTrudger t1_j7oz7j4 wrote

A "local" scale is specifically calibrated so that some measurable quantity (like the amplitude of seismic waves as measured on a seismometer) gives a somewhat repeatable estimate of earthquake size, but only for a specific area. This is because local scales, like the Richter scale, are effectively a measure of ground shaking. For a given magnitude of earthquake (in the moment magnitude sense, which is a measure of an intrinsic property of the earthquake, i.e., the seismic moment), the details of ground shaking will depend on distance/depth but also details of the rock that the seismic waves passed through between the source and the seismometer. So for the Richter scale and other local magnitude scales, if you try to transport it somewhere else, the magnitude won't be equivalent. I.e., a true Richter magnitude of X in one place won't actually be the same size earthquake of a Richter magnitude of X earthquake somewhere else. That's not a a very useful property for a scale to have.

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