USGS Earthquake Magnitude Policy

The U.S. Geological Survey (USGS) is responsible for providing information about earthquakes to other government agencies and to the public.  Information about earthquakes is used in many ways, including the response to felt earthquakes by the public, by Federal, State, and local government agencies, and by private organizations.  Appropriate response is promoted by the public’s immediate access to understandable, accurate estimates of earthquake size.  Because public safety is involved, the USGS herein states its policy to promote the rapid release of accurate, consistent, and understandable estimates of earthquake size.  All USGS press releases, websites, and documents releasing earthquake magnitude information to the public should conform to the guidelines described below.

In 1935, Charles Richter developed the local magnitude, ML scale for moderate-size (3 < ML < 7) earthquakes in southern California.  The ML scale is often called the “Richter scale” by the press and the public.  All of the currently used methods for measuring earthquake magnitude (ML, duration magnitude mD, surface-wave magnitude MS, teleseismic body-wave magnitude mb, moment magnitude M, etc.) yield results that are consistent with ML.  In fact, most modern methods for measuring magnitude were designed to be consistent with the Richter scale.  There is some confusion, however, about earthquake magnitude, primarily in the media, because seismologists often no longer follow Richter's original methodology.  Richter's original methodology is no longer used because it does not give reliable results when applied to M >= 7 earthquakes and it was not designed to use data from earthquakes recorded at epicentral distances greater than about 600 km.  It is, therefore, useful to separate the method and the scale in releasing estimates of magnitude to the public.

Different methods of estimating magnitude will be used, depending upon the time elapsed since the earthquake’s occurrence.  Preliminary estimates of magnitude should not be included in statements if revised, preferred estimates of magnitude exist.  If several different magnitude estimates are available, the reported magnitude should be moment magnitude, if available.  Moment magnitude is the preferred magnitude for all earthquakes listed in USGS catalogs.  All other magnitudes should be preserved in the database, but routine searches of the catalogs should list only the preferred magnitude.  For historical earthquakes, the preferred magnitude will be the best estimate of the moment magnitude (see attached Appendix 2 – Magnitudes of Significant Earthquakes).  This estimate, however, should not simply be a conversion from some other historical magnitude scale, but an estimate obtained using supplementary information (ground rupture, macroseismic data, etc.).

The least complicated, and probably most accurate, terminology is to just use the term “magnitude” and to use the symbol M (i.e., a capital M, plain text, without any subscripts or superscripts).  M should be expressed to the nearest 0.1.  If less precision is desired, M can be preceded by a tilde.  For example, “M ~ 7” or “magnitude ~6.8”.  If asked, more information can be provided.  Typical additional information can include that the magnitude was estimated using an extension of the concept originally developed by Richter, and/or that there are several different methods for estimating the size of an earthquake, all of which are consistent with the Richter scale, and a description of the measurement technique used.  However, most non-earth scientists are confused by this additional information, so it should be provided only if requested.

Press releases are a critical tool in communicating earthquake information to the public.  It is important that all USGS press releases adhere to the guidelines described in this policy.  An appropriate statement in a USGS press release might be:  “The magnitude of the earthquake was 6.5.  This magnitude estimate is preliminary and may be revised when more data and further analysis results are available.  A magnitude ~6.8 earthquake occurred in the same region on April 1, 1896.”

Another source of confusion is the form of the formula for converting from scalar moment M0 to moment magnitude, M.  The preferred practice is to use M = (log Mo)/1.5-10.7, where Mo is in dyne-cm (dyne-cm=10-7 N-m), the definition given by Hanks and Kanamori in 1979.  An alternate form in Hanks and Kanamori’s paper, M=(log M0-16.1)/1.5, is sometimes used, with resulting confusion.  These formulae look as if they should yield the same result, but the latter is equivalent to M = (log Mo)/1.5-10.7333.  The resulting round-off error occasionally leads to differences of 0.1 in the estimates of moment magnitude released by different groups.  All USGS statements of moment magnitude should use M = (log Mo)/1.5-10.7 for converting from scalar moment Mo to moment magnitude.

Within the United States, certain regional networks have been identified as the authoritative network for earthquakes occurring within their boundaries.  For example, the USGS/Caltech cooperative network is the authoritative network in southern California and the USGS/UC Berkeley cooperative network is the authoritative network in central and northern California.  The institutions operating these networks will take the lead on computing and/or identifying the authoritative location and magnitude for such earthquakes; other USGS institutions will follow their lead in reporting earthquake parameters to the public.  Since the possibility exists that a major earthquake could disable the authoritative network, the next most authoritative network, typically the USGS National Earthquake Information Center (NEIC), in Golden, CO, will report information to the public according to the rules established by the Council of National Seismic Systems (CNSS).