The computations are based on isoseismal maps or defined felt areas using various intensity-magnitude or felt area-magnitude formulas. Mb tends to saturate at about M 6.5 or larger.Īn estimate of body-wave (mb) magnitude based on the size of the area over which the earthquake was felt, typically assigned to widely felt earthquakes that occurred before the invention of seismographs and to earthquakes occurring in the early decades of seismograph deployment for which magnitudes calculated from seismographic data are not available. ![]() Only authoritative for global seismicity for which there is no Mww, Mwc, Mwb or Mwr, typically 4.0-5.5 range. ![]() Reported for most M4.0-4.5 to 6.5 EQs that are observed teleseismically. Waveforms are shaped to the WWSSN SP response. Where A is the amplitude of ground motion (in microns) T is the corresponding period (in seconds) and Q( D, h) is a correction factor that is a function of distance, D(degrees), between epicenter and station and focal depth, h (in kilometers), of the earthquake.īased on the amplitude of 1st arriving P-waves at periods of about 1 s. Ms_20 tends to saturate at about M8.3 or larger. Authoritative for 6), shallow events, providing secondary confirmation on their size. Source complexity and dimensions at larger magnitudes (~M7.0 or greater) generally limits applicability. Only authoritative if Mww and Mwc are not computed.īased on the scalar seismic-moment of the earthquake, derived from moment tensor inversion of the whole seismogram at regional distances (~10-100 s pass band based on size of EQ). Source complexity at larger magnitudes (~M7.5 or greater) generally limits applicability. Generally computable for all M5.5 or larger events worldwide. Only authoritative if Mww is not computed, not published otherwise.ĭerived from moment tensor inversion of long-period (~20-200 s pass band based on size of EQ) body-waves (P- and SH). Generally computable for all M6.0 worldwide using primarily the Global Seismograph Network. M W = 2/3 * (log 10(M O) - 16.1), where M O is the seismic moment.ĭerived from a centroid moment tensor inversion of the long-period surface waves (~100-2000 s pass band based on size of EQ). ![]() Authoritative USGS magnitude if computed. Provides consistent results to M~4.5 within a regional network of high-quality broadband stations. Computed for all M5.0 or larger earthquakes worldwide, but generally robust for all M5.5 worldwide. If using metric units (N.m), the constant is 9.1.ĭerived from a centroid moment tensor inversion of the W-phase (~50-2000 s pass band based on size of EQ). Note this is also unit-dependent the formula above is for moment in dyne-cm. Mww (Moment W-phase)(generic notation Mw)
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