Empirical determination of depth‐distance corrections for m b and M w from global seismograph network stations

Using P wave amplitudes from 52 deep earthquakes, recorded by the Global Seismograph Network (GSN) between 1991 and 1996, we empirically determined depth‐distance corrections for body wave magnitude m b and moment magnitude M w . After correcting vertical, broadband records from stations of the GSN for instrument response and source radiation factors, P wave amplitudes from velocity records were read for the case of m b and the surface area under the displacement pulse of the P wave for M w . Amplitudes were then normalized to a scalar moment M 0 =10 18 Nm, or m b =5.7, M w =5.9. Using the generic logarithmic relationship for each magnitude, correction factors Q (Δ, h ) were determined. A total of 973 observed values of Q in the plane of distance Δ vs. depth h were smoothed by local averaging, as well as fitted by a parameterized smooth surface. These empirical Q (Δ, h) surfaces differ in important detail from those historically established by Gutenberg and Richter, and markedly from the theoretical curves predicted by Veith and Clawson (1971). We present a first empirical correction factor for the determination of M w from P wave displacements.