The M-L scale in eastern North America

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Bulletin of the Seismological Society of America
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An attenuation curve for the local magnitude scale, ML, Of eastern North American (ENA) earthquakes was empirically determined using synthetic Wood-Anderson seismograms derived from newly available broadband recordings from the U.S. National Seismographic Network (USNSN) and from additional stations of comparable quality. Wood-Anderson peak amplitudes measured on approximately 210 three-component, broadband digital records from 38 earthquakes in the distance range of 50 to 800 km were inverted for the attenuation curve and magnitude of each event. The earthquakes ranged from M-L = 2.2 to 4.6 and were recorded at about 20 stations in the region. Separate attenuation curves were determined for the N-S, E-W, and vertical components, as well as for the mean of the two horizontal components. All curves had similar slopes indicating that M-L can be determined from any one of the three components available. The Wood-Anderson peak amplitude phases were predominantly Lg waves arriving with a mean group velocity of 3.40 +/- 0.23 km/sec and a mean period of 0.50 +/- 0.26 sec. M-L for earthquakes in ENA can be obtained from the horizontal-component Wood-Anderson peak amplitude in millimeters, A(Delta), using the formula M-L = log(10)A(Delta) (in mm) + 1.55 log(10) Delta (in km) - 0.22 + C, for distances 100 to 800 km and 2.2 less than or equal to M-L less than or equal to 4.6 and where C = station magnitude correction. The M-L is tied to Richter's (1935) M-L scale for southern California. A similar formula is given to determine M-L from the vertical-component synthetic Wood-Anderson seismograms. M-L is related to Nuttli's m(b)(Lg) by M-L = 0.976 m(b)(Lg) - 0.05 for earthquakes with 2.2 less than or equal to M-L less than or equal to 4.6 in ENA. Analysis of six additional large earthquakes with M-L greater than or equal to 5 indicates that M-L approximate to m(b)(Lg) - 0.15 in a wide magnitude range of 2 less than or equal to M-L less than or equal to 6.5 in ENA.


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