TheMLscale in eastern North America

Abstract

AbstractAn 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 ML = 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 ML 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. ML for earthquakes in ENA can be obtained from the horizontal-component Wood-Anderson peak amplitude in millimeters, A(Δ), using the formula ML = log10A(Δ) (in mm) + 1.55 log10 Δ (in km) − 0.22 + C, for distances 100 to 800 km and 2.2 ≦ ML ≦ 4.6 and where C = station magnitude correction. The ML is tied to Richter's (1935) ML scale for southern California. A similar formula is given to determine ML from the vertical-component synthetic Wood-Anderson seismograms. ML is related to Nuttli's mb(Lg) by ML = 0.976 mb(Lg) − 0.05 for earthquakes with 2.2 ≦ ML ≦ 4.6 in ENA. Analysis of six additional large earthquakes with ML ≧ 5 indicates that ML ≈ mb(Lg) − 0.15 in a wide magnitude range of 2 ≦ ML ≦ 6.5 in ENA.