LOCAL-MAGNITUDE AND ANOMALOUS AMPLITUDE DISTANCE DECAY IN THE EASTERN TENNESSEE SEISMIC ZONE

Seismic waveforms from the Eastern Tennessee Seismic Network are corrected to the nominal Wood-Anderson torsion seismometer to obtain a total of 11905 maximum trace amplitudes from 690 events seen on 50 different horizontal components to determine a local-magnitude scale for the Eastern Tennessee Seismic Zone (ETSZ). The distance-correction function –log₁₀(A₀) = 0.538(r/17)-0.0002516(r-17) + 2.0, where A₀ is the maximum amplitude measured in millimeters and r is the hypocentral distance measured in kilometers, better agrees with reported moment magnitudes for larger events in the ETSZ. Using the normal 100 km distance for M_L normalization severely over estimates M_L at distances greater than 17 km suggesting unusually low distance attenuation at local and near-regional distances from the ETSZ. Wood-Anderson response reported by the International Association of Seismology and Physics of the Earth’s Interior to the nominal response shows no significant difference in the distance-dependent factor for the log A₀ term although the revised response consistently yields M_L values that are 0.1 lower than those found using the nominal response. b-values estimates are obtained showing that the b-value for the currently reported duration magnitude is lower than the b-values obtained using the newly calculated M_L scale. The relationship between M_L and M_D can be expressed as M_L = 0.68093M_D + 0.64603. The catalog of events used in this study is seen to be complete for M_L greater than 1.3.