Local Magnitude and Anomalous Amplitude Distance Decay in the Eastern Tennessee Seismic Zone

Abstract

ABSTRACT Seismic waveforms from the Eastern Tennessee Seismic Network are corrected to the nominal Wood–Anderson (WA) torsion seismometer to obtain a total of 11,905 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). We use the following distance‐correction function −log 10 ( A 0 )=0.538( r /17)−0.0002516( r −17)+2.0, in which A 0 is the maximum amplitude measured in millimeters and r is the hypocentral distance measured in kilometers; this better agrees with reported moment magnitudes for larger events in the ETSZ. Using the normal 100 km distance for M L normalization severely overestimates M L , and we therefore chose to adopt the 17 km normalization technique. The −log 10 ( A 0 ) is very flat at distances >200 km, suggesting unusually low distance attenuation at local and near‐regional distances from the ETSZ. The WA 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 A 0 term, although the revised response consistently yields M L values that are 0.1 lower than those found using the nominal response. The b ‐values for the currently reported duration magnitude are 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 =0.68093 M D +0.64603. The catalog of events used in this study is complete for M L >1.3.