abstract

Local magnitudes (ML) are reported for 56 northeastern United States and Canadian earthquakes using records from a pair of Wood-Anderson torsion instruments which have operated at Weston Observatory since 1967. Corrections to the Richter ML formula are computed to take into account the difference in attenuation between southern California and the northeastern United States. The corrected ML values for these events were compared to the previously reported magnitudes which had been calculated either from the Nuttli (1973) mbLg formula applied to 5- to 10-Hz waves (called MN) or from a coda magnitude formula (Mc). In general, the ML determinations tend to underestimate the MN and Mc values by about 0.4 magnitude units. This confirms the fact that the MN scale is not appropriate when applied to high-frequency waves, as it is in the northeastern United States. The reason for this appears to be in the relation of the period of the wave used in the magnitude determination to the corner period of the earthquake and earth response.

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