A CODA-LENGTH EARTHQUAKE MAGNITUDE FORMULA FOR NORTHERN NEW YORK

A coda-length magnitude formula is developed to estimate magnitudes of earthquakes in northern New York. A total of 38 coda-length measurements from 38 earthquakes located in the area between 4059’ and 4626’ N Lat and 72 and 8000’ W Lat were made. In this study, short period earthquake records of the Potsdam Seismic Network and Lamont-Doherty Earth Observatory were used to measure coda lengths.

Coda-length is the duration of the earthquake signal in seconds from the “P” arrival time to a point where the signal disappears into the noise. The coda lengths versus the magnitudes of all the earthquakes are plotted. The known magnitudes of the earthquakes were obtained from Lamont-Doherty Earth Observatory. The magnitude of an earthquake is proportional to the logarithm of the coda length, we can write the general expression , where A and B are constants, T_c is coda length in seconds, and M_c is magnitude based on coda length. The constants A and B are determined from the data to derive the coda length formula for northern New York M_c = 2.24 Log T_c 2.24. This formula can be used to determine coda length magnitude in northern New York.

For a given event, it would be expected that a longer signal duration would depend on epicentral distance. However, data analysis indicates coda length changes very slightly with distance therefore distance dependence need not be taken into account.