SINGLE-STATION SPECTRAL DISCRIMINATION USING CODA WAVES

Spectral discrimination methods applied at regional distances are base d on the observation that there is a difference in the frequency conte nt of the seismic waves radiated from the source region of earthquakes and explosions. We have investigated a spectral discrimination method that exploits the scattered energy contained in the event codas of se ismograms to measure the relative source excitation of earthquakes and explosions as a function of frequency. Key strengths of the coda meth od include the following: (1) it is not affected by source radiation p attern because the scattering process averages the radiation over the focal sphere, (2) path corrections are not required with the coda meth od because coda waves are scattered throughout a common crustal volume , (3) it can be successfully applied using data from only a single sta tion, and (4) it can be successfully applied using data with clipped d irect arrivals. To implement the coda method, we first estimate a ''ty pe curve,'' which represents the average shape of all the individual c oda decay curves for a particular frequency band. The shape of the typ e curve (as well as the shape of all individual coda decay curves) is controlled by the scattering and attenuation properties of the crustal volume sampled by the coda waves (a common path effect). The amplitud e of the type curve is not defined, and we arbitrarily adjust it to ha ve a zero mean. The amplitude difference between an individual event c oda decay curve and the type curve provides a relative measure of the event's source size within the particular frequency band. We define th is amplitude difference as the ''relative source factor.'' Our approac h exploits all of an event's usable coda for each spectral measurement , thus providing a more stable measure of relative source size than co uld be obtained from using a predefined, fixed-length time window. Ana lysis of 27 Nevada Test Site explosions and 15 southern Great Basin ea rthquakes (3.1 less than or equal to M less than or equal to 4.7), sho ws that the source factor difference between the 0.5- to 1- and 2- to 4-Hz bands plotted against the 1- to 2-Hz source factor is a stable ev ent discriminant. The method reliably distinguishes small explosions f rom small earthquakes (3.0 < M < 3.5), even if the explosions are over buried. Because the coda method requires only one seismic station, we believe that it would be especially useful in identifying small, cland estine underground nuclear tests that occur far from established netwo rks.