Rg Waves as a Depth Discriminant for Earthquakes and Explosions in New England
Final rept. 24 Oct-31 Dec 1989
WESTON OBSERVATORY MA
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Fundamental mode Rayleigh waves with periods between about 0.4 and 2. 5 sec Rg are often observed on seismograms of explosions and very shallow- focus earthquakes. This study investigates Rg waves generated by small earthquakes and explosions in New England to evaluate the extent to which Rg waves can be used to estimate depth. The strongest Rg signals recorded in New England are generally in the period range of 0.5 to 1.5 sec. In that period range, Rg displacement is essentially confined to the upper 5 km of the crust, with most of the Rg wave energy in the upper 2 or 3 km. Sources deeper than about 4 km would not be expected to generate strong Rg signals at these periods and so, if Rg can be clearly identified on a seismogram, the source is most likely very shallow. Thus observed Rg waves can be used to discriminate very shallow-focus events from deeper events, provided that Rg can be identified and distinguished from other phases. A method for identifying Rg waves at distances up to about 170 km is presented in this report. Rg is identified and distinguished from other phases by measuring amplitudes at particular periods and arrival times using a narrow bandpass filter analysis. RgP and RgLg ratios are estimated by forming the ratios of amplitudes in the appropriate group velocity-frequency windows. An RgLg ratio appears to be a better measure of the presence of Rg on a seismogram than an RgP ratio.