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WESTON OBSERVATORY DEPARTMENT OF GEOLOGY AND GEOPHYSICS BOSTON COLLEGE, WESTON, MASSACHUSETTS 02193
Abstract
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. The Rg phase is particularly prominent on seismograms of quarry blasts. In this study, Rg waves generated by small earthquakes and explosions in New England are investigated to evaluate the extent to which Rg waves can be used to estimate depths of events in the upper crust. The data were recorded by the New England Seismic Network (NESN) operated by Weston Observatory. The strongest Rg signals recorded by the NESN 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. Observed Rg waves can, therefore, 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 paper. Rg is identified and distinguished from other phases by measuring amplitudes at particular periods and arrival times using a narrow bandpass filter analysis. Rg/P and Rg/Lg ratios are estimated by forming the ratios of amplitudes in the appropriate group velocity-frequency windows on vertical component seismograms. The phase referred to as "Lg" is actually a complicated wave train recorded on short-period seismograms that travels with group velocities appropriate for S and Lg waves. An Rg/Lg ratio appears to be a better measure of the presence of Rg on a seismogram than an Rg/P ratio. Observed Rg/Lg ratios are used to estimate depths of two earthquakes recorded by the NESN.
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