Coherence as a Measure of Body-Wave Signal to Noise Ratio in the Northeastern United States and Southeastern Canada

Abstract

Determination of the source parameters of a local earthquake from full seismic waveforms requires seismograms with clear body-wave signals from the earthquake source. Coherence of the earthquake body-wave seismograms recorded at two different receivers can be used to estimate the signal-to-noise ratio (SNR) of the body-wave energy radiated by the source. In this study, the coherence of earthquake body waves recorded in the Northeastern United States and Southeastern Canada (NEUSSEC) is measured as a function of frequency, interstation distance, and ambient SNR, and then used as an estimate of body-wave SNR. Seismograms from the CN, IU, LD, N4, NE, TA, and US arrays were used to measure coherence between stations with a mean separation of 70 km. Seismograms from the Acton Littleton Seismic Array (ALSA) were used to measure coherence at 5 km mean station separation. Coherence is measured at frequencies between 0.05-10 Hz for Pn and Sn phases from NEUSSEC earthquakes with magnitudes (M) between 0.0 and 4.7 at epicentral distances between 180-1800 km as well as at frequencies between 0.05-10 Hz for the first arrivals of P and S waves from earthquakes M>6 at distances >2500 km. The teleseismic P waves display values of coherence greater than 0.9 out to interstation distances of 1500 km at frequencies <0.8 Hz, but as frequency increases, the interstation distance at which coherence falls below 0.9 decreases. Teleseismic S and regional Pn and Sn waves display coherence values around 0.5, suggesting the amplitudes of the body-waves are smaller than those of the noise, which likely is the result of converted and reflected or refracted P waves and/or smaller signal amplitudes. These coherence values are compared to the coherence values of ambient noise. For any two P, S, Pn or Sn waveforms recorded in the NEUSSEC at 3-5 Hz there is a 50% or greater chance of those two waveforms containing coherent energy that is not ambient noise; these frequencies are where this percent chance is greatest for all seismic phases. At frequencies between 3-5 Hz the effects of scattering are most pronounced on the coherence values of regional seismic phases, suggesting that most scattering in the crust of the NEUSSEC takes place at these frequencies. Teleseismic seismic phases do not include as much scattered energy as the regional seismic phases at 3-5 Hz, and must therefore encounter fewer scattering heterogeneities along their travel path than the regional seismic phases.