Spatial Variations in the Coherence of Earthquake Body Waves in New England as a Measure of Signal to Noise

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Determination of the source parameters of a local earthquake from the full seismic waveforms requires seismograms with clear earthquake body wave signals. Noise from source energy scattering and from ambient microseismic sources can interfere with the body waveforms, especially for smaller earthquakes, and reduce the signal-to-noise ratio (SNR) of the observations. Thus, full-waveform analysis methods for studying local seismic sources require the seismograms with the highest SNR of the body waves. Coherence of the earthquake body waves at two different receivers can be used to estimate the SNR of the body wave energy radiated by the source. Coherent waveforms result from similar earth modifications to the propagating body waves while random noise affects the body waves differently at each station. Waveforms should be most coherent when the two seismic stations are spatially close. The SNR can be measured for the earthquake body waves propagating throughout a region by measuring coherence of the body waves at receiver pairs at different distances from the source. In this paper, a study of the coherence of earthquake body waves recorded in New England (NE) is performed to estimate the SNR of the body waves at different frequencies from small earthquakes within the region as functions of the source-receiver distance and the earthquake magnitude. Seismograms from the TA, NE, N4, CN, IU and US arrays are used to measure coherence between stations with mean separations of 70 km. Seismograms from the Acton Littleton Seismic Array are used to measure coherence at 5 km mean station separations. Coherence is measured at frequencies between 0.01-20 Hz for P and S first arrivals from earthquakes >M6 at distances >2500 km as well as Pn, Pg, Sn and Sg phases for NE earthquakes between M2-M4 at distances between 180-800 km. The results show that Pn waves from NE earthquakes at all frequencies show low coherence at interstation distances more than 70 km, suggesting at these interstation distances the body waveforms are dominated by scattered energy and microseismic noise.

Presenting Author: Ian P. Cooper

Authors