Using the single backscattering method, coda quality factor functions through coda window lengths of 20, 30, 40, 50, and 60 s have been estimated for the New Madrid seismic zone (NMSZ). Furthermore, geometrical spreading functions for distances less than 60 km have been determined in this region at different center frequencies exploiting the coda normalization method. A total of 284 triaxial seismograms with good signal‐to‐noise ratios (SNR>5) from broadband stations located in the NMSZ were used. The database consisted of records from 57 local earthquakes with moment magnitudes of 2.6–4.1, and hypocentral distances less than 200 km.
Q‐factor values were evaluated at five frequency bands with central frequencies of 1.5, 3, 6, 12, and 24 Hz. Vertical components were utilized to estimate vertical coda Q‐factor values. Horizontal coda Q‐factor values were determined using the average amount of the Q‐factor values estimated from two orthogonal horizontal components. The coda Q‐factor increases with increasing of the coda window length implying that with increasing the depth, the coda Q‐factor increases. The intermediate values of the Q‐factor and intermediate values of the frequency dependency indicate that the Earth’s crust and upper mantle beneath the entire NMSZ is tectonically a moderate region with a moderate to relatively high degree of heterogeneities.
The geometrical spreading factors of S‐wave amplitudes are frequency dependent and determined to be −0.761, −0.991, −1.271, −1.182, and −1.066 for center frequencies of 1.5, 3, 6, 12, and 24 Hz, respectively, at hypocentral distances of 10–60 km. The geometrical spreading factors for lower frequencies are not recommended to be used due to the greater impact of the radiation pattern and directivity effect on low frequencies, as well as the greater sensitivity of band‐pass‐filtered seismograms of small earthquakes to the noise in low frequencies.