Frequency dependent crustal scattering and absorption at 5–160 Hz from coda decay observed at 2.5 km Depth

A triaxial 10 Hz seismometer at 2.5 km depth in the Cajon Pass borehole near the San Andreas fault in southern California records shear‐wave coda motion from small local events for over 20 seconds duration. The passband of recorded seismic motion is 5 Hz to 200 Hz. To measure the rate of coda energy decay as a function of frequency, we filter the vector velocity seismograms of seven events into five octave‐wide frequency bands (mean frequencies ≈ 7, 14, 28, 56 and 112 Hz) and square the filtered seismograms. The observed energy decay in each passband is well approximated by first and second order scattering plus intrinsic attenuation as formulated by Zeng at al. ( JGR 1991). The fits determine two energy decay parameters expressed as inverse lengths, ß scat for scattering and ß intr for absorption. Because the source‐receiver distance is less than the thickness of the upper crust and the receiver is at depth, the direct body wave is uncomplicated by refracted energy and/or surface waves and allows accurate recording of coda energy relative to source pulse energy. The coda/source energy ratio directly defines the scattering attenuation parameter ß scat and voids the need for multiple offset observations. The observed frequency behavior of scattering is ß scat ≈f 1.3 ; the intrinsic attenuation parameter has frequency dependence ß intr ≈f 0.43 . Normalized to energy lost per unit cycle (quality factor Q), intrinsic attenuation decreases with frequency, Q intr ≈f 0.57 , while scattering attenuation increases with frequency, Q scat ≈1/f 0.3 with Q scat > 10Q intr at all observed frequencies.