Seismic attenuation tomography of the North-Western Himalaya using Coda waves

We use 4695 local waveforms from 1206 earthquakes (epicentral distance < 350 km and 2.0 ≤ Mw ≤ 5.5) recorded by IISER Kolkata network (IK) at 22 stations (32°N to 35°N latitude and 74°E to 77°E longitude), located within the North-Western Himalaya (28°N to 39°N latitude and 68°E to 81°E longitude). We study the coda waves which are generally the tail of a seismogram and arrive after the main seismic waves. We use the temporal decay of coda amplitude to calculate the coda quality factor (Q_c) from which we estimate the attenuation (Q_c^-1). We consider the single back-scattering model (Aki & Chouet, 1975) where both the scattering (Q_sc^-1) and intrinsic (Q_i^-1) component of the attenuation are included in the measurement. We use a lapse time of 2t_s (t_s is the S-wave arrival time) as the starting point of the coda window. Then, we consider multiple forward-scattering model, where the attenuation (Q_c^-1) is dominantly dependent on the intrinsic (Q_i^-1) component. In this model we use lapse time greater than 2t_s so that the coda waves encounter multiple scatterers and enter the diffusive regime. We calculate the frequency dependent quality factor for each earthquake-receiver path at frequencies 1 to 14 Hz using the linear least squares approach on temporal decay of coda amplitude. We calculate Q₀ (quality factor at a reference frequency f₀ which is chosen to be 1 Hz for the analysis) and its frequency dependence (η) using weighted least squares approach on the power law dependence of Q_c on frequency. To see the lateral variation of Q in our study area, we have produced 2-D maps by combining the Q_c measurements together in a tomography. For single back-scattering model we use the back-projection algorithm which is based on the calculation of area overlap of ellipses with the gridded region. For multiple forward-scattering model, the same back-projection algorithm is modified to calculate the length overlap of traces with the gridded region. To understand the spatial resolution of the 2-D Q_c maps, we use the point spreading function test which quantifies the recovery of Q_c perturbation. In addition to this, we also perform a standard checkerboard resolution test to ensure simultaneous recovery of Q_c perturbation. We observe low Q in the Kashmir basin and Lesser Himalaya and high Q in surrounding northeastern Higher Himalaya which clearly correspond to the coda wave attenuation signatures in the older Tethyan sedimentary rocks and crystalline igneous rocks in these regions respectively.