Seismic attenuation tomography of northwest (NW) Himalaya using local earthquake coda waves

Abstract:

We use 696 local waveforms from 119 earthquakes (epicentral distance ≤ 200 km; 2.0 ≤ M_w ≤ 5.5) recorded by JAKSNET to study the attenuation characteristics of NW Himalaya. We measure temporal decay of coda wave amplitude and compute the coda quality factor (Q_c), which is inversely proportional to attenuation. Q_c is composed of scattering (Q_s) and intrinsic (Q_i) attenuations. We model the coda energy as a composite of multiple forward-scattered energy, in a diffusive regime, and hence dominated by Q_i. In this phenomenon, the coda energy is primarily concentrated along the ray path connecting the source and receiver. Analyzing a range of lapse time and coda window length, we show that lapse time of 2t_s and coda window length of 90 s produces stable Q_c measurements. We calculate Q_c for every ray path at frequencies 1, 2, 3, 5, 8, 10, 12 and 14 Hz. Using power law dependence of Q_c on frequency, we perform linear regression, using weighted least squares approach, between log Q_c and logf, to obtain Q₀ (Q at 1 Hz) and frequency dependence of Q(η). To study the lateral variation of Q across NW Himalaya, we construct 2-D tomography maps of Q_c(f) using the back-projection algorithm. We parameterized the region into square grids and used length overlap between ray paths and grids to compute the contribution from each segment of the ray path. The spatial resolution of Q_c maps are obtained using checkerboard tests. From our results, we observe a strong frequency dependence in the lateral variation of Q_c in NW Himalaya. The Kashmir Basin, at all frequencies, has 10–20% higher than average Q_c, which is attributed to Higher Himalayan crystalline rocks underneath the Tethyan sediments in the basin. Immediately south of the Kishtwar window, at 1-2 Hz, we observe 20–30% lower than average Q_c, a result of the upwarped Lower Himalayan rocks, exposed within the geologic window. South of Kashmir Basin, the Pir Panjal Ranges are underlain by ~10% higher than average Q_c.