Seismic Attenuation of India, Himalaya and Tibet using Lg-coda waves

We present maps to show the lateral variation of Lg coda attenuation at 1-Hz across India, Himalaya and Tibet. We use vertical component waveforms from regional earthquakes (epicentral distance<3500 km and Mw>5) recorded by the IISER-K seismological network, ones operated by the Indian Meteorological Department, and data acquired from the IRIS-DMC. Lg-coda waves are modeled as single back-scattered energy, sampling an ellipsoidal volume. The attenuation of Lg-coda is quantified using the quality factor (Q), which is frequency dependent. We use the stacked spectral ratio (SSR) method to calculate the single-trace Lg-coda Q at 1 Hz (Qo) and its frequency dependence (η). A moving-window stack of scaled-logarithmic ratios of spectral amplitudes, for window length of 25.6 s and different central lapse time, is computed for each frequency. Through a linear regression of log (stacked spectral ratio) and log (frequency), using least-squares fitting, we obtain (1-η) and log(Qo), respectively. Lg-coda is selected in a frequency range of 0.2-5 Hz, with coda window starting at 3.15 km/s. Our total coda window lengths vary between 140 s to 780 s. Our preliminary results show low Q values (~200-400) in the Eastern and Western Himalaya - possibly because of scattering of seismic energy from structural heterogeneities. Most of the Indian Shield and the intraplate regions of Shillong Plateau and Brahmaputra valley are characterized by intermediate to high Q values (~600-800), indicating fairly efficient propagation of seismic energy. Intermediate values of Q (~400-500) occur in the Indo-Burman Ranges which may be due to the cold elastic subducting oceanic lithosphere. Patches of low Q in the Tibetan Plateau (~200) are possibly the result of high temperatures and partial melts present in the crust. Our results show how the nature of the Indian Plate changes as we go from an active continent-continent collision zone in the north to eastward subduction of transitional material at the Indo-Burma ranges. Our plots of Qo and η as a function of epicentral distance, coda length and magnitude show no systematic variations.