Seismic attenuation tomography of the North-Western Himalaya using Coda waves
Authors: Bera Amit , Agrawal Himanshu , Mitra Supriyo , Sharma Shubham 
Published: March 2020

&lt;p&gt;We use 4695 local waveforms from 1206 earthquakes (epicentral distance &lt; 350 km and 2.0 &amp;#8804; Mw &amp;#8804; 5.5) recorded by IISER Kolkata network (IK) at 22 stations (32&amp;#176;N to 35&amp;#176;N latitude and 74&amp;#176;E to 77&amp;#176;E longitude), located within the North-Western Himalaya (28&amp;#176;N to 39&amp;#176;N latitude and 68&amp;#176;E to 81&amp;#176;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&lt;sub&gt;c&lt;/sub&gt;) from which we estimate the attenuation (Q&lt;sub&gt;c&lt;/sub&gt;&lt;sup&gt;-1&lt;/sup&gt;). We consider the single back-scattering model (Aki &amp; Chouet, 1975) where both the scattering (Q&lt;sub&gt;sc&lt;/sub&gt;&lt;sup&gt;-1&lt;/sup&gt;) and intrinsic (Q&lt;sub&gt;i&lt;/sub&gt;&lt;sup&gt;-1&lt;/sup&gt;) component of the attenuation are included in the measurement. We use a lapse time of 2t&lt;sub&gt;s&lt;/sub&gt; (t&lt;sub&gt;s&lt;/sub&gt; 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&lt;sub&gt;c&lt;/sub&gt;&lt;sup&gt;-1&lt;/sup&gt;) is dominantly dependent on the intrinsic (Q&lt;sub&gt;i&lt;/sub&gt;&lt;sup&gt;-1&lt;/sup&gt;) component. In this model we use lapse time greater than 2t&lt;sub&gt;s&lt;/sub&gt; 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 Q&lt;sub&gt;0&lt;/sub&gt; (quality factor at a reference frequency f&lt;sub&gt;0&lt;/sub&gt; which is chosen to be 1 Hz for the analysis) and its frequency dependence (&amp;#951;) using weighted least squares approach on the power law dependence of Q&lt;sub&gt;c &lt;/sub&gt;on frequency. To see the lateral variation of Q in our study area, we have produced 2-D maps by combining the Q&lt;sub&gt;c &lt;/sub&gt;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&lt;sub&gt;c &lt;/sub&gt;maps, we use the point spreading function test which quantifies the recovery of Q&lt;sub&gt;c &lt;/sub&gt;perturbation. In addition to this, we also perform a standard checkerboard resolution test to ensure simultaneous recovery of Q&lt;sub&gt;c &lt;/sub&gt;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.&lt;/p&gt;