Earthquake Relocation and Lateral Variation of Coda Q in Sikkim-Himalaya

We use data from 29 local earthquakes recorded by a network of broadband 3-component stations in Sikkim Himalaya for estimation of frequency dependent attenuation of the crust. The events range in magnitude from 3.0-5.5 and had occurred between 2005 to present. First these events are relocated using phase data from both local and regional seismograms recorded by stations within a radius of 350 km. We then use the local records of event-receiver pairs to measure the decay of coda amplitudes at a range of central frequencies (eg. 1, 2, 3, 4, 8, 10 and 12 Hz). These measurements have then been combined to estimate the frequency dependence of coda Q of the form Q(f)= Q₀*f^η given as: Q(f)= 123.700 × 29.301 f^{(0.979 × 0.038)} For these 43 event-receiver pairs, the Q₀ value has been observed to range from 80-200 with an average of 123.700 × 29.301 and eta ranging from 0.92 - 1.04 with an average of 0.979 × 0.038. In order to unravel the lateral variation of Q₀, we regionalized the measured Q₀ values by combining all the event-receiver path measurements using a back projection algorithm. In order to stabilize the inversion a nine-point spatial smoothening (similar to spatial Gaussian filter) has been applied to our dataset. Our results reveal that the Himalaya is characterized by low Q₀ (80-100) values compared to the foreland basin to its south (150-200). Within the Himalaya we observe two E-W trending low Q₀ regions. A comparison with the crustal structure beneath the Sikkim Himalaya reveals that the low Q₀ regions correspond to the ramp on the Main Himalayan Thrust (MHT) beneath the Higher Himalaya. This is a region of high strain and profuse micro-seismicity corresponding to observed low quality factor of the crust.