Earthquake Relocation and Lateral Variation of Coda Q in Sikkim Himalaya

Thirunavukarasu, Ajaay (2013) Earthquake Relocation and Lateral Variation of Coda Q in Sikkim Himalaya. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS Dissertation of Ajaay Thirunavukarasu (08MS25)) AjaayT-08Ms25-MSThesis.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

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 3.0-6.5 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). 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.

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