Regional variation of coda wave attenuation in Northeast India: An understanding of the physical state of the medium

Abstract

In this study an attempt has been made to estimate spatial variation in attenuation characteristics in Northeast India using coda Q. The entire study region is divided into three sub-regions for this purpose. Estimated average frequency dependencies of coda wave attenuation for 30 s window length are Qc(f) = 135 ± 7f0.99±0.03, Qc(f) = 109 ± 7f1.10±0.03 and Qc(f) = 90 ± 2f1.04±0.02 for Shillong Plateau, Mikir hills and surrounding River valley, and Indo-Burma Ranges respectively. It is observed that Q0 is greater for the Shillong Plateau than the other sub-regions. This indicates lower attenuation due to more rigid high-density material present in this area than the other sub-regions. The depth variations of the Qc, Q0 and n values were also examined. It is observed that the rate of increase of Q0 with depth is not uniform for all the sub-regions. Indo-Burma Ranges has the smallest Q0 and the largest n values at all depth levels among the three sub-regions. These results indicate that central part of Indo-Burma Ranges is the most attenuative, seismically active and heterogeneous in nature. However, this region has smaller Qc values than the other two sub-regions for all window lengths up to the 6 Hz. This means at lower frequencies the subsurface beneath this area is more attenuative compared to the other two sub-regions. Similar trends are observed at 8, 10 and 12 Hz, up to 45 s window lengths. For window lengths ≥55 s, central part of Indo-Burma Ranges has higher Qc values at 10 and 12 Hz compared to Shillong plateau. Qc values are lower for Shillong Plateau compared to the other two regions for window length ≥ 55 s at 10 and 12 Hz, which corresponds to depth levels ≥90 km. Such a complicated variation in Qc values is a manifestation of complex nature of tectonic regime in Northeast India.