Coda Q and its Frequency Dependence in the Eastern Himalayan and Indo-Burman Plate Boundary Systems

Abstract:

We use broadband waveform data for 305 local earthquakes from the Eastern Himalayan and Indo-Burman plate boundary systems, to model the seismic attenuation in NE India. We measure the decay in amplitude of coda waves at discreet frequencies (between 1 and 12Hz) to evaluate the quality factor (Q_c) as a function of frequency. We combine these measurements to evaluate the frequency dependence of Q_c of the form Q_c(f)=Q_of ^η, where Q_o is the quality factor at 1Hz and η is the frequency dependence. Computed Q_o values range from 80–360 and η ranges from 0.85–1.45. To study the lateral variation in Q_o and η, we regionalise the Q_c by combining all source-receiver measurements using a back-projection algorithm. For a single back scatter model, the coda waves sample an elliptical area with the epicenter and receiver at the two foci. We parameterize the region using square grids. The algorithm calculates the overlap in area and distributes Q_c in the sampled grids using the average Q_c as the boundary value. This is done in an iterative manner, by minimising the misfit between the observed and computed Q_c within each grid. This process is repeated for all frequencies and η is computed for each grid by combining Q_c for all frequencies.

Our results reveal strong variation in Q_o and η across NE India. The highest Q_o are in the Bengal Basin (210–280) and the Indo-Burman subduction zone (300–360). The Shillong Plateau and Mikir Hills have intermediate Q_o (~160) and the lowest Q_o (~80) is observed in the Naga fold thrust belt. This variation in Q_o demarcates the boundary between the continental crust beneath the Shillong Plateau and Mikir Hills and the transitional to oceanic crust beneath the Bengal Basin and Indo-Burman subduction zone. Thick pile of sedimentary strata in the Naga fold thrust belt results in the low Q_o. Frequency dependence (η) of Q_c across NE India is observed to be very high, with regions of high Q_o being associated with relatively higher η.