3D Attenuation Tomography of the Kashmir ‘Seismic Gap’ in NW Himalaya

Abstract

Lateral variation in seismic-energy attenuation is necessary to unravel the tectonic and thermal structure of the lithosphere, and to quantify ground-motion from future earthquakes. We study the 3D variations in intrinsic, scattering and body-wave attenuation across the Kashmir ‘seismic gap’ in the NW Himalaya, which is among the least studied segments of the Himalayan arc. This region is situated between the rupture zones of the 1905 Kangra earthquake (Mw  ∼ 7.9) and the 2005 Muzaffarabad earthquake (Mw ~ 7.6), and spans the meizoseismal zone of the 1555 Kashmir earthquake (Mw ∼ 8.0). Over the last five centuries, this region has accumulated sufficient strain-energy to drive a future mega-thrust earthquake of similar magnitude.  We use 507 local earthquake (Mw ≥ 2) waveform data recorded by the Jammu And Kashmir Seismological NETwork between 2013 to 2017. These earthquakes have been re-located using the non-linear location algorithm. Intrinsic attenuation is calculated using coda waves modeled as a composite of multiple forward-scattered energies in a diffusive regime (Qc ~ Qi). The exponential decay of the coda-wave envelopes are used to invert for the intrinsic attenuation using sensitivity kernels, whose parameters like albedo and extinction length are computed using the Multiple Lapse Time Window Analysis (MLTWA). Scattering attenuation is imaged using peak delay-time method, which is a direct measure for multiple forward-scattering. The body wave (P- or S-wave) attenuation is computed using the coda-normalization method, by taking the ratio of the measured direct and coda wave energies, which depends only on Q, thereby using a linearized inversion.  The preliminary results show a spatial variation in frequency dependent 3D scattering, absorption, and body-wave attenuation, which are related to the different geologic/tectonic features across the NW Himalaya. These results will be jointly interpreted with local S-wave velocity models to understand the tectonic and thermal structure of the lithosphere.