Lateral variations of crustal seismic attenuation along the INDEPTH profiles in Tibet from <i>Lg Q</i> inversion

Abstract

Fourier spectra of crustal guided Lg waves are collected to study seismic attenuation, or Q , along the INDEPTH profiles. The 1 Hz Lg Q ( Q 0 ) values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying Q 0 models. Along the INDEPTH III profile in central Tibet, the Q 0 model consists of low and nearly constant values of ∼90. These low values are consistent with abnormally high temperature as well as partial melts in the crust. Along the INDEPTH II profile in southern Tibet the Q 0 values increase southward. They are extremely low (∼60) over the northernmost segment but increase by a factor of 2 over a distance of ∼100 km between the Indus‐Yalong Suture and the Kangmar Dome. Farther south and into the high Himalayas, Q 0 values are higher than 300. Regional Rayleigh waves observed along the northern INDEPTH II profile are used to infer a low‐velocity, low‐ Q layer at midcrustal depths (between ∼15 and 30 km) in southern Tibet. The aqueous fluid trapped in the upper crust and a midcrust partial melting zone, associated with the underthrusting of the Indian lithosphere, are the likely causes of the high Lg attenuation in southern Tibet. Low Q , rather then a systematic Lg conversion along a dipping Moho, is the main cause of the previously observed Lg blockage over paths crossing the southern Tibetan boundary.