Lg WAVE ATTENUATION CHARACTERISTICS OF DIFFERENT PARTS OF INDIA

Abstract

Love waves with high frequency that are observed at large distances of continental crust are Lg waves. These are one kind of guided waves. They can travel along large continental paths with a minimal energy loss but is cutoff on encountering an oceanic segment of length 2° or more along the path. Lg waves that are critically incident on the Mohorovicic′ (Moho) discontinuity travel by undergoing multiple reflections within the continental crust with a group velocity of 2.9 to 3.6 km/s. Horizontal components of seismograms are dominated by these waves. At epicentral distances > 50, Lg waves can generally be recorded. I have developed an algorithm in Matlab to estimate crustal Lg quality factor (QLg) to understand the attenuation characteristics of western Gujarat, the NW Himalayan region and NE India. The algorithm automatically calculates QLg values from records of earthquakes obtained from different stations efficiently. The attenuation properties of continental crusts can be studied using Lg waves that dominate seismograms at large distances and travel within Earth's crust. Crustal attenuation properties can be studied with help of the Fourier amplitude spectra of Lg waves. Strong attenuation is generally associated with the tectonically active regions and low Lg attenuation is found in stable regions with thin sediment cover, low heat flow and a lack of fluids. The measurements of QLg are obtained using reliable two-station method. The Lg spectra of 25th April, 2015 Nepal earthquake of Mw 7.6 and its aftershocks with focal depth ≤ 10 km and magnitude 5.1 - 7.6 in the epicentral distance range from 300-2000 km are used. Data from 56 broadband stations deployed across India is processed to obtain Q0 (QLg at 1-Hz) values between every possible station pair by least-square inversion of the spectral ratios of crustal guided Lg waves. Finally, high quality inter-station paths were selected from possible pairs of stations based on the criteria 1) stations are aligned within 15o of the source, 2) stations are far apart from each other to minimize the error in estimates of Q0 . The Q0 values obtained in NW Gujarat region vary from 30-70, 150-300 in NW part of the Himalayas and 80-400 in the NE India. The results are correlated with the geology and tectonic trends of the study regions.