Crustal Structure of Madras Block in Southern Granulite Terrain, South India: A Receiver Function Perspective

Renu M.S. and Rajarathnam S.

To map out the crustal structure of Southern Granulite Terrain (SGT), waveform data for teleseismic earthquakes recorded by 12 broadband stations located in and around the study area was analysed using receiver function technique. SGT can be divided into several blocks with respect to the shear zones present in the study area. The present study supports the major shear zones (Moyar Bhavani Shear Zone, MBSZ and Palghat Cauvery Shear Zone, PCSZ) as sutures connecting blocks with different age. The study reveals the identity of Madras Block which lies behind the curtain till recent days. Salem-Attur Shear Zone (SASZ) can be considered as the boundary zone separating Madras Block from the others. Among the stations 6 are in this Block and their results shows that the Moho discontinuity lies in the depth of 36-39km and has a thicker crust than the Northern Block in which a unique value of 35km was shown by all the 3 stations. This is supported by variation in geological formations and the corresponding Poisson’s ratio across Madras and Northern Blocks. The velocity contrast at Moho is very clear in most of the stations and it is evident from the receiver functions with sharp P-to-S conversions. The immediate lows after Moho conversions are observed in some of the stations.

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Determination of seismic wave attenuation: A Review

Kumar Abhishek and Banerjee Soham

Earthquake occurs when pent up energy is released during tectonic activities. This energy spreads in the form of waves and while propagating, these waves attenuate non-uniformly in different directions due to the variation in the elastic properties of the propagation medium. The recorded amplitudes of seismic waves are directly related to the attenuation properties of the medium. Compressional wave (P wave) and shear wave (S wave) are the primary waves (also known as direct waves) generated during an earthquake (EQ) and do significant damages within a certain range (200 km). Hence, attenuation studies for these waves are pretty much important in seismic hazard estimation. Various approaches had been developed worldwide to study the attenuation of these waves. In one of the approach, coda wave (backscattered waves generated when direct waves interact with the medium heterogeneities) amplitudes are used to normalize the direct wave amplitudes in order to determine the frequency dependent attenuation of direct waves. Obtained attenuation values can be used to understand the tectonic stability and medium heterogeneities of a region. Further, using these values intrinsic and scattering attenuations can be obtained separately in a region. In this study, a delineate discussion reviewing the properties of coda wave and coda normalization method (CNM) is given. In addition to CNM, two more methods in order to determine the attenuation of direct waves are also presented here. A detailed comparison in terms of assumptions made while developing each of the above methods is carried out here. Further, a detailed summary of various studies, addressing the attenuation characteristics of direct waves based on the above three methods is presented here. Regional characteristics such as medium heterogeneity, tectonic stability etc. evaluated by various researchers based on the direct wave attenuation are also discussed in this paper.

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