Role of site effect for the evaluation of attenuation characteristics of P, S and coda waves in Kinnaur region, NW Himal
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Role of site eAect for the evaluation of attenuation characteristics of P, S and coda waves in Kinnaur region, NW Himalaya RICHA KUMARI1,2, PARVEEN KUMAR1,*, NARESH KUMAR1 and SANDEEP2 1 Wadia 2
Institute of Himalayan Geology, Dehradun, India. Banaras Hindu University, Varanasi, Uttar Pradesh, India. *Corresponding author. e-mail: [email protected] MS received 13 November 2019; revised 18 May 2020; accepted 7 June 2020
The site eAect and attenuation studies are carried out for Kinnaur region of northwest Himalaya, India. A total of 109 local events happened in Kinnaur region of magnitude range 1.6–4.5, are utilized for present work. The earthquake records are inCuenced by the site eAect depending on soft sediment thickness beneath the recording sites. Therefore, in the present study, records are corrected for site eAects to estimate P (Qp), S (Qs) and coda (Qc) wave quality factor. The regional frequency dependent attenuation relations, i.e., Qp ðf Þ ¼ ð29 1Þf ð1:010:05Þ , Qs ð f Þ ¼ ð38 5Þf ð1:10:06Þ and Qc ðf Þ ¼ ð74 11Þf ð1:170:01Þ are established for the Kinnaur region. The Kinnaur Himalaya mainly belongs to Higher Himalaya Crystalline (HHC) and Tethys Himalaya, where these two geological units are differentiated by the South Tibetan Detachment System (STDS). The resonance frequencies and attenuation characteristics are estimated for both regions, i.e., HHC and Tethys Himalaya. A comparison is made between HHC and Tethys Himalaya in the form of resonance frequencies and attenuation properties. The low value resonance frequency and high rate of attenuation towards the northern side of STDS, i.e., Tethys Himalaya support the presence of low-grade metasedimentary rocks. It suggests that Tethys Himalaya has high seismic hazard potential zone compared to HHC. Keywords. Kinnaur Himalaya; attenuation; quality factor; site eAect.
1. Introduction Seismic energy produced due to an earthquake attenuates during its passage in the subsurface. Attenuation characteristics of the subsurface control the amplitude decay of seismic waves during its propagation. Attenuation relation is one of the simple relations to describe the decay rate of seismic energy. The seismological parameters used in the attenuation relation characterize the earthquake source, path of seismic wave, soil and geological formation beneath the site, i.e., local site eAect (Campbell 2001). Attenuation property of earth medium is
quantitatively deBned by a dimensionless parameter, known as quality factor ‘Q’ given by KnopoA (1964). The ‘quality factor’ is described as the ratio of total energy to the loss of energy per cycle (Johnston and Toksoz 1981). Attenuation coefBcient in any region can be quantiBed by the quality factor of P-wave (Qp); shear wave (Qs) and coda wave (Qc). Various techniques are available to compute the quality factor of different waves of the earthquake record. The most frequently used techniques comprise (1) that parameterize the source an
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