Frequency‐Dependent Coda Amplitude Decays in the Region of Himalaya, India

Abstract

Abstract An earthquake dataset consisting of 327 records from 67 earthquakes is used to study the frequency‐dependent amplitude decay of seismic coda waves in the Kumaon Himalayas, India. Coda amplitude decays with time t are measured by empirical t − α dependences, with exponent α being variable regionally with frequencies and lapse times. The results show that α varies from 1 to 3, with an average of about 1.93. The lapse‐time dependence of α is noticeable but weak, and no significant azimuthal dependence of α is obtained. Significant regional variations of α are found, ranging from α ≈2.1 for the Lesser Himalaya to α ≈1.76 for the Greater Himalaya. These variations correlate with the geology of the region and crustal structure. The values of α are mostly frequency independent, indicating low intrinsic attenuation within the crust. When interpreted according to the conventional coda‐ Q ( Q c ) model, these values of α lead to Q c ≈78 f 1.06 in the Kumaon Himalaya, Q c ≈90 f 1.09 in the Kumaon Lesser Himalaya, and Q c ≈90 f 0.92 in the Kumaon Greater Himalaya. A frequency‐dependent α is found within a localized area, giving an estimate of near‐surface Q ≈200. Three general conclusions of this study may be significant for coda studies in other areas. First, the values of α are variable regionally, whereas its average level is remarkably constant and correlates with rock types and upper‐crustal structure. Second, the traditionally assumed value of α =1 is much lower than the actual spreading rates, which shows that the coda cannot be viewed as body waves scattered within a uniform crust. Using the value of α =1 causes a systematic underestimation of coda amplitude decays in the data. Third, the dependence of α on frequency is relatively weak and occurs in localized areas. Combined with an underestimated α , this weak frequency dependence may cause biases in the estimation of secondary parameters, such as coda Q .