Attenuation characteristics of Garwhal-Kumaun Himalayas from analysis of coda of local earthquakes

Coda of local earthquakes that occurred during 2006-2007 are used to study the attenuation characteristics of the Garhwal-Kumaun Himalayas. The coda attenuation characteristics are represented in terms of coda Q or Q(c). It is observed that Q(c) increases with frequency. Q(c) also varies with increase in lapse time of coda waves. Q(c) increases up to an 85-s average lapse time. This is similar to observations around the world reported by many workers who have interpreted this as a manifestation of the fact that heterogeneity decreases with depth. However, around a 90-s average lapse time Q(c) is lower than its values for lower and higher average lapse times. This is interpreted as an indication of possible presence of a fluid-filled medium or a medium having partial melts at around a 160-km depth. Q(0), i.e., Q(c) at 1 Hz, increases, and frequency parameter n decreases with increasing lapse time, barring around a 90-s lapse time. This again shows that in general, heterogeneity decreases with increasing depth. The Q(0) and n values for smaller lapse times are similar to those for tectonically active areas. By comparing Q(c) values obtained in this study with those obtained by us using the 1999 Chamoli earthquake aftershocks, it is concluded that the crust is turbid and the mantle is more transparent. However, whether the variation in Q(c) values between 1999 and 2006-2007 is temporal or not cannot be definitely established from the available data set.