CRUSTAL INVESTIGATIONS IN GARHWAL-KUMAUN HIMALAYAS: VELOCITY TOMOGRAPHY AND Qc

Abstract

The Himalayan mountain belt is the consequence of collision between Eurasian and Indian plates. According to the most widely accepted tectonic hypothesis the Indian plate underthrusts the Eurasian plate along a low northerly dipping detachment surface (basal detachment) and a sedimentary wedge has formed owing to uplift along southerly propagating thrust system. This hypothesis was originally formed on the basis of seismicity recorded at regional and teleseismic distances. In this work, crustal image in the Garhwal-Kumaun Himalayas has been obtained with the help of 3-D tomography using local earthquake data in order to determine the crustal structure of this region and to decipher if the deformation is basement involved or basement detached. From the 3-D tomographic analysis it is concluded that in the Garwhal- Kumaun Himalayas crustal level folding and thrusting have occurred. Slices of lower crustal material are overthrusted over the midcrustal detachment surface and some upper crustal low velocity material are underthrusted below it. This shows that deformation here is basement involved and not basement detached as postulated by several workers. Most part ofthe crust is rigid. This means here the entire crust can be seismogenic. Occurrence of earthquakes at all depths confirms this finding. The attenuation characteristics of a region depend on the tectonic environment. It is also a major factor that controls the level of damage and destruction caused by an earthquake. I have used coda of local earthquakes that occurred during 2006-2007 to study the attenuation characteristics of the Garhwal-Kumaun Himalayas. The coda attenuation characteristics are represented in terms of coda Q or Qc. The S-wave attenuation characteristic is also studied. This is represented in terms of S-wave Q or Qj. An attempt has been made to quantify the relative contribution of in scattering and intrinsic attenuation for the study area. Knowledge of the relative contributions of intrinsic (Qf1) and scattering (Qs~l) attenuation is important for identification of subsurface material, interpretation of tectonic set up and quantification of the ground motion. The coda and S-wave attenuation characteristics are represented by coda and S-wave quality factors Qc and Qd respectively. It is observed that both increase with increasing frequency. Qc also varies with increase in lapse time of coda waves. It increases up to about 85 s to 90 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, between 90 s and 100 s average lapse times Qc 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 160 km depth. Q0, i.e. Qc at 1 Hz increases and frequency parameter n decreases with increasing lapse time, barring around 90 s lapse time. This again shows that in general heterogeneity decreases with increasing depth. The Qc, Q0 and n values for smaller lapse times are similar to those for tectonically active areas. Qfor S-wave (Qd) is also similar to that for tectonically active areas. The variation of Qc with lapse time also indicate that the crust is turbid and the mantle is relatively more transparent in the Garwhal-Kumaun Himalayas. The fact that the crust is highly heterogeneous in the study area is also supported by tiie results of 3-D tomographic analysis of the study region. However, whether the variation in Qc values between 1999 and 2006-2007 is temporal or not can not be definitely established from the available data set. Qc'! and Q,1 are comparable at all frequencies for all the lapse times considered. For smaller lapse times coda attenuation is controlled by intrinsic attenuation, whereas, for larger lapse IV ♦ * times, both intrinsic and scattering attenuation contribute almost equally for coda attenuation at lower frequencies. At higher frequencies for all lapse times intrinsic attenuation controls coda attenuation. This shows that the attenuation characteristics changes with depth.