LOCAL EARTHQUAKE TOMOGRAPHY AND ATTENUATION CHARACTERISTICS OF NORTHEAST INDIAN REGION

Abstract

Various conflicting views have been proposed regarding the seismotectonics of northeast India. Uyeda and Kanamori29 postulated that the Alpine-Himalayan orogenic belt and the Andaman-Indonesian Islands are linked by a transform fault and the ophiolites of the Indo-Burma ranges are the product of a leaky transform fault. Nandy22, on the other hand, postulated that the eastern sector of the Indian plate collided with the Burmese block, and the ophiolites were emplaced along the Benioff zone. Sitaram et al.27 explored the upper mantle velocity in the Assam valley by using teleseismic P-wave arrivals. Kayal and De10 examined the seismic velocity in the uppermost mantle in the Shillong massif by using Pn arrivals of local earthquakes recorded by temporary microearthquake (MEQ) networks. Molnar (1987) suggested that the Shillong massif shares with the eastern Himalaya the effect of convergence of India with Eurasia. Mukhopadhyay and Das Gupta19 claimed that the Indian plate actively subducts below the Burmese arc. De and Kayal6 and Mukhopadhyay et al.18 attempted to study the crustal velocity beneath the Shillong massif by applying a time-distance plot method to P-wave arrivals of microearthquakes. Khattri et al.13 Rao and Kumar have interpreted that the massif is overthrusting southwards towards the Bengal basin along a gently dipping thrust plane to the north. 23 brought the concept of Pop-up tectonics for the first time. They proposed that the massif is rising by a pop-up mechanism due to compressional stress from the four directions, N-S stress from the collision tectonics in the Himalaya and the E-W stress due to subduction tectonics in the Indo-Burma ranges. Bilham and England5, based on geodetic and GPS data argued that the 1897 great earthquake was produced by a south dipping hidden fault at the northern boundary of the Shillong Massif, they named it ‘Oldham fault’ that extends from a depth of about 9 km down to 45 km. They further suggested that the Shillong Massif earthquakes are caused by the ‘pop-up’ tectonics between the Dauki fault and the Oldham fault and they suggested that Dauki fault is north dipping. They interpreted Shillong Massif as a pop-up structure bounded by two reverse faults. Evans7 and Nandy21 on the other hand, argued that the Dauki fault is a near vertical or a south dipping strike-slip/ normal fault, not a north dipping thrust fault. Bhattacharya et al.4 Verma and Mukhopadhyay had studied statistical characteristics of seismicity, fractal dimension and mapped b-values in the NE India region using permanent microearthquake network data and teleseismic data. They identified seismogenic structures and the crustal heterogeneities, based on this study. 30 General Geology and Tectonics of the Region analyzed the gravity data in the region and observed that the whole northeastern area shows a large variation in gravity anomalies, Bouguer anomalies show a variation from +40 m Gal over the Shillong Massif to -250 mGal over the northwestern part of the upper Assam valley. Gravity data suggests that the crust underlying the Shillong Massif is probably denser as well as thicker than normal for its elevation. According to them the Assam valley possibly overlies a crust which is thicker than normal for its topography and the crystalline solid crust underlying a large thickness of sediments of Bengal Basin is possibly denser as well as thinner than the normal continental crust. The region consists of crystalline rocks that are partly covered by gently dipping Tertiary and younger sediments7,11. The Shillong and Mikir hill Massifs are composed of Pre-Cambrian basement rocks, which are considered equivalent to the formations of the Indian shield7. These Massifs are separated by Kopili valley, covered with alluvium except for the southeastern side, where Tertiary outcrops ranging in age from Paleocene to Mio-Pliocene are exposed. Sediments from Pre-Cretaceous to recent are expected to be present in the Kopili valley. Geologically the entire area has been active since the Mesozoic. The region comprises of distinct tectonic units namely-Eastern Himalaya, Shillong Massif and Mikir Hills, Kopili Graben, Brahmaputra Valley, Indo-Burmese Mobile Belt, Eastern Syntaxis Zone (Mishmi Block) and Bengal Basin Seismicity of the Region Northeast India region is one of the six most seismically active regions of the world; the other five are Mexico, Taiwan, California, Japan and Turkey. It falls in zone V, the highest zone in the seismic zonation map of India. Objectives of the study A large number of earthquakes of magnitude greater than 7 have occurred in this area in the last century. As the population of this area has increased manifold in last 50 years, occurrence of such earthquakes in future will cause terrible death and destruction. Keeping this in mind, I have carried out the following investigations – i) I have studied the seismicity pattern and carried out 3-D Tomography using P and S wave travel time data for the entire North Eastern region. Tomography was carried out only for P-wave velocity structure as number of S-phase data was not sufficient to give reliable estimate of S-wave velocity structure. ii) I have carried out S wave attenuation analyses to study how wave propagation is affected by it and what are the implications on tectonics and seismic hazard of the area. This investigation has been useful in characterizing the seismic hazard in the region. iii) I have carried out the separation of intrinsic (Qi) and scattering (Qs Scope of Work ) attenuation parameters and studied the tectonic implications. In this study I have tried to obtain a more reliable picture for the study area by carrying out Local Earthquake Tomography (LET) on some recent earthquake data consisting of arrival times of P and S phases obtained from NEIST (Northeast Institute of Science and Technology), Jorhat. In the present study S-wave attenuation analysis for the study area have also been done along with the separation of S-wave quality factor Qd into scattering quality factor Qs and intrinsic quality factor Qi Local Earthquake Tomography to see the contribution of scattering and intrinsic attenuation in S-wave attenuation which has not been done by earlier researchers. Travel time tomography of local earthquakes (LET) has been used here to analyze the data. In this method using only one dimensional (1-D) velocity model and local travel times of seismic waves, the inversion gives a picture of the 3-D structure consistent with independent geologic and geophysical information without any prior assumptions of the structure. Attenuation characteristics Coda Normalization Method as proposed by Aki2 has been used to study the attenuation characteristics of the study area. The method comprises of determination of Q for S-wave (Qd) by comparing S- and coda amplitudes of events at different hypocentral distances. The basis for coda normalization method is the fact that coda envelopes show a decay rate that is independent of source-receiver distance1,24,8. Direct S-wave amplitude is normalized by coda amplitude measured at a fixed time and at the same frequency. Hence the effect of source, site and instrument can be eliminated and data from many earthquakes can be combined to obtain a stable estimate of attenuation.