Seismotectonics and Seismic Attenuation of Eastern Himalayan System and Indo-Burman Convergence Zone Using Broadband Seismology

Kumar, Ajay (2015) Seismotectonics and Seismic Attenuation of Eastern Himalayan System and Indo-Burman Convergence Zone Using Broadband Seismology. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The Eastern Himalayan System (east of 88⁰E longitude) is distinct from the rest of the India-Eurasia continent-continent collision zone, due to a wider zone of distributed deformation, oblique convergence across two orthogonal plate boundaries and near absence of foreland basin sedimentary strata. To understand the seismotectonics of this region we study the spatial distribution and source mechanism of earthquakes originating within the Eastern Himalaya, northeast India and the Indo-Burman Convergence Zone (IBCZ). We compute focal mechanism of (a) 37 moderate-to-large earthquakes (mb >=5.4) by modelling teleseismic P- and SH-waveforms, from the Global Digital Seismic Network stations (IRIS-DMC), us- ing a least-squares inversion algorithm; and (b) 7 small-to-moderate earthquakes (3.5<=mb<5.4) by modelling local P- and S-waveforms, from the NorthEast India Telemetered Network (Indian Meteorological Department), using a non-linear grid search algorithm. We also include source mechanisms from previous studies, either computed by waveform inversion or by using first motion polarity from analog data. From the depth distribution of the modelled earthquakes, we observe that the seismogenic layer beneath northeast India is ~45km thick. From the source mechanisms we observe that moderate earthquakes in northeast India are spatially clustered in five zones with distinct mechanisms: (a) thrust earthquakes within the Eastern Himalayan wedge, on north dipping low angle faults; (b) thrust earthquakes along the northern edge of the Shillong Plateau, on high angle south dipping fault; (c) dextral strike-slip earthquakes along the Kopili Fault zone in between the Shillong Plateau and Mikir Hills, extending northeast beneath Eastern Himalayan wedge and southeast beneath the Naga Fold belts; (d) dextral strike-slip earth- quakes within the Bengal Basin, immediately south of the Shillong Plateau; and (e) deep focus (>50 km) thrust earthquakes within the Indo-Burman Convergence Zone. Combining with GPS geodetic observations, it is evident that the N20E convergence between India and Tibet is accommodated as elastic strain both within the eastern Himalaya and the regions surrounding the Shillong Plateau. The source mechanisms also attests to the dextral shear between the Shillong Plateau and Mikir Hills. We hypothesize that the strike-slip faults south of the Shillong Plateau, within the Bangladesh Plains, are re-activated continental rifts paralleling the Eocene hinge zone. The NE-SW nodal plane is the fault plane with left lateral motion and have been re-activated due to compression within the Indian plate. Distribution of earthquake hypocenters across the IBCZ reveal active subduction of the Indian plate beneath the Burma micro-plate. Source mechanism of moderate-to-large earthquakes show thrust faults with strike of fault planes at high angle to the dip of the downgoing Indian plate. This is indicative of north- south compressive stresses within the downgoing plate. The eastern margin of the Burma-micro plate is marked by the Sagaing Fault with dextral strike-slip motion. We use broadband data for 305 local earthquakes (d <= 350km) recorded by a networks of broadband 3-component stations operated in NE India by Cambridge University, IIT Kharagpur, Tezpur University and IISER Kolkata under various projects for last 15 years and NE Telemetered network operated by IMD, to estimate attenuation factor called CODA Q (Qc) and it's frequency dependence in the crust represented by parameter η. We then use the local records of event-receiver pairs to measure the decay of coda amplitudes at a range of central frequencies (1, 2, 3, 4, 5, 8, 10 and 12 Hz). These measurements have then been combined to estimate the frequency dependence of Qc of the form Q = Qofη. CODA Q value has been observed to range from 80-360 with an average of 156±29.301 and η ranging from 0.85 - 1.45 with an average of 1.14±0.038. To produce lateral variation of map of CODA Q we use a back projection algorithm. We observe high Qo (~160) in NE India with high frequency dependence, surrounded by low Qo regions including western and eastern part of Shillong plateau (~120) and lowest in Arunachal basin and Naga hills (~80). We observe high Qo (~210-280) in Bengal basin due to underlying Indian oceanic crust and very high Qo value (~300-360) with high gradients in increase of Qo east of IBCZ which is due to subducted Indian oceanic crust with high frequency dependence. This lateral variation pattern of CODA Q is same at all the central frequencies.

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