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# frequency of earthquakes in sikkim

The system of south-verging thrust faults within the Sikkim Himalaya exhibit distinct characteristics: (1) the Main Central Thrust (MCT), which emplace the Greater Himalayan sequence over the Lesser Himalayan sequence, and the Peling Thrust (PT) to its south, are both arcuate in shape with domal geometry (Ray 2000); and (2) within the arcuate MCT and PT lies the Rangit ‘geologic’ window which exposes mid-crustal rocks on the surface. Seismicity beneath the Sikkim Himalaya occur throughout the crust and are mostly concentrated south of the arcuate MCT (Fig. Stress drop (50 bars) is useful for simulation of earthquake strong ground motions. The estimated source parameters of the 2011 Sikkim earthquake have been found to be as - corner frequency (fc): 0.09 Hz, seismic moment (M0): (2.43 ± 0.5) × 1026 dyne-cm, stress drop (Δσ): 50.4 ± 9.28 bars, and the moment magnitude (MW): 6.87 ± 0.05. {\rm ln} A(f,t) = {\rm ln} S(f) - \beta {\rm ln}(t) - bt The sum of the two contributions constitute the seismic quality factor (⁠|$Q^{-1} = Q_{{\rm i}}^{-1} + Q_{{\rm sc}}^{-1}$|⁠). Q has been observed to have strong regional variation in the lithosphere and is more stable within the interior of the Earth. Region of our study is marked by the box in (a). Spatial sampling of coda waves enable in imaging the lateral variation of Q0 and η in the Sikkim Himalaya. 4a). Slope of the linear regression gives Qc at a given f. A comparison of all plots show that Qc increases with increasing f. Plot of ln(Q) versus ln( f ). The observed deviation in the Q0 for northeast India could be due to a combination of factors which include absence of foreland basin sediments, popped up Shillong Plateau, transitional crust beneath the Bengal Basin and higher Q values in the cold subducting Indian crust beneath the Indo-Burman plate boundary systems (Mitra et al.2014; Kumar et al.2015). © The Authors 2016. However, those from northeast India are significantly higher. Presented by, All earthquakes taken from the ISC reviewed catalogue are plotted as green circles in the map. (a) Plot of Q versus f replicating the exact points of measurement from these studies. The residual error between the observed and theoretical Q values for all grids is computed. In these studies it has been shown that coda wave attenuation represents combined effect of intrinsic and scattering attenuation and has been observed to be frequency dependent. Download : Download high-res image (250KB)Download : Download full-size image. Q0 value ranges from 80–200 with an average of 123 ± 29 and η ranges from 0.92–1.04 with an average of 0.98 ± 0.04. Attenuation of seismic energy is quantitatively described by the seismic quality factor (Q), which is the ratio of total energy in the wave to the energy lost in one cycle. 7). • The seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. All events are relocated using International Seismological Center (ISC) and local network phase arrival times from stations within a radius of 300 km, using an iterative least squares minimization algorithm (Lee & Lahr 1975). time relative to origin time) in the time domain. We use a total of 74 earthquake–receiver path measurements for the study. The high-frequency earthquake ground motions are governed by stress drop at the source. I. Based on these tests, we choose the 2ts coda lapse time and 20 s coda window length for the whole epicentral distance range of our study (Figs 2b and 3b). We use 74 earthquake–receiver paths to compute the Q0 and η values in Sikkim Himalaya. (a) Plot of elliptical sampling area of the coda waves. Finally, we compare our coda Q results with those from other Himalayan regions and northeast India (Fig. Abstract. , Crustal structure of the Darjeeling–Sikkim Himalaya and southern Tibet, Theory of earthquake prediction with special reference to monitoring of the quality factor of lithosphere by the coda method, Practical Approaches to Earthquake Prediction and Warning, Origin of coda wave: source, attenuation, and scattering effects, Attenuation of coda waves in western anatolia, Regional variation of Lg coda Q in the continental United States and its relation to crustal structure and evolution, A new kinematic evolutionary model for the growth of a duplex an example from the Rangit duplex, Sikkim Himalaya, India, Seismic attenuation of coda waves in the eastern region of Cuba, Lapse-time dependence of coda Q: Anisotropic multiple-scattering models and application to the Pyrenees, Lateral variation and frequency dependence of coda-q in the southern part of Iberia, Sac2000: Signal processing and analysis tools for seismologists and engineers, The IASPEI International Handbook of Earthquake and Engineering Seismology, Attenuation of coda waves in the Garhwal Himalaya, India, Adaptation of back projection tomography to seismic travel time problems, Depth-dependent seismic attenuation in the Granada zone (southern Spain), Spatial and temporal correlation between coda q and seismicity in china, Spatial and temporal correlation between coda Q- 1 and seismicity and its physical mechanism, Coda Q and its frequency dependence in the eastern Himalayan and Indo-Burman plate boundary systems, Seismotectonics of the eastern Himalayan system and Indo-Burman plate boundary systems, Paleoseismological evidence of surface faulting along the northeastern Himalayan front, India: timing, size and spatial extent of great earthquakes. This relation may be used for estimating the peak ground acceleration of similar magnitude for future earthquakes in the region. Q = Q_{0}(f/f_{0})^{\eta }, The active LHD, within this zone, controls the foreland-ward propagation of thrust sheets (Bhattacharyya & Mitra 2009) and is straddled by a number of steeply dipping layers. a spatial Gaussian filter) is applied to smooth and stabilize the inversion. (b) Map of Sikkim Himalaya with plot of relocated earthquake epicentres (stars) and stations (triangles) from which data have been used for this study. Additionally, using Q0 as a reasonable proxy for temperature, we conjecture that the actively deforming Himalayan crust and upper mantle is hotter than the region underlying the foreland basin, resulting in intrinsic attenuation. (b) Comparison plot of Q0 versus η with associated uncertainties for these regions. We follow the later assumption as high frequency coda waves at 2ts lapse time and 20 s coda window is composed of both scattering and intrinsic attenuation. The working principle of the algorithm is as follows: A starting Q value for each grid (Qg) is assigned. First, we use individual seismograms, corresponding to each earthquake–receiver path (henceforth referred to as single trace) (Fig. Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. Pavankumar. It has been found that a relation given by McGuire (1978) represents the attenuation characteristics of peak ground acceleration is relatively better. We use cookies to help provide and enhance our service and tailor content and ads. It had also been pointed out that the length of the coda window should be determined from the data (Mayor et al.2016). We use data from local earthquakes (mb ≥ 3.0) recorded by the Sikkim broadband seismograph network to study the frequency dependent attenuation of the crust and uppermost mantle. Higher the Q value, lower will be the attenuation and vice-versa.