A unified earthquake catalogue for South Asia covering the period 1900–2014

Seismicity analysis is very much pertinent for Indian subcontinent and its adjoining region which is seismically active including many great earthquakes associated with collision and subduction tectonics in the northern, north-eastern part of the subcontinent and in the Andaman and Nicobar Island. An earthquake catalogue has been generated for South Asia covering the period 1900–2014 by compiling the records of earthquake occurrences from International Seismological Center, Global Centroid Moment Tensor (GCMT), US Geological Survey, India Meteorological Department and published literature. The uniform magnitude scaling in moment magnitude M _W,GCMT is achieved through connecting relationships between different magnitude types. These relationships are derived by orthogonal standard regression analysis on available data pairs. The derived relationships have been compared with the existing equations already reported by others. The catalogue is subsequently subjected to a seismicity declustering algorithm to identify the foreshocks, main-shocks and aftershocks. The catalogue thus compiled is envisaged to be a useful resource for seismotectonic and seismic hazard studies in the region.

     

Modeling of source parameters and moment tensors of local earthquakes occurring in the eastern Indian shield

Earthquake source parameters and crustal Q are being estimated simultaneously through the inversion of S-wave displacement spectra from three-component recordings of ten local cratonic intraplate earthquakes from 3-6 broadband stations in the eastern Indian shield, wherein, an iterative Levenberg-Marquardt inversion technique is used. The estimated seismic moment (M_o) and source radii (r) vary from 7.4 x 10¹² to 7.1 x 10¹⁴ N-m and 144.2 to 211.3 m, respectively, while estimated stress drops (Δσ) and multiplicative factor (E_mo) values range from 0.11 to 4.13 MPa and 1.33 to 2.16, respectively. The corner frequencies range from 6.23 to 8.62 Hz while moment magnitudes vary from 2.44 to 3.57. The radiated seismic energy and apparent stresses range from 8.3 x 10⁶ to 2.0 x 10¹⁰ Joules and 0.06 to 0.94 MPa, respectively, wherein the estimated corner frequencies and seismic moment satisfy the relation Mo ∞ f _c ^–(3+ε) for ε = 12.7. Thus, the source scaling of these events clearly deviates from the self-similarity i.e. f^–3. Estimated Zuniga parameters reveal that all selected events satisfy the partial stress drop model, which is in good agreement with the global observations. Our estimated crustal S-wave quality factors vary from 1091 to 4926 with an average of 3006, suggesting a less heterogeneous crustal structure underlying the study region.We also perform moment tensor inversion of five selected local events using ISOLA software, which reveals that the dominant deformation mode for the eastern Indian shield is left-lateral strike slip motion with minor normal dip-slip component on an almost vertical plane. This observation suggests that neotectonic vertical movements might have played a key role in generating these earthquakes. Our modeling also depicts that the seismically mildly active Singhbhum shear zone and Eastern Ghats mobile belt are characterized by the left-lateral strike motion while two events in the Chotanagpur half graben belt suggest a normal dip-slip motion along a south dipping plane. A north-south orientation of P-axis is found to be dominant in the area, which is consistent with the prevailing north–south compression over the Indian plate.     

Estimation of static stress changes after the 2001 Bhuj earthquake: Implications towards the northward spatial migration of the seismic activity in Kachchh,Gujarat

Spatial-temporal patterns of aftershocks of the 2001 Mw7.7 Bhuj earthquake during 2001–2008 reveal a northward spatial migration of seismic activity in the Kachchh seismic zone, which could be related with the loading stresses caused by the continued occurrences of aftershocks on the north Wagad fault (NWF), the causative fault of the 2001-mainshock. Aiming at explaining the observed northward migration of activity, we modelled the Coulomb failure stress change (DCFS) produced by the 2001-mainshock, the 2006 Mw5.6 Gedi fault (GF) and the 2007 Mw4.5 Allah bund fault (ABF) events on optimally oriented plane. A strong correlation between occurrences of earthquakes and regions of increased DCFS is obtained on the associated three faults i.e. NWF, ABF and GF. Predicted DCFS on the GF increased by 0.9 MPa at 3 km depth, where the 7^th March 2006 Mw5.6 event occurred, whereas predicted DCFS on the ABF increased by 0.07 MPa at 30 km depth, where the 15^th December 2007 Mw4.5 event occurred. Focal mechanism solutions of three events on the ABF have been estimated using the iterative inversion of broadband data from 5–10 stations, which are also constrained by the first P-motion data from 8–12 stations. These focal mechanism solutions for the ABF events reveal a dominant reverse movement with a strike-slip component along a preferred northwest or northeast dipping plane (∼50–70°). Focal mechanisms of the events on all the three fault zones reveal an N-S oriented P- axis or maximum principal stress in the region, which agrees with the prevailing N-S compression over the Indian plate. It is apparent that the northward migration of the static stress changes from the NWF, resulting from the occurrence 2001 Bhuj mainshock, might have caused the occurrence of the events on the GF and ABF during 2006–08.     

The deadliest stable continental region earthquake occurred near Bhuj on 26 January 2001

A large destructive earthquake occurred on 26 January 2001 in the region of Kutch, Gujarat, in Western India, with magnitude Mw 7.7. The earthquake caused very heavy damage and a large number of casualties with more than 20,000 deaths. A preliminary study of ground deformation, damage pattern and aftershock distribution is presented.     

Aftershock Activity and Frequency-dependent Low Coda Qc in the Epicentral Region of the 1999 Chamoli Earthquake of Mw 6.4

— On 28 March, 1999 (19:05:10.09, UT) a significant earthquake of M _w 6.4 occurred in the Garhwal Himalaya (30.555°N, 79.424°E). One hundred and ten well-recorded aftershocks show a WNW-ESE trending northeasterly dipping seismic zone extending from a depth of 2 to 20?km. As the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip, it is inferred that the main-shock rupture nucleated on the detachment plane at a depth of 15?km and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust (MCT) takes a significant turn towards the north, which might be acting as an asperity in response to the NNE compression due to the underthrusting of Himalayan orogenic process prevalent in the entire region. Presence of high seismicity including five earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the Garhwal Himalaya, a site lying in the central gap zone of the Himalayan frontal arc. Attenuation property of the medium around the epicentral area of the 1999 Chamoli earthquake, covering a circular area of 61,500?km² with a radius of 140?km, is studied by estimating the coda Q _c from 48 local earthquakes of magnitudes varying from 2.5–4.8. These earthquakes were recorded at nine 24-bit REFTEK digital stations; two of which were equipped with three-component CMG40T broadband seismometers and others with three-component L4-3D short-period seismometers. The estimated Q _o values at different stations suggest on average a low value of the order of (30?±?0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q _c at lower frequencies (1–3?Hz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Q _c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected. An important observation is that the region north of MCT (more rigid highly metamorphosed crystalline rocks) is less attenuative in comparison to the region south of MCT (less rigid slightly metamorphosed rocks (sedimentary wedge)). The acceleration decays to 50% at 20?km distance and to 7% at 100?km. Hence, even 1g acceleration at the source may not cause significant damage beyond 100?km in this region.     

Gravity induced near-surface stresses in long-symmetric ridge-valley systems

Patterns of near-surface gravity and tectonically-induced stresses within ridge-valley systems greatly illuminate our understanding of important geodynamic processes as well as design of experiments to elucidate them. This paper presents analytical results derived for a number of geometrical configurations and range of mechanical properties of a ridge-valley system using the elastic solution of McTigue and Mei. The study reveals (i) the presence of non-zero compressive stresses near the ridge crests, which decrease with increasing Poisson's ratio (μ) and reduce to zero at ridge crests for μ=0·5 and that (ii) the central tensional regime characterizing a valley becomes narrower due to the presence of two ridges; and decreases with increasing μ, becoming compressive at depth. For all geometrical parameters considered, all components of stress show concentration at the outer flanks of the ridges and increase with depth approaching a standard state of stress.     

Was the July 1981 rainstorm over Rajasthan unprecedented?

The rainstorm of 17 to 21 July 1981 which caused exceptionally heavy rains and floods in arid Rajasthan, was analysed. It was observed that 7 stations in the rainstorm area broke their previous 80-year record of one-day rainfall, and two of these stations equalled their respective estimates of probable maximum precipitation (PMP). A comparison of areal raindepths of this rainstorm with the similar raindepths of past severe-most rainstorms of this region showed that this rainstorm gave unprecedented raindepths up to about 38 850 sq. km (i.e. 15 000 sq. miles) for a 3-day duration.     

Mode of development of sigmoidal en echelon fractures

The solution of stress distribution for a multicrack system and model experiments confirm that en echelon cracks mutually interact with each other during their growth. Such a mechanical interaction deviates the crack-tip stress axes orientations from that of the bulk stress field and leads to a continuous change in propagation direction of tension cracks, initially at a right angle to the bulk tension direction. The sigmoidal shape of en echelon fractures evolve through rotation and crack length increments with changing orientations. The theoretical analysis shows that the instantaneous fracture-tip stress orientation is a function of initial crack spacing, orientation of crack array with respect to the principal axes of far-field stress.     

Intraplate stresses along a section of the Kavali-Udipi DSS profile in the south Indian shield induced by density heterogeneities and topography

Occurrence of intraplate seismicity has been attributed to several causes. The perturbation in the local stress regime, either due to local strength weakening of rock mass or surface and subsurface loading, is considered as a plausible mechanism of intraplate earthquake occurrences. The present work is aimed at analysing the state of stress in a part of south Indian shield induced by existing topography and undulations in the subsurface interfaces. The stresses are computed along a section of well studied Kavali-Udipi profile. The general nature of the stresses is compressive in the upper lithosphere except for a small region of extensional regime at both the ends of the profile. The magnitude of these stresses in comparison to the plate tectonic stresses shows that it also forms a significant component of the prevailing stresses in the region. The computed results also show a lateral variation in the stresses along the profile section. Thus, the role of stresses due to density heterogeneities and topography should be taken into consideration in explaining the microseismicity of the region.