Ionospheric disturbances with the time of occurrence, magnitude and location of the earthquake (M6.5) near the Indian subcontinent

The present study purports the analysis of total electron content (TEC, which is one of the major ionosphere anomalies during the earthquake), sea surface temperature (SST) and outgoing long-wave radiation (OLR) during the earthquake event recorded on 10 March 2013 (M = 6.5). Global assimilative ionosphere modelled output TEC values have been used for this present study; the clear signature of TEC during the recorded earthquake has been noticed (i.e. increase in TEC 60–70 TECU during the event). The correlation between the magnitude and location of earthquake with TEC is around 0.9, and the least correlation between SST and OLR is due to concerned atmospheric effects; we tried to study the variations of SST and OLR prior during and after the event from Kalpana satellite image products archived by IMD.     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Estimation of Sedimentary Thickness in Kachchh Basin, Gujarat Using SP Converted Phase

An inexpensive method using natural earthquake data is utilized for determining the sedimentary thickness in Kachchh. The Institute of Seismological Research (ISR) is operating a network of broadband seismographs and strong motion accelerographs in Gujarat. We used data from 13 broadband seismographs and two strong motion accelerographs in the study. The stations are within 5 to 80?km from the epicenters. In this study the S-to-P converted phase, S_P, is used. This phase is generated due to large impedance contrast between sediments and basement. This phase is clear in the vertical component. The difference in the travel times of S and S_P phases and velocities of P and S waves is used for determining the sedimentary layer thickness. The thickness of sediments beneath each of these 15 stations was determined covering an area of 23,500?sq km.     

Probabilistic Assessment of Earthquake Recurrence in Northeast India and Adjoining Regions

Northeast India and adjoining regions (20°–32° N and 87°–100° E) are highly vulnerable to earthquake hazard in the Indian sub-continent, which fall under seismic zones V, IV and III in the seismic zoning map of India with magnitudes M exceeding 8, 7 and 6, respectively. It has experienced two devastating earthquakes, namely, the Shillong Plateau earthquake of June 12, 1897 (M _w 8.1) and the Assam earthquake of August 15, 1950 (M _w 8.5) that caused huge loss of lives and property in the Indian sub-continent. In the present study, the probabilities of the occurrences of earthquakes with magnitude M ≥ 7.0 during a specified interval of time has been estimated on the basis of three probabilistic models, namely, Weibull, Gamma and Lognormal, with the help of the earthquake catalogue spanning the period 1846 to 1995. The method of maximum likelihood has been used to estimate the earthquake hazard parameters. The logarithmic probability of likelihood function (ln L) is estimated and used to compare the suitability of models and it was found that the Gamma model fits best with the actual data. The sample mean interval of occurrence of such earthquakes is estimated as 7.82 years in the northeast India region and the expected mean values for Weibull, Gamma and Lognormal distributions are estimated as 7.837, 7.820 and 8.269 years, respectively. The estimated cumulative probability for an earthquake M ≥ 7.0 reaches 0.8 after about 15–16 (2010–2011) years and 0.9 after about 18–20 (2013–2015) years from the occurrence of the last earthquake (1995) in the region. The estimated conditional probability also reaches 0.8 to 0.9 after about 13–17 (2008–2012) years in the considered region for an earthquake M ≥ 7.0 when the elapsed time is zero years. However, the conditional probability reaches 0.8 to 0.9 after about 9–13 (2018–2022) years for earthquake M ≥ 7.0 when the elapsed time is 14 years (i.e. 2009).     

Estimation of Strong Ground Motions for 2001 Bhuj (M w 7.6), India Earthquake

The strong ground motions for the 2001 Bhuj (M _w 7.6) India earthquake have been estimated on hard rock and B/C boundary (NEHRP) levels using a recently modified version of stochastic finite fault modeling based on dynamic corner frequency (Motazedian and Atkinson in Bull Seismol Soc Am 95, 995–1010 2005). Incorporation of dynamic corner frequency removes the limitations of earlier stochastic methods. Simulations were carried out at 13 sites in Gujarat where structural response recorder (SRR) recordings are available. In addition, accelerograms were simulated at the B/C boundary at a large number of points distributed on a grid. The corresponding response spectra have also been estimated. The values of peak ground accelerations and spectral accelerations at three periods (0.4, 0.75 and 1.25 s) are presented in the form of contour maps. The maximum value of peak ground acceleration (PGA) in the center of meizoseismal zone is 550 cm/s². The response spectral acceleration in same zone is 900 cm/s² (T = 0.4 s), 600 cm/s² (T = 0.75 s) and 300 cm/s² (T = 1.25 s). The innermost PGA contour is on the fault plane. A comparison of the PGA values obtained at 13 sites in this study with those obtained in earlier studies on the same sites, but employing different methods, show that the present PGA values are comparable at most of the sites. The rate of decay of PGA values is fast at short distances as compared to that at longer distances. The PGA values obtained here put some constraints on the expected values from a similar earthquake in the region. A synthetic intensity map has been prepared from the estimated values of PGA using an empirical relation. A comparison with the reported intensity map of the earthquake shows the synthetic MMI values, as expected, are lower by 1 unit compared to reported intensity map. The contour map of PGA along with the contour maps of spectral acceleration at various periods permit the assessment of damage potential to various categories of houses and other structures. Such information will be quite important in planning of mitigation and disaster management programs in the region.     

Possibility of Large Deposits of Gas Hydrates in Deeper Waters of India

The available geological and thermodynamic data, essential for the formation and accumulation of gas hydrates, have been integrated and broadly interpreted for the deeper waters of India. The preliminary studies indicate that, in all probability, vast gas hydrate resources exist in the shallow sediments under deep waters. The area of the Bay of Bengal and Arabian Sea, off the coast of India and Andaman Islands, have accumulated thick sediments, over 22 and 10 km, respec tively, during collision of the Indian Plate with the Tibetan Plate. Bottom Simulating Reflectors (BSRs), indicating the likely presence of gas hydrates, have been observed from multichannel and single-channel seismic reflec tion data west of the Andaman Islands and Kerala-Konkan offshore. The Indian continental shelf, slope, and rise areas have, at places, shown the presence of gas-charged sediments and gas seeps through faults. There are commercial oil and gas fields in the shallow waters of both the east and west coasts of India. These are indicative of generation of both biogenic as well as thermogenic gases in the offshore areas of India. For the first time, an attempt has been made to estimate in-place gas hydrate resources under deep waters of India beyond 600 m water depth to the legal continental shelf boundary, and to the Andaman Islands. The gas hydrate resources appear to be vast, and require extensive exploratory efforts for their precise mapping and quantitative assessment.     

Total electron content of the equatorial ionosphere

Total electron content (N_t) variations in the ionosphere above the magnetic equator (Thumba dip 0.6°S) obtained by the Faraday rotation measurements of beacon signals from S66 satellites are described for the period December 1965–August 1968. The N_t value reaches a minimum around 05 hr and a broad maximum between 14–18 hr, the diurnal ratio being more than 20. During no-echo condition at pre-sunrise hours, N_t is found to be abnormally low. The equivalent slab-thickness at Thumba is between 150 and 250 km except around 14 hr when it reaches a high value around 500 km. The electrons are almost equally distributed above and below the peak for the daytime hours, but in the latter part of the night the ratio of top-side to the bottom-side electron content exceeds the value of 5. This high ratio is suggested as being due to very low value of maximum electron density which during the pre-sunrise period becomes comparable to the electron density at great heights where there is no diurnal change of electron density. Combining the data of Thumba and Ahmedabad, the diurnal development of the equatorial anomaly in N_t is described.     

Infrared and Raman spectra of lunar samples from Apollo 11, 12 and 14

We report the room temperature infrared reflectance spectra of several lunar surface rocks in the form of polished slices or butt ends. The spectra were obtained over the frequency range 20-2000 cm^–1 throughout the mid and far infrared (5-500µ) region of the electromagnetic spectrum where the fundamental internal and lattice vibrational modes of all minerals and rocks occur.Some fines samples were examined as pressed pellets and their reflectivities compared with the bulk samples. Several terrestrial minerals and rocks were also investigated. Kramers-Kronig analyses of these reflectance spectra were undertaken and the dispersion of the dielectric response ( and ) and the optical constants (n andk) have been determined over this frequency range. The low frequency and high frequency (infrared) dielectric constants were also calculated from the reflectance data.Raman light scattering measurements were made on all the samples supplied from the first three Apollo missions. Large background scattering proved to be the greatest experimental problem. Successful spectra in nearly all cases were obtained from small crystalline inclusions imbedded in the main ground mass. Some crystalline bulk rocks containing many very fine inclusions gave identifiable spectra and at least three different types were obtained.Supported by NASA Grant NGR 22-011-069 and by a Northeastern University Grant for Basic Research.     

Strain estimation from single forms of distorted fossils — A computer graphics and MATLAB approach

Most of the existing methods of strain analysis can estimate strain in a single form of distorted brachiopod, or trilobite provided independent evidence, such as the association of the fossil with cleavage and/or stretching lineation is available for inferring the direction of maximum principal strain. This article proposes a simple computer graphics based method and its MATLAB code that determine the minimum amount of strain in a single distorted fossil form even if data for inferring the maximum principal strain direction are lacking. Our method is a rapid computer-graphics alternative to some of the existing analytical methods. In a distorted fossil form of original bilateral symmetry, the relative senses of angular shears along the hinge line and the median line are mutually opposite to each other. It follows, therefore, that the maximum principal strain direction lies within the acute angle between the hinge and the median lines in the plane of the fossil. Using this principle, our method performs several simulations such that each simulation retrodeforms the distorted fossil by assuming a particular orientation, lying within the acute angle between the hinge line and the median line, as the potential direction of the maximum principal strain. Each simulation of retrodeformation yields a potential strain ratio. The distribution of all the potential strain ratios, obtained by assuming different orientations as the potential directions of the maximum strain, is typically a parabola-like curve with a distinct vertex that corresponds to the minimum amount of strain in the distorted fossil. An entirely computer graphical approach is somewhat time-intensive because it involves a large number of retrodeformational simulations. We, therefore, give a MATLAB code, namely, the Minstrain, that rapidly retrodeforms the fossil and determines the minimum strain with precision.     

The 2007 Talala,Saurashtra, western India earthquake sequence: Tectonic implications and seismicity triggering

A damaging and widely felt moderate (M_w 5.0) earthquake occurred in the Talala region of Saurashtra, Gujarat (western India) on November 6, 2007. The highly productive sequence comprised about 1300 micro earthquakes (M > 0.5) out of which 325 of M ? 1.5 that occurred during November 6, 2007–January 10, 2008 were precisely located. The spatial aftershock distribution revealed a NE–SW striking fault in accordance with the centroid moment tensor solution, which in turn implies left-lateral motion. The orientation and sense of shear are consistent with similarly orientated geological fault identified in the area from satellite imagery and field investigation.The aftershocks temporal decay, b-value of frequency–magnitude distribution, spatial fractal dimension, D, and slip ratio (ratio of the slip occurred on the primary fault to the total slip) were examined with the purpose to identify the properties of the sequence. The high b-value (1.18 ± 0.01) may be attributed to the paucity of the larger (M ? 4.0) aftershocks and reveals crustal heterogeneity and low stress regime. The high p-value (1.10 ± 0.39), implying fast decay rate of aftershocks, evidences high surface heat flux. A value of the spatial fractal dimension (D) equal to 2.21 ± 0.02 indicates random spatial distribution and source in a two-dimensional plane that is being filled-up by fractures. A slip ratio of 0.42 reveals that more slip occurred on secondary fault systems.The static Coulomb stress changes due to the coseismic slip of the main shock, enhanced off fault aftershock occurrence. The occurrence of a moderate earthquake (M_w 4.3) on October 5, 2008 inside a region of positive Coulomb stress changes supports the postulation on aftershock triggering. When the stress changes were resolved on a cross section including the stronger (M4.8) foreshock plane that is positioned adjacent to the main fault, it became evident that the activity continued there due to stress transfer from the main rupture.