Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data (～166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband (3–150 keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (\({\dot{m}}_{d}\)), sub-Keplerian accretion rate (\({\dot{m}}_{h}\)), shock location (\(r_{s}\)) and black hole mass (\(M_{\mathit{bh}}\)) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as \(L^{\mathrm{obs}}_{\mathrm{jet}} \sim3\mbox{--}6 \times10^{37}~\mbox{erg}\,\mbox{s}^{-1}\) during one of the observed radio flares which indicates that jet power corresponds to 8–16% mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate (～14%) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of 5.2–7.9 \(M_{\odot}\) with 90% confidence.     

Greatest observed one-day point and areal rainfall of India

The greatest one-day rain amounts recorded at individual stations in the country during the last 41-year period from 1940 onwards were examined for all observatories as well as State rain-gauge stations in an attempt to bring out up-to-date information on the greatest recorded point rainfall for the duration of one day. Outstanding one-day point rainfall amounts recorded prior to 1940 were also examined and have been included in this note along with their date and year of occurrence by way of comparison. A generalized chart has been prepared based on the percentage ratios of the greatest one-day rainfall to the mean annual rainfall of about 300 observatory stations distributed uniformly over the entire country. On the basis of Depth-Area-Duration (DAD) analyses of the most severe rainstorms which occurred over different plain areas of the country, it has been found that the 2 July, 1941, rainstorm gave the highest areal rain depths in the country for different areas. Comparison with similar areal rain depths of the tropical USA has shown that rain depths of the July, 1941, rainstorm were higher for all areas excepting the areas of 500 sq. miles (1295 sq. km) and 1000 sq. miles (2590 sq. km).     

Three-dimensional intraplate stress distributions associated with topography and crustal density inhomogeneities beneath the Deccan Volcanic Province

The generation of intraplate earthquakes has been attributed to perturbations in the stress regime, either due to surface and sub-surface loading or strength weakening of the rock mass. The present work aims at estimating the intraplate stresses associated with topography and crustal density inhomogeneities beneath the Deccan Volcanic Province (DVP). A layered crustal model with irregular interfaces of small amplitude has been used for elastostatic stress calculations. The computed principal stress differences show a significant concentration at 5–20 km depths beneath the western side of the region. The maximum magnitude of principal stress difference occurs beneath the Karad at a depth of 10 km with a value of 60 MPa. The deviatoric stress estimates are further superposed on inferred stresses due to the regional plate tectonic forces. These results show principal stress difference concentrations beneath the Koyna, Poona and Karad regions which may thus be more vulnerable to brittle failure. It is also seen that the principal total stress directions point to the strike slip motion at Koyna, similar to that which is associated with the 1967 Koyna earthquake.     

Estimation of Source Parameters for the Aftershocks of the 2001 Mw 7.7 Bhuj Earthquake, India

The source parameters for 213 Bhuj aftershocks of moment magnitude varying from 2.16 to 5.74 have been estimated using the spectral analysis of the SH- waveform on the transverse component of the three-componnet digital seismograms as well as accelerograms. The estimated stress drop values for Bhuj aftershocks show more scatter (Mo^{0.5 to 1} ∞ Δσ) toward the larger seismic moment values (log Mo ≥ 10^14.5 N-m, larger aftershocks), whereas, they show a more systematic nature (Mo³ ∞ Δσ) for smaller seismic moment (log Mo < 10^14.5 N-m, smaller aftershocks) values. This size dependency of stress drop has also been seen from the relation between our estimated seismic moment and source radius, however, this size-dependent stress drop is not observed for the source parameter estimates for the other stable continental region earthquakes in India and around the world. The estimated seismic moment (Mo), source radius (r) and stress drop (Δσ) for aftershocks of moment magnitude 2.16 to 5.74 range from 1.95 × 10¹² to 4.5 × 10¹⁷ N-m, 239 to 2835 m and 0.63 to 20.7 MPa, respectively. The near-surface attenuation factor (k) is found to be large of the order of 0.03 for the Kachchh region, suggesting thick low velocity sediments beneath the region. The estimated stress drop values show an increasing trend with the depth indicating the base of seismogenic layer (as characterized by larger stress drop values (>15 MPa)) lying in 22–26km depth range beneath the region. We suggest that the concentration of large stress drop values at 10–36km depth may be related to the large stress/strain associvated with a brittle, competent intrusive body of mafic nature.     

Simulation of Heavy Precipitation Associated with an Intense Western Disturbance over Western Himalayas

Northern India is comprised of complex Himalayan mountain ranges having different altitude and orientations. This highly variable terrain is responsible for complexity of the weather systems passing over the region. During winter season, large amount of precipitation is received in this region due to eastward moving low pressure synoptic weather systems called Western Disturbances (WDs). Such heavy precipitation over the region lead to landslides and trigger avalanches in snow bound regions. This causes heavy damage to properties and human lives and therefore poses a great natural hazard threat. In this study an attempt is made to simulate a heavy precipitation event associated with an intense WD using a state of the art mesoscale model. Some important model simulated fields are compared with verifying analysis. Precipitation and circulation features associated with the intense WD are well simulated by the model. Forecast errors indicate that the high resolution mesoscale model could simulate the weather associated with the WD with reasonable accuracy.     

Fracture and stress orientation from borehole image logs: A case study from Cambay basin,India

The Deccan trap basalt, laid down by multiple lava flows during upper Cretaceous to Paleocene times forms the basement of current study in Cambay basin. As such, there is great interest and value in fracture detection and evaluation of fractured basement reservoirs in the Cambay basin. The procedure for identification and evaluation of natural as well as induced fractures in basaltic basement of the Cambay basin is presented in this work. In this study formation micro-imager (FMI) and extended range micro-imager (XRMI) log data for fracture identification is used. The Deccan trap basaltic basement of the study area, comprising five wells in the Tarapur-Cambay block, has potential for holding commercial hydrocarbon due to the presence of fractures and weathered basement. Both image logs (FMI, XRMI) identify three types of fracture including open (conductive), partially open and closed (resistive) fractures, of which open and partially open fractures are important for hydrocarbon accumulation. Fracture dip ranges from 10° to 80°. Image logs have also identified washout, breakout and drilling-induced fracture zones. The strike direction of the open natural fractures for four wells varies from N60°E to N30°E whereas the strike direction of most natural fracture in the fifth well is oriented towards N20°W. The orientations of drilling-induced fractures and breakouts may be interpreted for the in-situ stress direction over the logged interval. Drilling-induced tensile fractures, identified over the depth interval of 1969–1972 m, and borehole breakouts over the interval of 1953–1955 m in one well, suggest an orientation of maximum in-situ horizontal compressive stress (S_H) lies in the north-south direction. The azimuths of open natural fractures in the same well vary from north-south to N30°E. It is expected that the direction of fluid flow will be controlled by open natural fractures and therefore would be in a direction parallel to the S_H direction, which is orthogonal to the minimum horizontal stress (S_h) direction. The orientations observed are consistent with the present day S_H direction in the study area of Cambay basin.     

Estimation of earthquake source parameters in the Kachchh seismic zone,Gujarat, India,using three component S-wave spectra

Earthquake source parameters and crustal \(Q_{0}\) values for the 138 selected local events of (\(\hbox {M}_{\mathrm{w}}{:}2.5{-}4.4\)) the 2001 Bhuj earthquake sequence have been computed through inversion modelling of S-waves from three-component broadband seismometer data. SEISAN software has been used to locate the identified local earthquakes, which were recorded at least three or more stations of the Kachchh seismological network. Three component spectra of S-wave are being inverted by using the Levenberg–Marquardt non-linear inversion technique, wherein the inversion scheme is formulated based on \(\omega ^{2}\) source model. SAC Software (seismic analysis code) is being utilized for calculating three-component displacement and velocity spectra of S-wave. The displacement spectra are used for estimating corner frequency (in Hz) and long period spectral level (in nm-s). These two parameters play a key role in estimating earthquake source parameters. The crustal \({Q}_{0}\) values have been computed simultaneously for each component of three-component broadband seismograph. The estimated seismic moment (\(M_{0}\)) and source radius (r) using S-wave spectra range from 7.03E+12 to 5.36E+15 N-m and 178.56 to 565.21 m, respectively. The corner frequencies for S-wave vary from 3.025 to 7.425 Hz. We also estimated the radiated energy (\(E_{S}\)) using velocity spectra, which is varying from 2.76E+06 to 4.07E+11 Joules. The estimated apparent stress drop and static stress drop values range from 0.01 to 2.56 and 0.53 to 36.79 MPa, respectively. Our study also reveals that estimated \(Q_{0}\) values vary from 119.0 to 7229.5, with an average \(Q_{0}\) value of 701. Another important parameter, by which the earthquake rupture process can be recognized, is Zuniga parameter. It suggests that most of the Kachchh events follow the frictional overshoot model. Our estimated static stress drop values are higher than the apparent stress drop values. And the stress drop values are quite larger for intraplate earthquakes than the interplate earthquakes.     

Self-organized Fractal Seismicity and b Value of Aftershocks of the 2001 Bhuj Earthquake in Kutch (India)

— The devastating intraplate earthquake of M_w 7.7 of 26 January, 2001 took place along the south-dipping reverse fault in the lower crust ( 23 km) of Kutch, Gujarat, India, obliterating some 14,000 people. The aftershock activity has ensued for three years. We analyzed 997 aftershocks of M 3.0 to study the b value and fractal correlation dimensions in time and space. The b value is found to be 0.8 ± 0.03 from the Gutenberg-Richter relation and 0.76 ± 0.02 from the maximum-likelihood, suggesting a typical value for the intraplate region. The spatial correlation is 1.71 ± 0.02, indicating that events are approaching a two-dimensional region. Further, the temporal correlation dimension is estimated to be 0.78 ± 0.02, confirming the structure is mono-fractal in time domain. The depth section of b value shows a peak at 15–35 km depth range coinciding with the maximum occurrence of aftershocks ( 47%), which is inferred as a fluid-filled highly fractured rock matrix with fractures of high density. It will be important to note that tomographic results also suggest a low Vp, low Vs and a large Poissons ratio for the same depth range, further confirming this inference. Additionally, we have studied the variation of D₂^s and b value with time. During the first two months of aftershock activity the results show a marked negative correlation between spatial correlation dimension D₂ (large) and b value (low), indicating the predominance of large events associated with weak clustering. The negative correlation means the stress release along faults of a larger surface area. After two months the fractal dimension (D₂^s) and b value suggests a positive correlation implying more numerous smaller shocks with stress release along faults of a smaller surface area. This would indicate a reduced probability of large magnitude earthquakes due to fragmentation of the fault zone.Acknowledgement. The authors thank Dr. V.P. Dimri, Director, NGRI for his encouragement and kind permission to publish this work. The Department of Science and Technology, New Delhi supported this study.     

Self-organized Fractal Seismicity of Reservoir Triggered Earthquakes in the Koyna-Warna Seismic Zone,Western India

— Analysis of the Koyna-Warna earthquake catalog (1968–1996) shows that on an average there is a positive correlation between the b value (decrease) and fractal dimensions (decrease in both D₂^s and D₂^t) of earthquake epicenters 0.5 and 2.5 years prior to 1973 (M5.2) and 1980 (M5.5) events, respectively, except a negative correlation for about five years (1988–1993) prior to the 1993/1994 sequence (M5.4). This positive correlation indicates a weaker clustering, or that the epicenters tend to fill the two-dimensional plane. While the origin of the negative correlation seems to be that during periods of large events (low b value), there is strong clustering around the main shock epicenter (high fractal dimension). Interestingly, during the last year (1995–1996) of the studied period both the b value and correlation dimensions rose significantly, suggesting that stress release occurs through increased levels of low magnitude and increasingly scattered seismicity, suggesting an increased risk of larger magnitude events. Incidentally, during 2000 three earthquakes of magnitude M 5.0, one earthquake of M 4.0, 45 earthquakes of magnitude M 3.0–3.9, and several thousand earthquakes of M < 3 have occurred in the region. Thus it can be inferred that at local scales the relationship yields both positive and negative correlation that appears to be controlled by different modes of failure within the active fault complex.Acknowledgement. The authors are grateful to Dr. B.K. Rastogi of NGRI for providing the catalog of Koyna earthquakes and for useful scientific discussions. The comments of Dr. I. G. Main have improved the quality of paper for which we extend to him our sincere thanks. One of the authors (AOM) thanks the Third World Academy of Science and the Council of Scientific and Industrial Research, India for the Postdoctoral Fellowship award under which this work was carried out.