National Geophysical Research Institute Research Articles

Transverse Tectonics in the Sikkim Himalaya: Evidence from Seismicity and Focal-Mechanism Data

Abstract In the present study, about 356 local earthquakes in the region of the Sikkim Himalaya have been accurately located and analyzed using 2181 P travel times and 2161 S travel times from a network of 11 broadband seismic stations operated by the National Geophysical Research Institute during January 2006 to November 2007. Further refinement of the hypocentral parameters using the hypoDD relocation program resulted in 198 well-constrained locations. Interestingly, this study reveals several characteristic features that distinguish Sikkim from the rest of the Himalaya. The seismicity distribution is found to be confined mostly between the main boundary thrust (MBT) and the main central thrust (MCT) but not quite associated with either. While the entire Himalayan front is generally characterized by shallow-angle thrust faulting, focal mechanisms in this region are predominantly of strike-slip type in conformity with a right-lateral strike-slip mechanism along the northwest-trending Tista and Gangtok lineaments. The P -axis trends of earthquake focal mechanisms are clearly oriented north-northwest, marking a clear transition from the ambient north-northeast trending direction of Indian plate motion with respect to the Eurasian plate all along the Himalayan front. Moderate-sized earthquakes occur down to 70 km depth in this region, compared to an average focal depth of 15–20 km in the rest of the Himalaya. Also, a high average crustal P velocity of 6.66 km/sec and a fairly low b value of 0.83±0.04 are obtained indicating the probability of occurrence of a higher magnitude earthquake in the future. A north–south section in the Sikkim region shows a relatively flat topography, unlike in the rest of the Himalayan mountain chain and suggestive of lower rates of convergence in the recent geologic past. It is proposed that crustal shortening in the Sikkim Himalaya has been substantially accommodated by transverse tectonics rather than underthrusting in recent times.

Anisotropy of the Indian continental lithospheric mantle

SUMMARY Due to the paucity of seismological data available in the public domain, the structure of the Indian lithosphere is still little known. We investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the Himalayan arc and underneath peninsular India using seismic anisotropy. Shear wave splitting measurements are performed on core-refracted phases. For each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast S wave (phi), assumed to be a proxy for the orientation of the a axis of olivine, and the delay (dt) between the arrival time of the fast and slow S waves. We present a very comprehensive data set recorded at 86 seismological stations, deployed from the Himalayas to the southern tip of the Indian peninsula, in a joint effort by the National Geophysical Research Institute, Hyderabad, India, the University of Cambridge and the Indian Institute of Astrophysics. The unprecedented data set we present sheds light on the mechanisms involved in the India–Eurasia continental collision in a region along the Himalayan arc, south of the Indus-Tsangpo suture zone. At the scale of the Indian plate, the majority of the stations show a NNE–SSW orientation of phi over hundreds of kilometres, from Sri Lanka to the northern part of the Dharwar craton. This direction closely parallels the trend of the Indian plate motion, with respect to a fixed Eurasian plate, as defined through the NUVEL1A plate model. Along the Himalayan arc, from Ladakh in the northwest, to Bhutan and the Shillong plateau in the east, the orientation of phi rotates to become ∼EW, perpendicular to the plate motion as defined through NUVEL1A. Unlike previous studies, we do find strong evidence for seismic anisotropy south of the Indus Tsangpo suture zone. A large number of null results have been computed, with consistent orientation of the two fast polarization directions (phi) across the subcontinent. We demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy. From these results, we infer the dominance, beneath the Indian lithosphere, of the asthenospheric flow in aligning minerals in the sheared lithosphere–asthenosphere boundary layer, masking any compression induced anisotropy expected to be normal to this direction. Closer to the collision front in northern India, the anisotropy may in part, be due to the foliation planes of the Himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis, but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting Indian lithosphere acting as a barrier. The continent-wide consistency of results strengthens the understanding that the Indian lithosphere has distinct anisotropic signatures, contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of Indo-Eurasian convergence.

Attracting, retaining, and engaging early career scientists

This young scientists event was organized to engage younger scientists with the International Union of Geodesy and Geophysics (IUGG) and to provide a specific forum to express their views at the General Assembly. It comprised a panel discussion chaired by Kate Heal and with three young geosciences panelists (Masaki Hayashi, University of Calgary, Canada; Kalachand Sain, National Geophysical Research Institute, Hyderabad, India; and Simona Stefanescu, National Meteorological Administration, Bucharest). The group, which had identified several topics relevant to young geoscientists, presented their views in open discussion session. Thirty IUGG conference attendees were present.

Source Parameters of the 8 October, 2005 Mw7.6 Kashmir Earthquake

During the last six years, the National Geophysical Research Institute, Hyderabad has established a semi-permanent seismological network of 5 broadband seismographs and 10 accelerographs in the Kachchh seismic zone, Gujarat, with the prime objective to monitor the continued aftershock activity of the 2001 Mw7.7 Bhuj mainshock. The reliable and accurate broadband data for the Mw 7.6 (8 Oct., 2005) Kashmir earthquake and its aftershocks from this network, as well as from the Hyderabad Geoscope station, enabled us to estimate the group velocity dispersion characteristics and the one-dimensional regional shear-velocity structure of peninsular India. Firstly, we measure Rayleigh- and Love-wave group velocity dispersion curves in the range of 8 to 35 sec and invert these curves to estimate the crustal and upper mantle structure below the western part of peninsular India. Our best model suggests a two-layered crust: The upper crust is 13.8-km thick with a shear velocity (Vs) of 3.2 km/s; the corresponding values for the lower crust are 24.9 km and 3.7 km/sec. The shear velocity for the upper mantle is found to be 4.65 km/sec. Based on this structure, we perform a moment tensor (MT) inversion of the bandpass (0.05–0.02 Hz) filtered seismograms of the Kashmir earthquake. The best fit is obtained for a source located at a depth of 30 km, with a seismic moment, Mo, of 1.6 × 1027 dyne-cm, and a focal mechanism with strike 19.5°, dip 42°, and rake 167°. The long-period magnitude (MA ~ Mw) of this earthquake is estimated to be 7.31. An analysis of well-developed sPn and sSn regional crustal phases from the bandpassed (0.02–0.25 Hz) seismograms of this earthquake at four stations in Kachchh suggests a focal depth of 30.8 km.

Source Parameters of the Deadly Mw 7.6 Kashmir Earthquake of 8 October, 2005

During the last six years, National Geophysical Research Institute, Hyderabad has established a semi-permanent seismological network of 5–8 broadband seismographs and 10–20 accelerographs in the Kachchh seismic zone, Gujarat with a prime objective to monitor the continued aftershock activity of the 2001 Mw 7.7 Bhuj mainshock. The reliable and accurate broadband data for the 8 October Mw 7.6 2005 Kashmir earthquake and its aftershocks from this network as well as Hyderabad Geoscope station enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the Peninsular India. Firstly, we measure Rayleigh-and Love-wave group velocity dispersion curves in the period range of 8 to 35 sec and invert these curves to estimate the crustal and upper mantle structure below the western part of Peninsular India. Our best model suggests a two-layered crust: The upper crust is 13.8 km thick with a shear velocity (Vs) of 3.2 km/s; the corresponding values for the lower crust are 24.9 km and 3.7 km/sec. The shear velocity for the upper mantle is found to be 4.65 km/sec. Based on this structure, we perform a moment tensor (MT) inversion of the bandpass (0.05–0.02 Hz) filtered seismograms of the Kashmir earthquake. The best fit is obtained for a source located at a depth of 30 km, with a seismic moment, Mo, of 1.6 × 1027 dyne-cm, and a focal mechanism with strike 19.5°, dip 42°, and rake 167°. The long-period magnitude (MA ~ Mw) of this earthquake is estimated to be 7.31. An analysis of well-developed sPn and sSn regional crustal phases from the bandpassed (0.02–0.25 Hz) seismograms of this earthquake at four stations in Kachchh suggests a focal depth of 30.8 km.

Sediment Thicknesses and Qs vs. Qp Relations in the Kachchh Rift Basin, Gujarat, India Using Sp Converted Phases

Delineation of the top sedimentary structure and its Qs vs. Qp relationship using the travel-time difference of direct S and converted Sp phase is key to understanding the seismic hazard of any sedimentary basin area. We constructed filtered displacement waveforms from local ETNA Episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 Bhuj earthquake recorded by the Kachchh seismological network of the National Geophysical Research Institute (NGRI), Hyderabad, India during 2001–2004. Stations are within 15–70km of epicenters, and the resulting displacement waveforms are generally simple, displaying prominent P, Sp, and S wave pulses. Particle motion of P and S waves suggest near-vertical raypaths consistent with preliminary depth estimates. The direct S wave on the horizontal component is characterized by lower frequency content than the converted Sp phase on the vertical component. This difference in frequency content between S and Sp phases can be explained in terms of different attenuation effects for P and S waves in the unconsolidated sediments. The Sp phase is generated by S-to-P phase conversion at the base of Mesozoic sediments of the Kachchh basin. Travel-time inversion (VELEST) of 2565 P and 2380 S arrivals from 658 well located aftershocks recorded at 8–14 three-component local seismic stations led to 1 D velocity models indicated very slow sediments in the upper 0–2 km depth range (Vp: 2.92 km/s and Vs: 0.90 km/s) and an increasing trend of velocities with depth at 2–40 km depth. The estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel-time difference between S and converted Sp phases reaches a maximum of (1.534 ± 0.117) km beneath Bandri (near the location of 2001 Bhuj mainshock) and attains a minimum sediment thickness of (0.858 ± 0.104) km beneath Ramvav and Burudia. The spectral ratios between Sp and S from 159 three-component accelerograms have been used to study seismic wave attenuation in the Kachchh rift basin. The estimated Qs vs. Qp relations for 12 accelerograph sites vary from Qs = 0.184 Qp (at Chobari) to Qs = 0.505 Qp (at Dudhai). For stations Chobari, Chopdwa, Jahawarnagar, Vondh and Tapar, the spectral ratio slopes and hence the calculated Qs vs. Qp relations are effectively the same, and the correlation coefficients are quite high (0.91–0.93). Stations Adhoi, Manfara, New Dudhai, Dudhai and Sikara have similar Qs vs. Qp relationships to each other and also have high correlation coefficients (0.78–0.87). The spectral ratios for stations Anjar and Ramvav are small and poorly constrained, resulting in less reliable Qs vs. Qp relations. This could be due to noisy data, fewer available waveforms, or scattering due to velocity heterogeneities and/or interface irregularities.

Relocation of Early and Late Aftershocks of the 2001 Bhuj Earthquake Using Joint Hypocentral Determination (JHD) Technique: Implication toward the Continued Aftershock Activity for more than Four Years

We employed layered model joint hypocentral determination (JHD) with station corrections to improve location identification for the 26 January, 2001 Mw 7.7 Bhuj early and late aftershock sequence. We relocated 999 early aftershocks using the data from a close combined network (National Geophysical Research Institute, India and Center for Earthquake Research Institute, USA) of 8–18 digital seismographs during 12–28 February, 2001. Additionally, 350 late aftershocks were also relocated using the data from 4–10 digital seismographs/accelerographs during August 2002 to December 2004. These precisely relocated aftershocks (error in the epicentral location<30 meter, error in the focal depth estimation < 50 meter) delineate an east-west trending blind thrust (North Wagad Fault, NWF) dipping (~ 45°) southward, about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake. The aftershock zone is confined to a 60-km long and 40-km wide region lying between the KMF to the south and NWF to the north, extending from 2 to 45 km depth. Estimated focal depths suggest that the aftershock zone became deeper with the passage of time. The P- and S-wave station corrections determined from the JHD technique indicate that the larger values (both +ve and -ve) characterize the central aftershock zone, which is surrounded by the zones of smaller values. The station corrections vary from −0.9 to +1.1 sec for the P waves and from −0.7 to +1.4 sec for the S waves. The b-value and p-value of the whole aftershock (2001–2004) sequences of Mw ≥ 3 are estimated to be 0.77 ± 0.02 and 0.99 ± 0.02, respectively. The p-value indicates a smaller value than the global median of 1.1, suggesting a relatively slow decay of aftershocks, whereas, the relatively lower b-value (less than the average b-value of 1.0 for stable continental region earthquakes of India) suggests a relatively higher probability for larger earthquakes in Kachchh in comparison to other stable continental regions of the Indian Peninsula. Further, based on the b-value, mainshock magnitude and maximum aftershock magnitude, the Bhuj aftershock sequence is categorized as the Mogi's type II sequence, indicating the region to be of intermediate level of stresses and heterogeneous rocks. It is inferred that the decrease in p-value and increase in aftershock zone, both spatially as well as depth over the passage of time, suggests that the decay of aftershocks perhaps could be controlled by visco-elastic creep in the lower crust.

Preparation and certification of high-grade gold geochemical reference material

One high-grade gold geochemical reference material-BND 3401.01 has been prepared by the National Geophysical Research Institute (NGRI), Hyderabad in collaboration with the Hutti Gold Mines Company Limited (HGML), Hutti and National Physical Laboratory (NPL), New Delhi, in India. The sample is a sheared gold-sulphide-bearing quartz vein emplaced in a sheared acid volcanic rock that was collected at the 2400-foot level from the middle reef of Hutti deposit located in the north-western periphery of the late-Archaean Hutti-Maski greenstone belt, Karnataka, India. Fifteen institutions in India, Canada, China and Tanzania having experience in geochemical analysis had participated in this collaborative analysis programme. The sample was extensively characterized for its major, minor, trace and ultra-trace element composition by using a variety of analytical techniques. Usable values were provided for major, minor and several trace elements including all rare earth elements (REE). The certified value for the gold concentration in BND 3401.01 gold ore is 12.1±0.7 mg/kg.

Magnitude estimation of Koyna-Warna earthquakes, India

The paper presents the current state of magnitude estimation for Koyna earthquakes exceeding magnitude 3.0. We estimate coda duration magnitude from analogue seismograms recorded on the short period vertical (SPZ) seismometer at Hyderabad seismic observatory HYB and determine moment magnitude using very broad-band (VBB) data from the Geoscope station (HYB)and short period digital data from the local seismic network of NationalGeophysical Research Institute (NGRI) around the Koyna and Warna reservoirs.Firstly, the seismograms of 97 Koyna earthquakes exceeding magnitude 4.0 havebeen used to establish a new empirical coda duration magnitude scale which includes the higher order terms of log10τ, where τ is the coda length in seconds. Four background noise levels (1, 2, 6 and 10 mm) areconsidered to estimate the coda duration. We found that the duration magnitudes for 1 mm background level are more stable than those for 2, 6 and 10 mm. The new coda duration magnitude (Mdnew) scale, for 1 mmlevel, is:Mdnew = –0.594 + 2.04 log10 τ – 0.0435 (log10τ)2The estimated Mdnew are compatible with the reported MS values of the NGRI observatory and the mb values of the United States Geological Survey (USGS). These magnitudes can be obtained within the standard deviation of ± 0.26 units of MS (NGRI). A new relatively homogeneous catalog for Koyna earthquakes of Mdnew ≥ 4.0 is obtained. The momentmagnitudes for 34 Koyna-Warna events of Mdnew ranging from 3.0 to 5.4 have been estimated using two techniques. The first utilizes amplitudes of band-pass filtered (between 15 and 30 sec) velocity traces of moderate Koyna-Warna earthquakes of MW} 4.4 to 5.4, we abbreviate the magnitude using MA. The second is based on the S-wave spectrum of short period seismograms of local earthquakes (MW 3.8). We recommend the use of both techniques whenever possible. The estimated moment magnitudes and Mdnew show an almost linear relationship with a standard deviation of ± 0.05.

Velocity inversion in the Lodhika area, Saurashtra peninsula, Western India

First BreakVolume 18, Issue 12 p. 499-504 Velocity inversion in the Lodhika area, Saurashtra peninsula, Western India M.M. Dixit, M.M. Dixit ,Search for more papers by this authorN. Satya Vani, N. Satya Vani ,Search for more papers by this authorD. Sarkar, D. Sarkar ,Search for more papers by this authorP. Khare, P. Khare ,Search for more papers by this authorP.R. Reddy, P.R. Reddy 5Search for more papers by this author M.M. Dixit, M.M. Dixit ,Search for more papers by this authorN. Satya Vani, N. Satya Vani ,Search for more papers by this authorD. Sarkar, D. Sarkar ,Search for more papers by this authorP. Khare, P. Khare ,Search for more papers by this authorP.R. Reddy, P.R. Reddy 5Search for more papers by this author First published: 24 December 2001 https://doi.org/10.1046/j.1365-2397.2000.00109.xCitations: 17 P.R.Reddy National Geophysical Research Institute, Hyderabad-500 007, India, E-mail: postmast@csngri.ren.nic.in Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume18, Issue12December 2000Pages 499-504 RelatedInformation

New Reference Material of Dunite Rock, NGRI‐UMR: Preparation and Evaluation

A dunite reference material has been prepared as part of the reference materials programme at the National Geophysical Research Institute, Hyderabad, India. The scarcity of reference materials for undertaking the chemical analysis of ultramafic rocks for major and trace elements justified the preparation of such a rock reference material. A 300 kg sample of starting material was collected from an active quarry near the town of Salem in Tamil Nadu, southern India. The sample was homogenized and an invitation for round robin participation was published in Geostandards Newsletter (October 1995). A total of twenty seven international laboratories participated in the study, providing information on the total element concentration of major, minor, trace and rare earth elements. Following data compilation, outlier rejection, and statistical analysis, proposed and indicative total element concentrations are reported for twenty four elements.

International Proficiency Tests for Analytical Geochemistry Laboratories: An Assessment of Accuracy and Precision in Routine Geochemical Analysis

Abstract The results of Second International Proficiency Test (GeoPT2) obtained at the Geochemical Laboratory of National Geophysical Research Institute are presented. Estimation of 33 trace elements including all the fourteen rare earths obtained by ICP-MS in Bardon Volcanic Tuff Samples (OU-1) are compared with the assigned values. Sample decomposition method and other relevant details of the analytical procedures adopted are presented. Depending upon the 'Z-scores' calculated, the fitness of the data for either "pure geochemistry" or "applied geochemistry" was assessed. An overview of 'Z-score' results indicates that about 2/3rds of the data contributed from our laboratory fell within the -2&amp;lt;z&amp;lt;2 range which included majority of the trace and rare earth elements used normally in several pure geochemical investigations. These results allowed us to further investigate the analytical discrepancies observed in the data and optimize necessary aspects such as sample preparation by the use of more effective dissolution methods like microwave digestion and minimise the interference effects by the use of matrix matching calibrations.

Gas-Hydrates - A Synoptic View

Abstract Large-scale occurrence of gas-hydrates in the marine sediments of outer continental margins is by now well-known. While the methods of tracing gas-hydrate enriched zones, like identifying markers such as bottom simulating reflectors and blanking zones on seismic reflection sections, are well-established, quantification and resource assessment of gas hydrates still remain hazy. Investigations over the classical gas-hydrate enriched Blake outer ridge on the U.S. Atlantic margin have provided new insights into the environs of gas-hydrate occurrences. Deep sea drilling results in this area indicate that the estimates of gas-hydrate from geophysical measurements could be less by an order of three in comparison with those obtained from drilling. Several factors like bathymetry, sediment thickness and sedimentation rates, sea bottom temperature and total organic carbon content, which control gas-hydrate formation, indicate that the shallow sediments in the offshore regions of India could be potentially enriched in gas hydrates. National agencies like the Oil &amp; Natural Gas Corporation, Gas Authority of India Ltd., National Geophysical Research Institute, National Institute of Oceanography and others are now actively involved in assessing the true potential of this anticipated future energy resource. The environmental aspects of gas hydrates like large-scale release of methane gas into the air and slope instabilities and submarine landslides in the continental margins resulting in catastrophic events, however, need to be kept in mind.

Source parameters of Koyna earthquakes, India

Abstract A seismic zone in southwestern India extending 30 km south from Koyna Dam has witnessed more than 150 earthquakes of M ≧ 4 and 10 of M ≧ 5 during the last 34 years. During August 1993 to December 1994, a spurt of earthquakes occurred in the vicinity of the newly impounded Warna reservoir, 25 km south of Koyna Dam. The epicenters are located between the two reservoirs but more near the new reservoir. During 1994, digital data acquisition systems were deployed by the National Geophysical Research Institute in the vicinity of the Koyna-Warna reservoirs to help in understanding the physical processes responsible for generating earthquakes in the area. Source parameters were estimated at three stations using S-wave spectrum for 193 selected earthquakes of M = 1.5 to 4.7 that were recorded during October 1994 to June 1995. Seismic moments of 1011 to 1016 N-m, source radii from 94 to 538 m, and stress drops of 0.03 to 19 MPa for earthquakes of magnitude 1.5 to 4.7 were obtained from this analysis. It is observed that earthquakes of M ≧ 3 are associated with larger stress drops of more than 2 MPa. The larger energy release (through more numbers of M ≧ 3 earthquakes) and higher stress-drop values are found in 0 to 1 km and 5 to 13 km depth ranges. Depth range of 1 to 4 km has low seismicity and low stress-drop earthquakes. Higher energy release and stress drops are possibly caused by higher stress concentration and incremental stress on rocks with fluids in subhydrostatic conditions in 0 to 1 km depth range and suprahydrostatic conditions in 5 to 13 km depth range. Spatially, the earthquakes of M ≧ 3 and high stress drop are now less near Koyna reservoir but frequent near the new reservoir.

Discussion on ‘An analytic signal approach to the interpretation of total field magnetic anomalies’ by Shuang Qin1

Geophysical ProspectingVolume 44, Issue 3 p. 495-497 Discussion on ‘An analytic signal approach to the interpretation of total field magnetic anomalies’ by Shuang Qin1 H.V. Ram Babu, H.V. Ram Babu National Geophysical Research Institute, Hyderabad 500 007, IndiaSearch for more papers by this author H.V. Ram Babu, H.V. Ram Babu National Geophysical Research Institute, Hyderabad 500 007, IndiaSearch for more papers by this author First published: May 1996 https://doi.org/10.1111/j.1365-2478.1996.tb00161.xCitations: 1 1 eceived November 1994, accepted October 1995. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume44, Issue3May 1996Pages 495-497 RelatedInformation

Travel times ofP-waves in North-East India

Based on 90 accurately localized earthquakes in and around North-East India and the local crustal velocity model of Gupta et al. [4], the travel times of P-waves have been determined from the foci of these earthquakes at arbitrarily selected depths of 5, 13, 25, 41 and 50 km to the sites of the seismic stations operated by the Regional Research Laboratory, Jorhat and the National Geophysical Research Institute, Hyderabad, and also to the sites of the Shillong and Tura seismic stations run by the India Meteorological Department, New Delhi. The travel times of P-waves fit a straight line very well with velocities of 5·97 ± 0·31, 6·18 ± 0·01, 6·41 ± 0·03, 7·82 ± 0·07 and 7·95 ± 0·01 km/sec at each of the depths under study. Similar investigations of P* and Pg-waves of 16 earthquakes at a depth of 10 km have revealed velocities of 6·53 ± 0·31 and 5·64 ± 0·34 km/sec, respectively. A simplified two-layered crustal model consisting of an average crustal thickness of 41·5 km with 22·2 and 19·3 km thick layers has been obtained.

Indian Geophysical Union celebrates 25th anniversary

The Indian Geophysical Union under its president A.P. Mitra, director‐general of the Council of Scientific and Industrial Research, is holding the seminar “Advances in Geophysical Research in India” at its 25th annual convention February 1–3 at the National Geophysical Research Institute (NGRI) in Hyderabad. Broad disciplines covered in the seminar are solid Earth geophysics, physics of the oceans, atmospheric sciences, solar‐terrestrial relations, space sciences and planetology, and instrumentation. An international symposium on structure and dynamics of the Indian lithosphere is also part of the convention program.

Gravity Bases Established by the Centre for Earth Science Studies in and Around Palghat Region, South India - Part I

Abstract Forty-one gravity bases were established with a Worden gravimeter in parts of Palghat gap area bordering Kerala and Tamil Nadu. The description of these base stations and the observed gravity values are presented. The observed gravity values at these stations are with reference to a value of 978.0715 gals at the Coimbatore Airport, established by the National Geophysical Research Institute, Hyderabad. The accuracy of the observed gravity values is estimated to be 0.07 mgal. The new base stations are being used for regional gravity surveys.

Crustal studies of the Koyna region using explosion data from deep seismic soundings

This study is based on the seismic data collected as a result of explosions carried out during the 1976 and 1978 Deep Seismic Sounding (DSS) field operations in the Koyna region. These shots were exploded from twelve shot points by the National Geophysical Research Institute along the Guhagar-Chorochi and Kelsi-Loni profiles. Refraction studies of the records reveal a two-layered crust. The top layer consists 17 km of granite and the second layer 19 km of basalt, giving the average depth of the Moho as 36 km in the region. The velocities of the phases P g, P∗ and P n have been computed as 5.82 ± 0.01, 6.61 ± 0.05 and 8.23 ± 0.05 km/sec respectively and those of S g, S∗ and S n as 3.41 ± 0.00, 4.09 ± 0.07 and 4.60 ± 0.08 km/sec respectively. The shear wave velocity in the basement rock has been found to be lower in comparison with other regions of the peninsular India. In some cases reflections were recorded both from the Moho as well as from the intermediate layer. These reveal a crustal thickness of 39 km with 19 km of granitic and 20 km of basaltic layers. Coda signal durations from DSS explosions recorded by microearthquake seismographs indicate a lateral heterogeneity in the crust on either side of Karad in an east-west direction.

COMMENT ON “BEDROCK DEPTH FROM SURFACE POTENTIAL MEASUREMENTS” BY A. ROY*

Geophysical ProspectingVolume 31, Issue 6 p. 1000-1002 COMMENT ON “BEDROCK DEPTH FROM SURFACE POTENTIAL MEASUREMENTS” BY A. ROY* A. APPARAO, A. APPARAO National Geophysical Research Institute, Hyderabad 500 007 AP, India.Search for more papers by this author A. APPARAO, A. APPARAO National Geophysical Research Institute, Hyderabad 500 007 AP, India.Search for more papers by this author First published: December 1983 https://doi.org/10.1111/j.1365-2478.1983.tb01104.x * Received February 1983. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume31, Issue6December 1983Pages 1000-1002 RelatedInformation