January, 2006 || Volume 10 No.1
1
Widespread Geologic Evidence of a large Paleoseismic event near the Meizoseismal Area of the 1993 Latur Earthquake, Deccan Shield, India
B.S. Sukhija1*, B.V.Lakshmi2, M.N. Rao1, D.V. Reddy1, P. Nagabhushanam1, Syed Hussain1 and H.K. Gupta3.
1. National Geophysical Research Institute, Uppal Road, Hyderabad, India.
2. Indian Institute of Geomagnetism, Navi Mumbai, India.
3. Secretary, Govt. of India, Department of Oceanic Development, Mahasagar Bhawan, Block-12, C.G.O. Complex, Lodhi Road, New Delhi, India
ABSTRACT
The occurrence of large to major earthquakes in Stable Continental Regions (SCR) is a rare phenomenon, generally associated with very long recurrence periods. The deadly seismic event (M-6.3) of 30th September 1993 in Latur district, Maharashtra, and the Jabalpur earthquake of 1999 (M 6.1), central India, challenge the earlier assumptions of the aseismic nature of the Deccan shield of India. Lack of historic seismic records for this region and the recent debate about the reactivation of a pre-existing fault in the basement beneath Deccan traps as the causative source for the 1993 Latur earthquake, led us to investigate the paleoseismicity of Latur, Osmanabad region. We present geological evidences, obtained from three sites, 60 km apart, of a paleoseismic event that took place during 190 BC-410 AD in the meizoseismal area of the 1993 Latur earthquake. The paleoseismic signatures like faults and liquefaction features (such as flame structures) are identified in four trenches made in the alluvial deposits of Tirna and Manjira river valleys. The timing of the paleoseismic signatures is constrained by radiocarbon dating of a number of organic samples from trenches at three sites, as well as through archaeological artifacts found in and around the disturbed horizon in one of the sites. The observed stratigraphic relations and concurrent 14C dates strongly suggest that the observed paleoseismic features resulted from a single prehistoric seismic event, the magnitude of which could be inferred to be greater than that of the 1993 Latur earthquake.
2
Seismicity in the Antarctic and surrounding ocean
Katsutada Kaminuma
C/O National Institute of Polar Research, 9-10, Kaga-1, Itabashi, Tokyo 173-8515, Japan and
Masaki Kanao, National Institute of Polar Research, 9-10,Kaga-1, Itabashi, Tokyo 173-8515, Japan
ABSTRACT
Seismicity in the Antarctic and surrounding ocean is evaluated based on the data compiled by the International Seismological Centre for the period 1964 - 2002. The Antarctic continent and surrounding ocean were believed to be one of the aseismic regions of the Earth for many decades. However, after the development of Global Seismic Networks and local seismic arrays, a number of tectonic earthquakes have been detected in and around the Antarctic Continent.
Antarctica and the surrounding ocean has been divided into a total of 13 seismic areas, 3 for the continental region and another 10 for the oceanic region. Wilks Land is the most active area in the Antarctic Continent; where several small earthquakes were detected and located. In the ocean surrounding Antarctica, the seismic activity in 120°-60°W sector is three times lager than the other oceanic areas. This is probably due to stress concentration towards the Easter Island Triple Junction among Antarctic Plate, Pacific Plate and Nazuca micro-Plate. Three volcanic areas, namely the Deception Island, Mt. Erebus and Mt. Melbourn, are seismically active.
3
Intraplate earthquakes in Scandinavia and Greenland Neotectonics or postglacial uplift
Soren Gregersen
Geological Survey of Denmark and Greenland, Ostervoldgade 10, DK-1350 Copenhagen K, Denmark. sg@geus.dk.
ABSTRACT
Even if the intraplate areas of Scandinavia and Greenland have only experienced small earthquakes within the time of human tale, we can learn 4 lessons from these regions.
In a small part of Scandinavia, where the present earthquake activity is not significantly different from its surroundings, large faults have been discovered. And these are interpreted to show the occurrence of large earthquakes about 10,000 years ago. Signs of this are coincident landslides as well as liquefaction in loose sediments, which are well dated through varve-counting. Many Scandinavian scientists interpret the cause to be the deglaciation after the last Ice Age. And since the present dominating stress field in the area follows the pattern of the World Stress Map Project, namely compression within the plate, oriented in the direction of the absolute plate motion, the glacier off-loading is a significantly different cause, 10,000 years ago. Stress reorientation clearly indicates that present-day earthquake activity is caused by neotectonics - plate motion.
Into this argument goes the observation from Greenland and Antarctica, that no earthquakes occur under the ice caps. For Scandinavia the argument is that no earthquakes occurred under the ice sheet of the Ice Age, and that the stored stresses were released, when the ice sheet melted 10,000 years ago. A third lesson comes from Greenland. Here we find that the compressional regime in the intraplate region is not reached until several hundreds of kilometers from the mid-ocean ridge. The few existing focal mechanisms show spreading and strike slip motion. This is supported by data from Iceland far out from the spreading zone. When a map became available a few years back of deep-reaching faults in the Denmark a comparison was attempted. The question asked was, whether we can now point to some of these faults and call them active? Can the regional earthquake distribution be well correlated with the mapped deep faults? And the answer was for part of the region YES; while for another it was NO. The earthquake activity adds its own component to the fault map of Denmark.
The region is full of faults. Can we feel rather safe, because the stress seems to have diminished since the end of the Ice Age 9,000 years ago? Or can we expect large earthquakes as observed 10,000 years ago?
4
Relocation, Vp and Vp/Vs Tomography, Focal Mechanisms and other related studies using aftershock data of the Mw 7.7 Bhuj earthquake of January 26, 2001
Prantik Mandal1, S. Horton2 and J. Pujol21. National Geophysical Research Institute, Uppal Road, Hyderabad-500007, India
2. Center for Earthquake Research and Information, Memphis, Tennesse, USA
Email: prantik_2k@hotmail.com; horton@memphis.edu
ABSTRACT
To comprehend the seismo-tectonic process of the aftershock zone of the 26 January 2001 Mw 7.7 Bhuj earthquake sequence of Mw 7.7, we relocated 999 aftershocks (Mw 2.0-5.3) using HYPODD relocation technique and the data from a close combined network (NGRI, India and CERI, USA) of 8-18 digital seismographs during 12-28 February 2001. These precisely relocated aftershocks (ERH < 30 meter, ERZ < 50 meter) delineate an east-west trending blind thrust dipping (~ 45o) towards south (named as North Wagad Fault, NWF), about 25 km north of Kachchh Main Land Fault (KMF), as the causative fault for the 2001 Bhuj earthquake of Mw 7.7. The aftershock zone confines to a 60 km long and 40 km wide region lying between the KMF to the south and NWF to the north, extending from 10 to 45 km depth. The P- and S-wave station corrections determined from the JHD technique show a consistent and clear pattern of positive and negative values associated with the SW and NE part of the study area. The station corrections vary from 0.20 to –0.24 sec for the P-waves and from -0.55 to 0.95 sec for the S-waves.
The tomographic inversion technique is used to invert 5516 P- travel times and 4061 S-P travel time differences from 600 aftershocks recorded at 8 to 18 stations. Tomographic results suggest a regional high velocity body (characterized by high Vp (7-8.5 km/s), high Vs (4-4.8 km/s) and small s (0.24-0.55)) with a head extending 60 km in N-S and 40 km in E-W at 10-40 km depths. This high velocity anomaly is inferred to be a mafic pluton/rift pillow, which might have intruded during the rifting time (~135 Ma). Another important result of our study is the detection of a low velocity zone (low Vp (6.5-7 km/s), low Vs (3.6–4 km/s) and high s (0.26-0.265)) within the mafic body at the hypocentral depth of mainshock (~18-25 km), which is inferred to be a fluid-filled (trapped aqueous fluid resulted from metamorphism) fractured rock mass. The depth-wise variation of anisotropic percentage, stress drops and b-values suggest an increase at 18-30 km depths corroborating the presence of a fluid filled fractured zone coinciding with low velocity zone (at 18-25 km depths). Thus, the observed increase in anisotropy, stress drops and b-values at 18-30 km depths could be explained in terms of alignment of numerous fractures present in the fractured layer.
The focal mechanism solutions of selected 444 significant aftershocks, which were recorded at 6-14 stations, suggest that the majority of the selected focal mechanisms ranged between pure reverse and pure strike slip; some pure dip slip solutions were determined. The stress inversion using the selected earthquake focal mechanisms in the aftershock zone of the 2001 Bhuj earthquake shows that the axis of maximum principal stress is oriented N181oE with a shallow dip (=14o). Stress ellipsoid is oblate (R = 0.4). Further, the crustal shear wave anisotropy study suggests that leading shear wave polarization directions (LSPDs) over the aftershock zone vary from N-S to N-E with a delay of 0.07 to 0.14 sec. The delays in the N-S to N-E direction suggest cracks parallel to the direction of maximum horizontal regional compressional stress prevailing in the region.
5
Seismotectonics of the 2001 Bhuj earthquake (Mw 7.7) in western India: Constraints from aftershocks
J.R. Kayal1 and S. Mukhopadhyay21. Geological Survey of India, 27, J.L. Nehru Road, Kolkata – 700 016
• email: jr_kayal@hotmail.com
2. Department of Earth Sciences, IIT Roorkee, Roorkee 247 667, India
ABSTRACT
More than 500 aftershocks (M > 2.0) are relocated to study the source processes of the January 26, 2001 Bhuj earthquake (MW7.7) in western part of the peninsular Indian shield. The maximum intensity reached to X on the MSK scale, but no primary surface rupture or fault was mapped. The aftershocks are relocated by simultaneous inversion with an average rms of 0.19s, and average error estimates of latitude, longitude and depth are 1.2 km, 1.1 km and 2.3 km, respectively. Most of the aftershocks occurred in an area of 70 x 35 sq km; the maximum activity was observed at a depth range of 12-37 km. A bimodal distribution of aftershocks indicates that the main shock rupture propagated both in the upward and downward directions. Further, the best located larger magnitude aftershocks show two trends, one in northeast, parallel to the isoseismal trend and to the major Anjar Rapar Lineament/Delhi – Aravalli trend, and the other in northwest parallel to the Bhachau Lineament and a 8 km long secondary rupture. Fault-plane solutions of the northeast trending aftershocks indicate reverse faulting with left-lateral strike-slip motion; these are comparable to the main shock mechanism. The northwest trending aftershocks, on the other hand, show reverse faulting with right-lateral strike-slip motion. 3D-velocity, gravity, magnetic, ground positioning system (GPS) and satellite observations suggest block uplift during the main shock. These observations are comparable to the earthquake locations and source mechanisms of the main shock and aftershocks.
Eos, Transactions, American Geophysical Union, Vol.79, No. 27, July 7, Pages 319-321
Stable Continental Regions Are More Vulnerable to Earthquakes than Once Thought
Harsh K.Gupta
National Geophysical Research Institute, Hyderabad, India
ABSTRACT
Seismic events at shield areas throughout the world suggest that the stable continental regions (SCR) are much more vulnerable to earthquakes than was once thought. Earthquakes have struck SCRs at a number of locations, including the New Madrid Zone, United States; Tennant Creek, Australia; Ungava, Canada; and Kachchh, Koyna, Latur, and Jabalpur, India. In several developing countries, such as India, the problems caused by SCR earthquakes have become very serious because of high population density and the proliferation of structures not built to withstand earthquake damage.
