Earthquakes In India Research Articles

Source parameters of two rift-associated intraplate earthquakes in peninsular India: the Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970

The Bhadrachalam earthquake of April 13, 1969 and the Broach earthquake of March 23, 1970 are two major intraplate earthquakes associated with paleorift zones in the stable continental region of peninsular India. These two events shook very large portions of the peninsula, and casualties were reported for the Broach earthquake. Ancient rifts have recently been identified as a major seismogenic tectonic feature for intraplate seismicity in stable continental regions. P-wave first motions and waveforms of long-period teleseismic P and SH waves have been modeled to constrain the focal mechanisms and to determine the source parameters for the earthquakes. Previous investigators proposed multiple fault-plane solutions for each of the events. In this paper, some of the early solutions are eliminated and the best source mechanism is derived for each earthquake. For the Bhadrachalam earthquake, the focal mechanism determined is predominantly strike-slip (strike = 245°, dip = 72°, rake = 358°). The strike direction of the NE-SW-trending nodal plane is in good agreement with the trend of the observed isoseismals and is probably the fault plane. The seismic moment is 4.59 × 10 24 dyne-cm and the moment magnitude M w is determined to be 5.7. The focal depth and the static stress drop are estimated to be 10 km and 17 bar, respectively. The strike-slip motion represents a reactivated fault motion under the present-day stress field. For the Broach earthquake, the focal mechanism determined is reverse faulting with a strike-slip component (strike = 273°, dip = 58°, rake = 130°). The direction of the E-W-striking nodal plane is in good agreement with the mapped Narmada fault, dislocation direction of fissures, and the trend of the isoseismals. The seismic moment and moment magnitude are determined to be 1.39 × 10 24 dyne-cm and 5.4, respectivety. The focal depth and the stress drop are estimated to be 11 km and 9 bar, respectively. The reverse motion of this event represents an inverted slip or tectonic inversion within a rift zone under the present-day compressional stress field. Some rift-associated earthquakes are characterized by large multiple shocks with significant strike-slip motions. The two Indian earthquakes, however, occurred as regular main shock-aftershock sequences-and represent a different mode of strain release. These two earthquakes also represent events with low stress drops and relatively long recurrence intervals. The interpretation of this last observation is uncertain and needs more study in the future.

The Pn wave velocities and the relief of Moho in the Tibetan Plateau

Eleven PASSCAL broadband digital seismic stations were employed in the Tibetan Plateau by the Sino-US team from September, 1991 to June, 1992. Seven of them were distributed along the Qinghai-Tibet highway, others in Maqin and Yushu of Qinghai Province, Linzhi and Xigatze of Tibet. The data included 31 local earthquakes recorded by these stations from July, 1991 to January, 1992. Considering the characters of digital data, we identified the seismic phases carefully in several methods using SAC softwares (Seismic Analysis Code) in SUN workstation. We compared the seismic phases after converting the seismograms of the single stations to the seismic profiles; analyzed the first arrivals of P waves in the incident planes by rotating 3 component seismic records; identified the seismic phases from the particle motion pictures. The Pn apparent velocities were calculated in the distance range of 230–1200 km from Linzhi earthquakes, western Changtang earthquakes, Xitieshan and Gonghe earthquakes and the earthquakes in India. The results show that the Pn velocities change slightly in the Tibetan Plateau (8.0–8.1 km/s). These values near the velocities at the uppermost mantle of the normal continents. The Moho undulation in the Tibetan Plateau was also studied by using Pn data by the time-term method. The primary results indicate that the Moho beneath the Tibetan Plateau is flat, and its depths are 67–70 km.

Geological contacts, thermal springs and earthquakes in Peninsular India

The occurrence of thermal springs in Peninsular India shows a distinct pattern that is related to geological contacts, tectonic units and earthquakes. It is observed that most of these thermal springs occur either near the contact of two geological units or along prominent tectonic units in the Peninsular Shield. The seismicity map of Peninsular India from historical times up to 1990 for earthquakes of magnitude > 3.5 indicates a striking correlation between seismicity and thermal springs. The b-values which characterize the thermal spring areas of the western coast are higher (0.8-1.1) than those obtained in similar areas in the rest of the Peninsular Shield (0.47-0.6). This indicates that the stresses are released by frequent micro- to moderate earthquakes in the thermal springs area along the west coast of India, whereas in Peninsular India the stress release is through infrequent moderate earthquakes.

New catalogue of felt Indian earthquakes during 1901-1971

A new catalogue of felt earthquakes in India has been prepared for the period 1901.1971 based on the reports published in three newspapers namely, Times of India, Hindu and Statesman.

Indian earthquake

Indian earthquake

Interpretation of observed ground level changes due to the 1905 Kangra earthquake, Northwest Himalaya

The Kangra earthquake of 1905 in the Northwest Himalaya was the first Indian earthquake for which coseismic ground elevation changes were measured through repeat precision levelling. The levelling Une from Saharanpur to Mussoorie via Dehra Dun was situated near the southeastern extremity of the NW-SE trending meizoseismal area. Maximum uplift of 14.3 cm was measured at a bench mark in Dehra Dun. Some details of the levelling data having been published recently, data for eight bench marks were considered for interpretation in this study. In principle, these are sufficient for us to estimate eight parameters of a rectangular rupture buried in a homogeneous Poisson solid. But because of some uncertainties regarding the bench mark positions and also the relatively small amounts of uplift measured at the bench marks, we have preferred to use the data as a means to check the reasonableness of rupture models proposed on other grounds rather than to propose a new model independently. The observed elevation changes are not in accord with the view that the Kangra earthquake was caused by a single 100 km long rupture along the Shahpur-Mandi sector of the Northwest Himalaya. However, the data can be explained if, in addition, a second smaller rupture extending on either side of the levelling Une near Dehra Dun is assumed. The data can also be explained if the rupture during the earthquake had (1) a length of 280 km parallel to the general strike of the Himalaya from around Kangra up to a short distance (about 10–15 km) northwest of the levelling Une near Dehra Dun, (2) a breadth of 80 km perpendicular to the strike of the Himalaya, (3) a northeasterly dip of about 5°, and (4) a depth of 10–15 km along its SW edge. Finally, the elevation data rule out any role for the Main Boundary Thrust in the occurrence of the Kangra earthquake. This major tectonic feature demarcating the geological boundary between the outer and the lesser Himalaya thus appears to be unimportant in the current seismotectonics of the region and the subduction of the Indian lithospheric plate under the mountains.

The Mexican earthquake of 1985 vis-à-vis great Indian earthquakes: Field and seismological aspects

The great Mexican earthquake or 19 September 1985 (Magnitude 8.1 ) and its aftershocks caused the worst disaster in Mexico city about 350 km away from its epicentre. Through field surveys undertaken by the author, typical damages particularly on taller buildings in the city area have been discussed in the light of strong motion results vis-a-vis engineering aspects. The damage in Mexico city has been attributed to the largest spectral accelerations, longest duration and occurrence of two shallow focus events with an interval of 27 sec besides efficient generation of surface waves (0.2 to 1 Hz) towards Mexico city. The urban reconstruction plan has also been briefly described.
 Comparison of the source dynamics of the Mexico earthquake near subduction boundary of the Cocos South American plate with the great Himalayan earthquakes near the Indian Eurasian plate boundary shows that in spite of tectonically different environments (low stress drop of the Mexical1 earthquake compared with high stress drops of great Indian earthquakes), there .was a similarity in development of secondary miezoseismal areas for the great earthquakes of 1905 and 1934 which can be potentially hazardous zones for taller structures/ buildings.

Observed amplitude changes and seismic activity prior to two recent earthquakes in the Himachal Himalayas, India

The amplitude ratio of P to S waves measured on a vertical component seismograph from foreshocks and aftershocks to two recent Himachal Himalayan earthquakes in India exhibits a characteristic change at the time of the main events. This change was accompanied by simultaneous first motion changes at nearby stations. The variation is a function of the orientations of the earthquake source. Migration of foreshocks activity prior to the occurrence of these earthquakes suggests the possibility of the presence of a number of micro tear faults in the region.

841081 Geoelectrical and geohydrological precursors of earthquakes in northeastern India : Nayak, P N; Saha, S N; Dutta, S; Rama Rao, M S V; Sarker, N C GeoexplorationV21, N2, April 1983, P137–157

841081 Geoelectrical and geohydrological precursors of earthquakes in northeastern India : Nayak, P N; Saha, S N; Dutta, S; Rama Rao, M S V; Sarker, N C GeoexplorationV21, N2, April 1983, P137–157

Geoelectrical and geohydrological precursors of earthquakes in northeastern India

Resistivity, S.P., earth current, spring discharge, chargeability, magnetic, water level in dug wells and microearthquake measurements were carried out at a site in a reserved forest area near Shillong from February to July, 1980 to make a preliminary assessment of the precursors of earthquakes in northeastern India. At the site, steeply dipping quartzites are exposed, which are characterised by abnormally high self-potential (−1900 mV) and high resistivity. It was observed that the earthquakes having magnitudes from 4 to 5 on the Richter scale with epicenters located at distances varying from 40 to 400 km from the site were preceded by significant variation in the geoelectrical and geohydrological parameters of rocks over a period of 7—20 days. The variation of resistivities of rocks as related to earthquakes was considered to be very interesting. These values gradually decreased by 25%, in some cases, and then gradually increased to attain the pre-anomaly level. Significant earthquakes were observed to occur when the resistivity values approached the normal level. It is envisaged that under influence of tremendous stresses at the hypocenters of earthquakes, the normal field of stress at the site of measurements was disturbed, which brought about two types of changes in fractured rocks. Firstly, the sturcture of the interconnected capillaries was deformed resulting in changes of electromechanical potential. Secondly, the porosity fraction of rocks changed. Slight changes in the porosity fraction of rocks resulted in considerable changes in the resistivity of rocks, as the formation factor was high. This work has shown that the resistivity of rocks may decrease or increase depending on the structural features of rocks and changes in field of stress. It has also been shown that the maximum change of resistivity value is proportional to the distances of hypocenters of earthquakes.

The 1886 Charleston, South Carolina earthquake and the Appalachian Detachment

Several lines of evidence from new and existing data are consistent with an extended fault slip associated with the 1886 earthquake in South Carolina. Back slip over a portion of the Appalachian detachment approximately corresponding to the Coastal Plain of South Carolina fits some of the data including coseismic and post seismic strain indicators. A small seismic source for the August 31, 1886, main shock cannot be ruled out, and a large, but only partially seismic fault slip is possible. During this event intensities of MM ≥ VIII occurred over a wide area of the Coastal Plain and are not limited to the Charleston‐Summerville area, where the traditional small source for this earthquake is located. Near‐source effects such as ground fissuring, liquefaction, changes in line‐of‐sight, foreshocks, and aftershocks occurred over a wide area of the Coastal Plain and neighboring Inner Piedmont, which we tentatively refer to as the meizoseismal area. This area is sharply bounded by the Savannah River toward the southwest and approximately by the North Carolina‐South Carolina state border toward the northeast. These boundaries are not associated with any known structure transverse to the strike of the Appalachians. During the 14 years prior to 1886 seismicity was concentrated around the extended meizoseismal area while the seismicity within that area was unusually low, suggesting a premonitory doughnutlike pattern.The intensity distribution in the far and near field of the 1886 event and of the great Indian detachment earthquakes, e.g., the 1905 M = 8 and the 1934 M= 8.3 events, are qualitatively and quantitatively similar. This is consistent with comparable source dimensions, since well‐known intermediate magnitude earthquakes in India and in eastern North America exhibit similar intensity patterns and indicate similar seismic attenuation in the two cratonic environments. Several meters of coseismic (1886) northwest compressive strain near Summerville are indicated by buckling of rails. A large postseismic normal displacement is suggested by releveling data (1935–1968) across the Brevard fault zone updip (northwest) of the presumed 1886 detachment slip. These strains are consistent with a southeastward slip of the detached crystalline sheet associated with the 1886 earthquake. Available data indicate that in the southern Appalachians the updip transition from maximum horizontal to least horizontal compressive stress in the dip direction (perpendicular to structural trends)occurs somewhere in the Piedmont and may coincide with the steepest portion of the detachment. The stress field may also contain a time‐dependent component. In the sediments of the Coastal Plain, orthogonal sets of subvertical clastic dikes oriented along and across the Appalachian strike direction suggest one or more interchange of the principal stress axes. We infer from coseismic dry fissuring and liquefaction that clastic dikes filled from above and from below were also generated during the 1886 event. Prehistoric clastic dikes along with other structures in the sediments may have been generated during prehistoric earthquakes similar to the 1886 event. Stress reversals corresponding to earthquake cycles may characterize the stress field within detached sheets or wedges that move primarily by great detachment earthquake ruptures.Appendices are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐008; $1.00. Payment must accompany order.

Source parameters of earthquakes in northeast India from spectra of Rayleigh waves

A quantitative description of eight earthquakes in northeast India, in terms of seismic moment, dislocation, apparent stress and apparent average strain is sought. The analysis is based on the theory of radiation pattern of surface waves from buried seismic sources proposed by Ben-Menahem and Harkrider (1964). Further, it makes use of the amplitude spectra of digitally recorded Rayleigh waves at the Central Seismological Observatory in Erlangen (GRF), F.R. Germany. The amplitude spectra of Rayleigh waves are corrected for instrument response, path propagation effects and effects of the radiation pattern. Published results on attenuation and focal mechanism solutions are utilized to arrive at reasonable estimates of the quantities under investigation. Comparison of the calculated values is also made with the estimates based on body-wave spectra.It is concluded that the earthquakes in northeast India are of low moment type. The faulting process is supposed to be non-homogeneous, which results in low estimates of seismic source parameters and a characteristic source time function (Upadhyay and Duda, 1980). Comparison of source parameter values with known estimates for intraplate earthquake is made. For equal-magnitude earthquakes, intraplate earthquakes are characterized by higher seismic moment. Two of the earthquakes (Ms = 6.3) considered here are compared with the Koyna earthquake of Dec. 10, 1967. Emphasis is laid on development of theoretical models to account for the strain build-up rates in inter- and intraplate earthquakes.

Recent crustal movements and intra-plate earthquakes in India

The peninsular shield of India in the interior of the Indian Plate is characterized by intra-plate earthquakes. The major earthquakes in the peninsula are associated with prominent fault zones in the margins of the peninsular shield, especially along the Bombay—Ratnagiri coast, the western parts of the Narmada Rift zone and the southeastern parts of the Precambrian shield. Mild seismicity is also observed in several parts in the interior portions of the shield. Prominent recent crustal movements have been observed in the peninsular shield as evidenced by surface geologic and geomorphic features as well as prominent features in the lineament maps, including aeromagnetic lineaments. While the major intra-plate earthquakes are associated with tectonic zones characterized by deep crustal faults, the mild seismicity in the interior of the shield is associated with epeirogenic movements involving the movement of crustal blocks. The regional gravity and magnetic maps of the Deccan-trap region of the shield indicate prominent zones of uplift and subsidence involving the crust. The movements and dislocation with time in the deeper parts of the crust in the major zones of subsidence, as typified by the Koyna earthquake region which is bounded by a major fault zone on its west, obviously also involve displacements in the superincumbent mass of the Deccan trap. This latter being of much higher density and magnetic susceptibility, causes appreciable changes in the observed gravity and magnetic fields on the surface in addition to surface level changes observed by high-precision geodetic levelling. These studies are now in progress as an aid for earthquake prediction. The role of thermal processes in the upper mantle in these crustal movements is also discussed.

PKP times from indian earthquakes

About 1000 PKP readings, as obtained from ISS and ISC bulletins from fifty earthquakes, have been studied. These provide evidence that J-B times for the Indian region need correction for the DEF branch of PKP. The existence of an intermediate branch ( GH), corresponding to the PKHKP phase of Bolt (1962), has been established. The possibility of its extending up to 153° epicentral distance has not been overruled. Times for this branch of PKP travel-time curve are always greater by more than 2 s than those given by Gogna (1968).

Travel times of P from deep-focus Indian earthquakes

In this study all the deep-focus earthquakes occurring in the Indian region (6°–37° N; 70°–100° E) during the years 1936–1969, and which were widely recorded, have been investigated. An iteration process was used in revising the epicentres and 87 of the 108 earthquakes converged. P times for the Indian region for 0° ⩽ Δ ⩽ 105° have been calculated and are generally greater than the Jeffreys-Bullen P times at almost all distances except for the range 9° ⩽ Δ ⩽ 16°. Anomalous regions have been found both in the upper mantle and in the lower mantle.

S times from deep-focus Indian earthquakes

Summary. One hundred and eight deep-focus earthquakes occurring in the Indian region (6–37° N, 70–100° E) were originally selected for the study of P residuals (Paper I). Of these, 61 earthquakes were found suitable for analysing S residuals. S times for this region are generally greater than J—B S times. Analysis of residuals reveals that anomalous regions are present both in the upper mantle as well as the lower mantle.

India's earthquake

India's earthquake

Focal Mechanism of the Koyna, India Earthquake of 1967, December 10

One of the two alternate fault plane solutions determined in this study for the Koyna earthquake of 1967 December 10 shows a component of normal faulting. This possibility remained unnoticed in previous focal mechanism studies. Both solutions show left lateral motion along the nodal plane striking towards north-north-east. The origin of the tectonic stress causing the earthquake is believed to be related to the collision of Indian and Eurasian continents, following which the entire Indian Peninsula may be under a state of left lateral shear.

An earthquake spectrum prediction technique

abstract Empirical equations for the prediction of earthquake psuedo-relative velocity response spectra ( PSRV ) are determined utilizing 41 spectra calculated from 23 earthquakes at 28 stations. The derived equations employ PSRV attenuation exponents obtained from regression analysis of data acquired in the underground nuclear explosion program. The resulting relations allow 5 per cent damped PSRV to be predicted for earthquakes, given a magnitude and epicentral distance. The procedure, which is used to predict earthquakes in India, Japan, Colombia and the United States which were not in the data sample, shows good comparison with measured spectra.

Focal mechanisms of Indian earthquakes

abstractFocal mechanism solutions are presented for two earthquakes that occurred in peninsular India. The first motions of P, PKP and the polarization (or first motions) of S waves were used for this investigation. Both of these mechanisms are found to be associated with strike-slip faulting.