Irene Sarkar

New seismological results on the tectonics of the Garhwal Himalaya

This paper reports data pertaining to 90 local earthquakes recorded during 1984–86 using seismographs in arrays of 5–7 stations deployed near the Main Central Thrust between Bhagirathi and Alakhananda valleys. The results which are also compared with 162 earthquakes recorded in 1979–80 provide a local view that refines and complements information recorded at distant seismic stations.

Seismicity and the state of stress from investigations of local earthquakes in the Kumaon Himalaya

Abstract Results of investigations of local earthquakes in the region of the Main Central Thrust (MCT) in the Kumaon Himalaya, between and adjacent to the valleys of the Bhagirathi and Yamuna rivers, are presented. Records of over 250 earthquakes were analysed and the following facts emerged: 1. (1) Earthquakes in the Himalayas occur in specific areas and belts. One such belt has been identified in the region under investigation, hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt, crossing the Yamuna river between the villages of Barkot and Syanachatti. 2. (2) All but a few epicentres in this middle segment, lie to the southwest of the surface trace of the MCT in a zone with a width of 10–30 km. 3. (3) Most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface, the maximum estimated depth being 32 km. Using observations of first motion for a composite focal mechanism solution, the nodal planes were observed to be near vertical and the compression axis near horizontal and normal to the local strike of the seismic belt and of the MCT. We conclude that although the Main Central Thrust itself is not seismically active in this region, there is considerable activity immediately to the southwest of it. Furthermore, the mode of faulting as inferred from the records of these earthquakes, is strike slip.

Estimation of causative fault parameters of the Rudbar earthquake of June 20, 1990 from near field SH-wave data

Abstract We analyze the strong motion accelerograms recorded for the large ( M S =7.7, M W =7.3, m b =6.4) Rudbar earthquake of June 20, 1990. The earthquake had a complex source process. We have identified the imprints of rupture of three localized asperities on the major causative fault on the accelerograms. These asperities are interpreted to correspond to (i) the main shock that initiated the rupture process and was located in the domino block between the Kabateh and Zard Goli faults, (ii) a foreshock that occurred about 10 s earlier in the Kabateh fault and (iii) a later shock, on the western end of the Baklor fault, which terminated the bilateral rupture process at the western end. We estimate the strike, dip and slip of these causative sub-event rupture planes using the SH spectral amplitudes, based on a point source representation of sub-events and a non-linear least square formulation for inversion of the amplitude data. The results of our inversion of the near field data are comparable to other studies based on teleseismic data.

The role of the 1999 Chamoli earthquake in the formation of ground cracks

Abstract The moderate magnitude Chamoli earthquake that occurred in the Garhwal Higher Himalaya, in the early hours of March 29, 1999, caused intense damage to the ground and mountain slopes of the Alaknanda–Mandakini river valley and adjoining region. A systematic survey of this induced damage was conducted immediately after the earthquake occurred. Prominent shallow cracks of significant length, negligible width and indeterminate vertical extent, conspicuously tensile in nature, with little or no slip across the crack planes, were observed in the ground at several places along the surveyed route. These cracks had formed in the dynamic phase of the Chamoli earthquake process that is in the period of time during which the earthquake-generated seismic waves were passing through the geographic region of interest. However, we use the theory of earthquake-induced static (or long time) stress changes to visualize such cracks at some selected sites where ground damage was relatively more intense and varied to suggest lower bound estimates of the dynamic stress contributions of the main shock for their formation. Based on the results of our analysis we conclude that, just prior to the earthquake occurrence, under the influence of the local ambient stress field, the ground at these sites was already near failure in tension. To this, in its dynamic phase, the Chamoli earthquake induced stress perturbations, having, across the planes of the cracks, (i) shear components which were nearly equal and opposite to similar components of the ambient stress field and (ii) normal (tensile) components, necessary for triggering tensile failure of the ground. The σ3 (or minimum principal stress) component of the resultant perturbed failure stress field thus became sufficiently tensile while the transverse stresses became sufficiently insignificant. This facilitated formation of major tensile cracks in the ground there. Our static estimates of the tensile stress changes at the different sites are, in essence, estimates of the minimal triggering stress perturbations that was provided by the Chamoli earthquake in the dynamic state for the formation of the tensile cracks there.

Upper crustal compressional wave velocity in the Garhwal Himalaya

A value of 5.2 km/s is obtained for compressional wave velocity in the upper crust in the vicinity of the Main Central Thrust where it crosses the Yamuna and Bhagirathi valleys in the Garhwal Himalaya. The data used consisted of the arrival times of compressional waves; recorded at five portable stations from nine earthquakes occurring in the region.

Role of static stress transfer in earthquake occurrence in the Himalaya

Abstract Coulomb failure hypothesis suggests that earthquake interaction can lead to earthquake sequences and clustering. This implies that the phenomenon should be considered a fundamental feature in any description of seismicity and evaluation of the seismic hazard and risk of a region. We translate this idea to the Himalaya and investigate how significantly the past earthquakes of the region may have influenced the present day seismicity and seismic hazard potential along its different segments. For this, we estimate separately the change in Coulomb failure stress at the source of the most recent moderate magnitude earthquake along the Kumaon Garhwal segment, the March 29, 1999 Chamoli earthquake, due to two possible major sources. These are (i) the process of subduction of the Indian plate beneath the Himalaya and (ii) some selected preceding moderate and small earthquakes in and around the region, during their post seismic phase. Our results indicate that the change due to the former source completely overshadows that due to the latter. The implication of our calculations is that the plate subduction processes in the Himalaya (i) actively promote large, moderate and small earthquake activity and (ii) also indirectly influence the regional seismicity through the occurrence of the past earthquakes.

On the aftershock sequence of a 4.6 mb earthquake of the Garhwal Himalaya

Locally recorded data for eighteen aftershocks of a magnitude(mb) 4.6 earthquake occurring near Ukhimath in the Garhwal Himalaya were analysed. A master event technique was adopted to locate seventeen individual aftershock hypocentres relative to the hypocentre of the eighteenth aftershock chosen as the master event. The aftershock epicentres define an approximately 30 km2 rupture zone commensurate with the magnitude of the earthquake. The distribution of epicentres within this zone and the limited amount of first motion data support the view that a group of parallel, sub-vertical, sinistral strike-slip faults oriented N46°, transverse to the regional NW-SE trend of the Garhwal Himalaya, was involved in this seismic episode. Since the estimated focal depth range for aftershocks of this sequence is 3–14 km, we infer that this transverse fault zone extends through the upper crustal layer to a depth of 14 km at least.

A simulation of earthquake induced undrained pore pressure changes with bearing on some soil liquefaction observations following the 2001 Bhuj earthquake

The Bhuj earthquake of January 26th, 2001, induced wide spread liquefaction within the Kachch peninsula. It has been pointed out that inundation due to soil liquefaction was short lived in some parts than in others in the affected region. Several geological, seismological and hydrological factors would have cumulatively contributed to these observed changes.We simulate in this article, undrained or short-term change in pore pressure in a poroelastic half space, in response to a simplified model of the Bhuj earthquake source. We find that the regions of relatively shorter lived inundation due to soil liquefaction may fall in the region where pore pressure responsible for soil liquefaction attributable to strong ground shaking was counteracted by pore pressure changes due to undrained poroelastic effect and vice versa.

Co-seismic spring flow changes attributed to the March 29, 1999 Chamoli earthquake of Garhwal Himalaya

The moderate magnitude Chamoli earthquake that occurred on March 29, 1999, in the Garhwal Higher Himalaya produced, among many other observable effects, changes in flow of several artisan springs. Qualitative observations of significant changes in the flow of ten springs located in regions of higher intensity show a strong spatial correlation with our preliminary estimates of perturbing pore pressure field induced in the water saturated shallow rocks of the region by the earthquake its coseismic phase. The results are significant for it is the first successful attempt in the Himalayan region to investigate the response pattern of the local groundwater flow system to perturbations induced to the ambient tectonic stress regime by a major earthquake.

Estimation of hypocentral parameters of local earthquakes when crustal layers have constantP-velocities and dipping interfaces

The paper describes an algorithm for estimating the hypocentral coordinates and origin time of local earthquakes when the wave speed model to be employed is a layered one with dipping interfaces. A constrained least-squared error problem has been solved using the penalty function approach, in conjunction with the sequential unconstrained optimization technique of Fiacco and McCormick. Joint confidence intervals for the computed parameters are estimated using the approach of Bard for nonlinear problems. These results show that when a hypocentre lies outside the array of recording stations and head waves from a dipping interface are involved, then its inclination must be taken into account for dip angles exceeding 5°.