Chamoli Earthquake Research Articles

Attenuation characteristics of Garwhal–Kumaun Himalayas from analysis of coda of local earthquakes

Coda of local earthquakes that occurred during 2006–2007 are used to study the attenuation characteristics of the Garhwal–Kumaun Himalayas. The coda attenuation characteristics are represented in terms of coda Q or Qc. It is observed that Qc increases with frequency. Qc also varies with increase in lapse time of coda waves. Qc increases up to an 85-s average lapse time. This is similar to observations around the world reported by many workers who have interpreted this as a manifestation of the fact that heterogeneity decreases with depth. However, around a 90-s average lapse time Qc is lower than its values for lower and higher average lapse times. This is interpreted as an indication of possible presence of a fluid-filled medium or a medium having partial melts at around a 160-km depth. Q0, i.e., Qc at 1 Hz, increases, and frequency parameter n decreases with increasing lapse time, barring around a 90-s lapse time. This again shows that in general, heterogeneity decreases with increasing depth. The Q0 and n values for smaller lapse times are similar to those for tectonically active areas. By comparing Qc values obtained in this study with those obtained by us using the 1999 Chamoli earthquake aftershocks, it is concluded that the crust is turbid and the mantle is more transparent. However, whether the variation in Qc values between 1999 and 2006–2007 is temporal or not cannot be definitely established from the available data set.

Impact of seismic factors on landslide susceptibility zonation: a case study in part of Indian Himalayas

Landslides are one of the most widespread natural hazards in high mountain terrains such as the Himalayas, which are one of the youngest tectonically and seismically active mountain ranges in the world. The crustal movements along the longitudinal thrusts and transverse faults give rise to earthquakes and in turn initiate landslides in the region. In fact, in addition to various static factors causing landslides, earthquakes are one of the major causes of landslides. It is thus imperative to incorporate seismic factor also while carrying out landslide susceptibility zonation map preparation in a seismically active areas like Garhwal Himalayas. In this paper, a study on the effect of earthquakes on landslide susceptibility zonation has been demonstrated by taking Chamoli earthquake as an example.

Attenuation of P- and S-waves in the Chamoli Region, Himalaya, India

The attenuation properties of the crust in the Chamoli region of Himalaya have been examined by estimating the frequency-dependent relationships of quality factors for P waves (Qα) and for S waves (Qβ) in the frequency range 1.5–24 Hz. The extended coda normalization method has been applied on the waveforms of 25 aftershocks of the 1999 Chamoli earthquake (M 6.4) recorded at five stations. The average value of Qα is found to be varied from 68 at 1.5 Hz to 588 at 24 Hz while it varies from 126 at 1.5 Hz to 868 at 24 Hz for Qβ. The estimated frequency-dependent relations for quality factors are Qα = (44 ± 1)f(0.82±.04) and Qβ = (87 ± 3)f(0.71±.03). The rate of increase of Q(f) for P and S waves in the Chamoli region is comparable with the other regions of the world. The ratio Qβ/Qα is greater than one in the region which along with the frequency dependence of quality factors indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of attenuation relation for S wave estimated here (Qβ = 87f0.71) with that of coda waves (Qc = 30f1.21) obtained by Mandalet al. (2001) for the same region shows that Qc > Qβ for higher frequencies (>8 Hz) in the region. This indicates a possible high frequency coda enrichment which suggests that the scattering attenuation significantly influences the attenuation of S waves at frequencies >8 Hz. This observation may be further investigated using multiple scattering models. The attenuation relations for quality factors obtained here may be used for the estimation of source parameters and near-source simulation of earthquake ground motion of the earthquakes, which in turn are required for the assessment of seismic hazard in the region.

Inversion of Seismogram Envelopes Using a Multiple Isotropic Scattering Model in Garhwal Himalaya

We estimated intrinsic attenuation ![Graphic][1] , scattering attenuation ![Graphic][2] , site amplification Z , and source energy S from inversion of three-component coda envelopes of the 1999 Chamoli earthquake of India for central frequencies 1.5, 3, 6, 12, and 24 Hz. The multiple isotropic scattering of S waves was numerically simulated by using a Monte Carlo method based on the radiative transfer theory. Isotropic sources and acoustic scattering in a full space were assumed. Adapting a grid search for scattering coefficient g and least-squares inversion for intrinsic attenuation parameter b , and source energy S , we inverted the observed envelopes of ten aftershocks ( M L≥3.5) in 1–24 Hz. Our results reveal that both ![Graphic][3] and ![Graphic][4] are weakly frequency dependent with the power-law forms of (0.006±0.004) f -(0.89±0.33) and (0.003±0.0005) f -(0.84±0.08), respectively. High scattering loss can be interpreted to be due to the presence of large lateral velocity heterogeneities in the crust. The total attenuation Q -1 decreases with frequency, taking the power-law form of (0.009±0.003) f -(0.87±0.19). The mean free path ranges from 30 to 300 km, with an average value of 100 km, and the intrinsic attenuation parameter b ranges from 0.01 to 0.05 sec-1, with an average value of 0.03 sec-1. Our estimates of source energy are in good agreement with the values obtained assuming an ω 2-source model. Site effects estimated using the fixed values of S k , b , and g exhibit less scatter, ranging from 0.73 to 2.54 with no significant frequency dependence consistent with the rock sites. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif

Soft-Sediment Deformation in Contemporary Reservoir Sediment: A Repository of Recent Major Earthquake Events in Garhwal Himalaya

Two discrete 10–20-cm-thick zones of soft-sediment deformation (SSD) structures in horizontal beds occur in a ∼2.35-m-thick sandbar in the Asan reservoir in the northwestern Sub-Himalaya. The sediment column shows uninterrupted fluviolacustrine deposition since 1975. The depositional and deformational features, boundary conditions, and sedimentation rate suggest that the discrete SSD features formed in an apparent reverse density gradient and were triggered by the 1991 Uttarkashi and 1999 Chamoli earthquakes. These triggering events occurred within 100–150 km of the reservoir, which lay in intensity zones V–VI. The observations correspond well to the empirical relationship between distances to a liquefaction zone from an epicenter. The highly liquefiable sediments in the Asan reservoir provide a unique depositional and tectonic setting to record SSD. Triggering of SSD by contemporary seismic events demonstrates the validity of interpretations of similar causes of SSD in older sediments. This is probably the first documented example of earthquake-triggered SSD from the seismically active Himalaya range.

Seismic hazard scenario and attenuation model of the Garhwal Himalaya using near-field synthesis from weak motion seismometry

In this paper, we present a seismic hazard scenario for the Garhwal region of the north-western Himalayan range, in terms of the horizontal Peak Ground Acceleration. The scenario earthquake of moment magnitude Mw 8.5 has a 10% exceedance probability over the next 50 years. These estimates, the first for the region, were calculated through a stepwise process based on: An estimation of the Maximum Credible Earthquake from the seismicity of the region and Global Seismic Hazard Assessment Program considerations, and four seismotectonic parameters abstracted from near field weak-motion data recorded at five stations installed in Chamoli District of the Garhwal region in the aftermath of the 1999 Chamoli earthquake. The latter include The frequency dependent power law for the shear wave quality factor, QS the site amplification at each station using horizontal-to-vertical-spectral ratio and generalized inversion technique source parameters of various events recorded by the array and application of the resulting relations between the scalar seismic moment M0 (dyne-cm) and moment magnitude Mw and the corner frequency, ƒc (Hz) and moment magnitude Mw to simulate spectral acceleration due to higher magnitude events corresponding to the estimated Maximum Credible Earthquake, and regional and site specific local spectral attenuation relations at different geometrically central frequencies in the low, moderate and high frequency bands.

Lapse-Time Dependence of Coda Q in the Source Region of the 1999 Chamoli Earthquake

Abstract In the present study the attenuation of seismic-wave energy in and around the source area of the Chamoli Earthquake of 29 March 1999 is estimated using aftershock data. Most of the analyzed events are from the vicinity of the main central thrust (MCT), which is a well-defined tectonic discontinuity in the Himalayas. The method of a single backscattering model is employed to calculate frequency dependent values of coda Q ( Q c ). A total of 30 aftershock events are used for Q c estimation at central frequencies 1.5, 3, 6, 9, 12, 18, and 24 Hz through five lapse-time windows from 10 to 50 sec starting at double the travel time of the S wave. The observed Q c is strongly dependent on frequency, which indicates that the region is seismically and tectonically active with high heterogeneities. The variation of Q c has also been estimated at different lapse times to observe its effect with depth. The variation of Q c with frequency and lapse time shows that the lithosphere becomes more homogeneous with depth. Q c -values for higher frequencies increase very fast with depth within about the top 63 km of the lithosphere and then become more or less constant beyond this depth. This indicates that turbidity at higher frequency decays very fast with depth, and the mantle may be transparent to high-frequency waves. The variation of Q c at 1.5 Hz with lapse time matches quite well with those predicted by Gusev (1995). However, the frequency parameter n in the relation Q c = Q 0 f n , where Q 0 = Q c at 1 Hz, does not follow the expected pattern given in his model. This could be due to faster depth decay of turbidity as mentioned previously.

Enveloping relief surfaces of landslide terrain

Two relief surfaces that envelop the rock fall region in a part of Garhwal Himalayas around Chamoli have been identified. Relative relief and absolute relief have been analyzed and the enveloping surfaces recorded at two levels of relief in the landscape. All landslide activity lies within these surfaces. The lower enveloping surface (800 m) dips due south by 7-8 degrees, due to an elevation rise of loo meters within 12 km from south to north, i.e., a gradient of 8 percent. The nature of the surface is smooth. The upper enveloping surface (> 2500 m) is almost parallel to the lower one but its surface is undulatory due to landslides and denudation. The area has been a seismically active region and has undergone seismic activity up until recently, as evidenced by the Chamoli earthquake Of 29(th) March 1999. The effects of earthquakes are seen at higher levels in the form of landslide imprints on the terrain.

Seismic microzonation of Dehradun City using geophysical and geotechnical characteristics in the upper 30 m of soil column

The understanding of geotechnical characteristics of near-surface material is of fundamental interest in seismic microzonation. Shear wave velocity (Vs), one of the most important soil properties for soil response modeling, has been evaluated through seismic profiling using the multichannel analysis of surface waves in the city of Dehradun situated along the foothills of northwest Himalaya. Fifty sites in the city have been investigated with survey lines between 72 and 96 m in length. Multiple 1-D and interpolated 2-D profiles have been generated up to a depth of 30–40 m. The Vs were used in the SHAKE2000 software in combination with seismic input motion of the recent Chamoli earthquake to obtain site response and amplification spectra. The estimated Vs are higher in the northern part of the study area (i.e., 200–700 m/s from the surface to a depth of about 30 m) as compared to the south and southwestern parts of the city (i.e., 180–400 m/s for the same depth range). The response spectra suggest that spectral acceleration values for two-story structures are three to eight times higher than peak ground acceleration at bedrock. The analysis also suggests peak amplification at 3–4, 2–2.5, and 1–1.5 Hz in the northern, central, and south-southwestern parts of the city, respectively. The spatial distributions of Vs and spectral accelerations provide valuable information for the seismic microzonation in different parts of the urban area of Dehradun.

Surface deformation and subsurface slip of the 28 March 1999 Mw = 6.4 west Himalayan Chamoli earthquake from InSAR analysis

We report InSAR observations of coseismic ground deformation caused by the Mw6.4 Chamoli earthquake of March 28, 1999 in the western Himalaya near the surface trace of the Main Central Thrust. Analysis of ERS1/2‐SAR data from ascending and descending tracks reveals surface deformation in a region 30 km by 40 km with a maximum coseismic uplift of ∼60 mm. Assuming that the rupture occurred as a planar uniform slip dislocation in an elastic half space reveals slip of 0.55 ± 0.1 m, strike of N300°E, 15° dip, with fault dimensions of 13 ± 3 km along‐strike and 10 ± 3 km downdip. The corresponding moment magnitude is Mw = 6.2, lower than that derived from seismological methods. With the exception of the recent 2005 Kashmir earthquake, the Chamoli earthquake studied in this paper is the only Himalayan earthquake for which surface deformation data are available from directly above the epicenter.

Use of Acceleration Spectra for Determining the Frequency-Dependent Attenuation Coefficient and Source Parameters

High-frequency strong-motion data of two recent major earthquakes of the Garhwal Himalayan region of India have been analyzed to determine the Qβ ( f ) for this region. Data from a local array of 24 stations, two main shocks, and several aftershocks were used to determine Qβ ( f ) and source parameters. An estimate of root mean square (rms) error is given. The data set in the present work includes the Uttarkashi earthquake of 20 October 1991 ( M s 7.0) and the Chamoli earthquake of 28 March 1999 ( M s 6.6) and its aftershocks. The source region of these earthquakes falls in the central seismic gap region of Himalayan frontal arc. We find the frequency-dependent Q -relationship for this region to be Qβ ( f ) = 112 f 0.97. The validity of the obtained Qβ ( f ) relationship is checked by comparing the source spectra at various stations with the theoretical spectra and found to be consistent. Low value of coefficient ( 0.8) in the Qβ ( f ) relationship suggest that the region is seismically and tectonically active, characterized by a large number of heterogeneities. The present inversion of strong-motion data gives the corner frequencies for the Uttarkashi and the Chamoli earthquake as 0.09 and 0.20 Hz, respectively, which correspond to the stress-drop values of 31 bars and 98 bars, respectively.

First Order Seismic Microzonation of Delhi, India Using Geographic Information System (GIS)

A first order seismic microzonation map of Delhi is prepared using five thematic layers viz., Peak Ground Acceleration (PGA) contour, different soil types at 6 m depth, geology, groundwater fluctuation and bedrock depth, integrated on GIS platform. The integration is performed following a pair-wise comparison of Analytical Hierarchy Process (AHP), wherein each thematic map is assigned weight in the 5-1 scale: depending on its contribution towards the seismic hazard. Following the AHP, the weightage assigned to each theme are: PGA (0.333), soil (0.266), geology (0.20), groundwater (0.133) and bedrock depth (0.066). The thematic vector layers are overlaid and integrated using GIS. On the microzonation theme, the Delhi region has been classified into four broad zones of vulnerability to the seismic hazard. They are very high (> 52%), high (38–52%), moderate (23–38%) and less ( < 23%) zones of seismic hazard. The “very high” seismic hazard zone is observed where the maximum PGA varies from 140 to 210 gal for a finite source model of Mw 8.5 in the central seismic gap. A site amplification study from local and regional earthquakes for Delhi region using Delhi Telemetry Network data shows a steeper site response gradient in the eastern side of the Yamuna fluvial deposits at 1.5 Hz. The ‘high’ seismic hazard zone occupies most of the study area where the PGA value ranges from 90 to 140 gal. The ‘moderate’ seismic hazard zone occurs on either side of the Delhi ridge with PGA value varying from 60 to 90 gal. The ‘less’ seismic hazard zone occurs in small patches distributed along the study area with the PGA value less than 60 gal. Site response studies, PGA distribution and destruction pattern of the Chamoli earthquake greatly corroborate the seismic hazard zones estimated through microzonation on GIS platform and also establishes the methodology incorporated in this study.

A Study of Source Parameters, Site Amplification Functions and Average Effective Shear Wave Quality Factor Qseff from Analysis of Accelerograms of the 1999 Chamoli Earthquake, Himalaya

The accelerograms of the 1999 Chamoli earthquake and nine of its aftershocks, which occurred in Uttaranchal Himalaya, have been analyzed to investigate their source parameters, the site amplification functions and the average effective shear-wave quality factor Qseff in the region. The fault plane solution of the main shock is obtained using the spectral amplitudes of SH waves (approximated by transverse components of accelerograms) of the high-energy packets observed in the accelerograms of the main shock. It is found to be comparable with the reported solutions in other studies. Similarly the other source parameters (viz., seismic moment = (5.03±1.7) × 1025 dyne-cm, stress drop = 65 bars, source duration = 5.2 s and moment magnitude = 6.4) estimated for the main shock are consistent with the values obtained in other studies. The stress drops estimated for the aftershocks vary from 23 bars to 153 bars and the seismic moment from 1.4 × 1023 dyne-cm to 2.9 × 1023 dyne-cm. The average estimated values of the effective shear-wave quality factor Qseff vary from 655±359 in the Uttaranchal sector of Himalaya and 1475±130 in the Delhi region. In general, the Qseff value increases with an increase in the epicentral distance reflecting the penetration of the waves into deeper layers of the crust as the epicentral distance of the observation point increases. These values of Qseff indicate that in general the curst is at low temperatures that will promote brittle behavior and conditions for episodic failure as compared to creep, under the accumulated strains from plate collision at the Himalaya plate boundary. The site amplification characteristics at sites have been identified from the frequency bands of significant amplification observed in the spectral ratios of the horizontal to the vertical component records. The decay of peak ground acceleration (PGA) values with distance has been investigated using the empirical regression curves vis-a-vis the site amplification factors.

Radon Precursory Signals for Some Earthquakes of Magnitude &gt; 5 Occurred in N-W Himalaya: An Overview

The N-W Himalaya was rocked by a few major and many minor earthquakes. Two major earthquakes in Garhwal Himalaya: Uttarkashi earthquake of magnitude Ms= 7.0 (mb = 6.6) on October 20, 1991 in Bhagirthi valley and Chamoli earthquake of Ms= 6.5 (mb = 6.8) on March 29, 1999 in the Alaknanda valley and one in Himachal Himalaya: Chamba earthquake of magnitude 5.1 on March 24, 1995 in Chamba region, were recorded during the last decade and correlated with radon anomalies. The helium anomaly for Chamoli earthquake was also recorded and the Helium/Radon ratio model was tested on it. The precursory nature of radon and helium anomalies is a strong indicator in favor of geochemical precursors for earthquake prediction and a preliminary test for the Helium/Radon ratio model.

Rupture parameters of the 1999 Chamoli earthquake in Garhwal Himalaya: Constraints from aftershocks and change in failure stress

A positive correlation between areas of increased coulomb stress changes, induced by an earthquake, and the spatial distribution in the occurrence of aftershocks have been reported in recent years. We consider such a correlation between the aftershocks of March 29, 1999 Chamoli earthquake (Ms 6.6) that occurred in the Garhwal Himalaya, India, and the mainshock induced coulomb stresses, to constrain some of the rupture parameters of the mainshock. Most of the aftershocks occurred above and near the updip edge of the rupture implying reactivation of pre-existing thrust fault of the Himalayan wedge. The 15 × 12 km 2 rupture appears to be lying above the detachment, the plate boundary fault between the underthrusting Indian shield rocks and the Himalayan wedge rocks, in the Basement Thrust Front (BTF) zone.

A Simplified Technique for Simulating Wide-band Strong Ground Motion for Two Recent Himalayan Earthquakes

A simplified technique for simulation of wide-band strong motion based on simple regression relations and Empirical Green’s Function (EGF) technique by Irikura (1986) is presented in this paper. The method uses the acceleration envelope as a shaping window for a filtered white Gaussian noise, to get the synthetic accelerogram from each subfault. Correction factors for slip of large and small events and transmission factors at each boundary of different layers are included in this synthetic accelerogram. The synthetic accelerogram obtained from each subfault is used as the Green’s function to get resultant records. Simulations are made for the confirmed models of the Uttarkashi and the Chamoli earthquakes at a number of stations to get wide-band strong ground motion. The comparison of synthetic with the observed records over a wide range of frequencies for two different Himalayan earthquakes establishes the efficacy of the present technique.

STRONG MOTION VERSUS WEAK MOTION: IMPLICATIONS FOR MICROZONATION STUDIES

A set of mainshock and aftershock data following the Chamoli earthquake of March 29, 1999, recorded at a single station viz. Gopeshwar, has been studied. Particularly, the utility of the use of aftershock/weak motion data for site characterisation in seismic microzonation studies is investigated. The analysis of aftershock and mainshock data indicates that the spectral shape and amplification is quite different during the main-shock and the aftershock. This, in turn, implies that the use of weak motion/aftershock records may lead to erroneous conclusions regarding the expected ground motion during a strong earthquake. Further, it has been shown that the site characteristics estimated from H/V ratios are not stable in the near field conditions, even for weak motion data.

Fractal Dimension of the 1999 Chamoli Earthquake from Aftershock Studies in Garhwal Himalaya

The Aftershock sequence of Chamoli earthquake (Mw 6.4) of 29 March 1999 is analyzed to study the fractal structure in space, time and magnitude distribution. The b value is found to be 0.63 less than which is usually observed worldwide and in the Himalayas. This indicates that the numbers of smaller earthquakes are relatively less than the larger ones. The spatial correlation is 1.64, indicating that events are approaching a two-dimensional region meaning that the aftershocks are uniformly distributed along the trend of the aftershock zone. Temporal correlation is 0.86 for aftershocks of M ≥ 1, indicating a nearly continuous aftershock activity. However, it is 0.5 for aftershocks of M ≥ 1.75, indicating a non continuous aftershock activity. From the assessment of slip on different faults it is inferred that 70% displacement is accommodated on the primary fault and the remainder on secondary faults.

Seismic Tomography Structure of the 1999 Chamoli Earthquake Source Area in the Garhwal Himalaya

Simultaneous inversion of about 5000 seismic phase data of 391 aftershocks of the 29 March 1999 Chamoli earthquake ( m b 6.3), recorded by 28 closely spaced temporary and permanent seismic stations, sheds new light on understanding the seismogenic structure of the source area. The best-located 160 aftershocks ( m d 2.0-4.0) with root mean square error less than 0.2 sec and epicentral and depth error less than 1 km are clustered at a depth range of 8-16 km, above the plane of detachment, the interface between the Himalayan sedimentary wedge and the Indian shield. The mainshock nucleated by thrust faulting at the “fault end,” where the Alakananda fault meets the plane of detachment at a depth of 15 km, and the aftershocks occurred by shear adjustment at the Alakananda fault. The 3D P - and S -wave velocity structures of the source area show an east-west-trending low-velocity zone (LVZ). The LVZ is comparable with the north-dipping or near-vertical Alakananda fault on the surface. It extends from the surface down to 15 km depth. It broadens in the north-south direction at shallower (0-10 km) depth and narrows at deeper depth (10-15 km). The P -wave tomography shows a high-velocity zone (HVZ) at the eastern margin of the LVZ. The mainshock and the aftershocks occurred at the contact of these two zones and within the HVZ. The S -wave velocity structure resembles that of the P wave, but it shows an LVZ at the mainshock source area at a depth of 15 km where the higher P -wave velocity structure (HVZ) is observed. The seismogenic source area is thus characterized by a higher V P/ V S or higher Poisson's ratio at the fault end. This anomaly may be due to fluid-filled fractured rock matrix, which might have contributed to the nucleation of the mainshock. Manuscript received 17 June 2002.

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

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.