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The role of the 1999 Chamoli earthquake in the formation of ground cracks
Journal of Asian Earth Sciences 22 (2004) 529–538 www.elsevier.com/locate/jseaes
The role of the 1999 Chamoli earthquake in the formation of ground cracks Irene Sarkar Department of Earth Sciences, Indian Institute of Technology, Roorkee 247 667, India Received 20 February 2001; revised 31 May 2002; accepted 25 February 2003
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 s3 (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. q 2003 Elsevier Ltd. All rights reserved. Keywords: Tensile ground cracks; Triggering stresses
1. Introduction The moderate magnitude (mb ¼ 6:3; MS ¼ 6:6; MW ¼ 6:4) Chamoli earthquake, which occurred on March 29, 1999, 05.35 UT, is the most recent, damaging earthquake in the Garhwal segment of the Himalayan tectonic zone (Fig. 1). According to estimates provided by the US Geological Survey, the parameters of hypocentral location are 30.58N, 79.408E, 15.0 km while those of the nodal planes are: NP1: strike ¼ 2828N, dip ¼ 98, slip ¼ 958; NP2: strike ¼ 978N, dip ¼ 818, slip ¼ 898. The earthquake caused intense havoc to the life and property of the local populace and also to the ground and mountain slopes of the Alaknanda – Mandakini river valley E-mail address: [email protected] (I. Sarkar). 1367-9120/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1367-9120(03)00093-2
and adjoining region (Fig. 1). Immediately after its occurrence, an extensive and systematic ground and satellite survey of the induced damage was conducted. The general details of that survey have been reported elsewhere (Sarkar and Saraf, 2000; Sarkar et al., 2001; Saraf and Sarkar, 2002). During the field survey, conspicuous surface cracks, several meters in length, a few centimeters in width and having indeterminate vertical extent, were also noted in the ground. These cracks were distinctly tensile in nature with only some minor slip component, at most of the order of a few millimeters, across their planes. Figs. 2 and 3 are two representative photographs of these cracks. Our main objective in this study is to assess the role of the Chamoli earthquake in their formation. The cracks had reportedly appeared in the ground during the occurrence of the Chamoli main shock. Hence these
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Fig. 1. A simplified geological map of the surveyed region adapted from Gansser (1964). The locations of the surveyed sites are shown with closed dots. Their names are denoted by abbreviations as follows: L-Lachauli, N-Nandaprayag, Cm-Chamoli, Gp-Gopeshwar, Gn-Gangalgaon, Gw-Gwar, D-Deoldhar, U-Ukhimath, Ch-Chandrapuri. The star ( p ) denotes the US Geological Survey location of the main shock epicentre.
must have formed due to near surface ground failure caused by the earthquake-induced ground vibrations that occurred during the dynamic phase of the perturbing stress field. Earthquake-induced dynamic stress changes in the ambient stress field is generally estimated from detailed knowledge of nucleation, initiation and termination stages of the heterogeneous rupture process of the main shock (see for example, Belardinelli et al., 1999; Voisin et al., 2000). Unfortunately such details for the Chamoli earthquake rupture process are not available with us. Hence to analyze and assess the role of the Chamoli earthquake in the formation of these cracks we use the theory of earthquakeinduced static stresses. We consider ground cracks observed at eleven selected sites, mostly situated within 15 km of the USGS located earthquake epicentre, where ground failure was relatively higher in intensity and of varied nature and estimate the earthquake-induced static stress changes there. These estimates are, in essence, the lower bound estimates of the stress perturbations provided by the Chamoli main earthquake, during the dynamic phase of its rupture process, for the formation of these cracks. In recent years, the possibility of second order stresses influencing first order stress fields of a region has received wide attention in the literature (e.g. Ellsworth et al., 1981; Stein and Lisowki, 1983; Johnston, 1989; Stein and Lisowki, 1983; Talwani and Rajendran, 1991; Zoback, 1992; Reasenberg and Simpson, 1992; Bodin and Gomberg, 1997; King et al., 1994; Harris, 1998; Belardinelli et al., 1999; Voisin et al., 2000; Toda and Stein, 2000; Ziv
and Rubin, 2000, etc.). Second order or local perturbations to the regional stresses may be associated with specific geologic and tectonic features e.g. lithospheric flexure, lateral contrasts in strength and density of earth material, deglaciation, continent collisions, large sedimentary loads, topography, pore-pressure variations (e.g. Hasegawa and Basham, 1989; Johnston, 1989; Talwani and Rajendran, 1991; Sonders, 1990; Zoback, 1992; Kalpna and Chander, 1997, etc.) and also with elastic strain energy released during occurrence of large or moderate magnitude earthquakes (e.g. Stein and Lisowki, 1983; Bodin and Gomberg, 1997; Belardinelli et al., 1999; Voisin et al., 2000, etc.). Several studies indicate that static and dynamic stress perturbations to the ambient stress field, induced by large and moderate earthquakes, can influence rates of aftershock activity (Stein and Lisowki, 1983; Gross and Kisslinger, 1997, etc.), back ground seismicity (Reasenberg and Simpson, 1992) and even occurrence of future earthquakes in nearby regions (Okada and Kasahara, 1990; Bodin and Gomberg, 1997; King et al., 1994; Singh et al., 1998, etc.). Although the magnitudes of these induced stress changes are small, often just a fraction of the total stress drop, these may be significant in terms of stress failure. For example, Stein and Lisowki (1983) used geodetic and local network data to show that the aftershocks of the 1979 Homestead earthquake sequence clustered on regions of the causative fault where the perturbed stresses were increased by 3 bar only. Toda and Stein (2000) suggest that, in the case of the 1992 Landers earthquake and also the 1998 Antarctica plate
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Estimates of stress changes generated by large natural earthquakes occurring in different parts of the world (in Africa, Chile, Italy, Japan, Mexico, Turkey, New Zealand, United States, etc.) have been used to identify locales and timings of future earthquakes (Harris, 1998). The Himalayan seismic belt is one of the more seismically active regions of the world where several large and moderate earthquakes have occurred in the recent past. Yet, while scientists endeavor to reliably estimate the state of stress in different segments of the Himalaya and identify possible locations of future hazardous earthquakes, few attempts have been made to calculate stress changes induced by these earthquakes. Mention may however be made of a study of aftershocks of the 1999 Chamoli earthquake whereby possible regions of stress shadow and stress trigger on the causative fault have been identified (Chander and Sharma, 2002). In this preliminary investigation of Chamoli earthquakeinduced ground cracks that we report here, minimal estimates of the dynamic stress perturbations induced by the earthquake in the Alkananda –Mandakini river valleys have been provided. Despite the approximation of analysis that we have adopted, in the context of studies pertaining to second order stress patterns in the Himalaya, we consider our effort significant.
Fig. 2. Photograph of one of several similar tensile cracks observed in the ground at Chamoli (Courtesy: Dr Kusala Rajendran, Centre of Earth Science Studies, Trivandrum, India).
earthquake, aftershocks occurred in regions where static Coulomb failure stress increases were of the order of 1 – 2 bar. However, from a study of earthquakes of magnitude $ 4.5, which occurred in Central California between 1969 and 1998, Ziv and Rubin (2000) conclude that in this region, static stress changes much less than 0.1 bar had a noticeable triggering effect.
Fig. 3. Photograph of one of several similar tensile cracks observed in the ground at Nandaprayag (Courtesy: Dr Kusala Rajendran, Centre of Earth Science Studies, Trivandrum, India).
2. Data We observed major, predominantly tensile cracks in the ground, at several places along the surveyed route (Fig. 4). However in this study we report and analyze the tensile fractures formed at eleven representative sites only where the magnitudes of these fractures were most prominently higher and varied. We have chosen to report observations from these sites only so as to highlight the comparison within the purview of our presented analysis in a concise and effective manner. Ten of these sites viz. Nandaprayag, Chamoli, Gopeshwar, Gwar, Deoldhar, Gangalgaon (situated in the Alkananda river valley), Birahi (situated in the Birahi river valley) and Chandrapuri and Ukhimath (situated in the Mandakini river valley), distributed over less than 200 km2 area (Fig. 1), are located within 15 km of the USGS located epicentre (Fig. 1). The eleventh site Lachauli, in the Alaknanda valley, is located more than 70 km away from the epicentre. While Chamoli, Gopeshwar, Gangalgaon, Deoldhar and Gwar lie within the meizoseismal zone where intensity VIII of the modified Mercalli scale has been assigned, (i) Nandaprayag, (ii) Birahi and Ukhimath, (iii) Chandrapuri and (iv) Lachauli lie within zones with assigned intensities VI, V, IV and III, respectively (Fig. 4) (Sarkar and Saraf, 2000; Sarkar et al., 2001; Saraf and Sarkar, 2002). During the survey we found that generally the magnitude of these cracks increased with increase in the assigned
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Fig. 4. The surveyed route (in dotted broken lines) and isosiesmal contours, adapted from Sarkar and Saraf (2000), Sarkar et al. (2001), are shown here. The major tectonic features are adapted from Gansser (1964). Closed circles show the locations of the surveyed sites. Their names are denoted by abbreviations as follows: L-Lachauli, N-Nandaprayag, Cm-Chamoli, Gp-Gopeshwar, Gn-Gangalgaon, Gw-Gwar, D-Deoldhar, U-Ukhimath, Ch-Chandrapuri.
intensity and were highest in regions nearer the epicentre. But there were notable exceptions. For example, although the assigned intensities were similar, the cracks in the ground at Birahi were of a distinctly minor nature compared to those at Ukhimath and Chandrapuri. Similarly Deoldhar is located in close proximity to Gwar and Gangalgaon, within the meisoseismal zone. But the magnitudes of the tensile fractures at Deoldhar were most spectacular. Despite these notable variations in epicentral distance, intensity of ground damage and magnitude of the ground cracks, we find that the results of our analysis are consistent at the different sites under consideration here in this study. The coordinates of location and general orientations of the cracks observed at the sites form the database of our analysis and are given in Table 1.
3. Method of analysis The following assumptions are made for the analysis: 1. The Chamoli earthquake occurred at the USGS estimated hypocentral location. 2. The causative fault (i) is rectangular, (ii) covers an area of 540 km2 (a value which is commensurate with the estimated magnitude), (iii) has dimensions 15 km in the down dip direction and 36 km in the strike direction and (iv) is oriented in a direction similar to that of the nodal plane NP1 of the USGS fault plane solution. 3. During the Chamoli earthquake process, slippage along the causative fault occurred in the direction of dip only. Closed analytical expressions for internal and surficial displacement and strain fields, due to different mechanisms
of slip on finite shear or tensile faults in an isotropic, semiinfinite earth medium, are available in literature (Mansinha and Smyllie, 1971; Okada, 1992). We use the expressions for a dip-slip strain field (Okada, 1992) to theoretically estimate the two-dimensional surficial perturbing strains at each of the eleven sites and also the principal strain components of the perturbing strain tensor there (e.g. Jaeger and Cook, 1969). Assuming that the earth medium is homogeneous and isotropic, the corresponding stress ðs1 ; s3 Þ field components are evaluated from generalized Hooke’s relations. The normal ðsÞ and shear ðtÞ components of the static perturbing stress field on the cracks at a particular site can be calculated from the following formulae.
s ¼ s3 sin2 u1 þ s1 cos2 u1 ; t ¼ ðs3 2 s1 Þsin u1 cos u1 ; Table 1 Database of analysis Location
Latitude (8N)
Longitude (8E)
General orientation of the cracks
Lachauli Nandaprayag Chamoli
30.17 30.34 30.41
78.84 79.31 79.34
Gopeshwar Gwar Deoldhar Gangalgaon Birahi Ukhimath Chandrapuri
30.41 30.44 30.43 30.43 30.40 30.60 30.52
79.32 79.31 79.30 79.32 79.40 79.18 79.13
N–S N–S N208W–S208E (in Lower Chamoli); NE–SW (in Upper Chamoli) NE–SW N–S N–S E–W NW– SE N208E –S208W N708E –S708W
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Table 2 Results of analysis Location
s1 ; s3 (in bar) of static perturbing stress field
Lachauli Nandaprayag Chamoli (Lower) Chamoli (Upper) Gopeshwar Gwar Deoldhar Gangalgaon Birahi Ukhimath Chandrapuri
s1 s1 s1 s1 s1 s1 s1 s1 s1 s1 s1
¼ 20:03; s3 ¼ 20:20 ¼ 22:77; s3 ¼ 28:28 ¼ 20:51; s3 ¼ 21:03 ¼ 20:51; s3 ¼ 21:03 ¼ 20:95; s3 ¼ 22:12 ¼ 21:21; s3 ¼ 25:24 ¼ 2:07; s3 ¼ 25:59 ¼ 0:65; s3 ¼ 22:54 ¼ 2:82; s3 ¼ 20:62 ¼ 2:71; s3 ¼ 20:60 ¼ 0:56; s3 ¼ 23:12
Direction of s3 of static perturbing stress field
s; t (in bar) on crack planes
Direction of t across crack planes
N548W N428E N408W N408W N178W N298W N288W N288W N508W N748E N648E
s ¼ 20:08; t ¼ 0:08 s ¼ 25:30; t ¼ 22:48 s ¼ 20:57; t ¼ 20:17 s ¼ 20:94; t ¼ 20:20 s ¼ 21:78; t ¼ 20:39 s ¼ 22:14; t ¼ 21:70 s ¼ 0:24; t ¼ 23:19 s ¼ 21:82; t ¼ 21:28 s ¼ 2:8; t ¼ 20:30 s ¼ 0:54; t ¼ 21:56 s ¼ 23:08; t ¼ 0:38
N N N208W NE NE N N E NW N208E N708E
where u1 denotes the angle of orientation of these cracks relative to the primary slip direction on the causative fault and is reckoned positive in the anticlockwise direction. The results of this analysis are detailed in Table 2.
4. Discussion of observations 4.1. Orientations of the ground cracks A noteworthy feature of the observed ground cracks is their wide range of orientation, even for those distributed over small areas. For example, at Nandaprayag, Chamoli, Gopeshwar, Gwar, Deoldhar, Gangalgaon and Birahi, which are located within 100 km2 area, the ground cracks were oriented in several directions viz. N –S, E – W, NE – SW and NW – SE. Similarly, although Ukhimath and Chandrapuri are located within 40 km2 area, the orientations of the ground cracks were NNE – SSW and ENE –WSW, respectively. We provide the following as an explanation for this phenomenon. Within the Lesser and Higher Garhwal Himalaya where these observation sites are located, large variations and complexity in the surrounding topography, geological structures and near surface geology is evident. The bent and buckled complex folded rock strata within the Alaknanda, Mandakini and Birahi valley exhibit large variations in strike and dip. Detailed geological mapping within the Alaknanda valley show that, while around Chamoli, Gopeshwar, Gwar, Deoldhar, Gangalgaon the rock strata generally dip between N108 and 358E, farther away at Nandaprayag and Lachauli, the dip varies between N258–458E and N308– 608E, respectively. Again while in the Birahi valley, close to Birahi, the dips of the folded beds vary between N458 and 608W, in the Mandakini valley these lie in the N158– 408E range (Valdiya, 1980). Also, in the overall region, numerous transverse lineaments and fracture traces with distinctly different trends and sense of
movements have been identified from different geological and geophysical studies. For example, in the Mandakini valley, several prominent N508E trending lineaments, exhibiting sinistral sense of displacement have been identified on satellite images (Jain, 1987). On the other hand, several lineaments trending NNE – SSW in the Alaknanda valley and NW – SE in the Birahi valley have been geologically mapped (Valdiya, 1980) and also identified on landsat imageries (A.K. Saraf, personal communication). Again, based on a local seismological study, a shallow, steeply dipping fault zone, oriented N468E, and exhibiting sinistral sense of slip has been recognized near Ukhimath (Sarkar et al., 1995). The alignment of the various tensile fractures that we observed in the Alaknanda, Mandakini and Birahi valley, especially those closer to the epicentre, seem to suggest that their orientations have been influenced, at least to some extent, by those of the outcropping rocks and the fracture lineaments. The elevations of the sites are also varied. For example, Deoldhar, Birahi are located at low elevation within steep valleys surrounded by extremely high topography. In contrast, the nearby sites Chamoli, Gopeshwar, Gwar, Gangalgaon are at higher elevations. Of the two sites in the Mandakini valley, compared to Ukhimath surrounded by steeply rising topography, Chandrapuri is located at very high elevation where the heights of the surrounding mountains appear to be far less in comparison. The local near surface geology is also distinctly varied. While Lachauli, Nandaprayag, Chamoli, Gopeshwar, Birahi, Gwar, Deoldhar and Gangalgaon, located in the Garhwal Lesser Himalaya, are situated within quarzites, slate and lime sequences, at Chandrapuri and Ukhimath in the Garhwal Higher Himalaya essentially schists, migmatites and gneisses are present. The Higher Himalayan crystalline rocks are separated from the Lesser Himalayan metamorphosed sedimentary rocks by the Main Central Thrust (MCT) and its splays. The south dipping North Almora Thrust separates the Proterozoic quartzite of inner
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Garhwal Lesser Himalaya from the predominantly argillocalcareous metasediments of middle Garhwal Lesser Himalaya (Gansser, 1964) (see Fig. 1). Although the exact location of the MCT in the region is liable to varied interpretation, it is evident at outcrop within a kilometer northeast of Gangalgaon and Gwar. The orientation and vector direction of displacement of the observed fractures in the ground at any particular site were controlled to a large extent by the local geology, topography, presence of fracture lineaments, etc. Hence, considering the marked variations in these parameters, the large range of orientation of the ground cracks that we observed even over small areas appears a natural consequence. Further, since the dynamic stress perturbations are affected by these local variations in the geological and tectonic features, the perturbing dynamic stress field in the Alkananda– Mandakini river valley and adjoining region may be expected to have been heterogeneous in nature during the occurrence of the Chamoli earthquake. A natural consequence of such a perturbing dynamic stress field is that during ground failure, cracks would form in various directions, even within small zones, similar to what we observed in the field during our survey. Ground fracturing was generally found to be more severe in the Inner and Middle Lesser Garhwal Himalaya amidst the Proterozoic quartzite and calcareous metasediments rather than in the Higher Garhwal Himalaya within the schist and gneisses. This could be due to various reasons such as the following. Mineralogy, grain size, porosity, etc. are intrinsic properties that control rock strength. In situ tests conducted on various rock types indicate that the static and dynamic modulus of elasticity of limestone and calcareous metasediments are far less (of the order of 2.0 £ 1011 dynes/cm2) compared to those of schists and gneisses (of the order of 10.0 £ 1011 dynes/cm2). The static and dynamic bulk modulus and Poisson’s ratio also is far less for the former type of rocks (Lama and Vutukuri, 1978). This could imply that, within our area of investigation, for failure, the weaker Lesser Himalayan rocks required much smaller perturbations in the ambient stress field than the comparatively stronger Higher Himalayan rocks. The following considerations may further be noted here. As in other parts of the Himalaya, the prevalent active tectonics in the Garhwal Himalaya is complex with high rates of plate convergence and strain accumulation, of the order of 10 ^ 5 mm/yr (Khattri, 1999) and 15 mm/yr (Peltzer and Saucier, 1996), respectively. Studies based on earthquake locations and fault plane solutions (e.g. Seeber and Armbruster, 1981; Ni and Barazangi, 1984; Gaur et al., 1985; Khattri et al., 1989; Molnar, 1990; Sarkar et al., 1993; Sarkar et al., 1995, etc.), gravity anomalies (Lyon-Caen and Molnar, 1985; Molnar, 1988), electrical conductivity distribution (Arora and Singh, 1992; Arora and Reddy, 1995) and geodetic leveling, triangulation and limited GPS observations (Chander, 1988, 1989; Gahalaut and Chander,
1992, 1997; Bilham et al., 1995; Bilham, 2001, etc.) also indicate the complexity of the ongoing geodynamic processes in this region. Due to the significant variations in the various geologic and topographic features within the region of our investigation noted above and also due to the overall complex active tectonics here, the ambient stress field here may be locally heterogeneous. However, on a broad scale, the ambient stress field in the Garhwal segment of the Central Himalayan tectonic zone is homogeneous, with SH max oriented N338E, generally perpendicular to the mountain ranges (Gowd et al., 1992). Our assumption of a homogeneous ambient stress field for the purpose of calculations (see Section 3 above) thus appears justified. Also in this scenario, we can safely assume that the perturbing dynamic stress field induced by the moderate magnitude (mb ¼ 6:3; MS ¼ 6:6; MW ¼ 6:4) Chamoli earthquake was heterogeneous to a small scale only and that the perturbed ambient field in the region of our investigation is overall homogeneous. 4.2. Tensile nature of the ground cracks In general, it appears more plausible that shear cracks will form in the ground during stress failure conditions due to a large or moderate earthquake. Yet most of the shallow ground cracks we investigated were prominently tensile in nature, significant in length and had negligible down slope movement. This implies that, during their formation, in the dynamic stage, when the cracking process was distinctly unstable and progressing, the s3 component of the perturbed failure stress field must have become predominantly tensile and directed normal to their orientation on the ground. Also at that time, the transverse stress components of the perturbed failure stress field became negligibly small (Bles and Feuga, 1986). In such a perturbed stress scenario, the general orientation of the tensile ground cracks formed at the various surveyed sites provide good estimates of the s3 direction of the tensile failure stress field locally existing there. We further suggest the following. If ðta ; sa Þ and ðtp ; sp Þ denote the components of the ambient and perturbing stress fields, resolved along and normal to the vertical crack planes, respectively, then during such ground failure in tension, ta and tp possibly became nearly equal in magnitude but acted in nearly opposite directions (see Fig. 5). Correspondingly, sa and sp became tensile and were related to s3f ; the magnitude of (tensile) s3 of the perturbed failure stress field, as
s3f ðtÞ ¼ lsa l þ lsp ðtÞl; here t denotes time and has been incorporated into the equation to suggest the transient nature of s3f and sp : We thus opine that the perturbing stress field induced by the Chamoli earthquake in its dynamic stage contributed to the formation of tensile ground cracks in the region by providing such suitable sp and tp :
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Fig. 5. ABCD represents the vertical plane of a tensile ground crack having large length, negligible width and indeterminate vertical extent. The possible orientation of the normal and shear components of the ambient ðsa ; ta Þ and Chamoli earthquake-induced perturbing ðsp ; tp Þ stress fields, across the vertical crack plane, at an arbitrary point S of the tensile crack, during formation on the ground, is schematically shown in the inset. The lengths of the arrows indicate the comparative magnitudes of the normal and shear components of the two stress fields.
5. Discussion of results Specific patterns in the variations of the estimated static perturbing stress field emerge from the results of our theoretical analysis (Table 2). We highlight and discuss these trends in the context of the observed ground cracks within different zones of the surveyed area. 5.1. Within the meisoseismal zone The surveyed sites located within this zone are Chamoli, Gopeshwar, Gwar, Gangalgaon and Deoldhar. The area lies generally within a geologically identified nappe, the Berinag nappe (Valdiya, 1980). Prominent anticlinally folded rocks, with NE –SW oriented axes, on either sides of the locally NE – SW flowing Alaknanda river valley are evident here (Valdiya, 1980). Chamoli, Gopeshwar, Gwar and Gangalgaon are located at comparable elevations, amidst comparable topography. In contrast, Deoldhar is located within a steep valley surrounded by very high topography. We note that, except Deoldhar, at the other four sites (viz. Chamoli, Gopeswar, Gwar, Gangalgaon) the component of the static perturbing stress field, resolved normal to the plane of the ground cracks ðsÞ; is tensile (see Table 2). This implies that here in the meisoseismal zone, the perturbing stress field was predominantly tensile in the static stage of the earthquake process. At Deoldhar situated within the valley, this tensile nature of the perturbing static stress field within the meisoseismal zone has been overcome by the large compressive stresses induced by the steep surrounding topography.
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We also find that at Chamoli, Gopeswar, Gwar and Gangalgaon the estimated magnitude of the shearing component ðtÞ of the static perturbing stress field, resolved along the crack planes are less significant than the estimated magnitude of the normal component ðsÞ (see Table 2). However these t values are not small enough to be completely ignored. We also find that along the cracks observed at Deoldhar, in comparison to s; the estimated magnitude of t is extremely high. Yet we did not notice any significant lateral displacements along any of the tension cracks at Deoldhar during our survey. We are of the opinion that in the dynamic stage of the earthquake rupture process, when these cracks were being formed, the magnitudes of the shear stresses ðtÞ along the crack planes were comparatively small. These values possibly increased in the static stage of the earthquake process. Induced static stress perturbations that are predominantly sheared in nature can cause reactivation of shear fractures and even major aftershock activity. In several regions within the valley around Deoldhar, such as Mawana and Sendhikhal, severe fracturing of the ground, intense ground damage and rock slides on the hill slopes were observed and noted during the survey (Sarkar and Saraf, 2000; Sarkar et al., 2001; Saraf and Sarkar, 2002). Also numerous large aftershocks have been located in and around the meisoseismal zone, especially in regions close to Chamoli and Gopeshwar, by US Geological Survey. Chander and Sharma (2002) have identified many of these aftershocks to occur within stress trigger zones of the causative fault of the Chamoli earthquake. 5.2. Within the Birahi river valley The village of Birahi is located within the river valley, at ground elevation less than 1000 m, amidst steep topography and severely folded rocks. Here, several NW – SE and NE – SW striking, steeply dipping prominent anticlinal and synclinal folds have been geologically mapped (Valdiya, 1980). It is worthy to note here that the tensional ground cracks observed at Birahi were far less significant than those that we observed at the other nine sites of investigation located close to the epicentre. Our theoretical analysis estimates the component of the static perturbing stress field, resolved normal to the plane of the ground cracks ðsÞ at Birahi to be distinctly compressive (see Table 2). Also the magnitude of the shearing component ðtÞ of the static perturbing stress field, resolved along the crack planes appears insignificant when compared to the magnitude of s: We note that within the Birahi valley there were no major shear fractures, rock slides or significant aftershock activity. The large compressive s and comparatively negligible t estimates on the tensional crack planes at Birahi may be attributed to the effect of large compressive stresses locally induced by the surrounding geology. It is also possible that these tensile fractures are secondary features that occurred after the primary features stabilized within the overall compressive stress regime.
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5.3. Within the Mandakini river valley Ukhimath and Chandrapuri are located within the valley, in the higher reaches of the northward flowing Mandakini river. Geological mapping of the prominent steeply dipping, N – S striking anticlinally folded rocks of this region (Valdiya, 1980) suggests that the river valley has possibly been carved out along the axis of a major anticline, due to the breaking and subsequent weathering of the rock strata near its crest. Compared to Ukhimath, which is surrounded on all directions by spectacularly towering hills, Chandrapuri village is located at a much higher elevation on relatively flat ground. Our calculations suggest that the component of the static perturbing stress field, resolved normal to the plane of the ground cracks ðsÞ; is prominently tensile at Chandrapuri but slightly compressive at Ukhimath (see Table 2). We prefer to suggest that in general the static perturbing stress field was tensile in the Mandakini valley; the small compressive nature of s at Ukhimath could be due to the surrounding topography. However, we observed no significant shear fractures, rock slides or lateral displacements along the ground cracks in regions in and around Ukhimath to explain the large estimates of t here. 5.4. Nandaprayag in the Alaknanda valley The computed results from Nandaprayag show that here also the earthquake-induced static perturbing stress field is distinctly tensile. A noteworthy feature is that the magnitudes of s and t; the normal and shearing component of the static perturbing stress field along the crack planes here, are the largest amongst similar estimates at other sites. Although the damage to the ground and several wellengineered structures at Nandaprayag was indeed intense, we did not observe any distinctiveness in this damage or in the near surface geology that can possibly explain the cause of these extremely high s and t estimates along the crack planes here. However we may mention that an analysis of digital remote sensing data from IRS-1B LISS-I has suggested that first order stress direction patterns near Nandaprayag and the region to its south are anomalous (Dash et al., 2000). 5.5. Lachauli in the Alaknanda valley Of all the sites of this study, Lachauli is the farthest, more than 70 km away from the epicentre. The nature of the tensile ground cracks here were far less in both length and width as compared to those observed at the other sites closer to the epicentral zone. The estimated magnitudes of s and t along these crack planes are extremely small and insignificant in comparison to those along ground cracks located at the other sites. Considering the large epicentral distance, this is expected. However, we note that our estimates suggest that, at Lachauli also, the perturbing stress field
was predominantly tensile in the static stage of the earthquake process. 5.6. Concluding remarks In our analysis, we have not addressed the possibility that some of these predominantly tensile ground cracks are primary features while several others are perhaps secondary features. However, considering the large linear dimensions on many of these major cracks, especially those that formed close to the epicentral zone, within the Alaknanda and Mandakini valley, we opine that here the process of primary ground cracking during the dynamic stage of the earthquake process was generally unstable. Such a consideration could imply that, just prior to that stage, the ground here was already close to failure in tension under the influence of local ambient stresses. To this, in the dynamic stage of the Chamoli earthquake process, suitable stress perturbations were induced to form these major primary tensile features. This earthquake-induced perturbing dynamic stress field was such that it could provide, across these vertical major primary tensile crack planes, (i) shear components, nearly equal in magnitude and opposite in direction, to those of shear components of the local ambient stress field and (ii) normal (tensile) components of such magnitude as was necessary for triggering tensile failure of the ground. Thus, in the resultant perturbed failure stress field, the transverse stress components became sufficiently insignificant while the minimum principal stress ðs3 Þ component became sufficiently tensile and thereby facilitated formation of the major primary tensile cracks in the ground at Lachauli, Nandaprayag, Chamoli, Gopeshwar, Gangalgaon, Gwar, Deoldhar, Ukhimath and Chandrapuri. The static estimates of the tensile s at Lachauli, Nandaprayag, Chamoli, Gopeshwar, Gangalgaon, Gwar and Chandrapuri are lower bound estimates of these crack-triggering dynamic stresses. The minimal perturbing dynamic stresses at Deoldhar and Ukhimath are of the same order as those at Gwar,Gangalgaon and Chandrapuri, respectively. The large compressive s across the comparatively minor natured tensile cracks at Birahi possibly suggest that these are subsidiary features that were formed in the later stages of the cracking process.
6. Conclusions The following are the conclusions of this study 1. Just prior to the occurrence of the Chamoli earthquake, the local ambient stress field in the Alkananda – Mandakini river valley region of Garhwal Himalaya was such that the ground there was near failure in tension. 2. To this, in the dynamic stage of the Chamoli earthquake process, a suitable perturbing stress field was induced. The earthquake-induced dynamic stress perturbations
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3.
4.
5.
6.
were such that, at each of these sites, across the vertical crack planes, (a) shear components nearly equal and opposite to similar components of the local ambient stress field and (b) normal (tensile) components of magnitude necessary for triggering tensile ground failure there, were provided. The minimum principal stress component ðs3 Þ of the resultant perturbed failure stress field thus became sufficiently tensile at these surveyed sites while the transverse stresses became sufficiently insignificant. This facilitated formation of major predominantly tensile cracks in the ground there. The observed orientation of the ground cracks, at each of these specific sites, provides estimates of the local direction of this s3 there. The static estimates of the tensile ðsÞ stress perturbations at the surveyed sites Lachauli, Nandaprayag, Chamoli, Gopeshwar, Deoldhar, Gangalgaon, Gwar, Ukhimath and Chandrapuri provide minimal estimates of the triggering tensile stresses induced there by the Chamoli earthquake during the dynamic stage of its rupture process. At Birahi, where the static perturbing stress is prominently compressive, the tensile crack features are subsidiary.
Acknowledgements I am indebted to Professor Ramesh Chander, Department of Earth Sciences, Indian Institute of Technology, Roorkee, for illuminating discussions and erudite comments during the research work. I thank the anonymous reviewers for their constructive criticisms on the manuscript.
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Sarkar, I., Chander, R., Chatterjee, D., 1995. On the aftershock sequence of a 4.6 mb earthquake of the Garhwal Himalaya. Proceedings of the Indian Academy of Sciences (Earth and Planetary Sciences) 104, 683 –691. Sarkar, I., Pachauri, A.K., Israil, M., 2001. On the damage caused by the Chamoli earthquake of March 29, 1999. Journal of Asian Earth Sciences 19, 129–133. Seeber, L., Armbruster, J.G., 1981. Great detachment earthquakes along the Himalayan arc and long term forecasting. In: Simpson, D.W., Richards, P.G. (Eds.), Earthquake Prediction: An International Review, Maurice Ewing Series, 4, AGU, Washington, DC, p. 215. Singh, S.K., Anderson, J.G., Rodriguez, M., 1998. Triggered seismicity in the Valley of Mexico from major earthquakes. Geophysical International 37, 3–15. Sonders, L.J., 1990. Effects of density contrasts on the orientation of stresses in the lithosphere: relation to principal stress directions in the Transverse ranges, California. Tectonics 9, 761–771. Stein, R.S., Liwoski, M., 1983. The 1979 Homestead Valley earthquake sequence, California, control of aftershock and postseismic deformation. Journal of Geophysical Research 88, 6477–6490. Talwani, P., Rajendran, K., 1991. Some seismological and geometric features of interplate earthquakes. Tectonophysics 186, 19–41. Toda, S., Stein, R.S., 2000. Did stress triggering cause the large off-fault aftershock of the 25 March 1998 MW ¼ 8:1 Antarctic plate earthquake? Geophysical Research Letters 27, 2301–2304. Valdiya, K.S., 1980. Geology of the Himalayas, Wadia Institute of Himalayan Geology, Dehra Dun, India. Voisin, C., Campillo, M., Ionescu, I.R., Cotton, F., Scotti, O., 2000. Dynamic versus static stress triggering and friction parameters: inferences from the November 23, 1980, Irpinia earthquake. Journal of Geophysical Research 106 (21), 647 (see also p. 659). Ziv, A., Rubin, R.S., 2000. Static stress transfer and earthquake triggering: no lower threshold in sight? Journal of Geophysical Research 105, 13631–13642. Zoback, M.L., 1992. First and second order patterns of stress in the lithosphere: the World Stress Map Project. Journal of Geophysical Research 97, 11703–11708.
The role of the 1999 Chamoli earthquake in the formation of ground cracks Irene Sarkar Department of Earth Sciences, Indian Institute of Technology, Roorkee 247 667, India Received 20 February 2001; revised 31 May 2002; accepted 25 February 2003
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 s3 (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. q 2003 Elsevier Ltd. All rights reserved. Keywords: Tensile ground cracks; Triggering stresses
1. Introduction The moderate magnitude (mb ¼ 6:3; MS ¼ 6:6; MW ¼ 6:4) Chamoli earthquake, which occurred on March 29, 1999, 05.35 UT, is the most recent, damaging earthquake in the Garhwal segment of the Himalayan tectonic zone (Fig. 1). According to estimates provided by the US Geological Survey, the parameters of hypocentral location are 30.58N, 79.408E, 15.0 km while those of the nodal planes are: NP1: strike ¼ 2828N, dip ¼ 98, slip ¼ 958; NP2: strike ¼ 978N, dip ¼ 818, slip ¼ 898. The earthquake caused intense havoc to the life and property of the local populace and also to the ground and mountain slopes of the Alaknanda – Mandakini river valley E-mail address: [email protected] (I. Sarkar). 1367-9120/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1367-9120(03)00093-2
and adjoining region (Fig. 1). Immediately after its occurrence, an extensive and systematic ground and satellite survey of the induced damage was conducted. The general details of that survey have been reported elsewhere (Sarkar and Saraf, 2000; Sarkar et al., 2001; Saraf and Sarkar, 2002). During the field survey, conspicuous surface cracks, several meters in length, a few centimeters in width and having indeterminate vertical extent, were also noted in the ground. These cracks were distinctly tensile in nature with only some minor slip component, at most of the order of a few millimeters, across their planes. Figs. 2 and 3 are two representative photographs of these cracks. Our main objective in this study is to assess the role of the Chamoli earthquake in their formation. The cracks had reportedly appeared in the ground during the occurrence of the Chamoli main shock. Hence these
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Fig. 1. A simplified geological map of the surveyed region adapted from Gansser (1964). The locations of the surveyed sites are shown with closed dots. Their names are denoted by abbreviations as follows: L-Lachauli, N-Nandaprayag, Cm-Chamoli, Gp-Gopeshwar, Gn-Gangalgaon, Gw-Gwar, D-Deoldhar, U-Ukhimath, Ch-Chandrapuri. The star ( p ) denotes the US Geological Survey location of the main shock epicentre.
must have formed due to near surface ground failure caused by the earthquake-induced ground vibrations that occurred during the dynamic phase of the perturbing stress field. Earthquake-induced dynamic stress changes in the ambient stress field is generally estimated from detailed knowledge of nucleation, initiation and termination stages of the heterogeneous rupture process of the main shock (see for example, Belardinelli et al., 1999; Voisin et al., 2000). Unfortunately such details for the Chamoli earthquake rupture process are not available with us. Hence to analyze and assess the role of the Chamoli earthquake in the formation of these cracks we use the theory of earthquakeinduced static stresses. We consider ground cracks observed at eleven selected sites, mostly situated within 15 km of the USGS located earthquake epicentre, where ground failure was relatively higher in intensity and of varied nature and estimate the earthquake-induced static stress changes there. These estimates are, in essence, the lower bound estimates of the stress perturbations provided by the Chamoli main earthquake, during the dynamic phase of its rupture process, for the formation of these cracks. In recent years, the possibility of second order stresses influencing first order stress fields of a region has received wide attention in the literature (e.g. Ellsworth et al., 1981; Stein and Lisowki, 1983; Johnston, 1989; Stein and Lisowki, 1983; Talwani and Rajendran, 1991; Zoback, 1992; Reasenberg and Simpson, 1992; Bodin and Gomberg, 1997; King et al., 1994; Harris, 1998; Belardinelli et al., 1999; Voisin et al., 2000; Toda and Stein, 2000; Ziv
and Rubin, 2000, etc.). Second order or local perturbations to the regional stresses may be associated with specific geologic and tectonic features e.g. lithospheric flexure, lateral contrasts in strength and density of earth material, deglaciation, continent collisions, large sedimentary loads, topography, pore-pressure variations (e.g. Hasegawa and Basham, 1989; Johnston, 1989; Talwani and Rajendran, 1991; Sonders, 1990; Zoback, 1992; Kalpna and Chander, 1997, etc.) and also with elastic strain energy released during occurrence of large or moderate magnitude earthquakes (e.g. Stein and Lisowki, 1983; Bodin and Gomberg, 1997; Belardinelli et al., 1999; Voisin et al., 2000, etc.). Several studies indicate that static and dynamic stress perturbations to the ambient stress field, induced by large and moderate earthquakes, can influence rates of aftershock activity (Stein and Lisowki, 1983; Gross and Kisslinger, 1997, etc.), back ground seismicity (Reasenberg and Simpson, 1992) and even occurrence of future earthquakes in nearby regions (Okada and Kasahara, 1990; Bodin and Gomberg, 1997; King et al., 1994; Singh et al., 1998, etc.). Although the magnitudes of these induced stress changes are small, often just a fraction of the total stress drop, these may be significant in terms of stress failure. For example, Stein and Lisowki (1983) used geodetic and local network data to show that the aftershocks of the 1979 Homestead earthquake sequence clustered on regions of the causative fault where the perturbed stresses were increased by 3 bar only. Toda and Stein (2000) suggest that, in the case of the 1992 Landers earthquake and also the 1998 Antarctica plate
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Estimates of stress changes generated by large natural earthquakes occurring in different parts of the world (in Africa, Chile, Italy, Japan, Mexico, Turkey, New Zealand, United States, etc.) have been used to identify locales and timings of future earthquakes (Harris, 1998). The Himalayan seismic belt is one of the more seismically active regions of the world where several large and moderate earthquakes have occurred in the recent past. Yet, while scientists endeavor to reliably estimate the state of stress in different segments of the Himalaya and identify possible locations of future hazardous earthquakes, few attempts have been made to calculate stress changes induced by these earthquakes. Mention may however be made of a study of aftershocks of the 1999 Chamoli earthquake whereby possible regions of stress shadow and stress trigger on the causative fault have been identified (Chander and Sharma, 2002). In this preliminary investigation of Chamoli earthquakeinduced ground cracks that we report here, minimal estimates of the dynamic stress perturbations induced by the earthquake in the Alkananda –Mandakini river valleys have been provided. Despite the approximation of analysis that we have adopted, in the context of studies pertaining to second order stress patterns in the Himalaya, we consider our effort significant.
Fig. 2. Photograph of one of several similar tensile cracks observed in the ground at Chamoli (Courtesy: Dr Kusala Rajendran, Centre of Earth Science Studies, Trivandrum, India).
earthquake, aftershocks occurred in regions where static Coulomb failure stress increases were of the order of 1 – 2 bar. However, from a study of earthquakes of magnitude $ 4.5, which occurred in Central California between 1969 and 1998, Ziv and Rubin (2000) conclude that in this region, static stress changes much less than 0.1 bar had a noticeable triggering effect.
Fig. 3. Photograph of one of several similar tensile cracks observed in the ground at Nandaprayag (Courtesy: Dr Kusala Rajendran, Centre of Earth Science Studies, Trivandrum, India).
2. Data We observed major, predominantly tensile cracks in the ground, at several places along the surveyed route (Fig. 4). However in this study we report and analyze the tensile fractures formed at eleven representative sites only where the magnitudes of these fractures were most prominently higher and varied. We have chosen to report observations from these sites only so as to highlight the comparison within the purview of our presented analysis in a concise and effective manner. Ten of these sites viz. Nandaprayag, Chamoli, Gopeshwar, Gwar, Deoldhar, Gangalgaon (situated in the Alkananda river valley), Birahi (situated in the Birahi river valley) and Chandrapuri and Ukhimath (situated in the Mandakini river valley), distributed over less than 200 km2 area (Fig. 1), are located within 15 km of the USGS located epicentre (Fig. 1). The eleventh site Lachauli, in the Alaknanda valley, is located more than 70 km away from the epicentre. While Chamoli, Gopeshwar, Gangalgaon, Deoldhar and Gwar lie within the meizoseismal zone where intensity VIII of the modified Mercalli scale has been assigned, (i) Nandaprayag, (ii) Birahi and Ukhimath, (iii) Chandrapuri and (iv) Lachauli lie within zones with assigned intensities VI, V, IV and III, respectively (Fig. 4) (Sarkar and Saraf, 2000; Sarkar et al., 2001; Saraf and Sarkar, 2002). During the survey we found that generally the magnitude of these cracks increased with increase in the assigned
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Fig. 4. The surveyed route (in dotted broken lines) and isosiesmal contours, adapted from Sarkar and Saraf (2000), Sarkar et al. (2001), are shown here. The major tectonic features are adapted from Gansser (1964). Closed circles show the locations of the surveyed sites. Their names are denoted by abbreviations as follows: L-Lachauli, N-Nandaprayag, Cm-Chamoli, Gp-Gopeshwar, Gn-Gangalgaon, Gw-Gwar, D-Deoldhar, U-Ukhimath, Ch-Chandrapuri.
intensity and were highest in regions nearer the epicentre. But there were notable exceptions. For example, although the assigned intensities were similar, the cracks in the ground at Birahi were of a distinctly minor nature compared to those at Ukhimath and Chandrapuri. Similarly Deoldhar is located in close proximity to Gwar and Gangalgaon, within the meisoseismal zone. But the magnitudes of the tensile fractures at Deoldhar were most spectacular. Despite these notable variations in epicentral distance, intensity of ground damage and magnitude of the ground cracks, we find that the results of our analysis are consistent at the different sites under consideration here in this study. The coordinates of location and general orientations of the cracks observed at the sites form the database of our analysis and are given in Table 1.
3. Method of analysis The following assumptions are made for the analysis: 1. The Chamoli earthquake occurred at the USGS estimated hypocentral location. 2. The causative fault (i) is rectangular, (ii) covers an area of 540 km2 (a value which is commensurate with the estimated magnitude), (iii) has dimensions 15 km in the down dip direction and 36 km in the strike direction and (iv) is oriented in a direction similar to that of the nodal plane NP1 of the USGS fault plane solution. 3. During the Chamoli earthquake process, slippage along the causative fault occurred in the direction of dip only. Closed analytical expressions for internal and surficial displacement and strain fields, due to different mechanisms
of slip on finite shear or tensile faults in an isotropic, semiinfinite earth medium, are available in literature (Mansinha and Smyllie, 1971; Okada, 1992). We use the expressions for a dip-slip strain field (Okada, 1992) to theoretically estimate the two-dimensional surficial perturbing strains at each of the eleven sites and also the principal strain components of the perturbing strain tensor there (e.g. Jaeger and Cook, 1969). Assuming that the earth medium is homogeneous and isotropic, the corresponding stress ðs1 ; s3 Þ field components are evaluated from generalized Hooke’s relations. The normal ðsÞ and shear ðtÞ components of the static perturbing stress field on the cracks at a particular site can be calculated from the following formulae.
s ¼ s3 sin2 u1 þ s1 cos2 u1 ; t ¼ ðs3 2 s1 Þsin u1 cos u1 ; Table 1 Database of analysis Location
Latitude (8N)
Longitude (8E)
General orientation of the cracks
Lachauli Nandaprayag Chamoli
30.17 30.34 30.41
78.84 79.31 79.34
Gopeshwar Gwar Deoldhar Gangalgaon Birahi Ukhimath Chandrapuri
30.41 30.44 30.43 30.43 30.40 30.60 30.52
79.32 79.31 79.30 79.32 79.40 79.18 79.13
N–S N–S N208W–S208E (in Lower Chamoli); NE–SW (in Upper Chamoli) NE–SW N–S N–S E–W NW– SE N208E –S208W N708E –S708W
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Table 2 Results of analysis Location
s1 ; s3 (in bar) of static perturbing stress field
Lachauli Nandaprayag Chamoli (Lower) Chamoli (Upper) Gopeshwar Gwar Deoldhar Gangalgaon Birahi Ukhimath Chandrapuri
s1 s1 s1 s1 s1 s1 s1 s1 s1 s1 s1
¼ 20:03; s3 ¼ 20:20 ¼ 22:77; s3 ¼ 28:28 ¼ 20:51; s3 ¼ 21:03 ¼ 20:51; s3 ¼ 21:03 ¼ 20:95; s3 ¼ 22:12 ¼ 21:21; s3 ¼ 25:24 ¼ 2:07; s3 ¼ 25:59 ¼ 0:65; s3 ¼ 22:54 ¼ 2:82; s3 ¼ 20:62 ¼ 2:71; s3 ¼ 20:60 ¼ 0:56; s3 ¼ 23:12
Direction of s3 of static perturbing stress field
s; t (in bar) on crack planes
Direction of t across crack planes
N548W N428E N408W N408W N178W N298W N288W N288W N508W N748E N648E
s ¼ 20:08; t ¼ 0:08 s ¼ 25:30; t ¼ 22:48 s ¼ 20:57; t ¼ 20:17 s ¼ 20:94; t ¼ 20:20 s ¼ 21:78; t ¼ 20:39 s ¼ 22:14; t ¼ 21:70 s ¼ 0:24; t ¼ 23:19 s ¼ 21:82; t ¼ 21:28 s ¼ 2:8; t ¼ 20:30 s ¼ 0:54; t ¼ 21:56 s ¼ 23:08; t ¼ 0:38
N N N208W NE NE N N E NW N208E N708E
where u1 denotes the angle of orientation of these cracks relative to the primary slip direction on the causative fault and is reckoned positive in the anticlockwise direction. The results of this analysis are detailed in Table 2.
4. Discussion of observations 4.1. Orientations of the ground cracks A noteworthy feature of the observed ground cracks is their wide range of orientation, even for those distributed over small areas. For example, at Nandaprayag, Chamoli, Gopeshwar, Gwar, Deoldhar, Gangalgaon and Birahi, which are located within 100 km2 area, the ground cracks were oriented in several directions viz. N –S, E – W, NE – SW and NW – SE. Similarly, although Ukhimath and Chandrapuri are located within 40 km2 area, the orientations of the ground cracks were NNE – SSW and ENE –WSW, respectively. We provide the following as an explanation for this phenomenon. Within the Lesser and Higher Garhwal Himalaya where these observation sites are located, large variations and complexity in the surrounding topography, geological structures and near surface geology is evident. The bent and buckled complex folded rock strata within the Alaknanda, Mandakini and Birahi valley exhibit large variations in strike and dip. Detailed geological mapping within the Alaknanda valley show that, while around Chamoli, Gopeshwar, Gwar, Deoldhar, Gangalgaon the rock strata generally dip between N108 and 358E, farther away at Nandaprayag and Lachauli, the dip varies between N258–458E and N308– 608E, respectively. Again while in the Birahi valley, close to Birahi, the dips of the folded beds vary between N458 and 608W, in the Mandakini valley these lie in the N158– 408E range (Valdiya, 1980). Also, in the overall region, numerous transverse lineaments and fracture traces with distinctly different trends and sense of
movements have been identified from different geological and geophysical studies. For example, in the Mandakini valley, several prominent N508E trending lineaments, exhibiting sinistral sense of displacement have been identified on satellite images (Jain, 1987). On the other hand, several lineaments trending NNE – SSW in the Alaknanda valley and NW – SE in the Birahi valley have been geologically mapped (Valdiya, 1980) and also identified on landsat imageries (A.K. Saraf, personal communication). Again, based on a local seismological study, a shallow, steeply dipping fault zone, oriented N468E, and exhibiting sinistral sense of slip has been recognized near Ukhimath (Sarkar et al., 1995). The alignment of the various tensile fractures that we observed in the Alaknanda, Mandakini and Birahi valley, especially those closer to the epicentre, seem to suggest that their orientations have been influenced, at least to some extent, by those of the outcropping rocks and the fracture lineaments. The elevations of the sites are also varied. For example, Deoldhar, Birahi are located at low elevation within steep valleys surrounded by extremely high topography. In contrast, the nearby sites Chamoli, Gopeshwar, Gwar, Gangalgaon are at higher elevations. Of the two sites in the Mandakini valley, compared to Ukhimath surrounded by steeply rising topography, Chandrapuri is located at very high elevation where the heights of the surrounding mountains appear to be far less in comparison. The local near surface geology is also distinctly varied. While Lachauli, Nandaprayag, Chamoli, Gopeshwar, Birahi, Gwar, Deoldhar and Gangalgaon, located in the Garhwal Lesser Himalaya, are situated within quarzites, slate and lime sequences, at Chandrapuri and Ukhimath in the Garhwal Higher Himalaya essentially schists, migmatites and gneisses are present. The Higher Himalayan crystalline rocks are separated from the Lesser Himalayan metamorphosed sedimentary rocks by the Main Central Thrust (MCT) and its splays. The south dipping North Almora Thrust separates the Proterozoic quartzite of inner
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Garhwal Lesser Himalaya from the predominantly argillocalcareous metasediments of middle Garhwal Lesser Himalaya (Gansser, 1964) (see Fig. 1). Although the exact location of the MCT in the region is liable to varied interpretation, it is evident at outcrop within a kilometer northeast of Gangalgaon and Gwar. The orientation and vector direction of displacement of the observed fractures in the ground at any particular site were controlled to a large extent by the local geology, topography, presence of fracture lineaments, etc. Hence, considering the marked variations in these parameters, the large range of orientation of the ground cracks that we observed even over small areas appears a natural consequence. Further, since the dynamic stress perturbations are affected by these local variations in the geological and tectonic features, the perturbing dynamic stress field in the Alkananda– Mandakini river valley and adjoining region may be expected to have been heterogeneous in nature during the occurrence of the Chamoli earthquake. A natural consequence of such a perturbing dynamic stress field is that during ground failure, cracks would form in various directions, even within small zones, similar to what we observed in the field during our survey. Ground fracturing was generally found to be more severe in the Inner and Middle Lesser Garhwal Himalaya amidst the Proterozoic quartzite and calcareous metasediments rather than in the Higher Garhwal Himalaya within the schist and gneisses. This could be due to various reasons such as the following. Mineralogy, grain size, porosity, etc. are intrinsic properties that control rock strength. In situ tests conducted on various rock types indicate that the static and dynamic modulus of elasticity of limestone and calcareous metasediments are far less (of the order of 2.0 £ 1011 dynes/cm2) compared to those of schists and gneisses (of the order of 10.0 £ 1011 dynes/cm2). The static and dynamic bulk modulus and Poisson’s ratio also is far less for the former type of rocks (Lama and Vutukuri, 1978). This could imply that, within our area of investigation, for failure, the weaker Lesser Himalayan rocks required much smaller perturbations in the ambient stress field than the comparatively stronger Higher Himalayan rocks. The following considerations may further be noted here. As in other parts of the Himalaya, the prevalent active tectonics in the Garhwal Himalaya is complex with high rates of plate convergence and strain accumulation, of the order of 10 ^ 5 mm/yr (Khattri, 1999) and 15 mm/yr (Peltzer and Saucier, 1996), respectively. Studies based on earthquake locations and fault plane solutions (e.g. Seeber and Armbruster, 1981; Ni and Barazangi, 1984; Gaur et al., 1985; Khattri et al., 1989; Molnar, 1990; Sarkar et al., 1993; Sarkar et al., 1995, etc.), gravity anomalies (Lyon-Caen and Molnar, 1985; Molnar, 1988), electrical conductivity distribution (Arora and Singh, 1992; Arora and Reddy, 1995) and geodetic leveling, triangulation and limited GPS observations (Chander, 1988, 1989; Gahalaut and Chander,
1992, 1997; Bilham et al., 1995; Bilham, 2001, etc.) also indicate the complexity of the ongoing geodynamic processes in this region. Due to the significant variations in the various geologic and topographic features within the region of our investigation noted above and also due to the overall complex active tectonics here, the ambient stress field here may be locally heterogeneous. However, on a broad scale, the ambient stress field in the Garhwal segment of the Central Himalayan tectonic zone is homogeneous, with SH max oriented N338E, generally perpendicular to the mountain ranges (Gowd et al., 1992). Our assumption of a homogeneous ambient stress field for the purpose of calculations (see Section 3 above) thus appears justified. Also in this scenario, we can safely assume that the perturbing dynamic stress field induced by the moderate magnitude (mb ¼ 6:3; MS ¼ 6:6; MW ¼ 6:4) Chamoli earthquake was heterogeneous to a small scale only and that the perturbed ambient field in the region of our investigation is overall homogeneous. 4.2. Tensile nature of the ground cracks In general, it appears more plausible that shear cracks will form in the ground during stress failure conditions due to a large or moderate earthquake. Yet most of the shallow ground cracks we investigated were prominently tensile in nature, significant in length and had negligible down slope movement. This implies that, during their formation, in the dynamic stage, when the cracking process was distinctly unstable and progressing, the s3 component of the perturbed failure stress field must have become predominantly tensile and directed normal to their orientation on the ground. Also at that time, the transverse stress components of the perturbed failure stress field became negligibly small (Bles and Feuga, 1986). In such a perturbed stress scenario, the general orientation of the tensile ground cracks formed at the various surveyed sites provide good estimates of the s3 direction of the tensile failure stress field locally existing there. We further suggest the following. If ðta ; sa Þ and ðtp ; sp Þ denote the components of the ambient and perturbing stress fields, resolved along and normal to the vertical crack planes, respectively, then during such ground failure in tension, ta and tp possibly became nearly equal in magnitude but acted in nearly opposite directions (see Fig. 5). Correspondingly, sa and sp became tensile and were related to s3f ; the magnitude of (tensile) s3 of the perturbed failure stress field, as
s3f ðtÞ ¼ lsa l þ lsp ðtÞl; here t denotes time and has been incorporated into the equation to suggest the transient nature of s3f and sp : We thus opine that the perturbing stress field induced by the Chamoli earthquake in its dynamic stage contributed to the formation of tensile ground cracks in the region by providing such suitable sp and tp :
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Fig. 5. ABCD represents the vertical plane of a tensile ground crack having large length, negligible width and indeterminate vertical extent. The possible orientation of the normal and shear components of the ambient ðsa ; ta Þ and Chamoli earthquake-induced perturbing ðsp ; tp Þ stress fields, across the vertical crack plane, at an arbitrary point S of the tensile crack, during formation on the ground, is schematically shown in the inset. The lengths of the arrows indicate the comparative magnitudes of the normal and shear components of the two stress fields.
5. Discussion of results Specific patterns in the variations of the estimated static perturbing stress field emerge from the results of our theoretical analysis (Table 2). We highlight and discuss these trends in the context of the observed ground cracks within different zones of the surveyed area. 5.1. Within the meisoseismal zone The surveyed sites located within this zone are Chamoli, Gopeshwar, Gwar, Gangalgaon and Deoldhar. The area lies generally within a geologically identified nappe, the Berinag nappe (Valdiya, 1980). Prominent anticlinally folded rocks, with NE –SW oriented axes, on either sides of the locally NE – SW flowing Alaknanda river valley are evident here (Valdiya, 1980). Chamoli, Gopeshwar, Gwar and Gangalgaon are located at comparable elevations, amidst comparable topography. In contrast, Deoldhar is located within a steep valley surrounded by very high topography. We note that, except Deoldhar, at the other four sites (viz. Chamoli, Gopeswar, Gwar, Gangalgaon) the component of the static perturbing stress field, resolved normal to the plane of the ground cracks ðsÞ; is tensile (see Table 2). This implies that here in the meisoseismal zone, the perturbing stress field was predominantly tensile in the static stage of the earthquake process. At Deoldhar situated within the valley, this tensile nature of the perturbing static stress field within the meisoseismal zone has been overcome by the large compressive stresses induced by the steep surrounding topography.
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We also find that at Chamoli, Gopeswar, Gwar and Gangalgaon the estimated magnitude of the shearing component ðtÞ of the static perturbing stress field, resolved along the crack planes are less significant than the estimated magnitude of the normal component ðsÞ (see Table 2). However these t values are not small enough to be completely ignored. We also find that along the cracks observed at Deoldhar, in comparison to s; the estimated magnitude of t is extremely high. Yet we did not notice any significant lateral displacements along any of the tension cracks at Deoldhar during our survey. We are of the opinion that in the dynamic stage of the earthquake rupture process, when these cracks were being formed, the magnitudes of the shear stresses ðtÞ along the crack planes were comparatively small. These values possibly increased in the static stage of the earthquake process. Induced static stress perturbations that are predominantly sheared in nature can cause reactivation of shear fractures and even major aftershock activity. In several regions within the valley around Deoldhar, such as Mawana and Sendhikhal, severe fracturing of the ground, intense ground damage and rock slides on the hill slopes were observed and noted during the survey (Sarkar and Saraf, 2000; Sarkar et al., 2001; Saraf and Sarkar, 2002). Also numerous large aftershocks have been located in and around the meisoseismal zone, especially in regions close to Chamoli and Gopeshwar, by US Geological Survey. Chander and Sharma (2002) have identified many of these aftershocks to occur within stress trigger zones of the causative fault of the Chamoli earthquake. 5.2. Within the Birahi river valley The village of Birahi is located within the river valley, at ground elevation less than 1000 m, amidst steep topography and severely folded rocks. Here, several NW – SE and NE – SW striking, steeply dipping prominent anticlinal and synclinal folds have been geologically mapped (Valdiya, 1980). It is worthy to note here that the tensional ground cracks observed at Birahi were far less significant than those that we observed at the other nine sites of investigation located close to the epicentre. Our theoretical analysis estimates the component of the static perturbing stress field, resolved normal to the plane of the ground cracks ðsÞ at Birahi to be distinctly compressive (see Table 2). Also the magnitude of the shearing component ðtÞ of the static perturbing stress field, resolved along the crack planes appears insignificant when compared to the magnitude of s: We note that within the Birahi valley there were no major shear fractures, rock slides or significant aftershock activity. The large compressive s and comparatively negligible t estimates on the tensional crack planes at Birahi may be attributed to the effect of large compressive stresses locally induced by the surrounding geology. It is also possible that these tensile fractures are secondary features that occurred after the primary features stabilized within the overall compressive stress regime.
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5.3. Within the Mandakini river valley Ukhimath and Chandrapuri are located within the valley, in the higher reaches of the northward flowing Mandakini river. Geological mapping of the prominent steeply dipping, N – S striking anticlinally folded rocks of this region (Valdiya, 1980) suggests that the river valley has possibly been carved out along the axis of a major anticline, due to the breaking and subsequent weathering of the rock strata near its crest. Compared to Ukhimath, which is surrounded on all directions by spectacularly towering hills, Chandrapuri village is located at a much higher elevation on relatively flat ground. Our calculations suggest that the component of the static perturbing stress field, resolved normal to the plane of the ground cracks ðsÞ; is prominently tensile at Chandrapuri but slightly compressive at Ukhimath (see Table 2). We prefer to suggest that in general the static perturbing stress field was tensile in the Mandakini valley; the small compressive nature of s at Ukhimath could be due to the surrounding topography. However, we observed no significant shear fractures, rock slides or lateral displacements along the ground cracks in regions in and around Ukhimath to explain the large estimates of t here. 5.4. Nandaprayag in the Alaknanda valley The computed results from Nandaprayag show that here also the earthquake-induced static perturbing stress field is distinctly tensile. A noteworthy feature is that the magnitudes of s and t; the normal and shearing component of the static perturbing stress field along the crack planes here, are the largest amongst similar estimates at other sites. Although the damage to the ground and several wellengineered structures at Nandaprayag was indeed intense, we did not observe any distinctiveness in this damage or in the near surface geology that can possibly explain the cause of these extremely high s and t estimates along the crack planes here. However we may mention that an analysis of digital remote sensing data from IRS-1B LISS-I has suggested that first order stress direction patterns near Nandaprayag and the region to its south are anomalous (Dash et al., 2000). 5.5. Lachauli in the Alaknanda valley Of all the sites of this study, Lachauli is the farthest, more than 70 km away from the epicentre. The nature of the tensile ground cracks here were far less in both length and width as compared to those observed at the other sites closer to the epicentral zone. The estimated magnitudes of s and t along these crack planes are extremely small and insignificant in comparison to those along ground cracks located at the other sites. Considering the large epicentral distance, this is expected. However, we note that our estimates suggest that, at Lachauli also, the perturbing stress field
was predominantly tensile in the static stage of the earthquake process. 5.6. Concluding remarks In our analysis, we have not addressed the possibility that some of these predominantly tensile ground cracks are primary features while several others are perhaps secondary features. However, considering the large linear dimensions on many of these major cracks, especially those that formed close to the epicentral zone, within the Alaknanda and Mandakini valley, we opine that here the process of primary ground cracking during the dynamic stage of the earthquake process was generally unstable. Such a consideration could imply that, just prior to that stage, the ground here was already close to failure in tension under the influence of local ambient stresses. To this, in the dynamic stage of the Chamoli earthquake process, suitable stress perturbations were induced to form these major primary tensile features. This earthquake-induced perturbing dynamic stress field was such that it could provide, across these vertical major primary tensile crack planes, (i) shear components, nearly equal in magnitude and opposite in direction, to those of shear components of the local ambient stress field and (ii) normal (tensile) components of such magnitude as was necessary for triggering tensile failure of the ground. Thus, in the resultant perturbed failure stress field, the transverse stress components became sufficiently insignificant while the minimum principal stress ðs3 Þ component became sufficiently tensile and thereby facilitated formation of the major primary tensile cracks in the ground at Lachauli, Nandaprayag, Chamoli, Gopeshwar, Gangalgaon, Gwar, Deoldhar, Ukhimath and Chandrapuri. The static estimates of the tensile s at Lachauli, Nandaprayag, Chamoli, Gopeshwar, Gangalgaon, Gwar and Chandrapuri are lower bound estimates of these crack-triggering dynamic stresses. The minimal perturbing dynamic stresses at Deoldhar and Ukhimath are of the same order as those at Gwar,Gangalgaon and Chandrapuri, respectively. The large compressive s across the comparatively minor natured tensile cracks at Birahi possibly suggest that these are subsidiary features that were formed in the later stages of the cracking process.
6. Conclusions The following are the conclusions of this study 1. Just prior to the occurrence of the Chamoli earthquake, the local ambient stress field in the Alkananda – Mandakini river valley region of Garhwal Himalaya was such that the ground there was near failure in tension. 2. To this, in the dynamic stage of the Chamoli earthquake process, a suitable perturbing stress field was induced. The earthquake-induced dynamic stress perturbations
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3.
4.
5.
6.
were such that, at each of these sites, across the vertical crack planes, (a) shear components nearly equal and opposite to similar components of the local ambient stress field and (b) normal (tensile) components of magnitude necessary for triggering tensile ground failure there, were provided. The minimum principal stress component ðs3 Þ of the resultant perturbed failure stress field thus became sufficiently tensile at these surveyed sites while the transverse stresses became sufficiently insignificant. This facilitated formation of major predominantly tensile cracks in the ground there. The observed orientation of the ground cracks, at each of these specific sites, provides estimates of the local direction of this s3 there. The static estimates of the tensile ðsÞ stress perturbations at the surveyed sites Lachauli, Nandaprayag, Chamoli, Gopeshwar, Deoldhar, Gangalgaon, Gwar, Ukhimath and Chandrapuri provide minimal estimates of the triggering tensile stresses induced there by the Chamoli earthquake during the dynamic stage of its rupture process. At Birahi, where the static perturbing stress is prominently compressive, the tensile crack features are subsidiary.
Acknowledgements I am indebted to Professor Ramesh Chander, Department of Earth Sciences, Indian Institute of Technology, Roorkee, for illuminating discussions and erudite comments during the research work. I thank the anonymous reviewers for their constructive criticisms on the manuscript.
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