The seismic hazard at Tehri dam

281

Tectonophysics, 218 (19931281-286 Elsevier Science Publishers B.V., Amsterdam

The seismic hazard at Tehri dam James N. Brune Se~rn~~gic~l Labor~~tory, U~uers~

of Nevada, Rena, NV 89577, USA

(Received May 29,1991; revised version accepted November 25, 1991)

ABSTRACT Bnme, J.N., 1993. The seismic hazard at Tehri dam. In: F. Lund (Editor), New Horizons in Strong Motion: Seismic Studies and Engineering Practice. Tectonophysics, 218: 281-286. The proposed 265 m high Tehri dam in India would lie in one of the most hazardous areas in the world with respect to earthquakes. A magnitude 8 or greater earthquake may be expected under the dam. Due to the unique tectonic setting and lack of detailed studies, little is known about the expected ground in motion, but indirect evidence suggests acceleration of over 1 g can be expected. In these circumstances the dam design should be subject to the most rigorous dynamic analysis.

Introduction Proposed construction of the 265 m high Tehri dam in a major Himalaya seismic gap capable of a M = 8.5 earthquake has highlighted the issues involved in predicting ground motion in this unique tectonic setting. In the initial stages of planning of the dam, engineers were not convinced of the validity of plate tectonic theory and the seismic gap theory and few of the detailed site investigations necessary for accurate prediction of ground motion were carried out. An early design for the dam was based on an effective peak horizontal acceleration of 0.25 g. The situation poses the following questions: (1) What is the appropriate peak ground acceleration for which the dam should be designed in an area of such extreme seismic hazard? (2) What studies should be carried out to make a more reliable estimate of the peak acceleration to be expected? The Tehri dam site is directly over the shallow dipping Main Boundary Thrust. Estimates of the distance from the site to the thrust vary from about 15 km to about 25 km, depending on the

Correspondence to: J.N. Brune, Seismological University of Nevada, Reno, NV 89577, USA.

0040-1951/93/$06.00

Laboratory,

dip. Study of waveforms of Himalaya earthquakes indicate shallow depths for moderate earthquakes, but the exact shape of the fault plane is not known, and there is the possibility of imbricate fault branches lying at shallower depths than the main fault, and thus nearer the dam. Strong motion attenuation curves for the region do not exist, since strong motion instruments have been in place for only a few years, and have not recorded enough earthquakes for statistical control of regression curves. A recent regression of existing world-wide strong motion data prepared by Dr. Kenneth Campbell (1989) for the Diablo Canyon Nuclear Power Plant seismic hazard study gives the 84th percentile vertical accelerations for large oblique and thrust earthquakes considerably over 1 g at a distance of 15 km and horizontal accelerations of somewhat over 1 g. However, regressions by other investigators give lower values, highlighting the un~e~ainties in the predictions and the lack of controlling data for large continental thrust earthquakes. Oldham (1899) gave graphic evidence that vertical accelerations of over 1 g occurred in the great Assam earthquake of 1897 and, although there has in the past been skepticism about his observations, they are now widely accepted as accurate, especially

0 1993 - Elsevier Science Publishers B.V. All rights reserved

282

since ground accelerations of over 1 g have been recorded on reliable modern instruments for a number of considerably smaller earthquakes. Given the above uncertainties, I believe that it is imperative to make full use of all methods available for placing constraints on the expected ground motion, including state-of-the-art studies of seismic sources in the region, study of attenuation properties of crust, and detailed studies of site effects which might effect the expected ground motion. Background From a geophysicist’s point of view the Himalayan region in the vicinity of Tehri dam is one of the most interesting on Earth, for a number of reasons: (1) It is the only place on Earth where two continents are crushing together at a high rate (about 5 cm/yr) and have been doing so for tens of millions of years. (2) The Himalayas are the highest mountain range in the world, a consequence of the intense effects of the collision, which has caused uplift of rocks at such a rate that erosion could not wear them down as it would in many other areas of the world. The Himalayas produce the greatest amount of erosional products of any region on Earth. These are delivered into the oceans, the Arabian Sea and the Bay of Bengal, by the Ganga, Brahmaputra and Indus rivers and produce the largest and most rapidly accumulating deposits of sediment in the world. (3) To keep the Indian plate moving into Asia, as it has for tens of millions of years, requires a tremendous expenditure of the energy which drives tectonic plates and a tremendous amount of energy is stored elastically in the crust at any given time. There is every reason to believe that the available rupture energies in the colliding continental structures of India and Asia are as high as anywhere on Earth. (4) The Himalayan thrust belt has, in historic times, produced some of the largest and most energetic earthquakes in the world. These great earthquakes can be expected to continue to occur indefinitely.

J.N. BKUNE

(5) A major seismic gap exists between the rupture zones of the 1905 and 1935 earthquakes (the Central Gap of Khattri and Tyagi, 19831, and this gap can be expected to be filled in the future with an earthquake of comparable great size. This gap includes the site of the proposed Tehri dam. A potential slip of considerably more than 10 m may have been stored up. (6) There is extreme danger not only from the major thrust fault, but also from numerous imbricate branching thrust faults which might rupture the crust much nearer the dam than the main fault itself, possibly producing peaks of ground acceleration even greater than produced by the rupture on the main thrust. (7) The Himalayan thrust belt produced the first clear evidence that the ground acceleration exceeded the acceleration of gravity, and objects were actually thrown into the air. This was documented by Oldham in his study of the great Assam earthquake of 1897. Although this evidence was considered with skepticism for many years, with the advent of modern strong motion instruments, which can reliably measure such strong accelerations, there is no longer any basis for skepticism, since we have recorded accelerations near and over 1 g for several large earthquakes. Considering the above points, we have to conclude that the proposed Tehri dam location is one of the most hazardous in the world from the point of earthquakes. There is little question that, in terms of the hazard rating of the International Commission on Large Dams (ICOLD), its hazard class is extreme. No large rockfill dam of the Tehri type has ever been tested by the shaking an earthquake in this area could produce and thus we have little basis for confidence as to how the dam would perform. Given the number of people who live downstream, the risk factor (in the ICOLD classification) is also extreme. In such circumstances ICOLD declares that a fully stateof-the-art dynamic design analysis of the dam, in response to specified acceleration time histories, is mandatory. Of course, we cannot predict with certainty what will occur during the lifetime of the Tehri dam. Further study might uncover reasons for

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lowering or raising the estimated hazard level. Estimates of ground acceleration and ground displacement should be evaluated from a probabilistic as well as a deterministic point of view. NevertheIess, in spite of major uncertainties in understanding the tectonics and seismology of the Tehri region, the above listed points seem strongly established, and there is no reason not to follow the recommendations of ICOLD for the case of extreme hazard. The Tehri dam seismic gap Twenty years ago there was still substantial debate about the basic tenets of plate tectonics: there no longer is. The well-known Soviet geologist V.V. Beloussov wrote his last fervent appeal against the hypothesis of ocean floor spreading in 1970. Almost all the substantive points raised by him in that article have subsequently been addressed, and results have turned out to further strengthen the arguments in favor of plate tectonics. The seismic gap hypothesis for indicating probable locations of future large earthquakes has evolved to be an intrinsic part of plate tectonic theory, even though it was originally proposed by Soviet and Japanese geophysicists Fedotov and Mogi before the main tenets of plate tectonics were firmly established. The seismic gap hypothesis states that, if a section of a long seismic belt has not ruptured recently; that is, if it is associated with a gap in major seismicity, and adjacent sections have had large earthquakes, the probabili~ of a large earthquake in the “gap” is proportionally increased. By 1988, thirteen identified circum-Pacific seismic gaps subsequentIy ruptured in large or great earthquakes (Nishenko, 1985). Although many engineers have been reluctant and late to accept the seismic gap hypothesis (up to and including the design years of Tehri dam), there can no longer be any doubt of its general validity. Use of the seismic gap theory led to the installation of the state-of-the-art digital array which recorded the 1985 Michoacan Earthquake (Anderson et al., 1986). There is also no longer any doubt that the Tehri dam site is located in a classical seismic

gap, between the great 1905 and 1935 Indian earthquakes (referred to as the Central Seismic Gap by Khattri and Tyagi, 1983). Thus, although we cannot precisely predict the time of the future gap-filling earthquake, there is little doubt that it will occur. Although the necessary neotectonic studies required to establish the average interval between large earthquakes on this section of the fault have not been carried out, we can certainly conchide that this earthquake might occur at any time in the next few hundred years, and has a high probability of occurring during the projected lifetime of the Tehri dam. Possibility of high ground accelerations, proaching or exceeding 1 g

ap-

Similar to the reluctance to accept the conclusions of plate tectonics, there has been a reiuctance among engineers, until recently, to accept the evidence that in the near field of large earthquakes, ground accelerations approaching or exceeding 1 g can occur. The graphic diagram of Oldham (18991, showing boulders thrown out of their seats without deforming the rims of the seats, during the great 1897 Assam earthquake was discounted with various subtle arguments. More recently, calculations based on our developing understanding of earthquake source physics were also discounted. However, with the advent of modern strong motion accelerographs capable of reliably recording ground accelerations in this range, the number of instrumental observations of ground accelerations of over 1 g began to accumulate. At first these observations were discounted or ascribed to special circumstances. However, it is now realized that accelerations of over 1 g can occur in the near field of only moderately large earthquakes, and are often to be expected in the near field of large earthquakes, depending on the particular local circumstances. AcceIerations of over 1 g have now been recorded for 6 earthquakes (Table 1). In this situation, for sensitive structures such the Tehri dam, the burden of proof has shifted onto those who would claim special circumstances preeluding such high accelerations. There are a number of circumstances which

2x4

TABLE

J.N. BRUNE

I

Earthquakes

with ground

motion

Place

exceeding Year

1g Motion (g)

Instrumental 1971

1.25

Gazli, USSR

1976

1.35

Imperial

Valley, Calif., USA

1979

Victoria,

B.C., Mexico

1987

> I.0

Nahanni,

N.W.T.,

Canada

1985

> 2.0

Calif., USA

1989

> 1.0

1897

> 1.0

San Fernando,

Loma Prieta,

Calif., USA.

I .75

Obserc>ationof Effects Assam,

India (Oldham,

1899)

suggest that accelerations near 1 g are likely for a major earthquake in the neighborhood of Tehri Dam: (1) Such high accelerations have occurred in an earlier Himalayan Thrust earthquake: the great 1897 Assam earthquake. (2) The high stresses thought to be required to keep the Indian subcontinent moving into Asia might indicate proportionately larger amounts of energy stored in the crust of the Earth waiting to be released during an earthquake. (3) There is currently debated evidence that continental thrust faults of the type associated with the Himalayan thrust are associated with larger stress changes (stress drops), and thus may generate higher ground accelerations than other types of earthquakes (e.g., the strike-slip earthquakes common in California, and normal faulting earthquakes common in Nevada). (4) Our current understanding of the effects of seismic wave attenuation, near-surface resonance and amplification, and the effects of focusing of energy by rupture propagation, all support the possibility of ground accelerations greater than 1 g from a large earthquake near the Tehri dam. (5) Recent statistical predictions based on extrapolations from the current data base of observations of ground accelerations, suggest that for earthquakes of magnitude 8 or above, as expected for the Tehri dam region, accelerations of over 1 g may be expected approximately 16% of the time (Cambell, 1989).

Comparison of the five points with respect to the 1985 Michoacan, Mexico, and the 1985 Chile earthquakes suggests that the probability of very high accelerations for these subduction zone earthquakes may be considerably less than for the Tehri dam region. Studies

required

to reduce uncertainties

in the

seismic hazard

The types of effects requiring study, in order to estimate seismic hazard at a particular site more accurately, can be divided into three types: source effects, propagation effects, and site effects. Source effects relate to the physical processes at the earthquake source. Propagation effects relate to the propagation of the seismic waves between the earthquake source and the site of the proposed structure (e.g., dam). Site effects refer to local structure, attenuation and impedance effects in the immediate vicinity of the structure (e.g., dam). The time required to obtain useful estimates of the above three types of effects is of the order of years for source effects, and a year for propagation and site effects. A brief discussion of each of these follows. Source effects

Earthquakes in a unique area such as the Himalayas may have unique source characteristics. The main parameters used to characterize seismic sources are the location and orientation of the rupture plane, the size of the rupture plane, and the average stress drop which occurs across the fault plane during the rupture. In addition, if the stress drop is not uniform in time, or not uniform across the rupture plane, we may have to distinguish the momentary dynamic stress drop from the average stress drop and distinguish local areas of greater than average stress drop (asperity stress drops), In general, the higher the stress drop during the earthquake, the higher the expected ground motion. Average stress drops during large earthquakes are of the order of 30 bar, but may be higher than 100 bar. Local asperity stress drops or local dynamic stress drops can exceed 1 kbar. There are very few detailed stud-

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DAM

ies of the source mechanism of earthquakes in the Himalayas using near-source data, but teleseismic data indicate the possibility of anomalously high stress drops, which might lead to higher accelerations than expected for other regions of the Earth. For example, Singh and Gupta (1980) inferred a stress drop of 275 bar for the 1934 Bihar-Nepal earthquake, much greater than typical values for large earthquakes (about 30 bar). Controversial data exist suggesting that thrust faults in general generate higher accelerations than other types of faults. On the other hand, subduction zone thrust earthquakes, such as the 1985 Michoacan, Mexico, earthqu~e, may have lower stress drops and generate lower accelerations. Chilean subduction thrust fault earthquakes may represent intermediate conditions. Heat flow data in the Himalaya region is very limited, and thus it is not known whether the depth to the most intense energy release is the same as that typical of the San Andreas fault (lo-15 km). Heat flow in subduction regions is generally reduced. It is possible that the fault below the dam lies in the depth range of most intense seismic energy release.

Propagation path effects

Propagation path effects include the geometric focussing and defocussing of energy, amplitude changes due to reflection and refraction of seismic waves, attenuation due to dissipation of seismic energy (Q) along the propagation path, and scattering of energy due to inhomogeneities along the propagation path. Detailed dete~ination of these effects are available from only a few specially studied areas in the world. Q values in the deeper part of the crust (deeper than a few km) are typically of the order of 1000 along the San Andreas fault. Shallow Q values are typicaliy of the order of lo-100 (in the upper kilometer of the crust). The attenuation properties of the crust in the Himalaya region are unknown. Most of the propagation path from the underlying fault to the surface would be at depths greater than a few kilometers, and thus the lithostatic pressure is expected to be high enough to close cracks tightly

and to reduce attenuation. The high seismic velocities in the underlying rocks suggests low attenuation. On the other hand, it is possible that intense fracturing of the rocks between the main thrust and the surface might lead to severe scattering of waves, thus reducing the high frequency energy arriving at the surface. A more precise determination of the Q and other propagation effects can be obtained in only a year or so using a modern digital seismic network, since the necessary analytical techniques have already been developed. Site effects

Once the energy from the source arrives at the region of the structure (e.g., dam), referred to as the site, it can be severely affected by attenuation and amplification caused by structures in the immediate vicinity of the site. These effects are referred to as site effects. Since Q is low in the vicinity of the Earth’s surface, especially when soft sediments such as alluvium are present, considerable attenuation can occur in only a few hundred meters of propagation. However, the effect of decreasing seismic impedance (primarily due to a decrease in shear wave velocity) can amplify the waves by more than a factor of two (in addition to the free surface amplification of a factor of two). Since these effects tend to counteract one another, the actual result of site effects needs to be determined for each site. Site effects have not been studied in detail in the Himalayas_ Due to the complex structure here, low velocity materials can lie at the surface immediately adjacent to materials with high velocities. Similarly, high velocity (low attenuation) materials may overthrust underlying low velocity (high attenuation) materials. Detailed studies of site effects along the San Andreas fault indicate that local site amplifications of greater than a factor of 2 can occur, but it is uncertain whether such conditions may be expected in the Himalayas. Fortunately, determination of site effects is relatively straightforward and can be accomplished by operating a digital seismic array at the

J.N. BRUNE

286

site, along with arrays of downhole seismometers (between the surface and a few hundred meters depth), for about 1 year. Additional testing should be made on rock samples taken from bore holes to constrain the values determined from seismic studies.

the crest, increasing the freeboard special design changes for the core.

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Given the circumstances cited above, there is no doubt in my mind that Tehri dam should be subjected the most rigorous state-of-the-art dynamic design analysis, and should be designed for peak ground accelerations of about 1 g, and correspondingly large ground velocities and displacements, exact values to be determined by a thorough analysis of the geologic and geophysical evidence available, as well as of the dam response. It is very likely that mitigating design changes will be required, including, decreasing the upstream and downstream slopes, widening

and making

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