Tectonophysics, 28 (1975) T19-T24 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Letter Section

Fault-plane solution and tectonic implications of the Pattan, Pakistan earthquake of December 28,1974

UMESH CHANDRA

Dames and Moore, Cranfor~, NJ. (USA.)

(Submitted May 21, 1975; revised version accepted Jufy 25, 1975)

ABSTRACT

Chandra, U., 1975. Fault-plane solution and tectonic implications of the Pattan, Pakistan earthquake of December 28, 1974. Tectonophysics, 28: Tl9-1’24.

A large destructive earthquake occurred on December 28, 1974 on the western bank of the Indus River near the village of Pattan. The earthquake reportedly killed 5,300 persons, injured 17,000 and left 60,000 peopie homeless. A seismicity map of the region is presented for the period January, 1963, to March, 1974 on a Mercator projection. Two main linear trends are recognized on the epicenter map. The northwest trend, beginning at 32.3”N, 76,6”E terminates at the southwest alignment of epicenters beginning at 36.O”N, 73.5% and ending at 33.O”N, ?f.O”E. The Pattan earthquake occurred near the junction of the two linear trends, A fault-plane solution for this earthquake has been determined from an analysis of teleseismic P-wave first-motion and S-wave polarization data. The strike and dip of the two nodal planes are N65”E, 68”SE and N50”E, 23”NW, respectively. The solution is compatible with and indicates underthrusting of the Indian plate in this region in the NNW direction along a thrust zone striking northeast.

INTRODUCTION

A destructive earthquake occurred on December 28, 1974 in a mountainous region in the Swat district of Pakistan. The earthquake killed about 5,300 people, injured 17,000 and left about 60,000 persons homeless. The worst affected area was the village of Pattan on the western bank of the Indus River, where nearly every house was demolished. Damage occurred in several villages on a 100 km strip near the Indus River from Pattan to Somar Nullah. The village of Jajal located at a distance of about 11 km from Pattan was severely affected.

The Karakorum Road, the region’s main artery, was blocked by rocks and landslides at several places along a llO-km stretch by the earthquake. A 40-km portion of the K~akorum Highway caved in. Inaccessibility of most of the earthquake affected region by road was a major problem in the distribution of relief supplies and nearly all the rescue work was done by army helicopters.

The hypocentral parameters determined by the U.S. Geological Survey are

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as follows:

Origin Time : 12h llm 43.8s Latitude : 35.1”N Longitude : 72.9”E Depth : 22km Magnitude : mb = 6.0, MS = 6.2

For an earthquake of this size the number of people killed was quite large. BSth (1968) introduced the notion of ‘specific destruction’, f, as a measure of the number of deaths, Nh, per unit seismic energy. It is related to the earth- quake magnitude, M, by the equation

f = log (& + 1) - 1.44 M + 12.82

For M = 6.2, it gives f = 7.6 for the Pattan earthquake. This value of ‘specific destruction’ is higher than those for all the seismically active regions reported by Bath (1968) and is next only to the Agadir, Morocco earthquake of February 29, 1960 (f = 8.5) in which a 5%magnitude earthquake killed 10,000 persons. The high concentration of several populated villages in the immediate vicinity of the epicenter, poor building structures (mostly huts and mud houses), sliding of loosened masses of rocks and boulders that plunged through villages, and the time of occurrence (5h 12m P.M.) of the earthquake on a cold winter evening when most people were indoors, un- doubtedly contributed to the great loss of human life.

This region has been subjected to a rather large number of destructive

34N

Fig. 1. Location of recent destructive earthquakes. The magnitude of each earthquake is shown in parentheses along with the name of the nearest locality. The number under the date gives an estimate of the number of people killed.

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earthquakes in the past. Figure 1 shows the location and other pertinent ~formation on the recent destructive e~hquakes for the period 1963-1974. The Kangra earthquake of 1905, which is the largest magnitude (8.6) earth- quake known to have occurred in this region, is also plotted. The Pattan earth- quake is located at a distance of about 115 km from the recent Gilgit earth- quake of September 3, 1972 in which 100 people were killed and 1,000 houses collapsed in the Tangin-Gulpis area. This earthquake is of concern also because it occurred at a distance of only about 110 km from the Tarbela Dam, a major construction project.

REGIONAL GEOLOGY, SEISMICITY AND TECTONICS

Although very little geological or tectonic information is available for the region close to the epicenter, it is known that the tectonics of the region is highly complicated. Southeast of the epicenter near the northwestern extremity of the Himalayas, the geological formations show a sharp deflection towards the south and begin to curve around towards the west. As one moves outward from the innermost part of the syntaxis, the geological formations consist of (youngest to oldest): (1) Siwalik (Upper Tertiary); (2) Murree ~Middle Tertiary); (3) Eocene and Cretaceous; (4) Triassic, Permian, and Carboniferous; (5) at some places Devonian, Silurian and Cambrian; (6) at places Dogra slate with unfossiliferous Palaeozoics; (7) Salkhala series (Pre- cambrian); and (8) gneiss and granitic rocks (Wadis, 1966).

Figure 2 shows a seismicity map of the region in the vicinity of the epi- center for the period January 1963 to March 1974, together with major tectonic features. The locations of the Main Boundary Thrust, the Main Central Thrust and the Counter Thrust have been taken from Valdiya (1973).

The Pattan earthquake is located at the northwest-trending linear alignment of the epicenters beginning at 32.3”N, 76.6”E and terminating just near the epicenter of this earthquake at 35.3”N, 728”E. Most of the earthquakes on this linear belt occur on the Main Central Thrust. Another ali~ment of epicenters, between 33.O”N, 71.O”E and 36.O”N, 73.5”E and which contains this earthquake, trends in the north-northeast direction and occurs very near and parallel to the Indus river. The northwestern Kashmir earthquakes of September, 1972 occurred at the northern end of this belt. Further northwest, a parallel north-northeast trending belt of earthquakes containing the Hindukush region occurs. There is a gap of about 100 km width in seismicity between these two parallel belts. A small east-west segment of earthquake epicenters between 72O”E and 73.2”E at a latitude of 33.8”N contains the western portion of the Main Central Thrust which curves towards the south and then west.

To determine the focal-mechanism solution for this earthquake, copies of

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‘4 ??C 71r ‘2k 7 3 E 74E 75E 76E 77E

‘)f 7 31 F 75L 76E

Fig.2. Seismicity map for the period January, 1963 to March, 1974, on a Mercator projection. The distribution of earthquake epicenters in the shaded part of the map, which corresponds to the Hindu Kush region, is very dense.

N N

2‘8 12 19711 35 1N 72 YE

Fig.3. P-wave first-motion and S-wave polarization data together with the P-nodal planes on an equal-area projection of the lower focal hemisphere. The diamond symbols indicate P, T, X- and Y-axes of the mechanism solution. The B-axis is determined by the inter- section of the two nodal planes shown by solid lines. Squares indicate compressional first-motion, and the crosses dilatational first-motion data. S-wave polarization directions are shown by small line segments.

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seismograms were requested from selected seismographic stations of the worldwide standardized stations (WWSSN), Canadian network and some other stations. P-wave first-motion directions were read from these seismo- grams and S-wave polarization angles were determined from the measure- ment of amplitudes on the N-S and E-W component records. In some cases in which copies of seismograms were not available, the P-wave first- motion data provided by the stations were used. Out of 43 P-wave first-motion directions thus obtained, 39 were compressions and the rest were all dilata- tions. S-wave polarization angles were determined at nine stations.

The focal-mechanism solution was determined by using the method described by Chandra (1971) and is plotted in Fig. 3. A compression at Hyderabad is inconsistent with the solution. The seismograms of this statibn were not seen by the author, but the data received indicated clear compression on long-period vertical records and doubtful dilatation on short-period vertical records. The standard deviation of S-wave polarization observations with respect to the solution shown in Fig. 3 is 20.5”. The orientation of the pressure, P, intermediate, B, tension, Z’, X- and Y-axes giving trend, measured clockwise from the north, and plunge, measured from the horizontal, is:

P B T X Y

Trend 151 243 345 140 335 Plunge 23 5 67 67 22

Two nodal planes are determined in the focal-mechanism solution, one of which is the fault plane and the othe’r the auxiliary plane. The seismic data in general do not distinguish between the two. Additional considerations based on geology and general tectonics of the region, alignment of the epicenters on a seismicity map, aftershock distribution etc., must be applied to identify which of the two nodal planes is most likely the fault plane. The epicenter of this earthquake is located on the southwestern flank of the Himalayan syntaxis. The general structural trend in this region is northeast. On a large scale, current plate-tectonics thinking suggests movement of the Indian plate towards the north. The nodal plane dipping gently towards the northwest is, therefore, suggested as the fault plane. The pole of the other nodal plane gives the direction of the slip vector. Because the focal mechanism gives a thrust solution, central quadrant compressional, we conclude that this earth- quake occurred along a thrust zone striking northeast and is related to the underthrusting of the Indian plate in the north-northeast direction in this region.

ACKNOWLEDGMENTS

I wish to express my gratitude to the directors of all the seismographic

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stations, who made their seismograms/data available for this study. My thanks are also due to Jim McWhorter and Carl Spiker for reading the manuscript and making helpful suggestions. Professor M. Bith brought to my attention the concept of ‘specific destruction’. This research was carried out as part of an Earth Sciences Research and Development Project of Dames and Moore.

REFERENCES

BHth, M., 1968. Earthquakes, Large, Destructive. In: S.K. Runcorn (Editor), International Dictionary of Geophysics. Pergamon Press.

Chandra, U., 1971. Combination of P- and S-data for the determination of earthquake focal mechanism. Bull. Seismol. Sot. Am., 61; 1655-1673.

Valdiya, KS., 1973. Tectonic framework of India: A review and interpretation of recent structural and tectonic studies. Geophys. Res. Bull., 11: 79-114.

Wadia, D.N., 1966. Geology of India. MacMillan, London, 3rd revised ed.