Bulletin of the Seismological Society of America

Bulletin of the Seismological Society of America This copy is for distribution only by the authors of the article and their institutions in accordance with the Open Access Policy of the Seismological Society of America. For more information see the publications section of the SSA website at www.seismosoc.org

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Bulletinofthe SeismologicalSocietyofAmerica,Vol.69,No.3, pp. 773-823,June 1979 HISTORICAL AND MODERN SEISMICITY OF PAKISTAN, AFGHANISTAN, N O R T H W E S T E R N INDIA, AND SOUTHEASTERN IRAN BY R. C. QUITTMEYER AND K. H. JACOB ABSTRACT Both historical (noninstrumental) and modern (instrumental) data are compiled and critically reviewed to document the seismicity of Pakistan, Afghanistan, northwestern India, southeastern Iran, and neighboring areas. Earthquakes occurring between 1914 and 1965 are systematically relocated and magnitudes are determined for these events when possible. For some of the larger earthquakes, in both historical and modern times, the orientation and length of the rupture zone, and an approximate value of the seismic moment, are estimated. The usefulness of the documented seismicity to locate the sites of future large earthquakes in this part of the world is limited. The restricted historical record, the occurrence of earthquakes over wide zones (i.e., less confined than at oceanic subduction zones), and the long recurrence intervals combine to make the identification of seismic gaps, with a significant potential for rupture in large earthquakes, a difficult procedure. Seismicity variations prior to the great earthquake in the Makran region along the southern coast of Pakistan, in 1945, appear to be consistent with patterns identified before large earthquakes elsewhere in the world. Recent patterns of seismicity farther west along the Makran coast may be consistent with those for a zone in preparation for a large future earthquake; however, this observation is based on a limited amount of data.

INTRODUCTION The growing interest in continental tectonics in general, and in the seismicity and tectonics of south-central Asia in particular, has created a need for accurate knowledge of the historical and modern seismicity of that area. Such knowledge is necessary to understand the tectonic regime, to provide a solid basis upon which a systematic appraisal of seismic hazards can be founded, and to aid in the interpretation of microseismicity data from local networks. In this paper we provide a thorough examination of the documented seismicity for one portion of south-central Asia. The area covered includes Pakistan, Afghanistan, northwest India, southeast Iran and small portions of the USSR, People's Republic of China, and the Arabian Sea. This region is bounded by latitudes 20 ° and 38°N and longitudes 60 ° and 80°E. Previous studies of seismicity in the region considered are inadequate for one of several reasons. Some neglected the historical record of activity (e.g., Nowroozi, 1971); others were concerned with a region of only limited extent (e.g., Heuckroth and Karim, 1970); and several were completed many years ago (Oldham, 1882; de Montessus de Ballore, 1904; Milne, 1912). In this study a systematic and uniform presentation of all available data is given. Noninstrumental data are interpreted in terms of the Modified Mercalli {MM) Scale, while instrumental data are systematically employed to determine locations and magnitudes. It is then possible to identify persistent patterns of seismic activity, as well as long-term variations in the style of seismicity. In the following sections various terms will be used to describe the magnitude of an earthquake. These terms and the magnitude ranges associated with them are summarized in Table 1. 773

774

R. C. QUITTMEYER AND K. H. JACOB DATA: SOURCES AND ANALYSIS

The region throughout which the seismicity is studied is shown in Figure 1. Events from all depths are considered except in the Hindu Kush and Pamir regions. There, the enormous number of events occurring at intermediate depth and the detailed consideration given these events by others (e.g., BiUington et al., 1977), lead us to TABLE 1 TERMS DESCRIBINGEARTHQUAKESIZE Magnitude Term Range Small Moderate Large Great

M 6.0 M M

< 6.0 _-_ M < 7.0 _-> 7.0 _-> 7.8 N

~@~!f'-~ ~'~'--~JH ; PAKIS TAN j~' ~,~

~K

,J

INDIA

0~0o ~

SEA

~ L~~

AND'-I /

'

ARABA I ~~"?/0 ~LA~

OFZ

60ON~

~ 'DA ' ATE Ns 20ON

~

(

~

80ON

FIG. 1. Index map of the area considered in this study. Mapped surface faults from the following sources are shown: Bakr and Jackson (1964), Gansser (1964), Closs et al. (1969}, Weippert et al. (1970), Terman (1974), StScklin (1974), Desio (1974), and Molnar and Tapponnier (1975). The 2-km sea-depth contour is also shown. The filled circles in Iran and Pakistan respresent centers of Quaternary volcanism. Geographic features indicated are as follows: AR, Aravalli Range; CB, Cambay Basin; CF, Chaman fault; CH, Chagai hills; CR, Central Brahui Range; GF, Gardez fault; HR, Hazara Range; HF, Herat fault; HH, Harboi hills; HK, Hindu Kush region; HM, Himalayas; HS, Hazara-Kashmir syntaxis; K, Kirthar range proper; KF, Kunar fault; KR, Karakorum region; M, Makran region; MR, Murray Ridge; OF, OrnachNal fault; OFZ, Owen fracture zone; P, Pamirs; QTR, Quetta transverse ranges; RK, Rann of Kutch; S, Sulaiman Range; and SR, Salt Range. Several cities are indicated by filled triangles: HRT, Herat; HYD, Hyderabad; KAR, Karachi; KBL, Kabul; LAH, Lahore; NDI, New Delhi; QUE, Quetta; RWP, Rawalpindi. The inset in the lower right-hand corner shows the plate tectonic setting of the region studied;

eliminate from consideration events located at depths greater than 85 km. The specific areas affected by this action are Flinn-Engdahl regions 715 through 720 (Flinn and Engdahl, 1965). The data used in this study can be classified as belonging to one of two categories: historical (noninstrumental) or modern (instrumental}. Historical data consist of

H I S T O R I C A L AND M O D E R N

S E I S M I C I T Y OF S O U T H - C E N T R A L A S IA

775

accounts and felt reports that can be interpreted in terms of intensities. All data concerning events prior to 1900 are of this type. Modern data consist of hypocenters and instrumentally recorded arrival times from worldwide stations that are reported in various catalogs and journals. Historical data

To obtain the longest possible record of documented seismicity for the area of interest, historical references to earthquakes were compiled. The following sources form the basis for these historical data: Oldham (1882), Jones (1885a, b), Griesbach (1893), de Montessus de Ballore (1904), Middlemiss (1910), Heron (1911), Milne (1912), the International Seismological Summary (1918-1963), Oldham (1926), West (1934, 1935), Crookshank (1939), Auden (1949), the Bulletin of the Bureau Central International de S~ismologie (1954-1964), Banerji {1957), Heuckroth and Karim (1970), Gupta et al. (1972), and Ambraseys et al. (1975). Historical data from the portions of the USSR and China shown in Figure 1 are not considered in this study. TABLE 2 EQUIVALENCE AMONG VARIOUS SCALES OF INTENSITY USED IN THIS STUDY Intensity Class

Modified Mercalli Intensity

Milne I

II III Rossi Forel I

II III IV V VI VII VIII IX X

VI-VII VIII-IX IX-X+ I-II II III IV IV-V V-VI VI VII-VIII VIII-IX X-XII

The time period for which historical data are available ranges from about 25 A.D. through 1972. The length of this time period, however, is misleading; the completeness and uniformity of these data in both a temporal and spatial sense are probably poor. Nevertheless, references to large earthquakes, which are most important when considering regional tectonics, are apt to be more complete than the record considered as a whole. All historical data are interpreted in terms of intensity using the MM scale as given by Richter (1958). In a few instances the data are already in this form and are used directly. In most cases, however, an intensity is either assigned to a historical felt report or converted to the MM scale from some alternate scale of intensity. The values used to convert from one intensity scale to another are detailed in Table 2. For the Milne scale, the equivalent Modified Mercalli values are based on Milne's description of the severity of damage each of his intensity classes was intended to represent (Milne, 1912, pp. 653 to 654). The conversion for the Rossi-Forel scale is taken from Richter (1958, p. 651}. Original reports of damage, when available,

776

R. C. Q U I T T M E Y E R AND K. H. JACOB

are used to verify and, if possible, to make more precise the equivalences listed in Table 2. For those cases in which an intensity must be assigned to a historical felt report, a number of difficulties are encountered. Intensities often must be assigned on the basis of very limited descriptions, in some cases only a sentence or two. Frequently an earthquake is characterized by an intensity at only one location. In addition, the possibility of embellishment or exaggeration must always be considered; this is especially true of reports recorded second-hand a number of years after an earthquake occurred. Furthermore, the positions are not always known exactly for some ancient towns to which intensities are assigned. Keeping the above-mentioned limitations in mind, the assignment of intensities to the felt reports was done in as systematic a manner as possible. Descriptions of the construction methods and building materials used in the study area (Jones, 1885b; Heron, 1911; West, 1934) indicate that the buildings should be considered, in TABLE 3 DESCRIPTIVE PHRASES AND THE INTENSITIES ASSOCIATED WITH THEM Words

Slight Felt slightly Felt Sharp Strongly felt Strong earthquake Severe Very severe Slight damage Very violent Damaging Buildings damaged Walls fell Several buildingsdestroyed Heavy damage Town destroyed

Intensity (MM)

III IV V VI

VII VII-VIII VIII+

general, as belonging to Richter's (1958) masonry D category. For all reports in which construction type was not mentioned, it was assumed to be equivalent to masonry D. Also, key words (e.g., slight, felt, severe} occur frequently in the descriptions of the earthquakes. In the absence of conflicting evidence these words are interpreted to indicate the same intensity whenever they occur. Table 3 is a list of these words and the intensity with which each is associated. The data that have been compiled and interpreted are presented in Appendix 3. This appendix is a complete list of the localities and intensities associated with earthquakes for which noninstrumental data are available. The date, location, assigned intensity, source, and a qualitative rating of each datum's reliability are given in this list. A more selective list, containing only earthquakes associated with an intensity _->8 (MM), is given in Table 4 for convenience. The intensity data are also summarized in Figure 2, a map of maximum observed intensity. In this figure, the maximum documented intensity at each location is shown; or, in the case of some of the larger earthquakes, isoseismal lines are

HISTORICAL AND MODERN SEISMICITY OF SOUTH-CENTRAL ASIA

777

TABLE 4 DOCUMENTED EARTHQUAKESWITH INTENSITY~ 8 (MM) Date Intensity ApproximateLocation -25 A.D. 9-10 Taxila (33.7°N, 72.9°E) ~50 A.D. 8-9 Aikhanum (37.2°N, 69.5°E) 818-819 7-9 Balkh (36.8°N, 66.9°E) 848-849 7-9 Herat (34.4°N, 62.2°E) 893-894 8-10+ Daibul (Daipul) (24.8°N, 67.8°E) 1052-1053 7-9 Urgun (32.8°N, 69.1°E) 06 Jul 1505" 9-10 Paghman (34.6°N, 68.9°E) 03 May 1668 8-9 Samawani (Delta of the Indus) 04 Jun 1669 6-9 Fort Mandran (33.4°N, 73.2°E) 23 Jun 1669 8-9 Attock (33.9°N, 72.3°E) 15 Jul 1720 7-9 Delhi (28.5°N, 77.2°E) 01 Sep 1803t 8-9 Mathura (27.4°N, 77.7°E) 01 Sep 1803~ 7-9 Badrinath (30.7°N, 79.5°E) 1809 7-9 Garhwal (30.0°N, 79.0°E) 16 Jun 1819" 9-10+ Rann of Kutch (24.0°N, 69.0°E) 24 Sep 1827 8-9 Lahore (31.6°N, 74.4°E) 06 Jun 1828 9-10 Srinigar (34.1°N, 74.8°E) 1831 8-9 Daraban (31.8°N, 70.4°E) 22 Jan 18325 8-9 Kalifgan (36.9°N, 69.8°E) 21 Feb 18325 8-9 Badakhshan Province (37.3°N, 70.5°E) 19 Feb 1842" 8-9 Aligar Valley (34.8°N, 70.3°E) 19 Jun 1845 7-8 Lakhpat (23.8°N, 68.8°E) 24 Jan 1852 8 Mnrree Hills-Kahan (29.3°N, 68.9°E) 1862 8 Kohn Valley (29.9°N, 69.2°E) 11 Aug 1868 7-8 Peshawar (34.0°N, 71.6°E) 10 Nov 1868 7-8 Bannu (33.0°N, 70.6°E) Apr 1869 7-8 Peshawar (34.0°N, 71.6°E) 20 Dec 1869 7-8 Campbellpore (33.8°N, 72.3°E) 22 May 1871 7-8 Gilgit (35.9°N, 74.3°E) 15 Dec 1872 9-10 Lahri (29.2°N, 68.2°E) 18 Oct 1874 9 Jabal al Siraj (34.1°N, 69.2°E) Nov 1874 8-9 Kabul (34.5°N, 69.2°E) 12 Dec 1875 7-8 Hazara (33.5°N, 73.0°E) 02 Mar 1878 7-8 Kohat (33.6°N, 71.4°E) 02 Mar 1878 7-8 Peshawar (34.6°N, 71.6°E) 30 May 1885 8 Sopor (34.3°N, 74.5°E} 06 Jun 1885 9-10 Kashmir (34.0°N, 74.5°E) 28 Dec 1888 8-9 Quetta (30.2°N, 67.0°E) 1889 8 Jhalawan (27.8°N, 67.2°E) 20 Dec 1892" 8-9 Chaman (31.0°N, 66.4°E) 13 Feb 1893 8-9 Quetta (30.2°N, 67.0°E) 1900 8 Quetta-Pishin (30.4°N, 67.0°E) 04 Apr 1905" 10 Kangra (32.0°N, 76.3°E) 08 Jul 1909 7-8 Kalam (35.4°N, 72.7°E) 21 Oct 1909" 8-9 Kachhi Plain (29.0°N, 68.2°E) 02 Jun 1931 7-8 Pashghur (35.3°N, 69.4°E) 27 Aug 1931" 7-8 Mach (29.9°N, 67.3°E) 30 Mar 1934 8 Pashtunkot (35.9°N, 64.8°E) 30 May 1935" 9-10 Quetta (30.2°N, 67.0°E) 09 Sep 1937 7-8 Gulmarg (34.1°N, 74.4°E) 20 Oct 1937 7-8 Dehra Dun (30.3°N, 78.1°E) 07 Nov 1937 7-8 Srinigar (34.1°N, 74.8°E) 29 Sep 1941 8 Quetta (30.2°N, 67.0°E) 27 Nov 1945" 10+ Makran Coast (25.2°N~ 63.5°E) 10 Jul 1947 7-8 Bhadarwah (32.9°N, 75.8°E) 05 Aug 1947 8 Makran Coast (25.2°N, 63.5°E) 18 Feb 1955 7-8 Quetta (30.2°N, 67.0°E) 13 May 1956 8 Fort Munro (29.9°N, 69.9°E) 09 Jun 1956" 8-9 Sayghan (35.2°N, 67.7°E) 26 May 1959 7-8 Rustak (37.1°N, 69.8°E) 01 Aug 1966 7-8 Loralai (30.4°N, 68.6°E) 03 Sep 1972 6-8 Tangir-Darel Valley (36.3°N, 76.7°E) * These events are discussed in more detail in the text. t These two reports probably represent a single event. There is a good possibility these intensities are related to intermediate-depth Hindu Kush-Pamir events, but as this cannot be ascertained, they are included in this table for completeness.

778

R. C, QUITTMEYER AND K. H. JACOB

presented. It should be remembered that large intensifies at some locations, from earthquakes during gaps in the historical record, may be missing from Figure 2. Modern data

Beginning with the year 1914, instrumentally recorded arrival times, and hypocenters determined from these data, are readily available. They are collected in the regular bulletins of the British Association for the Advancement of Sciences, Seismological Committee; the International Seismological Summary (ISS), the Bureau International de S~ismologie (BIS), the International Seismological Centre 60"

8O" e

20"

20" 6v

80 a

Fro. 2. Map of maximum documented intensity (Modified MercaUi scale) at any given location. Data for the time period ~25 A.D. to 1972 are shown; however, mapped portions of the U.S.S.R. and China are not considered, and data from Iran are insufficient for quantitative description. Isoseismal lines (dotted where inferred) are plotted for some of the larger events. The year of occurrence for each such large event is indicated. The intensity value associated with a given isoseismal line is indicated in the box near each date. T h e fucst value given is for the innermost isoseismal line, etc. A few locations for which a documented intensity is known are not plotted so t h a t isoseismal lines will be more clearly visible. For complete intensity data, see Appendix 3. Isoseismal lines for the 1905 event are after Middiemiss (1910), those for the 1909 event are after Heron (1911), and those for the 1931, 1931b, and 1935 events are after West (1934, 1935). The 1931 and 1931b events are, respectively, the Sharigh and Mach earthquakes of Figure 6. The open triangles represent the same cities shown in Figure I.

(ISC), and various publications of U.S. government agencies (e.g., Earthquake Data Reports (EDR) and the U.S. Geological Survey epicenter data tape). Relocations. Events recorded in the time period 1914 through 1965, which meet certain criteria, were relocated using a computer program similar to the one described by Bolt (1960}. The criteria used to select events for relocation are: (1) at least 10 stations reported a P-arrival time (at least 5 stations prior to 1931), and (2) the event had a preliminary hypocentral location within the study area. After each event was relocated, it was given a qualitative grade from A (good) to D (no relocation possible). These grades are based upon the consistency of the data with the new hypocenter, the number of arrivals used in the relocation, and the distri-


779

bution of stations in azimuth and distance. Depths are usually not well constrained by station distribution or reported depth phases, and thus, in general little significance should be attached to them. The results of the relocations are shown in Figure 3 and are listed in Appendix 4. Figure 3 and Appendix 4 also include hypocenters determined from at least 10 P arrivals for the period January 1966 to April 1975, which were compiled from the ISC and EDR bulletins but not relocated. Table 5 is a list of the largest earthquakes that occurred from 1905 to 1975 in the region of interest. This list contains several events for which an instrumental

60*

20* 6~

80*

20 ° 80*

Fro. 3. Epieentral map of modern seismicity for the region studied. The filled squares are proportional

to magnitude and represent earthquakes recorded at teleseismic distances from January 1914 through April 1975. D-graded earthquakes are not shown. The symbol labeled UD in the magnitude scale denotes events with undetermined magnitude. Events from 1914 to 1964 are relocated. No distinction is made between surface-wave and body-wave magnitudes in this figure. For such distinction and additional information, see Appendix 4. Four large earthquakes (Ms > 7) t h a t occurred from 1905 to 1914 are represented by open circles.

magnitude is available, but that do not meet the criteria for a relocation attempt. These latter events are plotted as large circles in Figure 3. Magnitudes. An instrumental magnitude was determined for each earthquake for which sufficient data existed to do this. Gutenberg and Richter (1954) and Roth~ (1969) assigned magnitudes to many of the earthquakes considered in this study, and their values are used when available. For other earthquakes ground-displacement amplitudes reported in the station bulletins of Uppsala (UPP), DeBilt (DBN), Prague (PRA), and Granada (CRT), during the years 1914 to 1959, are used to compute magnitudes. Most magnitudes determined in this manner are based on data from only Uppsala and/or DeBilt. The magnitudes are computed from the displacement amplitudes reported for the

780

R. C. Q U I T T M E Y E R A N D K. H. J A C O B

vertical or horizontal c o m p o n e n t s of surface waves with periods in the range 14 to 24 sec. T h e f o r m u l a used, except for a m i n o r correction discussed below, is M = log ( A / T ) + 1.66 log h + 3.3

( K a r n i k et al., 1962)

w h e r e A is the a m p l i t u d e of ground d i s p l a c e m e n t in microns (zero to peak), T is the period in seconds, and h is the epicentral distance in degrees. Geller and K a n a m o r i (1977) point out t h a t for surface waves of 20-sec period, the m a g n i t u d e s d e t e r m i n e d f r o m the f o r m u l a of K a r n i k et al. (1962) are greater t h a n those d e t e r m i n e d f r o m the f o r m u l a of G u t e n b e r g (1945a) by an additive constant of 0.18. As m a n y of the m a g n i t u d e s compiled f r o m o t h e r sources for this s t u d y were d e t e r m i n e d using G u t e n b e r g ' s formula, m a g n i t u d e s d e t e r m i n e d from the a b o v e f o r m u l a were decreased b y 0.18 so c o m p a r i s o n s could be made. TABLE 5 LARGEMAGNITUDEEARTHQUAKESSINCE 1905 Approximate Epicentral Location

Date

04 21 20 01 06 24 27 13 30 27 05 09

Apr Oct Oct Jan Feb Aug Aug Jun May Nov Aug Jun

1905 1907 1909 1911 1914 1931 1931 1934 1935 1945 1947 1956

32.2°N, 38°N, 29.0°N, 38°N, 29.7°N, 30.4°N, 29.9°N, 27.7°N, 28.9°N, 25.2°N, 25.0°N, 35.I°N,

76.1°Et 69°E$ 68.2°Et 66°E$ 63.8°E 67.7°E 67.3°E 62.7°E 66.4°E 63.5°E 63.5°E 67.5°E

Magnitude* (Ms)

8.0 8.0 7.2 7.2 7.0 7.0 7.4 7.0 7.5 8.0§ 7.3 7.611

* Magnitudes are from Gutenberg and Richter (1954) unless otherwise noted. t Epicentral location estimated from intensity data. Epicentral location from Gutenberg and Richter. § Magnitude from Geller and Kanamori (1977). It PAS station magnitude. As a check on the validity of this p r o c e d u r e to d e t e r m i n e magnitudes, m a g n i t u d e s c o m p u t e d b y G u t e n b e r g a n d R i c h t e r {1954) were c o m p a r e d with those c o m p u t e d f r o m the U p p s a l a a n d DeBilt d a t a for those cases in which b o t h are available. T h e results are s h o w n in Figure 4. M o s t events lie within _+~ m a g n i t u d e unit of the line r e p r e s e n t i n g equality b e t w e e n the two scales. All of the events lying significantly outside this range are f r o m t h e s a m e geographic a r e a - - t h e M a k r a n coast of s o u t h e r n P a k i s t a n (Figure 1). T h e large discrepancies for these e a r t h q u a k e s m a y be related to s o m e typical source m e c h a n i s m or focal d e p t h effect, or to s o m e characteristic of t h e source area. I n fact, the M a k r a n region h a s r e c e n t l y b e e n i n t e r p r e t e d as an a r e a of active s u b d u c t i o n (e.g., F a r h o u d i and Karig, 1977; J a c o b and Q u i t t m e y e r , 1979), with s o m e r e c e n t events as deep as 80 kin. G u t e n b e r g and R i c h t e r (1954) assigned similar d e p t h s to two of t h e a n o m a l o u s events s h o w n in Figure 4. In a few cases w h e r e it was not possible to c o m p u t e surface-wave magnitudes, it


781

was possible to compute a body-wave magnitude. These values were determined using the formula M

--

log (A/T) + Ao

(Gutenberg, 1945b)

where Ao is an empirically determined epicenter-distance/hypocenter-depth factor, and A and T are as before. P-wave displacements with periods of 0.5 to 2.0 sec, as reported in the Uppsala bulletin, were used. For events after 1959, magnitudes are obtained from the earthquake data sources mentioned previously, and also from Roth~ (1969). Magnitudes are included in

'

I

'

I

'

o- UPPSALA

8.0 --

o - DE B I L T

I-,.J

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be,-

~o - -

r-io m

0 ~n o. o.

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0 m

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i

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7.0 MAGNITUDE AND RICHTER

1

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B.O (1954)

Fie. 4. Comparison of surface-wave magnitudes determined by Gutenberg and Richter (1954) with those computed in this study using reported ground-displacement amplitudes at Uppsala and DeBilt. Most events plot within +_.~ 1 magnitude unit from equivalence between the two scales. The box in the lower l e f t - h a n d corner represents the fact that Gutenberg and Richter (1954) used a single designation (class D) for all earthquakes in the magnitude range 5.3 to 5.9. The data points labeled "m" represent earthquakes that occurred in the Makran region of southern Pakistan. Several of these Makran events were reported as occurring at depths of 80 to 100 km by Gutenberg and Richter (1954).

Appendix 4. Th e type of magnitude (body- or surface-wave) and the source from which it was determined are indicated. When both a surface-wave and a body-wave magnitude are known, only the surface-wave magnitude is reported. LARGE EARTHQUAKES Large earthquakes are important in understanding the tectonic regime in a given region for they account for most of the seismically released portion of the total tectonic strain release. Often these large events are also responsible for severe damage over wide areas. It is thus important to learn as much as possible about those large shocks that occurred in the past.

782

R. C. QUITTMEYER AND K. H. JACOB

The quantity and quality of information that can be gathered concerning large earthquakes in the region studied here, varies considerably. For some events little is known other than isolated damage reports or an epicentral location and magnitude; others may have escaped documentation completely. Fortunately, however, the effects of a number of large earthquakes are more adequately described. In some cases it is even possible to estimate values for rupture length and seismic moment. In Appendix 1 some of these large events are discussed in detail. Pertinent information concerning them is summarized in Table 6. OBSERVATIONS OF R E G I O N A L SEISMICITY

Tectonic framework Although tectonic interpretation is not the main purpose of this paper, a presentation of the tectonic framework for the region studied will facilitate the discussion of the results. The dominant feature within this area is the extensive fold and thrust belt that extends from northwestern India to southern Pakistan (Figure 1). This feature, which includes the Himalayas, the Hazara and Salt Ranges, the Sulaiman Range, and the Kirthar ranges, has developed as a consequence of the collision between the continental portions of the Indian and Eurasian plates (e.g., Molnar and Tapponnier, 1975). Deformation along the northern collisional boundary (the Himalayas and the Hazara and Salt Ranges) has involved folding and thrusting of the upper crustal layers along the edge of the Indian plate and the development of extensive d6collement surfaces (Gansser, 1964; Seeber and Jacob, 1977; Seeber and Armbruster, 1979). Along the western collisional boundary (the Sulaiman and Kirthar ranges), in addition to folding and thrusting, evidence of a large amount of left-lateral shear is also observed (Abdel-Gawad, 1971; Hemphill and Kidwai, 1973). Thus, while the northern boundary is characterized by convergence, the western boundary appears to show relative movement in a left-lateral sense in addition to some convergence. Deformation also involves considerable portions of the Eurasian plate. The Karakorum, Pamir, and Hindu Kush ranges located along the southern edge of the Eurasian plate (Figure 1), are examples of such deformation. These mountain ranges occur to the north of the Indus suture line and its extensions, lines of ophiolitic material that mark the original zone of convergence between the Indian and Eurasian plates. At the margins of and within the Eurasian plate there are several major strikeslip features that are morphologically prominent. The Chaman fault in Pakistan and Afghanistan, the Herat fault in Afghanistan, and the Karakorum fault northeast of the Hazara-Kashmir syntaxis are all such features (Figure 1). Molnar and Tapponnier {1975) and Tapponnier and Molnar (1976, 1977) interpret these faults as equivalent to slip-lines produced in a rigid-plastic body {Eurasia) by a rigid indenter (India). The faults are interpreted as surfaces along which movement occurs as material situated to the north of the Indian indenter is squeezed out to the east and west. Thus, although not related directly to the deformation at a plate boundary in their interpretation, these features are a consequence of India's collision with Eurasia. To the west of the Indian-Eurasian plate collision, the Eurasian plate interacts with the Arabian plate. This interaction has produced an active zone of subduction in the Makran region of southern Pakistan and southeastern Iran (White and Klitgord, 1976; Farhoudi and Karig, 1977; Jacob and Quittmeyer, 1979). The oceanic

HISTORICAL

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MODERN

SEISMICITY

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SOUTH-CENTRAL

ASIA

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