Patterns of Historical Earthquake Rupture in the Iranian Plateau

Patterns of Historical Earthquake Rupture in the Iranian Plateau 121historical interaction of earthquakes in the Iranian plateau isthe subject of this article. The southern plate-boundary marginof Iran is discussed separately by Berberian (1995a) forthe Zagros Mountains and by Byrne et al. (1992) for theMakran Ranges of both Iran and Pakistan.In earthquake hazard analysis, there has been a tendencyto treat each fault as a separate entity for probabilistic forecasting,thereby considering a potential earthquake on agiven fault as unrelated to other faults. Yet there is increasingevidence that neighboring faults, even those with differingsenses of displacement, are interactive in sequences of earthquakesor in multiple events that are part of a single earthquake.For example, the 1927 Tango, Japan, and 1930Tanna, Japan, earthquakes involved simultaneous surfacerupture on both left-lateral and fight-lateral strike-slip faults(Yamasaki and Tada, 1928; Ihara and Ishii, 1932; TFTRG,1983). The 1957 Gobi-Altai, Mongolia, earthquake involvedsimultaneous rupture of reverse-slip and left-lateral strikeslipfaults, and in the 1967 Mogod, Mongolia, earthquakes,rupture on a strike-slip fault was followed 15 days later byrupture on a reverse fault (Baljinnyam et al., 1993; Bayarsayhanet al., 1996). The best-known interactive earthquakesequence occurred from 1939 to 1967 in Turkey, when adjacentsegments of the North Anatolian fault, some separatedby large en-echelon stepovers, ruptured from east to west(Ambraseys, 1970; Barka, 1992). Stein et al. (1992), Harrisand Simpson (1996), and Harris et al. (1995) have suggestedthat interaction is to be expected. Rupture on one fault loadsadjacent faults and brings them closer to Coulomb failure.The Iranian plateau, bounded by the Turan platform onthe north and the Zagros collision zone and Makran subductionzone with the Arabian plate on the south, is a collageof different fragments of Gondwanaland that accreted to themargins of Eurasia during several collisional orogenies (Berberian,1981, 1983a). Continental convergence of about 35mm/yr in a N-S to N15°E direction between Arabian andEurasia (DeMets et al., 1990) is taken up in part by distributeddeformation in a diffuse zone across the Iranian plateaumore than 1000 km across (McKenzie, 1972; Berberian,1981; Jackson and McKenzie, 1984; Jestin et al., 1994; Jacksonet al., 1995; Fig. 1). Active deformation includes intercontinentalshortening and thickening in most parts of theplateau, subduction of the oceanic crust of the Arabian plateunder the Makran of southeast Iran, and strike-slip and reversefaulting elsewhere in the plateau.The active deformation is not uniformly distributed, andno single fault accommodates a large percentage of plateconvergence. Seismicity is mainly concentrated beneath theZagros active thin-skinned fold-thrust belt in the southwest,the Kopeh Dagh active thin-skinned fold belt in the northeast,partly in Turkmenistan, the Alborz thrust belt borderingoceanic crust of the South Caspian Depression, and the CentralIranian thick-skinned range-and-basin province (cf.Yeats et al., 1997) with reverse-slip and strike-slip earthquakes(Fig. 1). Oblique convergence between Arabia andEurasia is partitioned into shortening perpendicular to strikeof folds and reverse faults, right-lateral strike slip on NW-SE- to N-S-striking faults in the northwestern, western, andeastern parts of the plateau, and left-lateral strike slip on E-W-striking faults in the east and northwest (Jackson, 1992;Berberian et al., 1992; Fig. 1).Active faults have not been mapped and trenched in Iranto the degree that they have in more developed countries.But Iran has an advantage lacking in most regions; an abundantand diverse archaeological record and a long periodduring which written records were kept. These may be combinedwith geological and seismological evidence to correlatemany historical earthquakes with their source faults.The concentration of population in restricted fertileregions does not allow a uniform treatment of Iranian seismicityover the past 2500 yr. However, regional case historiesare possible. Here we present seven case histories andconsider their implications for hazard analysis elsewhere.One of the case histories includes the capital megacity ofTehran, where 12 million people live, one-fifth of the populationof Iran.Methods"For two farsangs [-12km] between Nauzad and Maskvillages [located along the Nauzad active fault zone] theground was fissured to such a depth that the bottom ofthe fissures was invisible ..." (Esfezari, 1493, describingthe 1493.01.10 Nauzad earthquake of IoM-- 7.0. The first known description of a surface ruptureaccompanying an earthquake by a contemporaryobserver in Iran).Accurate evaluation of reports of historical earthquakesand the attribution of these earthquakes to specific faults isan important goal in studies of active tectonics. Most earthquakecatalogs and reports concerning Iranian historicalearthquakes have not considered adequately the Iranian writtenand archaeological record and have not been able to relatelarge-magnitude historical earthquakes to mapped activefaults (for Iran, see Ambraseys and Melville, 1982; fornorthern Iran and southern Caucasus, see Kondorskaya andShebalin, 1982). In this study, we combine first-hand descriptionsof earthquake effects with the recognition of activefaults and folds from air photos and field surveys.Records from original sources in Persian, Arabic, andArmenian were reviewed by Ambraseys and Melville (1982)and Berberian (1995b). Both references discuss analyticalmethods in detail, including the method of elimination ofsecond-hand reports and mislocated or duplicated earthquakes.In some cases, such as the 1493 Nauzad earthquakein east Iran (Esfezari, 1493, previously quoted), and the 1721Shebli (Brydges, 1834) and 1780 Tabriz (Zonuzi, 1801;Donboli, 1813) earthquakes in northwestern Iran (Fig. 1),surface faulting was reported in contemporary accounts. Inother cases, the long axes of reliably determined meizoseismalzones of historical large-magnitude earthquakes align

122 M. Berberian and R. S. YeatsEURASIAICASPIANSEATURI~MENISTANQAZVIN,IFIGS.3&4;IALI~~IANISTAN!~LUTi AFGHAN\ I! BLOCK/ ’.,! SAUDI AR/~BIAPERSIANGULFGULF iOF OMANFigure l. Active faults of Iran and vicinity, modified from Berberian (1979, 1981,1983a,b, 1995a,b, 1997). Reverse faults shown with teeth on hanging-wall side. Strikeslipfaults shown with arrows. Faults without teeth or arrows: sense of slip unknown.Inset: Map of Iran showing boundary with Arabian plate (line with teeth). AZ, Azarbaijan;KP, Kopeh Dagh; MA, Makran deformation zone; S, Sistan suture.with Quaternary faults and folds that are clearly visible inair photos and can be followed on the ground. In this way,most large-magnitude earthquakes can be correlated to causativesurface fault zones and blind faults, although, as shownbelow, it may be ambiguous in distinguishing which of severalparallel, closely spaced faults in an elongate meizoseisrealzone was responsible for a particular earthquake. Insome cases, elongate meizoseismal zones of some largeearthquakes, such as the 1641 Tabriz-Dehkharqan earthquake(10 - VIII +, M - 6.8), could not be correlated toparticular surface fault.It is usually impractical to assign Modified Mercalli(MMI) intensities greater than VIII for Iranian earthquakesbecause adobe houses are completely destroyed at intensityVIII (Tchalenko and Ambraseys, 1973; Ambraseys and Melville,1982). Exceptions are larger structures such asmosques, churches, or palaces, and of wood-framed buildings,but such structures are much less common in Iran.Therefore, the meizoseismal areas of historical destructiveearthquakes shown in this article cover areas with MMI intensityVIII and higher for earthquakes with estimated equivalentsurface-wave magnitude of Ms > 6.5 from felt effects,using an empirical equation from Ambraseys and Melville(1982).Case Histories"They say, in the War of the Religion [the war betweenVishtaspa, king of Iran in the time of Zoroaster, and

Patterns of Historical Earthquake Rupture in the Iranian Plateau 123Arjaspa the king of Hiyun; approximately 4000 yearsago] ..... Mount Matan Faryad [Kumes Mountain, inthe epicentral area of the 856.12.22 earthquake of Io --IX +, M -- 7.4] broke off from the PatishkhvargarMountain, and slid down into the middle of the plain[’Miyandasht’, possibly referring to the Miyandasht village11 km north of the Meyamay active fault, or themiddle of the plain in the epicentral region of the 856Kumes/Damghan earthquake] ..." (Bundahishn, XII/32-33: Original/Primal Creation, "Cosmogony"; an ancientreligious text of the Zoroastrians, re-written in theMiddle Iranian, Pahlavi, language of Sassanian DynastyA.D. 224 to 642)."The earthquake effects were not confined to the city[Neyshabur]; one village was transferred bodily from itsplace and came down on another village, of which notrace was left ..." (al-Meqrizi, 1436, describing the1389 Neyshabur earthquake of I o -- IX +, M - 7.3 + )At least four historical earthquakes with M > 7 struckin less than 200 yr (1209-1405) near the city of Neyshaburin northeastern Iran (Melville, 1980; Berberian, 1981,1995b; Ambraseys and Melville, 1982; Berberian and Qorashi,1989, 1999a; Fig. 2). This region is characterized byactive reverse faults that follow the structural grain of theBinalud zone and the Neyshabur magmatic arc, an easterncontinuation of the orogenic belt of the Alborz Mountains(Berberian, 1981). The Neyshabur magmatic arc was formedby subduction of oceanic crust northward beneath Eurasiain Jurassic and Cretaceous time (Berberian and King, 1981;Berberian, 1983a,b).The damage distribution suggests that two of the earthquakes(1270 and 1405) ruptured the 55-km-long Neyshaburreverse fault to the west; whereas the other two (1209 and1389) ruptured the 95-km-long Binalud reverse fault to theeast (Fig. 2). The meizoseismal zones of these earthquakescorrespond to the mapped surface traces of these two faults.Historical records show that in 1209, the district of Neyshaburfrom Neyshabur City in the west to Daneh village in theeast (SE Neyshabur in the Zebarkhan district; see Fig. 2)was totally destroyed (Jovaini, 1260), and in 1389, landslidesfrom the Binalud Mountains in the east destroyed severalvillages in the same region. On the other hand, the 1270and 1405 earthquakes caused soil to be "scattered in the air"in the Neyshabur plain to the west (Fazlollah Hamedani,1304; Hafez-e-Abru, 1414). An ob server, Maulana LotfallahWe focus on earthquakes with M _--> 6.5 and especiallythose with M _--> 7 because these account for most of themoment release and are most likely to be recorded accuratelybecause of their size. An M < 6 event may go unrecorded, Neyshaburi in the village of Esfaris (modern Qadamgah easteven in a civilized region, if losses did not warrant a special of Neyshabur, located in Fig. 2) felt the 1405 shock but didreport, but an M > 7 event in a settled region would probably not report any damage.produce sufficient losses to be recorded and described.The correlation of meizoseismal zones with the mappedThe case histories presented here, all in parts of Iran surface traces of the Binalud and Neyshabur faults carrieswith a long historical record, have faults that appear to interact,resulting in earthquake sequences. The zones of earth-entire length during each earthquake. However, these faultsthe assumption that one of these faults ruptured along itsquakes commonly continue along strike to fault segments are part of a structural zone including the North Neyshabur,that have not sustained large-magnitude historical earthquakes.Historically, quiet zones may indicate (1) an alongmentsof these faults may have ruptured instead or that rup-Esfaris, and Buzhan faults, raising the possibility that segstrikechange in the method of strain release, or (2) Iran’s ture may have occurred on unknown blind faults underlyingrecorded history, long as it is, is still too short to encompass young folds in the Neyshabur plain, or partly blind faultsa complete earthquake cycle, and the quiet zones may be such as the Kal-e-Shur thrust (Fig. 2).segments that will sustain large-magnitude earthquakes in These uncertainties aside, it seems clear that since thethe future.late seventh century A.D., most moment release occurred inThe first two case histories discuss regions along the less than 200 yr on two adjacent zones of faulting that maymountainous northern margin of the Iranian plateau, where have included the Binalud and Neyshabur surface faults. Aftera quiet period of at least 500 yr, a rupture on the Binaludmost faults are characterized by reverse slip. The last fivecase histories cover regions farther south and west within zone (1209) was followed 61 yr later by a rupture on thethe Iranian plateau, where strike-slip faulting is dominant, Neyshabur zone (1270). The Binalud zone ruptured againbut reverse-fault earthquakes are locally significant.180 years after its first rupture (1309), followed 16 yr laterby a second rupture on the Neyshabur zone (1405). SinceNeyshaburthat time, these zones have been relatively quiet, althoughan earthquake of M 6.6 in 1673 may have ruptured the Binaludzone (or another fault to the northeast) damaging onethirdof Neyshabur and half of Mashhad (Fig. 2). Continuationsof this zone of reverse faults along strike to thesoutheast and northwest have not produced major momentrelease in historic time; the Neyshabur Jame’ Mosque of1494, the Shadyakh Mohammad Mahruq Shrine of the fifteenthcentury, and the Qadamgah Shrine built in 1680 arestill standing today. However, the southeastern end of theRivand fault may have sustained an M 6.9 earthquake in1851, damaging the ancient Ma’dan-e-Firuzeh [’TurquoiseMine’].Tehran"Not long ago, an earthquake in Ruyan [mountainsnorth-northwest of Tehran; Figure 3] caused two moun-

124 M. Berberian and R. S. Yeats..... \~:STANDING MONUMENTS (1- 1494 A.D.JAME MOSQUE;2-15th C. MAHRUQ SHRINE;3-1680 A.D.QAD~GAH SHRINE;4- 1433 A.D. TOR~ ~USALLA)1928.08.21 (VI I .5.2)- 1 900 TURQUOISE MINE NEYSHABUR 1 900 --185~.03.21,1857.05.001851.06.00(IX,6.9)- TURQUO I SE M I NE/QUCHAN-1500ESTIMATED 0 6.0-6.9MAGNITUDE ?~ 1673.07.30(V111+,6.6)_~)?~....(Ms) 5.0-5.9 N ? QADAMGAH? MASHHADIi7.0--7.9NOT ESTIMATED? QUESTIONABLE DAMAGEC.LATE 7th CENTURY--~)--NEYSHABUR- I 000100[R4]."~--1405.11.23( i x+, 7.4) NEYSHABUR[IN THE PLAINS, THE SOIL N [LANDSLIDES DESTROYED ANDWAS SCATTERED I N THE A I R;D I SPLACED V I LLAGES]MAULANA LOTFALLAH NEYSHABURIFEL THE EVENT IN QADAMGAH4 ,N SHADYAKH1270.10.07( I X, 7. _~ ~__ ,~[R2] _ .¢_--0--!251 (Vl !+,5.3)1209(IX,7.3)RUPTURE 1 [RI]¯--¯----N SH Q DANEH1145(VI I ,5.3) BANASK--n -- El--[ ZEBARKHANNEYSHABURSHADYAKH/[DISTANCE(km)]SABZPUSHAN/V I NEYARDD I STRICT]80 60 40 20 0 20 40 60J l L /~1 J L1800-1700-1600-1 500-1400-1,300-(j,)13:::1200-1100-800 -700 -8OI__100Figure 2. (Top) Meizoseismal areas (outer edge of Intensity VIII) of large-magnitudeearthquakes striking the Neyshabur-Binalud region in 1209, 1270, 1389, and 1405A.D. superimposed on a map of late Quaternary faults. Fault symbols as Figure 1. Solidtriangles locate monuments that have not undergone significant damage since constructionafter the 1405 earthquake. (Bottom) Time-space diagram of earthquakes in theNeyshabur-Binalud region from 700 A.D. to present. Zone of extensive damage shownby dashed line, queried where uncertain; specific sites damaged or destroyed shown insolid circle (M => 7.0), blank circle (M 6.0-6.9), and square (M 5.0--5.9). Wheredate of the earthquake is shown, it is given by year.month.day. Distances are alongstrike with respect to Neyshabur. The metropolitan area of Mashhad, the capital cityof the Khorasan (lit., The Rising Sun) province is hatched.

Patterns of Historical Earthquake Rupture in the Iranian Plateau 125tains to collide and tumble down; the debris of the collisionblocked the course of the rivers that ran between,and the waters of the rivers receded and formed a lake..." (ebn al-’Amid, 961, describing the 958.02.23Ruyan-Taleqan, northwest Tehran, earthquake of Io --IX, M -- 7.4; quoted by Biruni-Kharazmi, 1025).Active reverse faults of the Tehran region are parallelto the northwest-trending structural grain of the AlborzMountain belt (Fig. 3). The historical earthquake sequenceoccurred over a time period longer than 1100 yr, muchlonger than other case histories presented here. Four earthquakesof M > 7 devastated the Tehran region in a fourcenturyperiod from 743 to 1177, but only one earthquakethat large has struck in the last 800 yr, in 1830 (Ambraseys,1970; Ambraseys and Melville, 1982; Berberian et al., 1985;Berberian, 1995b; Fig. 3). At least three damaging historicalearthquakes ruptured adjacent segments of the Mosha faultfor a continuous distance of nearly 200 km: 958 (westernsegment), 1665 (eastern segment), and 1830 (central1830.03.27( IX.7.1 )-- ----I----I~------TEHRAN JAJRUD DAMAVAND1900-1800-- 1 200 1177--1119.12.10(Vl I I ,6.5)QAZV I N1177.05.00(IX,7.2)RAY1665.06-07 (V I I, 6.5) 1700?--~). ?DAMAVAND1600-1200-- I 000A.D.MAGNITUDE 6.0-6.9(Ms)958.02.23(X,7.7)S. 0-5.90 ? QUEST I ONABLE DAMAGERAY864.01-02(Vl I ,5.3)~ RAYB.C. --~[DISTANCE (km)]200 180 140 120 100 80 60 40 20 0 20L i t 1 l l l l__l__l__l"-" 1100-1000-~ 900 -743.03-06(~111+,7.2)800 -TSD 700 -A.D...,’312--280 BC(Xg;6)?40[60 80 100L~Figure 3. (Top) Meizoseismal areas of large-magnitude earthquakes striking theTehran metropolitan area (hatched). Symbols same as in Figures 1 and 2. Guidoboniand Traina (1995) dispute the location of the March to June 743 (exact date is unknown)earthquake and suggest that it is located in the Caspian Gates area of Derbent in Daghestan(750 km NW of Tehran, not shown in Fig. 1). (Bottom) Time-space diagram ofearthquakes in the Tehran area; symbols same as in Figure 2. Meizoseismal area of958 earthquake not well defined. Distances are along strike with respect to Tehran.

126 M. Berberian and R. S. Yeatsment, north of Tehran; Berberian et al., 1985). The Moshafault has a youthful scarp. The boundary between the 958and 1830 ruptures is marked by an abrupt change in faultstrike, but the boundary between the 1830 and 1665 rupturesis not marked by an obvious geological discontinuity. Thecontinuation of the Mosha fault east of the 1665 rupture hasnot ruptured historically in a large earthquake (Fig. 3; Berberianet al., 1985).We are unable to assign the 855 to 856 (exact year uncertain)and 1177 earthquakes to specific faults. These earthquakescould have ruptured the North Tehran thrust (NTT),which marks the range front at the north edge of the Tehranplain (Fig. 4). In contrast to the Mosha fault, however, theNTT has a less distinctive fault scarp. An additional complicationis the presence of south-dipping reverse faults, inpart blind, such as the Bagh-e-Feyz, Davudieh, T.V., Mahmudieh,Shian, and other faults in the Tehran plain (Fig. 4).South-dipping reverse faults may account for uplift of theNTT footwall block, analogous to the 1994 Northridge earthquakeon a blind thrust that uplifted the footwall of the SantaSusana fault (Yeats and Huftile, 1995). If this is the case,the south-dipping faults could have produced the 855 to 856and the 1177 events. If such an earthquake occurred today,it would wreak a catastrophe on the Tehran metropolitanregion. A complicating factor is the poor documentation ofthe location of the 855 to 856 earthquake, which could bethe mislocated Kumes earthquake (M-- 7.4 + ) of 856.12.22in the Damghan region (located in Fig. 1). These eventscould also have been located along the Ray-Kahrizak faultsystem south of Tehran (Figs. 1 and 3). The Qazvin earthquake(M-- 6.5) of 1119.12.10 on the North Qazvin thrustto the west may be a continuation of the zone of large earthquakeson the Mosha fault but separated from the surfacetrace of that fault by a historical gap (Fig. 3).Dasht-e-Bayaz/Abiz FaultsThe first two study regions are part of the reversefaultedranges that extend across northern Iran, including theAlborz Range (Fig. 1). To the southeast, across the centralKavir depression, the structural style changes dramatically~ulti0 0 0 0TehronFigure 4. Map of Tehran metropolitan areas of 1970 (diagonal dashed lines) showingNorth Tehran thrust, smaller faults, and partly covered faults (numbered by a letterfollowed by a number), some range facing within the Tehran plain. Contour intervalsin 200 m. Open circles indicate anomalies in groundwater elevation (dropping to thesouth). Long and short broken lines are the existing highways and roads, respectively,in Tehran.

Patterns of Historical Earthquake Rupture in the Iranian Plateau 127to strike-slip faulting: left-lateral faults striking east-westand right-lateral faults striking north-south to NNW-SSE(Fig. 1). Subordinate to the strike-slip faults are northweststrikingreverse faults, either bounding individual ranges orsplays of larger left-lateral strike-slip fault zones. The Abiz,East Neh, and Nosratabad right-lateral faults bound this regionon the east, following the older Sistan suture, east ofwhich is the more stable Afghan continental block.South of the northward-convex Doruneh fault (Fig. 1)is a group of faults highlighted by a sequence of earthquakesof which the best known is the 1968.08.31 earthquake of Ms7.4 on the Dasht-e-Bayaz fault (Tchalenko and Ambraseys,1970; Tchalenko and Berberian, 1975; Fig. 5). Earthquakeinteraction and triggering between the Dasht-e-Bayaz andthe Abiz faults, bordering the northern and the eastern margins,respectively, of the Sistan suture zone (Tirrul et al.,1983; Berberian, 1981, 1983a; Berberian and King, 1981;Berberian et al., 1998) are discussed in this section.The Dasht-e-Bayaz left-lateral strike-slip fault consistsof an -70-km-long west segment that ruptured in 1968 (Ms7.4) and an --50-km-long east segment that ruptured 11 yrlater on 1979.11.27 (Ms 7.1). The 1979 earthquake and twosmaller events on the Abiz fault struck during a period ofcivil disturbance, and they were not studied shortly after theearthquakes in as much detail as the 1968 earthquake. Thetwo segments of the Dasht-e-Bayaz fault are separated bythe north-south-trending Mahyar right-lateral strike-slipfault. The intersection is marked by structural complexity,including a local change of strike, a zone of splays of theDasht-e-Bayaz fault, and a right step of about 1 km on theMahyar fault (Fig. 5). The Mahyar fault extends farthersouth with a left stepover to the Pavak fault. West of thisnorth-south fault, an earthquake of Ms 6.4 with a strike-slipfocal mechanism struck north of the city of Qa’en in1976.11.07, near the Dasht-e-Bayaz segment boundary in aregion containing left-slip and reverse-slip faults, all trendingeast-west (Fig. 5).The 1979 Dasht-e-Bayaz earthquake was the third in awest-to-east progression of strike-slip earthquakes within an11-yr time period. In addition, the 1979 earthquake was partof a shorter-term sequence that began in 1979.01.16 with anearthquake of M s 6.8 with a reverse-fault focal mechanism;this event has not been assigned to a mapped fault. The 16January event was followed on 14 November by an earthquakeof Ms 6.6 with surface faulting along 20 km of thenorth-trending Abiz fight-lateral strike-slip fault (Berberianet al., 1999). Next came the main Dasht-e-Bayaz event ofMs 7.1 on 27 November. A large aftershock of Ms 6.1 on theAbiz fault on 7 December produced surface rupture alongthe remaining 15 km of the northern tip of the Abiz faultsouth of its intersection with the Dasht-e-Bayaz fault thathad not ruptured during the 14 November event. The ISCepicenter of the 1979.11.27 earthquake, with 411 readings,is located on the eastern tip of the Dasht-e-Bayaz fault at thejunction with the Abiz fault. This suggests loading of theeastern segment of the Dasht-e-Bayaz fault by the Abiz eventon 14 November; the rupture started from the junction ofthe two faults and propagated toward the west. Thus, the1979 cluster included one on a left-lateral fault, two on aright-lateral fault, and one on a reverse fault (Fig. 5).Prior to the two, 1979 earthquakes along the northernmostAbiz fault, an earthquake of M s 6.0 took place on1936.06.30 south ofthe 1979.11.14 meizoseismal area. Atleast 12 km of surface rupture accompanied the 1936 event,but approximately 15 km of the Abiz fault between the 1936and 1979 earthquakes did not rupture at the surface (seearrows in Fig. 5). On 1997.05.10, almost 18 yr after the 1979earthquakes along the northern portion of the Abiz fault, anearthquake of Ms 7.3 produced right-lateral strike-slip rupturealong the entire 125-km length of the Abiz fault, includingthose portions that had ruptured in 1936 and 1979(Berberian et al., 1999; Figs. 5 and 6). The 1997 epicenterwas located at the southern end of the November 1979 surfacerupture and north of the 1936 rupture.The 1968 earthquake on the west segment of the Dashte-Bayazfault was followed 20 h later by an earthquake ofMs 6.4 on the Ferdo.ws reverse fault, possibly due to loadingof that fault by the Dasht-e-Bayaz earthquake (see discussionlater).The Dasht-e-Bayaz earthquakes comprise a twentiethcentury cluster, but the length of the pretwentieth centuryinterseismic interval is uncertain. Scattered pretwentiethcentury data in this region are not sufficient to correlate pretwentiethcentury damage to specific active faults. Leftlateraloffset of as much as 10 m across the Dasht-e-Bayazfault (Ambraseys and Tchalenko, 1969) has been measuredalong old qanats (traditional Iranian system of irrigationcomposed of underground canals drawing water from mountainsources by gravity). Unfortunately, it is not possible atpresent to establish the age of the major displaced and disusedqanats. The oldest known qanat in Iran was constructedin the second millennium B.C. in the Semnan areaat the northern edge of the central Kavir depression (Mehryarand Kabiri, 1986). Assuming the qanats offset by theDasht-e-Bayaz fault are as old as 4000 yr, the minimum sliprate on the Dasht-e-Bayaz fault would be 2.5 mm/yr.The 250-cm left-lateral slip released during the 1968earthquake (Tchalenko and Berberian, 1975) could have accumulatedin about 1000 yr at a rate of 2.5 mm/yr. Holoceneoffsets of 8 to 28 m are documented at Khidbas Creek (Fig.5) as well as other drainages crossing the fault.Four pretwentieth century historical earthquakes are recordedin the Dasht-e-Bayaz area, although none is part ofa cluster comparable to the twentieth century sequence. (1)An earthquake in the summer of 1238 apparently damagedGonabad (Fig. 5) and completely destroyed the Seljuq Jame’(Friday) mosque of Qa’en constructed in the period 10001218 (Tabandeh, 1969; Naderi, 1980). The damage distributionsuggests that the 1238 earthquake could be the penultimateearthquake before the twentieth-century clusteroccurred on the Dasht-e-Bayaz fault, an interpretationconsistent with the approximate 1000-yr average recurrence

128 M. Berberian and R. S. Yeats!

Patterns of Historical Earthquake Rupture in the Iranian Plateau 1291968.9.1~.8 6.£::68.9.~IFIG. 5"~’~194120 40 60I I Ikn1978.09.16 ’~¯ 199"/.5.10\¯33.004.4AFGHANBLOCK-32.00N1984.8.6LUTBLOCKKAVIR-E- LUT1981.7.287.1I~EPRESSIONNEH2.23 ..... ~6.1.~ZABOL;1994.2.24;.11981.6.117N OSRATA BAO,~:i1989.11.20OOP Z AH EDIO5 8.00E 59.00E //6 0.00E 61.00EFigure 6. Earthquake-fault map of the eastern Iranian plateau between Dasht-e-Bayaz and northern Makran on the south, showing major N-S fight-lateral strike-slipfaults and NW-oriented reverse faults (teeth on hanging-wall side). Fault-plane solutionsas in Figure 5.

: 130 M. Berberian and R. S. Yeatsdows] used to be a large town but when I visited it wasin ruins..." (Nasser Khosrau, 1052 A.D.)The following discussion focuses on the earthquake historyof a region between the Dasht-e-Bayaz fault and theNayband-Gowk fault to the west (Fig. 1). This provinceessentially the Shotori Range bounded on the west by theTabas reverse fault, the Ferdows area to the east, and thebroad depression in between (Figs. 1 and 6).The 1968 Dasht-e-Bayaz earthquake on a strike-slipfault was followed 20 h later by two large aftershocks(1968.09.01, s 6.4, a nd 1968.09.04, M5. 2) with re versefaultfocal mechanisms on the Ferdows reverse fault at thewestern end of the Dasht-e-Bayaz fault. The Ferdows earthquakescould be interpreted as a separate mainshock andaftershock, just as the 1992 Big Bear, California, left-lateralstrike-slip earthquake is considered as an event separatefrom the right-lateral strike-slip Landers earthquake to theeast (Hauksson et al., 1993). The mainshock on 1 Septemberwas 60 km west of the mainshock of the Ms 7.4 Dasht-e-Bayaz earthquake.Ten years after the 1968 Dasht-e-Bayaz/Ferdows earthquakes,on 1978.09.16, the Tabas-e-Golshan earthquake ofM s 7.4 ruptured the Tabas reverse fault at the western frontof the Shotori Range. This earthquake was preceded by threesmall reverse-fault events in 1973 and 1974, but aside fromthose events, the Shotori Range had not been struck by alarge earthquake for at least 1100 yr. The eleventh-centuryMenar-e-Kabir Seljuk Tower, 40 m high, the EmamzadehHossein structure built in 1057, and the twelfth-centuryMadreseh-ye-Dau Menar (Meshkati, 1970) remained standinguntil they were destroyed during the 1978 earthquake(Fig. 6; Berberian, 1979, 1982).The Tabas-e-Golshan earthquake was accompanied bysurface rupture along the entire Shotori range-front fault,with surface rupture corresponding reasonably well with thelength of the mainshock subsurface rupture. In contrast, thesmaller Ferdows earthquakes of 1968 ruptured only thenorthwestern third of the Ferdows reverse fault system,based on aftershock distribution. The Ferdows earthquakeswere part of another sequence that began in 1941 with anearthquake of M s 6.1, followed by a larger earthquake of Ms6.8 in 1947 (Figs. 5 and 6). It is not clear, however, thateither of these events occurred on the Ferdows reverse-faultsystem. The 1941 meizoseismal zone is elongated parallelto a north-south-trending fight-lateral strike-slip fault. The1947 meizoseismal zone also has more of a north-south orientationthan the NW-SE strike of the Ferdows reverse-faultsystem, which extends southeast beyond the 1947 meizoseismalzone. Local people reported a ground fracture strikingNW-SE along the Ferdows fault, between Badamuk andGurab (B and G along the Ferdows fault in Fig. 5), fordistance of at least 6 km with the northeastern hanging-wallblock uplifted (Ambraseys and Melville, 1977, 1982). Thus,the 1947 event appears to have ruptured part of the Ferdowsfault system, but other faults may have been involved aswell. The timing and overlap of meizoseismal zones indicatean earthquake sequence progressing to the northwest.The Tabas and Ferdows earthquakes are examples ofcross-strike pairs of reverse-fault earthquakes, in which onefollows the other in a time span of decades. Other examplesare the 1929 Murchison and 1968 Inangahua earthquakes innorthwest Nelson, New Zealand, the 1971 Sylmar and 1994Northridge earthquakes in the San Fernando Valley, California,and the 1944 San Juan and 1977 Caucete earthquakesin the Pampean Ranges of northwest Argentina. All theseexamples occurred in thick-skinned range-and-basin provinces(Yeats et al., 1997). As discussed earlier, two additionalexamples may be the 1209 and 1389 Binalud earthquakesand the 1270 and 1405 Neyshabur earthquakes innortheastern Iran, although that region has not been shownto be characterized by thick-skinned deformation.Tabriz"The Sorkhab Mountain [in north Tabriz] was faultedby deep fissures of 2 zars [-2 meters] wide and 2 farsangs[--12 km] long trending southeast [along theNorth Tabriz fault]..." (Donboli, 1813 A.D., describingthe 1780 Tabriz earthquake).The Tabriz region is located in the Araxes structuralblock of northwestern Iran, southwest of the continuation ofthe western Alborz Mountains toward the Caucasus. TheNorth Tabriz fault (NTF) is a complex northwest-trendingstructure that contains evidence of fight-lateral strike-slipdisplacement observed on aerial photographs, and verticaldisplacement with the north side up (Fig. 7; Berberian andArshadi, 1976; Berberian, 1997; Berberian and Qorashi,1987, 1999b). The NTF merges northwest with a zone ofreverse faults that turns to the west-southwest in the areanorth of Lake Urumiyeh (Sufian and Tasuj faults; Fig. 7),and it merges southeast with another zone of reverse faults,which turns to the east-northeast (South and North Bozqush,Duzduzan, and South Sarab faults; Fig. 8).The NTF system and adjacent reverse faults rupturedfrom southeast to northwest in three earthquakes in 65 yr:the Shebli earthquake (M-- 7.3) in 1721 on the southeasternNTF with a surface rupture >35 km long as reported byBrydges (1834); the Tabriz earthquake (M-- 7.4) inon the northwestern NTF, with surface rupture >42 km longand 2- to 4-m vertical separation as reported by al-Omari(1795), Zonuzi (1801), Donboli (1813), and Drouville(1825); and the Marand-Mishu earthquake (M -- 6.3)1786 on the Mishu reverse fault and the Sufian segment ofthe NTF. A fourth earthquake of M -- 5.5 struck the Tasujreverse fault farther west in 1807. A fifth earthquake of M-- 6.7 took place along the South Bozqush reverse fault farthersoutheast in 1879 (Fig. 8; see discussion later). Theearthquake segment boundaries correspond to geometric discontinuitiessuch as stepovers (overlaps, offsets) and abruptchanges in strike. The boundary between the Tabriz and

Patterns of Historical Earthquake Rupture in the Iranian Plateau 131^~.~-/,,// \LAKE URU~.~ ~,_+1275o.-- E~ 1965.02.10 -~ (v~,~.~)- 1 900 N ~ ~ 900 -- E~I~ -1807.07.1 I(>Vl 1+,5.5)- -~ ~786.~o.oo(v~ ,6.~) ~7~O.O~.08(~X+,7.~+,L>~2T"F~ R ~A ~.=) ~ ~800-_ -- 1500M G D TABRIZTABR I ZRUPTURE 1 [R1]~~~~~~ -:~-- 1717.03.12(VI IT5.9) BASMENJ, SHEBL I BOSTANABADUJAN1700-1641.02.05(VI I 1+,6.8)1600-1550 O00Q r-~(>V~ r,>5". 3)~ 1 500 -1304.11 .07(VI I 1+,6.7) ----’J7~-~~ u~ 1 400 -~ -?~0~? ~ ~1300TABRIZ CLUSTER "B" ? [31 YRS] -?~01273.01 .18(Vl I 1+,6.5)~0T¯ 1200--- 1000_ FO 7.0--7.9EST I MATEDMAGN I TUDE 6.0-6 9- (Ms)5.0-5 9--100 80 60[DISTANCE(km)]40 201042.11 .04( I X+,7.3) z.L.TABR I Z,,=,1.", 11 oo-~,°1’ 1000-~’858.oo.oo(vl ~+,6.0)"°._L.~900?~o ~ ?oTABRIZ 20J J40l60 "180Figure 7. Meizoseismal area of large-magnitude earthquakes and space-time diagramsof nearly 1140 yr of seismicity of the Tabriz region, northwest Iran. Symbolsas in Figure 2. Surface rupture associated with the 1721 and 1780 earthquakes shownin bold lines. Distances are along strike with respect to Tabriz.Marand-Mishu earthquakes corresponds to the boundary betweenthe NTF and the Mishu fault (Fig. 7).The Tabriz-Dehkharqan earthquake (M-- 6.8) in 1641(Melville, 1981) was south of the boundary between theShebli and Tabriz earthquake. The earthquake was witnessedand documented by the Armenian celibate priest Arakel VartabedTavrizetsi (1670) and also by Mohammad Nassir(1641), who was assigned by the government to evaluate thedamage and death toll. The northeast-trending meizoseismalzone of the earthquake does not correspond to any mappedfault. The southeastern continuation of the NTF has had historicalearthquakes only smaller than M 6, which is also thecase for the northwestern continuation of the Mishu-Sufianreverse faults (Fig. 7).

132 M. Berberian and R. S. Yeats+4814mCASPIAN-28 MSEASABALANVOLCANO1883.05.03(3Z~, 621593.00.00(3ZI~,SARAB1980.05.04(62)1981.08.04(5.6) 38.00=N1978.11.041844.05.13( 13r, 6.9)MIYANEH¯ )’ANZALI ’1621.05,211990.06.:>2(4.6)RASHT1367(>5.537.00°N ~400 ~0 60 KmFigure 8. Meizoseismal areas (stippled; destruction sites marked with bold dots)earthquakes along active faults west of the Caspian Sea in northwest Iran. Active faultsassociated with known earthquakes shown by heavy lines; reverse faults with solidteeth on hanging-wall side. Other faults shown by thin lines and open teeth for reversefaults. Fault-plane solutions shown as in Figure 5. The 1983 and 1990 earthquakes atsoutheast comer of map are not discussed. Open squares: towns. Modified from Berberian(1997).Prior to the 1721 to 1786 earthquake sequence, Tabrizwas struck by earthquakes in 858 (M -- 6.0), 1042 (M7.3; with ground fracture witnessed by Qatran Tabrizi,1042), 1273 (M - 6.5; witnessed by Bar-Hebraeus, 1286),and 1304 (M -- 6.7; Fig. 8). However, the meizoseismalzones of these older earthquakes are not known well enoughto speculate on which faults were responsible for them. The1273 and 1304 earthquakes are a temporal cluster like the1721 to 1786 events, but the direction of propagation is notknown (Berberian, 1997).Three Earthquake Pairs"This glorious and heavenly temple [the Saint ThaddeusArmenian Monastery] was built with solid foundationon the grave of the Apostle Thaddeus which collapsedby an earthquake [in 1319 AD] because of our numeroussins. I, Bishop Zakaria, the son of Mr. Manuel as thehumble servant of the Apostle Thaddeus with the helpof God started to repair it; now that times are harderthan before when there was persecution against Christiansand many churches were destroyed. This is whywe established this memorial so that our parents, cousins,and our successor priest Diratzoo are rememberedbefore great God and bloodless lamb of God; in addition,our genuine brother Bedros and Sarkis who in accordancewith their capacity contributed with their belongings.It was finished in 778 [1329]". (Abbot Zakaria1329; stone plaque, in the old Armenian language ’Grabar’,on the northern wall inside the Monastery locatedin the area south of Maku or ancient Her in northwesternIran).The three examples that follow involve only two earthquakeseach. These are (1) two earthquakes at the southwesternedge of the Caspian Sea, (2) two earthquakes near

Patterns of Historical Earthquake Rupture in the Iranian Plateau 133the [ran-Turkey border, and (3) the Gowk-Sirch earthquakesin southern [ran. Other possible earthquake sequencesin which all magnitudes are M < 6 are not discussedhere.Rocks of the Talesh Mountains, the northwest continuationof the Alborz Mountains, have been thrust northeastwardand eastward over rocks of the south Caspian depression.An earthquake of Ms 6.0 in 1978 yielded a focalmechanism consistent with a low-angle thrust (Berberian,1983b; Fig. 8). To the west, the Sangavar fault consists ofnorthwest-striking reverse fault segment and a NNE-strikinghigh-angle fault segment that ruptured in two earthquakes.The first earthquake, of M -- 6.1, ruptured the northern partof the NNE-striking segment in 1863. The second earthquakeof M - 6.7 ruptured the southern part of this segment32 yr later, in 1896 (Fig. 8). This larger earthquake did notcontinue south into the reverse-fault segment (Berberian,1983b, 1995b, 1997; Berberian et al., 1992).Farther west, in the direction of Turkey, the 1844.05.13(I ~ IX, M -- 6.9), the 1879.03.22 (I ~ VIII, M -- 6.7),the 1883.05.03 (I ~ VIII, M ~ 6.2) earthquakes comprisesequence separated by 34 and 4 yr, respectively (Fig. 8).Available macroseismic data are insufficient to assign theseearthquakes to specific faults, although the 1879 event mayhave ruptured the South Bozqush fault.The Kelissa Kandi-Shadlu-Pambukh right-lateral strikeslipfault on the Iran-Turkey border sustained an earthquakeofM ~ 7.4 on 1840.07.02 (Fig. 9). Less than 3 yr later, theKhoy earthquake of M ~ 5.9 ruptured the Kamar Kassanfault (Fig. 9; Berberian, 1997). The meizoseismal zonesthese two earthquakes are separated by a gap more than 60km long that contains right-lateral strike-slip faults, includingthe Badalan fault. The 1319 St. Thaddeus earthquake(quoted earlier) might have ruptured this gap; however, sufficientdata are not available. West of the gap, the Chalderanright-lateral strike-slip fault in Turkey ruptured in1976.07.02 with an earthquake ofM s 7.1 (Arpat et al., 1977),280 yr after an earthquake on 1696.04.14 A.D. of M ~ 7.0in the same region, but not necessarily on the same fault.South of the Tabas thrust and Shotori range, a rightlateralstrike-slip fault zone, principally known as the Nayband/Gowkfault, extends south for more than 400 km. Mostof this fault zone has not sustained large-magnitude historicalearthquakes, but in its southern half, two earthquakesoccurred close together in space and time (Fig. 6).1981.06.11, the Gowk fault ruptured in an oblique strikeslipand reverse-slip earthquake with M s 6.7 (Berberian etal., 1984). Less than 2 months later, the Sirch earthquake ofMs 7.1 occurred on 28 July with a reverse-fault focal mechanism.This earthquake occurred where the Nayband-Gowkfault turns to the SSE into a zone of reverse faults (Fig. 10).The two meizoseismal zones are separated by the Zamanabadgap, 7 km long, which did not rupture. Smaller earthquakesstruck the meizoseismal zone of the 1981.07.28earthquake in 1877 (M 5.0), 1909 (M not established),(M 5.6), 1948 (M 5.9), 1969 (m 5.2), and (M s 6.9).:::44~00°E" " " ’ TURKEY!~Okm~TURKEYIRAN4/,.30OE" ’\/~ ""~.. ARMENIA/ \//"¯ .~.~ JAZERBAIJAN/ IRAN/1319.00.00(>5.3)°S.Thaddeus,~ _39.00°N-Figure9. Meizoseismal areas of earthquakesalong active faults in northwest Iran and adjacent Turkeymodified from Berberian (1997). Symbolsameas preceding figure.Noninteractive Large-Magnitude Earthquakes"I have visited most of the regions that are famous fortheir frequent occurrences of great earthquakes; amongthem are the district of Siraf [the present Taheri port]on the Persian Gulf situated between the mountains andthe sea; Saimareh in the Mehrjan Qozaq [MehrganKadeh = Saimareh and Dareh Shahr in the westernZagros] and Masbazan district [Mehrgan Qozaq +Masbazan = Ilam in the western Zagros] of Jebal[’Mountain’ in Arabic: referring to the Jebal province/Eraq-e-AjarrdMedia/Mada, the territory of the MedianTribes] at the foot of Mount Kabr [Kabir Kuh in Zagros];and the city of Antakiyeh [Antioch] in the Qonsorindistrict; and ’Avasem of Sham [Syria] at the mountainfoot; and velayat of Qumes [Kumes/Damghandistrict in northern [ran] which is highly seismic, theearthquakes are so violent that the springs dry up andgush out in another place. It is a very disturbed city ....and Amol is another earthquake-prone city which I visited;it is on the foot of the big Mount Damavand, andit is said that Damavand [in Alborz Mts] is the highestmountain in the world!" (Mas’udi, 943).The 1493.01.10 Nauzad earthquake of M -- 7.0 (withsurface rupture witnessed by Esfezari, 1493) and the 1838Nosratabad earthquake of M - 7.0 (with surface rupture)

134 M. Berberian and R. S. Yeats1981.07.28(IX ,7.1) 1998.03.14(V111+,6.9)-1 950 ~CHAHAR 1969.09.02 JOSHAN FANDOGHAFARSAKH1948.07.05(VIII,5.9}1950 -< ~ 1911 .04.29(VI I ,5.6) 1909.11 .06(VI t ,>5.6)--1 900 ~1909.10.27(VI I ,5.5)1 900 -~ 1877(V11+.5.6),18501854.11.00(Vl]l,5.8) ?ESTIMATEDMAGNITUDE0 6.0-6.9(M~) [] 5.0-5.9rY1 850 -[DISTANCE90 80 70 60 50 40 30 20 1010 20 30 40I I I I ,5OFigure 10. Meizoseismal area of large-magnitude earthquakes and space-time diagramof nearly 143 yr of seismicity along the Gowk fault, southeast Iran. Symbols asin Figures 1 and 2. Modified from Berberian et al. (1984), Berberian and Qorashi(1994), and Berberian (1995b).within the Neh-Nosratabad fault system are isolated singleearthquakes (Fig. 6). Several additional noninteractive earthquakesare identified below in two groups: (1) in the foldedrocks of the Alborz Mountains and related structural zonesand (2) farther south on the central Iranian plateau (Fig.Several large-magnitude earthquakes have occurredalong the southern margins of the Alborz structural zonebetween Neyshabur and Tehran. These include an earthquakeon 1052.06.02 of M -- 7.0 on the Sabzevar reversefault, an earthquake on 1695.05.11 of M -- 7.0 on the Esfarayenreverse fault, and an earthquake on 856.12.22 ofM ~ 7.4 on the Damghan/Astaneh fault (Berberian, 1981).Southwest of Tehran, the Ipak fault ruptured in 1962.09.01with an earthquake of Ms 7.2 (Bo’in Zahra earthquake).previous earthquake of M > 6.5 ruptured the Ipak fault inthe third millennium B.C., giving an earthquake recurrenceinterval on the same fault segment of between 4000 and5000 yr (Berberian and Yeats, 1999).South of the Doruneh fault and north of the Dasht-e-Bayaz fault, the Jangal reverse fault sustained an earthquakeof M ~ 7.3 on 1336.10.21. No large earthquakes have occurredthere since this event.Discussion"and I shall remember that I shall not violate the law ofthe Earth!" (Sohrab Sepehri, 1967)Difference between Plate-ConvergenceSeismic Moment Release RateRate andThe seismic moment release rate between the Arabianand Eurasian plates in the Iranian plateau since 1911 is ableto account for only about 10% (2 to 3 mm/yr) of the convergencerate between Arabia and Eurasia (North, 1974;Ambraseys and Melville, 1982; Jackson and McKenzie,

Patterns of Historical Earthquake Rupture in the Iranian Plateau 1351988; Jackson, 1992). Only about 15% of the long-term convergencerate across the Zagros Mountains, estimated at 20mm/yr, can be attributed to seismic moment release duringthis period. Seismic moment release can account for morethan 50% of the shortening in northeast Iran, but only about10% in the Alborz Mountains (Jackson and McKenzie,1984; Jackson et al., 1995). In northwest Iran, twentiethcenturyearthquakes account for all of the expected fightlateralcomponent of convergence but less than 25% of theexpected shortening by dip slip (Jackson, 1992). Unfortunately,there are no geodetic estimates, either space based orterrestrial, of active deformation rates within the Iranian plateau.Accordingly, the question about whether the historicalslip deficit is due to aseismic slip or to an inadequate samplingof the seismic moment release rate in the historicalperiod cannot be answered. We return to this question belowwhen we consider the seismic hazard of long, straight strikeslipfaults with geological evidence of Holocene deformationthat have not sustained large-magnitude earthquakes in thehistorical period.Temporal Clustering and Loading of Adjacent FaultsAlthough earthquakes release accumulated elasticstrain, they also load faults in nearby and even more distantregions that will be the location of future events (Stein etal., 1992; Stein and Lisowski, 1983; Hudnut et al., 1989;Reasenberg and Simpson, 1992; Du and Aydin, 1993; Kinget al., 1994; Bennett et al., 1995; Harris and Simpson, 1996;Jacques et al., 1996). The geometry of interactions betweengroups of active faults is critical for characterizing seismicsources of large earthquakes.The earthquake cluster in the Dasht-e-Bayaz area ineastern Iran (Figs. 5, 6, and 7) consisted of 11 earthquakesofM > 6.0 in 61 yr (1936 to 1997) in an area of 160 by 110km, approximately the same size as the Los Angeles metropolitanarea. The Dasht-e-Bayaz/Abiz earthquake sequenceruptured left-lateral and fight-lateral faults at approximatelyfight angles to one another, consistent withCoulomb stress changes predicted for the 1992 Landers,California, earthquake, although in detail, some of the earthquakesmay not fit this simple pattern. This shows that interactingsmaller faults may contribute a large part of thehazard in Iran and may make up the slip deficit in a limitedregion in a short period of time.The Tehran and Tabriz sequences illustrate loading offault tips by earthquakes, in which the most likely site forthe next earthquake would be the fault segment just beyondthe segment that last ruptured. At Tabriz, the sequence propagatedfrom east to west in a sequence of four earthquakesfrom 1721 to 1786. If an earthquake farther west in 1807 ispart of this sequence, it left a gap. The Tehran sequenceruptured from west to east from 958 to 1830, but the westand east ends of the Mosha fault ruptured before the centralsection did. Also, the Qazvin earthquake of 1119 is west ofthe 958 rupture, leaving an unruptured gap. These relationssuggest that although fault loading is important, strength ofindividual fault segments may allow an earthquake to skipa segment and fill it in later.The Sangavar earthquakes of 1863 and 1896 rupturedadjacent segments, but the Pambukh and Badalan earthquakesleft unruptured segments, as did the Gowk and Sirchearthquakes in southeastern Iran.Slip Rates and Recurrence IntervalsIntraplate deformation of the Iranian plateau, which isa result of the collision of the Arabian and Eurasian plates,is nonuniformly distributed and is concentrated along severalbelts that surround more stable, relatively aseismicblocks. The rates of shortening and uplift are highest in theZagros belt, close to the Arabian plate boundary, where theNE-SW shortening between Arabia and central Iran is accommodatedpredominantly by blind reverse faults with slipvectors oriented in a direction similar to that of the plateconvergence vector, and decreases northeastward. The Zagrosis characterized by rapid uplift, high seismicity, andmoderate-magnitude earthquakes on several blind reversefaults with historical recurrence intervals of 300 to 500 yrin the Lar and Qir region and medium- to large-magnitudeearthquakes on strike-slip faults around the edges of the Zagros,especially the Zagros Main Recent fault (Berberian,1995a).In contrast, lower slip rates are characteristic of the Iranianplateau away from the plate boundary. Areas such asthe Alborz Mountains and Kopeh Dagh are characterized bylarge-magnitude, shallow earthquakes accompanied by surfacefaulting separated by quiescent periods of 3000 to 5000yr on frontal reverse faults and some strike-slip faults, basedon historical recurrence intervals on the Ipak fault and theRudbar fault (Baklor-Kabateh-Zard Geli fault segments)southwest of the Caspian Sea (Berberian and Yeats, 1999).Active faults have reactivated ancient collisional zones,and the intervening blocks are more stable. Based on offsetqanats and Holocene offsets of streams, a minimum slip rateof 2.5 mm/yr and a recurrence interval of 1000 yr were estimatedfor the Dasht-e-Bayaz fault.Within central Iran, there are temporal variations in recurrenceintervals of large-magnitude earthquakes. The Tabrizearthquakes of 1273 to 1304 and 1721 to 1786, the 1209to 1405 Neyshabur-Binalud earthquakes, and the 1968 to1997 Dasht-e-Bayaz/Abiz earthquakes were temporal clustersof interactive earthquakes, although pretwentieth centuryearthquakes at Dasht-e-Bayaz/Abiz show no evidenceof clustering. However, the Sabzevar, Esfarayen, Kumes,Ipak, Nauzad, and Nosratabad earthquakes were not interactive.The presence of clustering has been demonstrated inseveral parts of the Iranian plateau, but the variability inearthquake recurrence patterns is greater than had been assumed.Could an M 8 Earthquake Strike the Iranian Plateau?Most large earthquakes in Iran for which magnitude estimatescan be made are in the range M 6.6 to M 7.2, with

136 M. Berberian and R. S. Yeatsthe largest historical earthquake M 7.7. Given a historicalrecord of several thousand years, one might conclude thatno earthquakes larger than M 7.7 could be expected. Onereason for this could be the structural complexity of thoseareas with large historical earthquakes, preventing meizoseismalzones to be longer than 75 to 125 km. Where thereare earthquake sequences, earthquake segment boundariescommonly (but not always) correspond to geometric discontinuities.For example, the Tabriz sequence formed in threeearthquakes rather than one large one, and the two Dasht-e-Bayaz, the two Gowk, and the two Sangavar earthquakesalso did not rupture in single events. The strain buildup waslarge enough for earthquakes to rupture a segment but notlarge enough for earthquakes to rupture across segmentboundaries.Could a larger event strike the Iranian plateau? Analogsfrom Mongolia (Baljinnyam et al., 1993; Bayarsayhan et al.,1996; C. Prentice, personal comm., 1997) suggest that thisis possible. Slip rates on Mongolian strike-slip faults arerelatively low. Earthquakes that occurred on relatively shortfaults were of M < 7.3, but three great earthquakes of M _-->8 ruptured long, continuous strike-slip faults: Bulnay in1911, Fuyun (across the border in China) in 1933, and Gobi-Altai on the Bogd fault in 1957. Preliminary work fromtrench excavations on the Bogd fault suggests that the earthquakerecurrence interval on the Bogd fault may be severalthousand years.The Nayband-Gowk fault (Fig. 6) has not had a largehistorical earthquake on the northern 250 km of its trace,which is also structurally the simplest part of the fault. The1981 Sirch earthquake, M 7.1, occurred at the southern endof the straight trace of the Nayband-Gowk fault where itintersects a set of northwest-striking reverse faults. East ofthe Nayband-Gowk fault, the East Neh, West Neh, and Kahurakfaults have not produced a large historical earthquakealong 360 km of the fault system north of the Nosratabadearthquake (M 7.0) of 1838 (Fig. 6). The longest strike-slipfault in the Iranian plateau is the broadly curved Dorunehfault, 600 km long, which has not produced a large historicalearthquake, although nearby shorter faults to the north andsouth have sustained large-magnitude earthquakes (Neyshabur,Jangal, Dasht-e-Bayaz; Fig. 1).If earthquake recurrence intervals on these long faultsare several thousand years long, then we conclude that allof them have been accumulating strain because the platetectonicslip deficit has not been relieved by historical earthquakesin this region. Paleoseismological investigations willbe necessary to determine the Holocene earthquake historyof these faults, but until that is accomplished, Iran shouldplan on the possibility of M 8 earthquakes on these longfaults.Implications for Southern CaliforniaLike Iran, southern California is cut by many activestrike-slip and reverse-slip faults, some of which have resuitedin interactive earthquakes. The main difference is thethroughgoing San Andreas fault, which has generated earthquakesof M -- 8. As in Iran, there are historical earthquakesoff the San Andreas fault with M > 7, including the 1992Landers earthquake, the 1952 Arvin-Tehachapi earthquake,and the poorly located 1927 Lompoc earthquake at the farwestern end of the Transverse range.However, the active faults in and adjacentto the heavilypopulated regions of the Transverse ranges, northernmostPeninsular ranges, and Los Angeles basin have generatedonly three moderate-magnitude earthquakes, all with M 7 are to be expected in the Los Angelesmetropolitan region, like those that have occurred in northwestNelson (New Zealand), northwest Argentina, and Japan.Paleoseismic excavations on the Sierra Madre faultnorth of Los Angeles revealed evidence for two events witha total displacement of 10 m, consistent with earthquakes ofM > 7 (Rubin et al., 1998).Conclusions"As the evil spirit rushed in, the earth shook and thesubstance of mountains was created in the earth. FirstMount Alborz arose; afterwards, the other ranges ofmountains of the middle earth; for as Alborz grew forthall the mountains remained in motion, for they have allgrown forth from the root of Alborz..." (Bundahishn,VIII/1:29).The long historical and archeological record of earthquakesin Iran allows unique insights into earthquake magnitudes,recurrences, segmentation, and clustering. The Neyshabursequence of four earthquakes between 1209 and 1405respected the geometric segment boundary between the Neyshaburand Binalud fault systems and may consist of twopairs of earthquakes comparable to the 1971 and 1994 SanFernando Valley, California, earthquakes. The Dasht-e-Bayaz sequence consisted of 11 earthquakes between 1936and 1997 that ruptured both strike-slip and reverse-slipfaults; pretwentieth century (historic) earthquakes in this regiondid not occur in clusters. The North Tabriz fault systemruptured from southeast to northwest in three earthquakesfrom 1721 to 1786; a previous earthquake cluster may haveoccurred in the same region (although not necessarily on thesame faults) in 855 to 856 to 958. The fault system north ofTehran ruptured in earthquakes in 958, 1665, and 1830. Pairsof earthquakes include a strike-slip reverse-slip pair in theTalesh Mountains near the Caspian Sea in 1863 and 1896,a pair of earthquakes near the Turkish border in 1840 and1843, and two earthquakes at the southern end of the Nayband-Gowkstrike-slip fault in 1981, one strike-slip and onereverse-slip.

Patterns of Historical Earthquake Rupture in the Iranian Plateau 137Other historical earthquakes have not occurred in sequences.These include earthquakes on the Sabzevar reversefault in 1052, the Esfarayen reverse fault in 1695, and anevent on the Damghan fault in 856. The Ipak reverse faultruptured in 1962 with an earthquake of M 7.2. The penultimatehistorical event probably occurred in the third millenniumB.C.The long historical record suggests that the areas of historicalearthquakes are not likely to sustain earthquakes ofM -- 8 where earthquakes rupture segments individuallyrather than several segments at once. Nonetheless, severallong, straight strike-slip faults (Doruneh, Nayband, WestNeh, East Neh, Kahurak) have not sustained historical earthquakesand, by analogy with large earthquakes in Mongoliain the twentieth century, may be capable of earthquakes ofM8.The seismic moment release rate in I_ran accounts foronly a small part of the convergence rate between the Arabianand Eurasian plates. The deficit may be due to aseismicslip, or it may be because the Iranian historical record, longas it is, is not sufficient to sample long-term deformationacross the plateau. To resolve this question for purposes ofseismic hazard assessment, a national program should beestablished to (1) determine the paleoseismological historyof faults that have produced earthquakes, as well as longstrike-slip faults that have not, and (2) establish a GPS network,including permanent stations around major metropolitanareas, to monitor the present-day strain accumulationacross the Iranian plateau.AcknowledgmentsThis article was first presented in September 1994, at the Marshall, California,workshop on paleoseismology sponsored by the Inter-Union Commissionon the Lithosphere (ICL) and the U.S. Geological Survey, withadditional support from the U.S. Nuclear Regulatory Commission. M.B.thanks UNESCO, Paris, especially Badauoi Rouhban, and the InternationalInstitute of Earthquake Engineering and Seismology (IIEES), especiallyMohsen Ghafuri-Ashtiani, Tehran, for support for his work published asBerberian (1995b), as well as Manuchehr Qorashi and other colleaguesthe Geological Survey of Iran for valuable discussions. Many of the fieldinvestigations were carried out while M.B. was working for the GeologicalSurvey of Iran. M.B. acknowledges travel support from ICL, GeologicalSurvey of Iran, and IIEES to investigate surface rupture on the Abiz faultas a result of the 1997 earthquake of Ms 7.3. R.S.Y. received support fromNational Earthquake Hazards Reduction Program Grant 1434-94-G-SC5090 from the U.S. Geological Survey. We thank James A. 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