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