Patterns of Historical Earthquake Rupture in the Iranian Plateau

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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
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Patterns of Historical Earthquake Rupture in the Iranian Plateau

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