Patterns of Historical Earthquake Rupture in the Iranian Plateau

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