Abiz earthquake surface rupture
A tree offset 1.2 m right-laterally by coseismic faulting in the 1997 Zirkuh earthquake in eastern Iran. The view is to the west near Extend, about 5 km S of Korizan in Fig. 66. (Photo courtesy M. Ghorashi.)
Part of the 1997 Zirkuh earthquake ruptures near Zeydan, about 30 km S of Ardekul in Fig. 66. The view is to the SE. The offset here is 2 m right-lateral strike-slip and up to 2 m vertical, down to the east. The ruptures occur within the high mountains of Sepestan Kuh and Zardalu Kuh, and cannot be on a fault responsible for the mountains’ formation. Note the pond in the valley, formed by damming of a small stream flowing from right to left.
Surface rupture of 1997 earthquake near Abiz.
Abiz earthquake surface rupture and the new small lake.
Dasht-e-Bayaz/Abiz Faults
The first two study regions are part of the reverse faulted ranges that extend across northern Iran, including the Alborz Range. To the southeast, across the central Kavir depression, the structural style changes dramatically Patterns of Historical Earthquake Rupture in the Iranian Plateau 127 to strike-slip faulting: left-lateral faults striking east-west and right-lateral faults striking north-south to NNW-SSE. Subordinate to the strike-slip faults is northwest striking reverse faults, either bounding individual ranges or splays of larger left-lateral strike-slip fault zones. The Abiz, East Neh, and Nosratabad right-lateral faults bound this region on the east, following the older Sistan suture, east of which is the more stable Afghan continental block.
South of the northward-convex Doruneh fault is a group of faults highlighted by a sequence of earthquakes of which the best known is the 1968.08.31 earthquake of Ms 7.4 on the Dasht-e-Bayaz fault (Tchalenko and Ambraseys, 1970; Tchalenko and Berberian, 1975). Earthquake interaction and triggering between the Dasht-e-Bayaz and the 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-Bayalz left-lateral strike-slip fault consists of an -70-km-long west segment that ruptured in 1968 (Ms 7.4) and a 50 km-long east segment that ruptured 11 yr later on 1979.11.27 (Ms 7.1). The 1979 earthquake and two smaller events on the Abiz fault struck during a period of civil disturbance, and they were not studied shortly after the earthquakes in as much detail as the 1968 earthquake. The two segments of the Dasht-e-Bayaz fault are separated by the north-south-trending Mahyar right-lateral strike-slip fault. The intersection is marked by structural complexity, including a local change of strike, a zone of splays of the Dasht-e-Bayaz fault, and a right step of about 1 km on the Mahyar fault. The Mahyar fault extends farther south with a left step over to the Pavak fault. West of this north-south fault, an earthquake of Ms 6.4 with a strike-slip focal mechanism struck north of the city of Qaen in 1976.11.07, near the Dasht-e-Bayaz segment boundary in a region containing left-slip and reverse-slip faults, all trending east-west.
The 1979 Dasht-e-Bayaz earthquake was the third in a west-to-east progression of strike-slip earthquakes within an 11-yr time period. In addition, the 1979 earthquake was part of a shorter-term sequence that began in 1979.01.16 with an earthquake of Ms 6.8 with a reverse-fault focal mechanism; this event has not been assigned to a mapped fault. The 16 January event was followed on 14 November by an earthquake of Ms 6.6 with surface faulting along 20 km of the north-trending Abiz fight-lateral strike-slip fault (Berberian et al., 1999).
Next came the main Dasht-e-Bayaz event of Ms 7.1 on 27 November A. large aftershock of Ms 6.1 on the Abiz fault on 7 December produced surface rupture along the remaining 15 km of the northern tip of the Abiz fault south of its intersection with the Dasht-e-Bayaz fault that had not ruptured during the 14 November event. The ISC epicenter of the 1979.11.27 earthquake, with 411 readings, is located on the eastern tip of the Dasht-e-Bayaz fault at the junction with the Abiz fault. This suggests loading of the eastern segment of the Dasht-e-Bayaz fault by the Abiz event on 14 November; the rupture started from the junction of the two faults and propagated toward the west. Thus, the 1979 cluster included one on a left-lateral fault, two on a right-lateral fault, and one on a reverse fault. Prior to the two, 1979 earthquakes along the northernmost Abiz fault, an earthquake of Ms 6.0 took place on 1936.06.30 south of the 1979.11.14 meizoseismal area. At least 12 km of surface rupture accompanied the 1936 event, but approximately 15 km of the Abiz fault between the 1936 and 1979 earthquakes did not rupture at the surface (see arrows in Fig. 5). On 1997.05.10, almost 18 yr after the 1979 earthquakes along the northern portion of the Abiz fault, an earthquake of Ms 7.3 produced right-lateral strike-slip rupture along the entire 125-km length of the Abiz fault, including those portions that had ruptured in 1936 and 1979 (Berberian et al., 1999). The 1997 epicenter was located at the southern end of the November 1979 surface rupture and north of the 1936 rupture.
The 1968 earthquake on the west segment of the Dashte-Bayaz fault was followed 20 h later by an earthquake of Ms 6.4 on the Ferdows reverse fault, possibly due to loading of that fault by the Dasht-e-Bayaze Earthquake (see discussion later). The Dasht-e-Bayaz earthquakes comprise a twentieth-century cluster, but the length of the twentieth-century interseismic interval is uncertain. Scattered pre-twentieth-century data in this region are not sufficient to correlate pre-twentieth-century damage to specific active faults. Left lateral offset of as much as 10 m across the Dasht-e-Bayaz fault (Ambraseys and Tchalenko, 1969) has been measured along old qanats (traditional Iranian system of irrigation composed of underground canals drawing water from mountain sources by gravity). Unfortunately, it is not possible at present to establish the age of the major displaced and disused qanats. The oldest known qanat in Iran was constructed in the second millennium B.C. in the Semnan area at the northern edge of the central Kavir depression (Mehryar and Kabiri, 1986). Assuming the qanats offset by the Dasht-e-Bayaz fault are as old as 4000 yr, the minimum slip rate on the Dasht-e-Bayaz fault would be 2.5 mm/yr. The 250-cm left-lateral slip released during the 1968 earthquake (Tchalenko and Berberian, 1975) could have accumulated in about 1000 yr at a rate of 2.5 mm/yr. Holocene offsets of 8 to 28 m are documented at Khidbas Creek as well as other drainages crossing the fault.
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