Dasht-e Bayaz Earthquakes of 31 August  1968  Mw.7.1

The Dasht-e Bayaz earthquake of 31 August 1968 killed about 10,000 people in a remote, arid area of eastern Iran, left 70,000 people homeless; and destroyed or damaged about 160 villages with a total population of 112,100. Out of 23,000 houses in the area, about 9600 (41%) were completely destroyed. The earthquake was associated with at least three sets of coseismic surface fault breaks: (i) the 70-km-long E–W-trending fault break (the western segment of the Dasht-e Bayaz fault), (ii) the NW–SE-trending fault branch (the Golbiz fault), and (iii) the NE–SW trending fault branch (the Mozdabad fault). This earthquake was associated with a set of coseismic left-lateral strike-slip surface fractures over a length of 70 km caused by the reactivation of the western segment of Dasht-e Bayaz fault. The trace of the earthquake fault extends from the western Kuh-e SiahRange across the Nimboluk plain, to the Kuh-e Meykay Mountains, and east of Chah Zighan in the east. The Khidbas section to the east, where the fault passes through the Mesozoic bedrock, starts where the trace enters the mountains and extends about 18 km eastward. Beyond this section, the fault continues for another 30 km and dies out in the Rud-e Shur Desert. The maximum components of displacement that accompanied the 1968 earthquake were measured in the Nimboluk plain and amounted to 4.5-m left-lateral and 2.5-m vertical. All horizontal displacements were left-lateral, but vertical displacements varied along the trace. The longest sections, however, and the entire Khidbas section in particular, showed a relative lowering of the southern block (the Nimboluk Plain). Coseismic surface deformation along straight segments of the Dasht-e Bayaz fault in alluvial deposits of the Nimboluk plain was made up of a systematic array of Riedel shears interspersed with compressional ridges. The width of the surface rupture zone ranged from 2 m to about 100 m but broadened in the vicinity of major dilational strep-overs (at the SW of Dasht-e Bayaz and the west of Khidbas creek) with cross-strike dimensions greater than 1 km. Where the near-linear surface rupture trace was exposed in the Mesozoic bedrock (the Khidbas section), field mapping revealed that most of the strike-slip displacement was restricted to a shear zone less than 40 cm wide. Horizontal displacement along the coseismic surface rupture decreases in the area between the eastern Nimboluk plain section and the western Khidbas section, and a gap of about 4 km becomes apparent between the two sections. Furthermore, the general alignment of the Khidbas fault trace is displaced about 1 km to the north with respect to the Nimboluk trace. Measured vertical and horizontal earthquake displacement decreases consistently as the gap is approached from the east or the west, suggesting a genuine discontinuity in the displacement field. A remarkable feature about this gap is that the step from one trace to the other is to the left; that is, it is opposite to the one taken when proceeding from one Riedel shear to the next in a leftlateral, en-echelon set of fractures. This “anti-Riedel” configuration is therefore contrary to the one characterizing a natural evolution of a wrench fault. Two step-overs (dilational jogs) with cross-strike dimensions of about 1 km are particularly prominent within the broad infrastructure of the Dasht-e Bayaz left-lateral strike-slip fault. These features developed in (i) the gap area between the Bedrock (Khidbas, in the east) and the alluvium (Nimboluk, in the west) segments and (ii) the area to the SW of Dasht-e Bayaz. Abrupt changes in the measured slip occur in the vicinity of these features. An area of intense sandblows immediately adjacent to one of the jogs is also noteworthy. The Dasht-e Bayaz left-lateral strike-slip fault consists of a 70-km-long west segment that ruptured during the 31 August 1968 Mw7.1 Dasht-e Bayaz earthquake and a 55-km long east segment that ruptured 11 years later during the 27 November 1979 Mw7.1 Koli earthquake. The 1979 earthquake and two smaller events on the N-S Abiz fault to the east occurred during a period of civil disturbance and were not studied soon 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–southtrending, 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 1968 earthquake was part of an unusual 13 large- to medium-magnitude earthquake cluster during a short period of 61 years from 1936 to 1997 on complex fault systems.