Seismological and field observations from the 1990 November 6 Furg (Hormozgan) earthquake: a rare case of surface rupture in the Zagros mountains of Iran

Abstract

S U M M A R YWe document a unique example of a co-seismic surface rupture in the Zagros mountains of SE Iran. Using P-and SH-waveform inversion, aftershock distributions and field observations we show that the M w 6.4 1990 November 6 Furg (Hormozgan) earthquake was associated with ∼15 km of south-facing surface ruptures with an average vertical displacement of ∼1 m. Earthquakes of M w 6-7 are common in the Zagros mountains of Iran. However, no co-seismic surface ruptures associated with thrust faulting have been reported before now, and blind thrusting appears to be more common. Source parameters determined from seismology and measurements from the surface ruptures indicate slip on a fault dipping ∼30 • N. Although there is likely to be considerable uncertainty in at least some locations, the aftershock activity measured in the month following the earthquake appears to be broadly confined to a diffuse area around the northward-dipping fault. The geomorphology of the epicentral region contains indications of both long-term Quaternary uplift and cumulative fault scarps from repeated earthquakes. The Furg earthquake probably occurred on a reactivated section of the High Zagros fault, a geological suture from the early history of the Iran-Eurasia collision. The 1990 earthquake is likely to have ruptured to the surface because of its unusual location in the High Zagros zone, away from the present-day focus of seismic activity in the Simply Folded zone of the Zagros. It is possible that other sections of the High Zagros sutures still have the potential to generate earthquakes.

Seismotectonics of the Zagros (Iran) From Orogen‐Wide, Calibrated Earthquake Relocations

“…The Zagros itself is often divided into two structurally and tectonically distinct domains (Figures b, , and ): (1) The High Zagros is a zone of high topography (maximum elevations of ∼4,000 m), features steep, NE dipping, surface‐breaking reverse faults and exposes older and more deformed Arabian plate rocks (Authemayou et al, ; Berberian, ; Falcon, ; Talebian & Jackson, ); (2) the SFB is lower in elevation and characterized by long (up to ∼200 km), parallel arrays of anticlines and synclines, mostly concentric in shape (though some verge toward the SW) and with typical half‐wavelengths of ∼10 km. The NW striking High Zagros Fault marks the boundary between these two structurally distinct domains and is associated with the only record of cosesimic surface rupture in the Zagros, in the 6 November 1990 M w 6.4 Furg earthquake (Walker et al, ; Figure b). The SFB is further subdivided along strike into the higher elevation Lurestan arc, Izeh zone, and Fars arc, and the lower elevation Kirkuk embayment, Dezful embayment, and Oman syntaxis.…”

“…The resulting minimally biased locations help us investigate whether seismicity is localized along large basement faults, in agreement with Berberian (), or distributed more diffusely among shallow faults and folds as some newer studies have proposed (e.g., Allen et al, ; McQuarrie, ). A related test is whether earthquake faulting is concentrated within the outermost part of the SFB, as has been suggested on the basis of sparse campaign GPS measurements (Walpersdorf et al, ), or whether many interior parts of the Zagros are also still seismically active (Walker et al, ). Finally, with minimally biased depth resolution now possible for many newer events, we provide new estimates of the seismogenic depth range which helps distinguish the relative importance of basement and cover faulting (e.g., Barnhart & Lohman, ; Nissen et al, , ).…”

New views on earthquake faulting in the Zagros fold-and-thrust belt of Iran

“…Jackson & Fitch 1981; Berberian 1995; Talebian & Jackson 2004). Partly because of the very rare (Walker et al 2005) and usually absent indications of coseismic ruptures at the surface, these earlier studies concluded that the larger earthquakes in the Zagros involve faulting mostly within the basement. However, more recent geodetic studies, which use radar interferometry (InSAR) to determine the depth extents of coseismic faulting, have revealed that moderate‐sized earthquakes (up to M w ∼ 6) also occur within the sedimentary cover, rupturing between depths of ∼4 and ∼9 km (Nissen et al 2007, 2010; Roustaei et al 2010).…”

Intermediate-term variations in 200 years seismicity of south of Iran

“…Hence, the earthquake data was mainly extracted from the IIEES database and other documented earthquake data (Berberian & Yeats 1999;and Berberian 2005; for the eastern Iran; Walker et al 2005;Nemati et al 2012 andBHRC 2014 for the Zagros; Ambraseys and Melville 1982;Berberian 1995aBerberian , 1995bEngdahl 2013;ISC 2014;and CMT catalogue 2013 for both areas) in 26 NÀ36 N and 45 EÀ64 E. Totally, 703 moderate and large earthquakes (M 5.0) were investigated for the seismicity rate assessment in the eastern Iran (235 events) and Zagros (468 events) seismotectonic provinces in which 73 earthquakes in the eastern Iran and also 78 earthquakes in Zagros have macroseismic data.…”