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Palaeoseismic evidence for a medieval earthquake, and preliminary
estimate of late Pleistocene slip-rate, on the Firouzkuh strike-slip fault in
the Central Alborz region of Iran
H. Nazari
a,b,
⇑
, J.-F. Ritz
b
, R.T. Walker
c
, R. Salamati
d
, M. Rizza
b
, R. Patnaik
e
, J. Hollingsworth
f
,
H. Alimohammadian
d
, A. Jalali
d
, A. Kaveh Firouz
d
, A. Shahidi
d
a
Research Institute for Earth Sciences, Geological Survey of Iran, P.O. Box 13185, 1494 Tehran, Iran
b
Laboratoire Géosciences Montpellier, UMR 5573, Université Montpellier 2, 34095 Montpellier Cedex 05, France
c
COMET+, Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
d
Geological Survey of Iran, P.O. Box 13185, 1494 Tehran, Iran
e
Centre for Petroleum and Applied Geology, Panjab University, Chandigarh 160014, India
f
Géoazur, Les Lucioles, UMR 7329, Sophia-Antipolis, Nice, France
article info
Article history:
Received 23 July 2013
Received in revised form 13 December 2013
Accepted 16 December 2013
Available online 27 December 2013
Keywords:
Paleoseismology
Slip-rate
Medieval earthquake
Firouzkuh Fault
Iran
abstract
The 55 km-long Firouzkuh fault is located in the Central Alborz Mountains of Iran. It is a left-lateral
fault, which dips to the south, and possesses a small dip-slip component of motion that we interpret
to result from extension. The ratio of horizontal to vertical displacement across the fault, calculated from
the cumulative displacement of landscape features, is 7.6. We provide constraints on the timing of the
last earthquake on the Firouzkuh fault from two trenches (T1 and T2) across the fault zone, excavated
in 2004, and located east of Firouzkuh city. The trenches expose faulted sedimentary deposits. Two opti-
cally-stimulated luminescence (OSL) ages from sediments in the lower part of trench T1 date from the
late Pleistocene (15.9 ± 0.9 ka and 27.1 ± 1.7 ka). The younger of the two dated units in T1 is displaced
vertically across the fault by 2.2–4.4 m, from which we estimate a strike-slip displacement of 18.2–
33.4 m, and hence a average horizontal slip-rate of 1.1–2.2 mm/yr. The sediments exposed in T1 do
not yield constraints on the most recent earthquake history. In trench T2, however, human skeletal
remains of a middle aged male, which yield a radiocarbon age of 1159 ± 28 BP (corresponding to a mean
calendar age of 791 AD), were found within a faulted alluvial layer at a depth of 60–70 cm from the sur-
face. The existence of these medieval human places shows that a surface-rupturing earthquake occurred
at some time after 1159 ± 28 BP. The amount of slip in each earthquake on the Firouzkuh fault is difficult
to estimate, but assuming the entire 55 km fault length ruptures in each event, they will have had a
maximum magnitude of 7.1. At our estimated late Quaternary slip-rate of 1.1–2.2 mm/yr magnitude
7.1 earthquakes, involving 1.2 m average displacement, would be expected to occur every 1100–
540 years. As the last earthquake on the Firouzkuh fault may be up to 700 years in age we suggest that
the Firouzkuh fault is a major hazard for earthquakes in the near future.
Ó2013 Published by Elsevier Ltd.
1. Introduction
The active faults within the Alborz Mountains of northern Iran
pose a continuous hazard to local populations (Fig. 1a). Many ma-
jor population centers – including the capital city of Tehran with a
population of 12 million – are situated along the southern margin
of the Alborz range and hence are at risk from future seismic
events. Constraining the late Quaternary seismic history on
individual faults within the Alborz is thus a high priority for under-
standing seismic hazard within Iran. In addition, the major faults
are also important elements in the deformation caused by the Ara-
bia–Eurasia continental collision and details of their rate and style
of slip are vital for understanding how they accommodate tectonic
strain (e.g. Ritz et al., 2006; Nazari et al., 2009; Solaymani Azad,
2009; Landgraf et al., 2009;; Hollingsworth et al., 2010; Rizza
et al., 2011).
In this paper we investigate palaeoseismic evidence for the
most recent surface-rupturing earthquake on the Firouzkuh left-
lateral strike-slip fault (Fig. 1b and c). The Firouzkuh fault is one
of a system of left-lateral strike-slip faults that run through the
middle of the Alborz mountains and are thought to accommodate
1367-9120/$ - see front matter Ó2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.jseaes.2013.12.018
⇑
Corresponding author at: Research Institute for Earth Sciences, Geological
Survey of Iran, P.O. Box 13185, 1494 Tehran, Iran. Tel.: +98 2166070518; fax: +98
2166070511.
E-mail address: hamidnazari@hotmail.com (H. Nazari).
Journal of Asian Earth Sciences 82 (2014) 124–135
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes
the westward motion of the South Caspian block relative to sur-
rounding parts of Iran (e.g. Jackson et al., 2002; Ritz et al., 2006).
The Firouzkuh fault trends NNE–SSW and extends for almost
55 km in the Central Alborz Mountains (Fig. 1b). It connects to
the Mosha Fault in the west and to the Asteneh fault in the east.
The Astaneh fault may have generated an extremely destructive
historical earthquake in 856 AD (e.g. Ambraseys and Melville,
1982; Berberian and Yeats, 1999; Hollingsworth et al., 2010; Rizza
et al., 2011). The Mosha fault has also been suggested as the source
of earthquakes in 1665 and 1830 AD, (Ritz et al., 2003; Nazari,
2006; Solaymani Azad, 2009).
The Firouzkuh fault is easily traceable on satellite images and
aerial photographs, suggesting that it has been active in the late
Quaternary (Berberian, 1976; Jackson et al., 2002; Nazari, 2006;
Ritz et al., 2006). The Morphotectonic and palaeoseismic study of
the Mosha fault, which lies immediately west of the Firouzkuh
fault, yielded an average left-lateral slip-rate of 2 mm/yr. (Ritz
et al., 2003; Nazari, 2006; Solaymani Azad, 2009). GPS studies of
present-day crustal deformation indicate 2.5 ± 1.5 mm/yr of
left-lateral shear across the Alborz Mountain range at the longitude
of the Firouzkuh fault (Vernant et al., 2004). Despite the clear
expression of the Firouzkuh fault in the geomorphology there is
no record of large-scale seismic activity in either historic or instru-
mental periods of observation. The largest recorded seismic event
in the area was the Gadok earthquake of 1990 with a surface-wave
magnitude of 5.9 (Berberian et al., 1996;Fig. 1b). A number of
Fig. 1. (a) Map of the South Caspian region (modified after Ritz et al., 2006) with active faults and focal mechanisms (larger spheres with dates correspond to earthquakes
having magnitude > 6). Focal mechanisms are from Jackson et al. (2002) for the Apsheron sill and Kopet Dagh regions, and from Ashtar et al. (2005) for the Alborz. Gray
spheres are from McKenzie (1972) for 1957 earthquake, from Jackson et al. (2002) for the 1962 and 1990 earthquakes and from the U.S. Geological Survey for the 2004
earthquake. Abbreviations: R – Rudbar fault, T – Taleghan fault, M – Mosha fault, F – Firuzkuh fault, A – Astaneh fault. The red rectangle outlines the region shown in part ‘b’.
(b) Seismotectonic sketch map of the Central Alborz. The light brown circles represent the meisoseismal areas of historical earthquakes and a single instrumental earthquake;
the Gadok earthquake in 1990 AD (after Berberian et al., 1993; Berberian and Yeats, 1999, 2001). Focal mechanisms are from the Harvard CMT catalogue. Red circles show
epicenters of earthquakes with Mw > 5 (from the catalogue of Engdahl et al., 1998). Continuous GPS stations (2008) and velocity vectors (in mm/yr) relative to Eurasia are
shown by red arrows (http://www.ncc.ir). Rivers are shown as blue lines. Dashed thin and thick brown lines represent inferred, minor and major faults. Our
paleoseismological sites are marked by green rectangles. (c) A close-up view of seismic activity along the Firouzkuh fault shown by blue rectangle in Fig. 1b, with focal
mechanisms obtained from full inversion of waveforms modeling from small locally-recorded earthquakes (Momeni, 2012). Normal focal mechanisms are well consistent
with a normal component that seen after morphotectonics observation (Ritz et al., 2006; Nazari et al., 2011b) along the Firouzkuh fault. Green circle shows first class and
yellow one means second order of the seismic waveform modeling, locality of the Firouzkuh city shown by Fi in white rectangle. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135 125
small-magnitude earthquakes (body-wave magnitude < 4.8) oc-
curred in 1969, 1973, 1975, 1979, 1985, 1989 2008 and 2010 AD
(http://www.iiees.ir), (Fig. 1).
The seismicity recorded by Iranian National Seismic Network
(Institute of Geophysics, University of Tehran, IGUT) during 2006
and 2012 indicates on seismic activity of the Firouzkuh fault
(Fig. 1c). Distribution of local microearthquakes recorded by IGUT
is well consistent with activity of the Firouzkuh fault, and a very
clear alignment of seismicity can be related to this fault (Fig. 1c).
Regional moment tensor solution of earthquakes (Zahradnik
et al., 2008) recorded by Iranian National Broadband Seismic
Network (International Institute of Earthquake Engineering and
Seismology, IIEES) indicates on fault-parallel strike-slip motion
with considerable extensional component perpendicular to the
Firouzkuh fault (Momeni, 2012). The observed extension is due a
left-stepping offset on Firouzkuh sinistral fault which formed a
pull-apart basin along the mentioned fault in the region (Fig. 1a
and b). The computed focal mechanism solutions for small size
earthquakes (Ml < 4) are consistent with observed extension of
previous morphotectonic studies (Ritz et al., 2006; Nazari et al.,
2011a,b). The focal mechanism solutions of earthquakes computed
applying regional moment tensor inversion reveal clearly the left-
lateral strike-slip motion of two other segments of en echelon left-
stepping strike-slip fault system of Astaneh–Firouzkuh–Mosha.
In contrast to the Firouzkuh fault, large earthquakes are known
from other parts of the Alborz Mountains. The most recent destruc-
tive earthquake within the Alborz region is the 1990.06.20 Mw 7.2
Rudbar earthquake in the western part of the range (Berberian
et al., 1992; Tatar and Hatzfeld, 2009; Berberian and Walker,
2010), (Fig. 1a and b). Large historical earthquakes, including
events in 958 AD, 1485 AD and 1608 AD, are also known from
the western Alborz. These events may be associated with left-lat-
eral strike-slip faults displaying late Quaternary activity in the
mountainous regions between the Taleghan fault and the Rudbar
fault (e.g. Nazari et al., 2009; Berberian and Walker, 2010).
Destructive earthquakes are also known from the eastern Al-
borz, including the 856 AD Qumis earthquake (Io = X (MMI), Mod-
ified Mercalli intensity scale, M = 7.9 after Ambraseys and Melville,
1982), which killed an estimated 200,000 people (Berberian and
Yeats, 1999). The most likely source of the Qumis earthquake is
the Astaneh left-lateral strike-slip fault (Hollingsworth et al.,
2010). One of the aims of our present study is to examine whether
rupture in the 856 A.D. earthquake may have continued as far west
as the Firouzkuh fault.
Fig. 2. A view of the Firouzkuh Fault and adjacent area using imagery from Google Earth (http://www.earth.google.com). The Firouzkuh Fault shown as a red line. The white
rectangles indicate areas shown in Fig. 3 (Harandeh village), Fig. 4 (the sag pond behind shutter ridge formed by left lateral movement on Firouzkuh Fault) and Fig. 5 (our
palaeoseismological sites). Yellow filled rectangles are TF1 and TF2; blue lines show principal drainage channels; orange regions represent scarps; and violet colors highlight
terraces. White abbreviation letters are captions for the geological units: C, carboniferous shale and limestone; P, Permian limestone; TR, Triassic calcareous deposits; J,
Jurassic deposits; K, Cretaceous limestone; PE, Paleocene rocks; E, Eocene volcano-sedimentary deposits; M, Miocene marl and limestone; Qt, Quaternary (undifferentiated
young deposits); Qal, alluvium deposits. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Left lateral displacement (100 m) of a Cretaceous ridge and river due to left
lateral strike-slip faulting. Approximate Fault trace shown by red line on field photo
of Harandeh village (see Fig. 3 for location). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
126 H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135
The absence of historical records of earthquakes on the Fir-
ouzkuh fault may be a result of the low population density (prior
to the 20th Century population explosion in Iran) and the relatively
remote location of this mountainous region. However, the Fir-
ouzkuh fault is situated close to the cities of Firouzkuh, Damavand,
Semnan and Tehran (with a combined population of nearly 13 mil-
lion people, Fig. 1). Our aim in this paper is to constrain both the
timing, and the probable magnitude, of the last seismic event on
the Firouzkuh fault. We provide a minimum age on this event from
radiocarbon dating of human bones recovered from young
alluvial deposits that postdate the most recent identifiable fault
displacement. We also provide the first estimate of the average late
Pleistocene slip-rate on the fault from optically-stimulated lumi-
nescence dating of deposits that have been displaced by multiple
earthquake events.
2. Geology and active tectonics of the Firouzkuh region
Structural analyses carried out in the southern part of the
Central Alborz show that the evolution of this belt has been
strongly conditioned by the inversion of pre-existing xtensional
Fig. 4. (a) Aerial photo (scale 1:55000) taken in 1955 of the western part of the Firouzkuh fault. The fault trace is highlighted by red arrows. (b) A perspective view of a large
scale Digital elevation model (DEM) produced from kinematic GPS surveying. (c) The GPS DEM shown in plain view with structural and drainage features highlighted. The
fault is represented by a red line; blue lines represent drainage channels; brown and violet lines delineate a topographic crest that we use to calculate the ratio of vertical and
horizontal displacement across the fault. The vertical displacement is shown by the topographic profile in the lower left corner of the image. The total horizontal displacement
is shown by the distance H–H
0
. (d) to (f) show restorations of increasing amounts of left-lateral slip, each of which realign elements of the drainage and topography. See text
for further details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135 127
faults (Zanchi et al., 2006; Nazari et al., 2007). Recent deformation
within the Alborz is transpressional, with sinistral strike-slip faults
present along the entire length of the chain, and running parallel
with reverse faults and overthrusts (Jackson et al., 2002; Allen
et al., 2003; Nazari, 2006; Ritz et al., 2006; Hollingsworth et al.,
2008). The occurrence of a recent change in the kinematics of fault-
ing in the internal parts of the Central Alborz is inferred from struc-
tural observations along several of the active faults including the
Firouzkuh, Mosha and Taleghan faults (Ritz et al., 2006); (Fig. 1a).
The Firouzkuh fault trends northeast-southwest and generally
dips at 55°–70°to the SSE. It separates Jurassic–Cretaceous depos-
its in the NNW from Quaternary deposits in the SSE (Aghanabati
and Hamedi, 1994); (Fig. 2). A slight vertical component in addition
to the predominant left-lateral sense of motion indicates that the
Firouzkuh fault may possess a small normal component of slip
(N060E 70S 05E) (Nazari et al., 2007); (Figs. 3 and 4). We generated
a high-resolution digital elevation model (DEM) of a section of the
fault where it laterally displaces a prominent NE–SW trending
ridge. The ridge is displaced laterally by 434 m (distance H–H
0
in
Fig. 4b), and vertically by 57 m (distance A
0
B), allowing us to esti-
mate a ratio (H/V) of 7.6 between the horizontal and vertical com-
ponents. The Alborz Mountains accommodate 5 ± 2 mm/yr of N–S
shortening (e.g. Vernant et al., 2004; Masson et al., 2005) and the
vertical component might be expected, therefore, to arise from
shortening along the fault (Berberian et al., 1996). However, it is
possible that the strike-slip faults of the high Alborz accommodate
extension resulting from the western motion of the South Caspian
relative to Central Iran and Eurasia; with the overall Arabia–Eur-
asia convergence instead accommodated by reverse faults at the
margins of the range (e.g. Ritz et al., 2006; Nazari and Ritz,
2008). Understanding the overall structure of the Firouzkuh fault
is complicated by bending, which might locally introduce vertical
components not representative of the overall fault motion.
3. Palaeoseismic trenching of the Firouzkuh fault
Palaeoseismological research on the Firouzkuh fault was carried
out between 2004–2006. We report the findings from two trenches
(TF1 and TF2) that were excavated northeast of Firouzkuh city in
2004 (Fig. 2). The trenches were dug perpendicular to the general
trend of the Firouzkuh fault (Figs. 2 and 5). Trench TF1 is 65 m
long and TF2 is 40 m, each one has a width of 1.5 m and an aver-
age depth of 4 m. The eastern trench (TF1), has coordinates of
35°47
0
20
00
N and 52°51
0
05
00
E, and is situated on lacustrine deposits
that we interpret to have formed due to localized ponding at the
scarp of the Firouzkuh fault (Fig. 6). The TF2 trench is situated
500 m west of the TF1 trench at 35°47
0
12
00
N, 52°50
0
43
00
(Fig. 5).
TF2 is excavated across the gouge zone of the Firouzkuh fault
and exposes heavily deformed and faulted Eocene–Miocene sedi-
ments covered by <1 m thickness of Quaternary colluviums.
Two sediment samples were collected from trench TF1 and da-
ted using the Optical Simulated Luminescence (OSL) method at the
Luminescence laboratory of the US Geological Survey in Denver.
Radiocarbon dating of human bones (performed at the Oxford
radiocarbon Accelerator Unit) provided valuable age control from
trench TF2.
3.1. Interpretation of trench TF1
Our paleoseismological investigation of Trench TF1 (Fig. 7b and
c), which included detailed logging of the 260 m
2
surface of the
eastern wall, allows us to propose at least five paleo-earthquakes
from top to base of the trench:
Event 0 (the most recent event) – Unit 5 corresponds to the sur-
face soil that is cut by some fine joints in north part of the trench
TF1, (Figs. 6,7a and 8a). Although no more than 4 cm of vertical
displacement was observed within unit 5, the existence of minor
faulting that reaches the surface with a small vertical displace-
ment, allows us to interpret unit 5 as the most recent event horizon
in TF1. Unit 5 could correspond to a deposit following one of the
strong historical earthquakes (856 AD, 958 AD, 1301 AD, 1177
AD, 1665 AD and 1830 AD) that occurred in the region. On the
bases of paleoseismological studies on other active faults in Central
Alborz, such as the Taleghan fault (Nazari, 2006; Nazari et al.,
2009); the Mosha fault (Ritz et al., 2003; Solaymani Azad, 2009)
and the Astaneh fault (Nazari et al., 2007; Hollingsworth et al.,
Fig. 5. Aerial photograph (scale 1:55000; taken in 1955) showing the two trench locations. Red arrows indicate Firouzkuh fault trace along the boundary between highlands
to the southeast and flat plains to the Northeast. Trenches TF1 and TF2 are shown by filled yellow rectangles. They were excavated perpendicular to the fault. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
128 H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135
2010), the reported historical earthquakes in 958 AD on the Tale-
ghan fault, in 1665 AD and 1830 AD along the Mosha fault and
856 AD took place on the Asteneh fault. We infer that unit 5 in
trench TF1 was deposited synchronously with the uppermost unit
in trench TF2 (Unit 1), which yielded an age younger than a C
14
age
of 1159 ± 28 BP.
Event 1 – We interpret unit 8 as a colluvial deposit in the north-
ern part of trench TF1 that is isochronous with unit 12 in the cen-
tral and southern parts of the trench (Figs. 7a and 9). Units 8 and 12
cover, and have eroded into the underlying Units 3, 4 and 7, which
themselves appear to be a colluvial wedge in the northern part of
the trench. Unit 12 has a thickness of 1.2 m and is cut and dis-
placed by faulting between horizontal grid line 56 and 59 m on
the log of the eastern wall of the trench TF1 (Figs. 8a and 10).
The top of Unit 12 constitutes the event horizon of Event 1. A fis-
sure of Event 1 within Unit 12 was subsequently in filled with Unit
14 in the southern part of trench TF1 (Figs. 7a and 9).
As we cannot trace the continuation of Unit 12 immediately
south of the fault plane (Fig. 9) it is difficult to estimate the vertical
displacement in Event 1 with accuracy. Extrapolation of the dip of
the base of Unit 12 between metres 44 and 48 of the trench wall to
the fault indicates at least 2.4 m of vertical displacement across the
fault (points A
0
and A
00
in Fig. 10). Using a horizontal to vertical slip
ratio of 1–7.6, a vertical displacement of 2.4 m should correspond
to horizontal displacement of 18 m. As such, it is likely that Event
1 is actually a composite of several indistinguishable seismic
events. The base of Unit 12 is dated with a single OSL sample at
15.9 ± 0.9 ka. It is likely that Units 12 and its Suggested corre-
sponding unit 8 have been displaced by several earthquakes since
15.9 ± 0.9 ka (Table 2).
Event 2 – Unit 4: We interpret the colluvial Unit 4, which over-
lies and has eroded into the underlying Units 1, 2, 3, 6 and 7, as a
colluvial wedge developed following surface rupture in Event 2.
Unit 4 is cut by two faults (Figs. 6–8). The northernmost of the
two faults has not cut unit 4 completely, so it is possible that this
unit is actually composed of two superimposed colluvial wedges.
In this hypothesis, Unit 8 would be deposited between two earth-
quake events and Unit 9 would post date the younger event. If we
consider the occurrence of only a single event, the total observed
accommodated distributed displacement on two fault planes be-
tween A–A
0
and B–B
0
is 95 cm.
The northern exposure of Unit 4 is dated at 27.1 ± 1.7 ka from
one OSL sample (Fig. 7). The overlying Unit 8 is likely to be syn-
chronous with Unit 12, which is dated at 15.9 ± 0.9 ka (Table 2).
Event 2 is therefore likely to have occurred in the interval between
15.9 ± 0.9 ka and 27.1 ± 1.7 ka.
Event 3 – Unit 7 is a colluvial wedge originating from the deg-
radation of a fault scarp in the northern part of the trench (Figs. 7
and 8d–e). Unit 7 is covered by Unit 4, which is a colluvial wedge
dated 27.1 ± 1.7 ka, (Table 2). We cannot make a direct estimation
of the vertical displacement associated with seismic event 3, but
the maximum thickness of Unit 7 allows us to evaluate a minimum
scarp height of 60 cm (distance c–c
0
in Fig. 8e).
Event 4 – Colluvial Unit 3 is present in both the northern and
southern parts of trench TF1 (Fig. 7). It is cut by the northern fault
zone and we suggest that it may have originated as a colluvial
wedge from the degradation of a fault scarp in this northern zone
(Figs. 7and 8f). The base of Unit 2 was not uncovered in the trench
and so we cannot make an estimate of the minimum scarp height
in Event 4. We have no age control on these deepest exposed sed-
iments, other than that they must pre-date the deposition of Unit 4
at 27.1 ± 1.7 ka.
3.2. Interpretation of Trench TF2
The TF2 trench is situated 500 m west of the TF1 trench (at
35°47
0
12
00
N, 52°50
0
43
00
;Fig. 5). Trench TF2 was excavated across
the gouge zone of the Firouzkuh fault (Fig. 10). An interpretation
of the western wall of the TF2 trench is provided in Fig. 11a and
b, a
0
and b
0
. The lower part of the trench exposes brecciated hard
sediments trending parallel to the Firouzkuh fault (Unit 7 in
Fig. 11). The brecciated sediments also form elongated outcrops
at the surface, which are folded, and are thought to be of
Eocene–Miocene age (Figs. 10 and 11).
The intensely faulted Eocene–Miocene sediments are overlain
by a relatively thin sequence (1 m thick) of young alluvial
deposits (Unit 1, 3, 4 and 5 in Fig. 11). The thinness of these
younger sediments makes it difficult to determine a detailed anal-
ysis of sedimentary structures and seismic histories from Trench
TF2. The young, surface sediments are clearly faulted as shown
by the presence of fissures that are present from the base of
the trench to the surface (Fig. 11c). This event, which cuts the
uppermost Unit 1, must postdate the deposition the Unit 4. We
assume that Unit 1 in trench TF2 correlates with Unit 5 in trench
TF1.
The alluvium in Unit 4 was found to bear the partial skeleton of
an adult human male at a depth of 60–70 cm from the surface
(Figs. 10 and 11a and a
0
). The human bones include a mandible
(jaw bone, Fig. 12A and B), the proximal part of a femur
Fig. 6. (a) Digital topographic map generated from a GPS kinematics survey
showing location of trench TF1 (green rectangle) within a shallow basin between
two active branches of the Firouzkuh fault (fault zones represented by red lines).
Yellow shading highlights areas that are covered by lacustrine deposits. Blue lines
show drainage channels. (b) Block diagram (modified after Nazari, 2006) of the
approximate area shown in part ‘a’ (red lines indicate the fault; yellow rectangle
shows paleoseismological trench (TF1) on Firouzkuh fault; blue lines show active
drainage and dashed white line shows uplifted and abandoned drainage). The map
is shown in a local zone UTM projection. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135 129
(Fig. 12C), two arms and many small finger bones. Unfortunately
the skull was crushed during excavation of the trench. The skeletal
elements do not show any evidence of rolling or abrasion that
might be expected during long periods of transportation. The
bones were subsequently covered by several tens of centimeters
of alluvial sediments. The high concentration of relatively well-
preserved and intact bones suggests that they were relatively fresh
when incorporated into these sediments.
The presence of human bones provides a means of constraining
the age of Unit 4, and hence of providing a maximum limit on the
age of the last earthquake on the Firouzkuh fault. Radiocarbon
measurements were made at the Research Laboratory for Archeol-
ogy and the History of Art, Oxford University on samples of the
mandible (Fig. 12A and B). Dates are given in radiocarbon years
BP (Before Present – AD 1950). Isotopic fractionation has been cor-
rected for using the d
13
C values measured by AMS. The quoted d
13
C
values are measured independently by mass spectrometry (to ±0.3
per mil relative to VPDB). Calibrated calendar ages were generated
using the Oxcal computer program (v4.0) of C. Bronk Ramsey, using
the ‘INTCAL04’ dataset (Reimer et al., 2004), and are presented in
Fig. 13 and Table 3. The calibrated age for the samples is in the
range 1159 ± 28 years BP (A.D. 763–819), (Table 3). As the alluvial
Unit 4, which yielded the human bone, is covered by recent faulted
sediments (Unit 1) the date of the earthquake event must be youn-
ger than A.D. 763–819.
4. Discussion
4.1. Estimate of late Pleistocene horizontal slip-rate
Although the age constraints on Units 4 and 12 do not provide a
detailed chronology of past earthquake events on the Firouzkuh
fault - as discussed in Section 4.2 – they are useful in providing
an initial estimate of the average late Pleistocene left-lateral slip-
rate on the Firouzkuh fault from the vertical displacement of the
base of Unit 12 across the southern fault zone exposed in trench
TF1 (Fig. 9). Extrapolation of the dipping base of Unit 12 to the fault
plane provides estimates of vertical displacement between 2.4 m
and 4.4 m (the extrapolations are represented by lines connecting
to points A
0
and A
00
in Fig. 9). Using the horizontal to vertical slip
ratio of 7.6 calculated in Section 2, the estimates of vertical dis-
placement of Unit 12 corresponds to horizontal displacements of
18.2 m to 33.4 m. As the age of the mid- to top part of Unit 12
is constrained from a single OSL sample at 15.9 ± 0.9 ka (Table 2
and Fig. 9), we estimate the horizontal slip-rate for the Firouzkuh
fault at 1.1–2.2 mm/yr.
Fig. 7. (a) Log of the eastern wall of the TF1 trench (location shown in Figs. 2 and 6). Unit 1: Red to light brown gouge materials, silty-clayed matrix, 10% clasts (5 mm–10 cm),
poorly sorted and rounded, non-stratified; Unit 2: Buff to white silt with gypsiferous matrix, 60% clasts (5 mm–10 cm), poorly sorted, moderate rounded, non-stratified,
colluvial wedge; Unit3: Light brown silt with silty-sandy gypsiferous matrix, 30% clasts (3 mm–4 cm), moderate sorted and rounded, poorly stratified; Unit 4: Light brown
gypsiferous silt, clay matrix, 10% clasts, poorly rounded, non-stratified, suggested as colluvial wedge in north part of TF1; Unit 5: Brown silt to sandy silt, gypsiferous in places,
40% clasts (3 mm–10 cm), moderate to poorly sorted and rounded, non-stratified, humus soil with roots, last event horizons in trench TF2; Unit 6: Light brown gypsiferous
silt, silty-clay matrix, 10% clasts, poorly rounded, non- stratified, similar to unit 15 but with much less gypsum; Unit 7: Buff, gypsiferous sandy silt, silt matrix, 70–80% clasts
(5 mm–5 cm), moderate sorted and poorly rounded; Unit 8: Cream to buff silt, muddy, sandy clayed matrix, 80% clasts (1–10 cm), poorly sorted and rounded, non-stratified,
the unit is interpreted as colluvial wedge in the north part of the trench; Unit 9: Light brown muddy-silt with sandy-clayed matrix, 30% clasts (1–10 cm), poorly sorted and
rounded, non-stratified; colluvial wedge; Unit 10: Greenish brown silt and sand, sandy-silt matrix, 5–10% clasts (5 mm–1 cm), well sorted and stratified; Unit 11: Brown
gypsiferous silt with clay matrix, 5–10% clasts, poorly sorted and rounded, non-stratified; Unit 12: Light grey to white sand, gypsiferous silt matrix, 5% clasts (P5 cm), poorly
rounded, non-stratified, isochronous with unit 8; Unit 13: Brown to dark brown silt, clayed-silt matrix, 40% clasts (7–15 cm) in middle part, moderate sorted, poorly rounded
and poorly stratified; Unit 14: Light cream silty clay, gypsiferous clay matrix, 5% clasts, non-stratified, gouge, fault zone; Unit 15: brown gypsiferous silt, silty-clay matrix, 5–
10% clasts (1–7 cm) with a thin layer (5 cm) of basal conglomerate, similar to unit 4. Sampling location shown by stars and rectangles indicate position of Fig. 8 in north of
trench TF1 and Fig. 9 in south of TF1. (b) A field photo of the trench TF1, red triangles show Firouzkuh fault trace, Magenta rectangle: Trench position (T), L: lake deposit, D
shown dry valley and Q: quaternary deposits, Q
al
: alluvium recent deposits, C: Cenozoic train, M: Mesozoic rocks and Pz: Paleozoic formations. (c) A close up view of the TF1
trench. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
130 H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135
Our estimate of 1.1–2.2 mm/yr for the rate of left-lateral
strike-slip on the Firouzkuh fault can be compared with existing
constraints on the slip-rate on the main faults both to the east
and west of Firouzkuh. The estimated left-lateral slip rate of
the Mosha fault is estimated at 1.7–2.7 mm/yr (Ritz et al.,
2003; Solaymani Azad, 2009) and the Astaneh fault is estimated
to slip at 1.7–2.2 mm/yr, (Hollingsworth et al., 2011) and 1.3–
2.5 mm/yr (Rizza et al., 2011;Fig. 1). Our 1.1–2.2 mm/yr average
late Quaternary slip-rate for the Firouzkuh fault is consistent
with these estimates, and it is also consistent with the
2 mm/yr of left-lateral shear measured across the Alborz with
GPS adjacent to the Astaneh, Firuzkuh and Mosha faults
(Djamour et al., 2010).
Fig. 8. This figure shows a back stripping scenario for the northern part of the
provided log of eastern wall of TF1 trench. In this scenario the northern zone of
faults present in TF1 represent Ev0 in stage a, Ev2 in stage b, Ev3 in stage e and Ev4
in stage f (for detail see Table 1). Ev1 is inferred from the southern fault zone.
Fig. 9. Southern part of the paleoseismological log made from eastern wall of trench TF1, dotted lines show vertical displacements of the base of Unit 12 across the fault
plane. Extrapolation of the base of Unit 12 at slightly different dips yields estimates of vertical displacement ranging from 2.4 to 4.4 m. A small block diagram above the log
shows geometric value respectively on a fault plane such Firouzkuh fault, in this block diagram H: horizontal displacement, V: vertical displacement, Vf: vertical displacement
along the fault plane,
q
: Pitch angle and dshows the fault dip angle.
Fig. 10. (a) Photograph showing the trenching site (TF2) prior to excavation of the
trench. The fault zone exposes Eocene–Miocene rocks covered by a thin sequence of
young faulted deposits. Red arrows show fault trace on the field and the blue
shaded rectangle indicates trench position. (b) Field photo of trench TF2 showing
the position of the photographs and logs presented in Fig. 11. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135 131
4.2. Magnitude and timing of past earthquakes on the Firouzkuh fault
The C
14
age of human bones recovered from trench T2 indicates
an age of A.D. 763–819 for the alluvial deposits bearing these
bones. The alluvium (Unit 4) is overlain by sediments (Unit 1),
which have been displaced due to the recent activity of the Fir-
ouzkuh fault. As the human bones were found in fluvial deposits
(rather than in a grave) it is possible that the skeletal remains
are substantially older than the deposits in which they have been
found, but the high degree of preservation of the bones indicates
that they have not been transported for long distances. Therefore,
we interpret the timing of the last surface-rupturing earthquake
on the Firouzkuh fault as close to the age of the bones (calibrated
calendar age of A.D. 763–819).
Table 2
Ages obtained from Trench TF1 (Fig. 7), using the OSL method.
Unit Sample number Water content
a
(%) K
b
(%) Th
b
(ppm) U
b
(ppm) Cosmic dose
c
(Gy/Ka) Total dose (Gy) N
d
Age
e
(ka)
4 2004/F3 5(34) 0.49 ± 0.03 2.2 ± 0.12 0.93 ± 0.05 0.28 ± 0.02 1.08 ± 0.04 24(24) 27.1 ± 1.7
12 2006/FT.H7 5(31) 0.96 ± 0.04 4.96 ± 0.14 1.31 ± 0.05 0.33 ± 0.02 1.84 ± 0.03 24(28) 15.9 ± 0.9
a
Field measurements of moisture content, with figures in parentheses indicating the full sample saturation %. Ages were calculated using 30% of the saturation value.
b
Dosimetry obtained using laboratory Gamma Spectrometry (low resolution Nal detector).
c
Cosmic doses depth attenuation calculated using the methods of Prescott and Hutton (1994).
d
Number of replicated equivalent dose (De) estimate used to calculate the mean. The figures in parentheses indicate total number of measurements made including failed
runs.
e
Dose rate and age for fine-grained
l
m quartz sand, Linear + exponential fit used on age, errors to one sigma.
Fig. 11. A panel of the field’s photos and its corresponding sketch logs of the western wall of the trench TF2. (a and b) field photographs are shown in the left-hand panels and
a
0
,b
0
are its corresponding sketch logs in the right-hand panels. Unit 1: Brown to buff modern soil with roots, covered by limestone cobbles, moderately rounded; Unit 2: Light
brown to buff, fine-grained deposits, with silty- clay matrix, 5% clasts (5–10 mm), inter fingers with unit 3; Unit 3: Light Brown alluvium with silt matrix, 75% clasts (3–
10 cm), rarely with bigger cobbles (P15 cm), and moderate rounded and sorted, well stratified, with channelised erosion at its base; Unit 4: Brown channel deposit, with clay
matrix, 60% clasts (4–15 cm), contains human skeletal remains; Unit 5: Buff to light grey fine deposits with silty matrix, 20% clasts (1–5 cm), interfingers with unit 3; Unit 6:
Light grey altered calcareous tuff and silt; Unit 7: Dark greenish to Buff brownish gouge zone with purple intercalations of volcanic tuff from Eocene–Miocene formations,
with roots in upper part. Red lines: fault ruptures. The yellow star in unit 4 is the sampling locality of human bone; superficial soils (unit 1) indicate the last event horizons in
trench TF2. Brown color shows ground surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1
Estimated values of Magnitude (Mw) after Wells and Coppersmith (1994) from observed events and measured average displacement (AD) on the log in trench TF1.
N Event horizon Vertical displacement
on the fault plane, Vf (m)
Average displacement
on the fault plane, AD (m)
Mw (AD), general fault Mw (AD), normal fault Mw (AD), strike slip fault
Ev 0 5 – – – – –
Ev 1 12 & 8 2.40–4.40 13.82–25.34 7.86–8.08 7.52–7.69 8.05–8.29
Ev 2 4 0.95 5.47 7.53 7.26 7.76
Ev 3 7 0.60 3.40 7.36 7.13 7.51
Ev 4? 2 – – – – –
132 H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135
Parts of Iran adjacent to the Alborz Mountains have a long re-
cord of historical seismicity (e.g. Ambraseys and Melville, 1982).
The occurrence of a large-magnitude earthquake on the Firouzkuh
fault within the last 1000 years is thus likely to be present in the
historical record. The epicentral regions of known historical earth-
quakes in the Firouzkuh region are shown in Fig. 1. The largest
event is the 856 A.D. Qumis earthquake, which took over 200,000
lives and caused almost complete destruction of towns and villages
in a region 150 km in length (Ambraseys and Melville, 1982; Hol-
lingsworth et al., 2010). A smaller event, with an estimated magni-
tude of 6.7, is also reported from northeast of Firouzkuh in A.D.
1301. The Qumis earthquake occurred 50 years after our calcu-
lated C14 AMS age of sediments displaced by the Firouzkuh fault.
Therefore, the Firouzkuh fault may be a potential source of the
Qumis earthquake. However, palaeoseismic trenching presented
by Hollingsworth et al. (2010)) suggests that the Astaneh fault
was responsible for the Qumis earthquake. The reported distribu-
tion of damage, which is close to the Astaneh fault rather than
the Firouzkuh fault, provides additional support for the Astaneh
fault as the source of this major historical earthquake (Fig. 1).
Fig. 11c. A close up view of the uppermost part of the fault zone in the western wall of the TF2 trench shown in Fig. 10b. Red arrows indicate a linear fracture that cuts the
sediments to the ground surface (unit 1). A close-up view of the fissure in shown in the right-hand panel. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Fig. 12. Human bones that were found in trench TF2 (Fig. 11a and a
0
), Homo sapiens
sapiens (A and B) mandible showing occlusal view (A) and left lateral view (B), (C)
proximal end of the femur. Scale bar represents 1 cm.
Fig. 13. Probabilitiy distribution obtained from C14 dating on the mandible bone
shown in Figs. 12A and 10B, using the Oxcal computer program (v4.0) of C. Bronk
Ramsey, and using the ‘INTCAL04’ dataset (Radiocarbon 46 (3), 2004) to generate
calibrated calendar ages.
Table 3
Radiocarbon ages from human bones excavated from Unit 4 in trench TF2 presented
in Fig. 11a0and Fig. 12, (cal BP = cal BC + 1950 and cal BP = 1950 – cal AD).
Unit Sample d
13
C Age
14
C BP Age
14
C (calAD) Age
14
C (cal
BP)
4 2006/HN/
1
19.06 1159 ± 28 763–819 1187–1131
H. Nazari et al. / Journal of Asian Earth Sciences 82 (2014) 124–135 133
The relatively minor vertical displacement of 4 cm in the
uppermost sedimentary layers corresponds to only 30 cm of
strike-slip displacement in the most recent surface rupturing event
on the Firouzkuh fault. The co-seismic slip on the Firouzkuh fault
may, therefore, represent a sub-event, and relatively minor part
of the overall Qumis earthquake rupture, which had an estimated
peak slip of 5–8 m (Hollingsworth et al., 2010). A second possibility
is that the Firouzkuh event represents a separate earthquake that is
unreported in the historical records and which is likely to have had
a maximum magnitude of 7.1. In this second scenario, and given
that the surface sediments are not likely to postdate the Qumis
earthquake by a long time, we suggest that it might have been trig-
gered by the main Qumis earthquake in 856AD.
Our interpretation of the eastern wall of the TF1 trench pre-
serves evidence for four palaeo-earthquakes in addition to the
most recent one described above (Nazari, 2006; Nazari et al.,
2007;Fig.7 and Table 1). The late Pleistocene ages that we have ob-
tained on silty-sand deposits of Units 4 and 12 in trench TF1 from
two OSL samples are not sufficient to provide detailed chronologies
and interval recurrence times for seismic activity on the Firouzkuh
fault.
The apparent vertical offsets associated with the events, from
the most recent to the oldest, are 0.04 m, >2.4 m, 0.95 and
0.60 m (note that the vertical displacement in the oldest event in
TF1 is not constrained). We can estimate the likely net slip dis-
placement in each of these ground-rupturing events by applying
the horizontal to vertical ratio of 7.6 that we calculated from the
displacement of a prominent ridge close to the trench sites (see
Section 2) and geometrical estimated values of the fault plane such
as dip (70°S) and pitch angle (5°E) . We can then calculate the
approximate moment magnitude (Mw) for each of events using
the empirical relationships Mw = 7.04 + 0.89 log AD (where AD is
the average displacement) for strike-slip faulting by Wells and
Coppersmith (1994). The magnitudes calculated in this way are
listed in Table 1. The large variation in vertical displacement in
the five events interpreted from trench TF1 indicates either that
the vertical component in each event was variable, or that a num-
ber of palaeoearthquakes are not recorded in the sedimentary se-
quence exposed in the trench walls, such that the larger
displacements are actually a composite of several ground ruptures.
It is also possible that the vertical displacements measured at any
point along the fault rupture, which are likely to be only a minor
component of the total displacement, might vary locally.
The record of past earthquakes exposed in our trenches across
the Firouzkuh fault is, therefore, too fragmentary to directly esti-
mate the average recurrence interval between surface-rupturing
events. Given these shortcomings, we use the scaling relationship
between moment magnitude and fault length of Wells and Copper-
smith (1994), to estimate the likely maximum magnitude of Mw
7.1 for an event on the Firouzkuh fault (assuming that surface rup-
ture occurred along the entire 55 km fault length). Applying the
relationship between magnitude and average slip (Wells and Cop-
persmith, 1994) then allows us to estimate an average strike-slip
displacement of 1.2 m in each event. Combining this value of aver-
age displacement with our slip-rate estimate of 1.1–2.2 mm/yr
(Section 4.1), suggests that the recurrence interval between earth-
quakes with magnitude 7 would be 1100–540 years. As the last
earthquake on the Firouzkuh fault may be up to 700 years in
age we suggest that the Firouzkuh fault is a major hazard for earth-
quakes in the near future.
5. Conclusion
Our trenching investigations on the Firouzkuh fault have
allowed us to estimate an average horizontal slip-rate of
1.1–2.2 mm/yr and to show that a surface-rupturing earthquake,
with a likely maximum magnitude of 7.1, occurred at some time
after 1159 ± 28 BP. We interpret this most recent surface rupturing
event on the Firouzkuh fault either as a western continuation of
the 856AD Qumis earthquake rupture, which had an estimated
peak slip of 5–8 m on the Astaneh fault to the east (Hollingsworth
et al., 2010), or a separate earthquake that is unreported in the his-
torical records, and which may have been triggered by the main
Qumis earthquake. These two scenarios are not easily distinguish-
able, but the correct interpretation has important implications in
the assessment of seismic hazard posed by the Firouzkuh fault to
regional population centers including the large cities of Tehran
and Semnan. Further palaeoseismic studies on the Firouzkuh fault
are therefore required to estimate the seismic hazard in northern
Iran.
Acknowledgements
This study was mainly supported by the Geological Survey of
Iran (GSI) and scientific collaboration of Géosciences Montpellier
(GM). For Optically Simulated Luminescence dating (OSL), the
authors benefited from the support of S. Mahan of the USGS. We
thank A. Ghassemi, A. Shafei, M. Ghorashi , A. Saidi from GSI and
M. Tatar and M. Momeni from IIEES for their participation in field
observation and fruitful discussion and useful comments. We
appreciate the help of Diane Baker and her colleagues in the Re-
search Laboratory for Archeology and History of Art in Oxford
(UK) for providing dating data. We acknowledge also constructive
comments of R. Cox, A. Avagyan and M. Talebian.
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