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The depth of seismicity in the Kermanshah region of the Zagros Mountains (Iran)
Earth and Planetary Science Letters, 40 (1978) 270-274 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
270
[6]
THE DEPTH OF SEISMICITY IN THE KERMANSHAH REGION OF THE ZAGROS MOUNTAINS (]RAN) M. NIAZI, I. ASUDEH i
School o f Sciences, Ferdowsi University, Mashad {Iran) G. BALLARD, J. JACKSON, G. KING and D. McKENZIE
Department o f Geodesy and Geophysics, Madingley Rise, Madingley Road, Cambridge, CB3 0EZ (England)
Received December 24, 1977 Revised version received March 7, 1978
A detailed microearthquake survey in a small region of the Zagros Mountains in Iran failed to detect any shocks whose depths were greater than 20 km. One third of the shocks in the same area have depths greater than 50 km when located using teleseismic observations. Because of poor azimuthal coverage and lack of local stations these teleseismic locations are probably in error. There is therefore no reliable seismic evidence for the existence of oceanic lithosphere beneath the Zagros fold belt.
1. Introduction The Zagros mountain range, running from eastern Turkey to the Gulf of Oman, has long been thought of as a classic fold and thrust system, apparently resulting from the collision of the Arabian continental plate to the southwest with Central Iran to the northeast. Present-day seismicity is intense and appears to be confined within the structural boundaries of the range ; with few events to the northeast of the Zagros Thrust Line (of Falcon [1] and others) or in the interior of the Arabian plate, which suggests that tectonic deformation is still continuing. The apparent presence of a small number of intermediate depth earthquakes (located by USCGS, NOAA, ISS, and Nowroozi [2] has caused a tendency for the compressive plate margin drawn along the Zagros to be interpreted as a subducting system complete with downgoing slab. Nowroozi [2] relocated events from 1950 to 1965 and shows that the seismicity apparently increases with depth to the northeast, which he took as evidence for the existence of a 1 Present address: Department of Geodesy and Geophysics, Madingley Rise, Cambridge CB3 0EZ, England.
k...~'(k e T A
' "(k
~_Caspian
fl
• Bo ".'...
......-..,, _x,
(
..-..............................
CENTRAL IRAN
("":',,?,poX,,..
PLATE •
..........
- - Zagros Thrust Line 0I " . . . . - P o l i t i c a l boundQry
km
500
Fig. 1. Area A shows the regions of this study. Main cities are marked: TA = Tabriz, TE = Tehran, KM = Kerman, BA = Bandar Abbas, SH = Shiraz, BD = Badhdad, ES = Esfahan, K = Kermanshah.
271 slab about 60 km thick dipping northeast at 1 0 - 2 0 °. If such a slab exists it is presumably a relic o f oceanic lithosphere, and is obviously important in understanding the tectonic evolution o f the Zagros ranges. However, because o f the lack o f nearby stations and poorly known velocity structure beneath the Zagros, focal depths are not well controlled. The depth o f seismicity therefore needs careful examination. In September 1976 a joint expedition from Cambridge (U.K.) and Ferdowsi (Iran) Universities operated a set of portable seismic stations to determine the depths of events in the Kermanshah area o f the northwestern Zagros (Fig. 1). The area was chosen both because it was accessible and because a third of the shocks in the region were claimed to be deeper than 50 kin.
2. Local seismicity in the Kermanshah area Instrument sites are shown in Fig. 2. Noise levels were generally low and most onset times could be read to within 0.1 second. Differences in station altitude are small and no corrections were applied. The Zagros Thrust Line is indistinct in this area, which is much imbricated. A large Quaternary rightlateral strike-slip fault (Fig. 2) described in detail by Tchalenko and Brand [3] and called by them "the Main Recent F a u l t " is the clearest structural feature and coincides with the line normally taken as the northeast front o f the Zagros. Over 140 events were recorded of which about 100 were close to or within the station network. All of these were too small to be felt (magnitudes M L < 1.2), and many which occurred on the periphery of the network were only large enough to be seen on the nearest one or two stations, and could not be located with any accuracy. 43 earthquakes were seen on enough instruments to make accurate locations possible, and their arrival times could be clearly read. These shocks divided into three groups;
Group 1: seen on five or more instruments with at least one S phase identified. Residuals and standard errors in this group are low. (12 events)
Group 2: not seen on enough instruments to allow the deter-
47 E
48 E
+
+
oL\-')---. ~ ~
K i t rnonshc~h
~ . ~ i ........... iiiiiiiii .~"
~
\
MA
~
"~
.....
° " ~
,~:.
TS •
35N
~-\-
•CH
o
03
~i ~"
o
.., : : :
::
::~.~\ o
"Vo\
%
PA•
•% 0
'km
5'0
0
0 O
O
Fig. 2. Map of area A in Fig. 1. Recording stations are marked by triangles, instrumental epicentres from Nowroozi [ 2] by closed circles. Macroseismal areas of 1957-1963 events are shaded. Fault lines show only recent and active structure on the Main Recent Fault System (from Tchalenko and Braud [3]: nf = Nehavand Fault, sf = Sahneh Fault, mg = Morvarid Fault. Open circles are epicentres located in this paper. The network operated for 26 days, GA replacing TS after day 7. The crustal model used was: Layer
Vp
Vs
Thickness (km)
1 2 3
3.80 5.70 7.10
2.10 3.30 4.10
4.00 16.00
mination o f standard errors. Residuals are low and depths do not conflict with S-P times at nearby stations and are therefore thought to be reliable. (10 events)
Group 3: do not give sensible depth results (negative or hundreds o f kilometers) and were located at a fixed depth o f 12 km to find the approximate epicentres. (2t events) Groups 2 and 3 are generally of smaller magnitude than group 1, or else occurred during periods when noise obscured readings at some stations.
3. Method o f location and reliability o f results The location program starts with an initial approximate location and an assumed three-layered crustal
272 15-
"o"
///' ,/'/
I'O
O_ I
OD
~o o
RMS Residuol
i
-------._------~\ //
/,
,, .'7
0
I
I
/'/
I
IOO
Dtstance
./
,~\
66
i
Fig. 4. S-P times in seconds plotted against diStance in kilometres of the epicentre from GA. Note the lack of points of high S-P time at small delta, and the small variation in S-P for any particular delta value. Closed circles are group l ; open circles, group 2; stars, group 3.
I
50 DEPTH
Fig. 3. Plot of rms residual in seconds vs. fixed depth in kilometres for events 64, 66 (group 1)and 6,132 (group 2). No. 6 is the worst located of group 2 with one S and three P arrivals; its depth, however, is restricted by an S-P time of 5.7 seconds at station MA, 36 km from the epicentre.
model, and improves the position by iteration. The crustal model used for local events is shown in the caption to Fig. 2 and was selected to give low residuals and standard errors for those shocks with reliable and clear arrival times. The epicentre locations are not very model-dependent, especially when there is a good azimuthal distribution of stations relative to the epicentre. For instance changing Vp in layer 2 from 5.7 to 5.9 moves most o f the epicentres in group 1 less than a kilometre. Sensitivity to depth is more difficult to determine as origin time or depth can be adjusted to similar effect by the location program if only P arrivals are used. However, if S phase arrivals are also available they apply a considerable depth constraint. Fig. 3 shows plots of rms residual vs. fixed depth for a number o f events which were located with different fixed depths. A sharp increase in the rms residual is seen when focal depths are constrained to deeper than 20 km, suggesting that the model and network are sensitive to
depth if sufficient S phases are available. A further check on the depths of groups 1 and 2 can be made by looking at the S-P times at stations near to the epicentres. These times are all too small to allow focal depths greater than 2 0 - 3 0 km. They do not show a great spread o f S-P times at particular delta values when they are plotted against delta for station GA (Fig. 4), which would be expected if some o f the events were deeper. The narrow band of recorded S-P times at particular delta values suggests a similar depth for many events. Although the depths o f groups I and 2 are o f the
I,
,I,I
,
I0
20
DEPTH Fig. 5. Histogram of focal depths (kin) in groups 1 and 2.
273 right order, reasonable changes in the model can move these by 1 - 2 km. The histogram in Fig. 5 shows a cluster of events at 4 km (a boundary in the crustal model) and 12 km (the starting depth for iterations in the location program) which also indicates slight model dependence.
4. Results and discussion All the events whose focal depths have been located with any confidence are shallower than 20 kin. 50% of the epicentres located are close to the Main Recent Fault (Fig. 2) and GA was by far the most active station, showing many events of small ( 1 - 3 seconds) S-P times which were too weak to be seen at other stations. This fault is known to be active and field evidence [3] and a fault plane solution [4] suggest strike-slip motion to be important. The activity we found also has a NW-SE trend, parallel to the strike of the fault (Fig. 2).
TABLE 1
In 1957, 1958, and 1963 earthquakes of magnitude 7.1,6.6, 5.8 respectively occurred on the Main Recent Fault (Fig. 2), that of 1958 being associated with surface faulting [3]. Focal depths calculated by Nawroozi [2] and the ISS for these and some of their aftershocks are shown in Table 1. Assuming the depths located in this paper to be accurate to within 10 kin, possible reasons for the deeper hypocentres in Table 1 are either that activity in September 1976 on the Main Recent Fault was shallower than that of 1 9 5 7 1963, or that the focal depths in Table 1 are inaccurate for the reasons mentioned in the introduction. Temporal migration of focal depths is possible, but such a complete change in the focal depths seems unlikely. Most epicenters in Fig. 2 lie near the macroseismal regions of the 1957 and 1963 events [3] to the northeast of the Main Recent Fault and 2 0 - 3 0 km from the 1957 and 1963 instrumental epicentres. Tchalenko and Braud [3] postulated a southwest-dipping fault plane with strike-slip and thrust motion (Fig. 6) to explain this, though such a southwest dip is not needed to explain the distribution of microseismic epicentres shown here. It is possible that the 1957, 1958, and
Hypocentral parameters calculated by Nowroozi [2] for events on or near the Main Recent Fault. Focal depth of 1957 Dec. 13 also reported by ISS
1957
1958
Dec
Jan
Aug
Sep 1961
Oct
1963
Mar
Latitude
L o n g i - Depth tude
Magnitude ML
13
34.35
7.1
16 25 2 2 6 16 17 17 17 17 19 24 3 10 21 3 14 24
34.33 34.35 34.49 34.25 34.23 34.29 34.25 34.25 34.50 34.71 34.19 34.37 34.64 34.33 34.52 34.38 34.11 34.37
47.67 40 42 (ISS) 47.67 68 47.72 53 47.84 53 47.56 85 47.87 89 48.01 58 47.91 51 47.91 50 48.17 92 47.34 143 48.18 86 48.08 79 47.89 86 48.01 73 47.42 71 47.81 69 48.46 84 47.80 37
6.6
5.8
Fig. 6. Tchalenko and Braud's [3] model for the 1957 earthquake of magnitude 7.0. The dip and strike of the fault plane, marked F, are those of one of the planes of the fault plane solution. Tchalenko and Braud argued that the hypocentre of the 1957 shock determined from teleseismic observations, marked with a solid circle, was displaced from the surface fault because the fault was inclined to the vertical. Abbreviations sf, nf, and mf as in Fig. 2.
274 1963 epicentres determined from teleseismic observations are themselves displaced to the southwest azimuthal distribution of stations for the microearthquakes located here is relatively good, and even a substantial change in velocity structure across the fault is unlikely to cause the 1 5 - 2 0 km shift o f epicentres to the northeast as observed. Control o f the 1 9 5 7 1963 epicentres is less good, and even with good azimuthal distribution teleseimic locations o f epicentres are rarely in error by less than 1 0 - 2 0 kin. Similar results have been reported by Savage et al. [5] who carried out a microearthquake survey south of Shiraz, and by Von Dollen et al. [6] in the region near Bandar Abbas. Both groups found that all the shocks that they recorded had focal depths o f less than 20 km. A.A. Nowroozi (personal communication) has also failed to find any subcrustal events in the Bushehr area on the Persian Gulf. Like the Kermanshah region, depths determined from teleseismic observations are as great as 100 km in all three areas. Therefore these studies all suggest that the depths determined teleseismically are in error, though, unlike the Kermanshah area, all three regions are somewhat remote from the Zagros Thrust, and hence from the probable location o f subduction.
5. Conclusions No focal depths were found deeper than 20 km, and though the sample is not sufficiently large to be statistically conclusive, several deeper shocks might have been expected to occur in the period o f operation if deep and shallow shocks occur in the same ratio for small as for large events. Most o f the observed activity was on the Main Recent Fault, and it might be argued that activity on this strike-slip fault is not typical of the Zagros as a whole. However, earthquakes on or
near this fault were a major part of the seismicity defining the deeper part of Nowroozi's downgoing slab. The large discrepancy between the depths of events on the Main Recent Fault as calculated by Nowroozi and those reported here strongly suggests that previous depth estimates in the Zagros are in error, and that tectonic models which use these focal depths to support the presence o f a subducting slab should be questioned.
Acknowledgements We would like to thank M. Barazangi for bringing the papers by Savage et al. and by Von Dollen et al. to our attention. This project was supported by a grant from the Natural Environmental Research Council and by Ferdowsi University.
References 1 N.L. Falcon, Problems of the relationship between surface structure and deep displacements illustrated by the Zagros Range, in: Time and Place in Orogeny, Spec. Publ. Geol. Soc. Lond. 3 (1969) 9-22. 2 A.A. Nowroozi, Seismo-tectonics of the Persian plateau, eastern Turkey, Caucasus, and Hindu-Kush regions, Bull. Seismol. Soc. Am. 66 (1971) 317-341. 3 J.S. Tchalenko and J. Braud, Seismicity and structure of the Zagros (Iran): the Main Recent Fault between 33 and 35 degrees north, Philos. Trans. R. Soc. London, Ser. A, 277 (1974) 1-25. 4 D.P. McKenzie, Active tectonics of the Mediterranean region, Geophys. J.R. Astron. Soc. 30 (1972) 109-185. 5 W.U. Savage, J.N. Alt and A. Mohajer-Ashari, Microearthquake investigations of the 1972 Qir, Iran, earthquake zone and adjacent areas, Geol. Soc. Am. Abstr. 9 (1977) 496. 6 F.J. Von Dollen, J.N. Alt, D. Tocher and A. Nowroozi, Seismologie and geologic investigations near Bandar Abbas, Iran, Geol. Soc. Am. Abstr. 9 (1977) 521.
270
[6]
THE DEPTH OF SEISMICITY IN THE KERMANSHAH REGION OF THE ZAGROS MOUNTAINS (]RAN) M. NIAZI, I. ASUDEH i
School o f Sciences, Ferdowsi University, Mashad {Iran) G. BALLARD, J. JACKSON, G. KING and D. McKENZIE
Department o f Geodesy and Geophysics, Madingley Rise, Madingley Road, Cambridge, CB3 0EZ (England)
Received December 24, 1977 Revised version received March 7, 1978
A detailed microearthquake survey in a small region of the Zagros Mountains in Iran failed to detect any shocks whose depths were greater than 20 km. One third of the shocks in the same area have depths greater than 50 km when located using teleseismic observations. Because of poor azimuthal coverage and lack of local stations these teleseismic locations are probably in error. There is therefore no reliable seismic evidence for the existence of oceanic lithosphere beneath the Zagros fold belt.
1. Introduction The Zagros mountain range, running from eastern Turkey to the Gulf of Oman, has long been thought of as a classic fold and thrust system, apparently resulting from the collision of the Arabian continental plate to the southwest with Central Iran to the northeast. Present-day seismicity is intense and appears to be confined within the structural boundaries of the range ; with few events to the northeast of the Zagros Thrust Line (of Falcon [1] and others) or in the interior of the Arabian plate, which suggests that tectonic deformation is still continuing. The apparent presence of a small number of intermediate depth earthquakes (located by USCGS, NOAA, ISS, and Nowroozi [2] has caused a tendency for the compressive plate margin drawn along the Zagros to be interpreted as a subducting system complete with downgoing slab. Nowroozi [2] relocated events from 1950 to 1965 and shows that the seismicity apparently increases with depth to the northeast, which he took as evidence for the existence of a 1 Present address: Department of Geodesy and Geophysics, Madingley Rise, Cambridge CB3 0EZ, England.
k...~'(k e T A
' "(k
~_Caspian
fl
• Bo ".'...
......-..,, _x,
(
..-..............................
CENTRAL IRAN
("":',,?,poX,,..
PLATE •
..........
- - Zagros Thrust Line 0I " . . . . - P o l i t i c a l boundQry
km
500
Fig. 1. Area A shows the regions of this study. Main cities are marked: TA = Tabriz, TE = Tehran, KM = Kerman, BA = Bandar Abbas, SH = Shiraz, BD = Badhdad, ES = Esfahan, K = Kermanshah.
271 slab about 60 km thick dipping northeast at 1 0 - 2 0 °. If such a slab exists it is presumably a relic o f oceanic lithosphere, and is obviously important in understanding the tectonic evolution o f the Zagros ranges. However, because o f the lack o f nearby stations and poorly known velocity structure beneath the Zagros, focal depths are not well controlled. The depth o f seismicity therefore needs careful examination. In September 1976 a joint expedition from Cambridge (U.K.) and Ferdowsi (Iran) Universities operated a set of portable seismic stations to determine the depths of events in the Kermanshah area o f the northwestern Zagros (Fig. 1). The area was chosen both because it was accessible and because a third of the shocks in the region were claimed to be deeper than 50 kin.
2. Local seismicity in the Kermanshah area Instrument sites are shown in Fig. 2. Noise levels were generally low and most onset times could be read to within 0.1 second. Differences in station altitude are small and no corrections were applied. The Zagros Thrust Line is indistinct in this area, which is much imbricated. A large Quaternary rightlateral strike-slip fault (Fig. 2) described in detail by Tchalenko and Brand [3] and called by them "the Main Recent F a u l t " is the clearest structural feature and coincides with the line normally taken as the northeast front o f the Zagros. Over 140 events were recorded of which about 100 were close to or within the station network. All of these were too small to be felt (magnitudes M L < 1.2), and many which occurred on the periphery of the network were only large enough to be seen on the nearest one or two stations, and could not be located with any accuracy. 43 earthquakes were seen on enough instruments to make accurate locations possible, and their arrival times could be clearly read. These shocks divided into three groups;
Group 1: seen on five or more instruments with at least one S phase identified. Residuals and standard errors in this group are low. (12 events)
Group 2: not seen on enough instruments to allow the deter-
47 E
48 E
+
+
oL\-')---. ~ ~
K i t rnonshc~h
~ . ~ i ........... iiiiiiiii .~"
~
\
MA
~
"~
.....
° " ~
,~:.
TS •
35N
~-\-
•CH
o
03
~i ~"
o
.., : : :
::
::~.~\ o
"Vo\
%
PA•
•% 0
'km
5'0
0
0 O
O
Fig. 2. Map of area A in Fig. 1. Recording stations are marked by triangles, instrumental epicentres from Nowroozi [ 2] by closed circles. Macroseismal areas of 1957-1963 events are shaded. Fault lines show only recent and active structure on the Main Recent Fault System (from Tchalenko and Braud [3]: nf = Nehavand Fault, sf = Sahneh Fault, mg = Morvarid Fault. Open circles are epicentres located in this paper. The network operated for 26 days, GA replacing TS after day 7. The crustal model used was: Layer
Vp
Vs
Thickness (km)
1 2 3
3.80 5.70 7.10
2.10 3.30 4.10
4.00 16.00
mination o f standard errors. Residuals are low and depths do not conflict with S-P times at nearby stations and are therefore thought to be reliable. (10 events)
Group 3: do not give sensible depth results (negative or hundreds o f kilometers) and were located at a fixed depth o f 12 km to find the approximate epicentres. (2t events) Groups 2 and 3 are generally of smaller magnitude than group 1, or else occurred during periods when noise obscured readings at some stations.
3. Method o f location and reliability o f results The location program starts with an initial approximate location and an assumed three-layered crustal
272 15-
"o"
///' ,/'/
I'O
O_ I
OD
~o o
RMS Residuol
i
-------._------~\ //
/,
,, .'7
0
I
I
/'/
I
IOO
Dtstance
./
,~\
66
i
Fig. 4. S-P times in seconds plotted against diStance in kilometres of the epicentre from GA. Note the lack of points of high S-P time at small delta, and the small variation in S-P for any particular delta value. Closed circles are group l ; open circles, group 2; stars, group 3.
I
50 DEPTH
Fig. 3. Plot of rms residual in seconds vs. fixed depth in kilometres for events 64, 66 (group 1)and 6,132 (group 2). No. 6 is the worst located of group 2 with one S and three P arrivals; its depth, however, is restricted by an S-P time of 5.7 seconds at station MA, 36 km from the epicentre.
model, and improves the position by iteration. The crustal model used for local events is shown in the caption to Fig. 2 and was selected to give low residuals and standard errors for those shocks with reliable and clear arrival times. The epicentre locations are not very model-dependent, especially when there is a good azimuthal distribution of stations relative to the epicentre. For instance changing Vp in layer 2 from 5.7 to 5.9 moves most o f the epicentres in group 1 less than a kilometre. Sensitivity to depth is more difficult to determine as origin time or depth can be adjusted to similar effect by the location program if only P arrivals are used. However, if S phase arrivals are also available they apply a considerable depth constraint. Fig. 3 shows plots of rms residual vs. fixed depth for a number o f events which were located with different fixed depths. A sharp increase in the rms residual is seen when focal depths are constrained to deeper than 20 km, suggesting that the model and network are sensitive to
depth if sufficient S phases are available. A further check on the depths of groups 1 and 2 can be made by looking at the S-P times at stations near to the epicentres. These times are all too small to allow focal depths greater than 2 0 - 3 0 km. They do not show a great spread o f S-P times at particular delta values when they are plotted against delta for station GA (Fig. 4), which would be expected if some o f the events were deeper. The narrow band of recorded S-P times at particular delta values suggests a similar depth for many events. Although the depths o f groups I and 2 are o f the
I,
,I,I
,
I0
20
DEPTH Fig. 5. Histogram of focal depths (kin) in groups 1 and 2.
273 right order, reasonable changes in the model can move these by 1 - 2 km. The histogram in Fig. 5 shows a cluster of events at 4 km (a boundary in the crustal model) and 12 km (the starting depth for iterations in the location program) which also indicates slight model dependence.
4. Results and discussion All the events whose focal depths have been located with any confidence are shallower than 20 kin. 50% of the epicentres located are close to the Main Recent Fault (Fig. 2) and GA was by far the most active station, showing many events of small ( 1 - 3 seconds) S-P times which were too weak to be seen at other stations. This fault is known to be active and field evidence [3] and a fault plane solution [4] suggest strike-slip motion to be important. The activity we found also has a NW-SE trend, parallel to the strike of the fault (Fig. 2).
TABLE 1
In 1957, 1958, and 1963 earthquakes of magnitude 7.1,6.6, 5.8 respectively occurred on the Main Recent Fault (Fig. 2), that of 1958 being associated with surface faulting [3]. Focal depths calculated by Nawroozi [2] and the ISS for these and some of their aftershocks are shown in Table 1. Assuming the depths located in this paper to be accurate to within 10 kin, possible reasons for the deeper hypocentres in Table 1 are either that activity in September 1976 on the Main Recent Fault was shallower than that of 1 9 5 7 1963, or that the focal depths in Table 1 are inaccurate for the reasons mentioned in the introduction. Temporal migration of focal depths is possible, but such a complete change in the focal depths seems unlikely. Most epicenters in Fig. 2 lie near the macroseismal regions of the 1957 and 1963 events [3] to the northeast of the Main Recent Fault and 2 0 - 3 0 km from the 1957 and 1963 instrumental epicentres. Tchalenko and Braud [3] postulated a southwest-dipping fault plane with strike-slip and thrust motion (Fig. 6) to explain this, though such a southwest dip is not needed to explain the distribution of microseismic epicentres shown here. It is possible that the 1957, 1958, and
Hypocentral parameters calculated by Nowroozi [2] for events on or near the Main Recent Fault. Focal depth of 1957 Dec. 13 also reported by ISS
1957
1958
Dec
Jan
Aug
Sep 1961
Oct
1963
Mar
Latitude
L o n g i - Depth tude
Magnitude ML
13
34.35
7.1
16 25 2 2 6 16 17 17 17 17 19 24 3 10 21 3 14 24
34.33 34.35 34.49 34.25 34.23 34.29 34.25 34.25 34.50 34.71 34.19 34.37 34.64 34.33 34.52 34.38 34.11 34.37
47.67 40 42 (ISS) 47.67 68 47.72 53 47.84 53 47.56 85 47.87 89 48.01 58 47.91 51 47.91 50 48.17 92 47.34 143 48.18 86 48.08 79 47.89 86 48.01 73 47.42 71 47.81 69 48.46 84 47.80 37
6.6
5.8
Fig. 6. Tchalenko and Braud's [3] model for the 1957 earthquake of magnitude 7.0. The dip and strike of the fault plane, marked F, are those of one of the planes of the fault plane solution. Tchalenko and Braud argued that the hypocentre of the 1957 shock determined from teleseismic observations, marked with a solid circle, was displaced from the surface fault because the fault was inclined to the vertical. Abbreviations sf, nf, and mf as in Fig. 2.
274 1963 epicentres determined from teleseismic observations are themselves displaced to the southwest azimuthal distribution of stations for the microearthquakes located here is relatively good, and even a substantial change in velocity structure across the fault is unlikely to cause the 1 5 - 2 0 km shift o f epicentres to the northeast as observed. Control o f the 1 9 5 7 1963 epicentres is less good, and even with good azimuthal distribution teleseimic locations o f epicentres are rarely in error by less than 1 0 - 2 0 kin. Similar results have been reported by Savage et al. [5] who carried out a microearthquake survey south of Shiraz, and by Von Dollen et al. [6] in the region near Bandar Abbas. Both groups found that all the shocks that they recorded had focal depths o f less than 20 km. A.A. Nowroozi (personal communication) has also failed to find any subcrustal events in the Bushehr area on the Persian Gulf. Like the Kermanshah region, depths determined from teleseismic observations are as great as 100 km in all three areas. Therefore these studies all suggest that the depths determined teleseismically are in error, though, unlike the Kermanshah area, all three regions are somewhat remote from the Zagros Thrust, and hence from the probable location o f subduction.
5. Conclusions No focal depths were found deeper than 20 km, and though the sample is not sufficiently large to be statistically conclusive, several deeper shocks might have been expected to occur in the period o f operation if deep and shallow shocks occur in the same ratio for small as for large events. Most o f the observed activity was on the Main Recent Fault, and it might be argued that activity on this strike-slip fault is not typical of the Zagros as a whole. However, earthquakes on or
near this fault were a major part of the seismicity defining the deeper part of Nowroozi's downgoing slab. The large discrepancy between the depths of events on the Main Recent Fault as calculated by Nowroozi and those reported here strongly suggests that previous depth estimates in the Zagros are in error, and that tectonic models which use these focal depths to support the presence o f a subducting slab should be questioned.
Acknowledgements We would like to thank M. Barazangi for bringing the papers by Savage et al. and by Von Dollen et al. to our attention. This project was supported by a grant from the Natural Environmental Research Council and by Ferdowsi University.
References 1 N.L. Falcon, Problems of the relationship between surface structure and deep displacements illustrated by the Zagros Range, in: Time and Place in Orogeny, Spec. Publ. Geol. Soc. Lond. 3 (1969) 9-22. 2 A.A. Nowroozi, Seismo-tectonics of the Persian plateau, eastern Turkey, Caucasus, and Hindu-Kush regions, Bull. Seismol. Soc. Am. 66 (1971) 317-341. 3 J.S. Tchalenko and J. Braud, Seismicity and structure of the Zagros (Iran): the Main Recent Fault between 33 and 35 degrees north, Philos. Trans. R. Soc. London, Ser. A, 277 (1974) 1-25. 4 D.P. McKenzie, Active tectonics of the Mediterranean region, Geophys. J.R. Astron. Soc. 30 (1972) 109-185. 5 W.U. Savage, J.N. Alt and A. Mohajer-Ashari, Microearthquake investigations of the 1972 Qir, Iran, earthquake zone and adjacent areas, Geol. Soc. Am. Abstr. 9 (1977) 496. 6 F.J. Von Dollen, J.N. Alt, D. Tocher and A. Nowroozi, Seismologie and geologic investigations near Bandar Abbas, Iran, Geol. Soc. Am. Abstr. 9 (1977) 521.