Bulletin of the Seismological Society of America, Vol. 71, No. 4, pp. 1201-1213, August 1981

SOURCE PARAMETERS OF 1978 TABAS AND 1979 QAINAT, IRAN, EARTHQUAKES FROM LONG-PERIOD SURFACE WAVES

BY MANSOUR N1Az1 AND Hrnoo KANAMORI

ABSTRACT

The teleseismic records of three strong earthquakes which inflicted heavy damage and loss of many thousands of lives in northeastern Iran during 1978 and 1979 are analyzed. The observed R 2 and R 3 phases of the 16 September 1978 Tabas earthquake across the IDA network are inverted to derive the gross source parameters of the main shock. The synthetic seismograms computed for the inferred source model are compared with observations. The agreement is generally good at most of the stations. However, unexpected asymmetrical observations at ESK, CMO, KIP, and PFO stations cannot be explained by directivity alone. Possible factors responsible for these observations may include regional heterogeneities or the departure of fault plane from simple planar geometry. The resulting focal mechanism agrees. with nearly pure reverse mechanism with a NW-SE strike. The seismic moment estimate for this earth­quake is about 1.5 x 1027 dyne-cm (Mw = 7.4). The seismic moment of the same earthquake obtained from long-period P-wave observations at six WWSSN stations is smaller than the above estimate by a factor of 2.

Starting with the geologic field data of the Qainat earthquakes of 14 and 27 November 1979, the R 2 and G 2 phases on the ultra-long-period instruments at Pasadena and Berkeley are analyzed to estimate their seismic source moments. The estimated values are 1.0 x 1026 dyne-cm for the 14 November and 8.0 x 1026 dyne-cm for the 27 November 1979 events. Their corresponding moment magnitudes are 6.6 and 7.2, respectively.

INTRODUCTION

On 16 September 1978, a major earthquake (Ms 7.4 to 7.7) destroyed the town of Tabas, east-central Iran, and caused extensive damage and destruction in surround­ing villages. The earthquake death toll is not exactly known, but it has been estimated to exceed 15,000. Accounts of field observations are given by Mohajer­Ashjai and Nowroozi (1979) and Berberian (1979). This earthquake has been the largest earthquake of instrumental era in Iran and certainly the most destructive regional event of this century. Prior to the recent outburst of seismic activity which began with the 31 August 1968 Dasht-e Bayaz earthquake (Ms 7.3) and continued with nearly ten earthquakes of magnitude 6 and above, a few decades of relative quiescence prevailed in east-central Iran, interrupted now and then by events of moderate intensity. The latest major earthquake sequence occurred in November 1979 in Qainat, just to the east of the 1968 Dasht-e Bayaz epicenter (Haghipour and Amidi, 1980). The hypocentral parameters of the three earthquakes studied here are given in Table 1. Here we discuss the results of the analysis of the R 2 and R 3

phases recorded digitally by long-period gravimeters at eight stations of the IDA network (Agnew et al., 1976) regarding the source mechanism of the Tabas earth­quake. General description of the source geometry and seismic moment of the larger events of the 1979 sequence as revealed from the observed amplitudes of the R 2 and G2 phases at Pasadena and Berkeley and their tectonic environment are also given.

1201

1202 MANSOUR NIAZI AND HIROO KANAMORI

REGIONAL TECTONICS

Tectonics of east-central Iran as part of the Alpine-Himalayan orogenic belt has been the subject of numerous discussions in recent years (Stocklin, 1968, 1974; Takin, 1972; Dewey et al., 1973). The region has a complex tectonic history (Mohajer-Ashjai et al., 1975) of repeated folding, magmatism, and metamorphism (Stocklin, 1974). With dimensions of several hundred to over a thousand kilometers and surrounded by a fault-dissected ophiolite ring, the region approximately coin­cides with what Takin (1972) termed the central and east Iranian Microcontinent. The nearest prominent te.ctonic boundary of global significance is the main Zagros thrust (ZMT) line to the southwest, which marks the plate boundary between two continental masses of varied tectonic environment (Figure 1). There are differing views as to whether, prior to the latest phase of collision, the lithospheric plates colliding along this boundary had ever been in contact. The collision was apparently brought about by the closure of the Tethys Sea and the commencement of the opening of the Red Sea some time between the Oligocene (McKenzie and Sclater, 1971) and the early Miocene (Wells, 1969). Stonely (1974) contends that folding in Zagros had already started in the late Miocene. Stocklin (1974) has drawn upon his extensive field experience in Iran to present strong evidence for close correlation of Paleozoic sections on either side of this boundary and to suggest that Iran was part of a "continuous undivided" Arabian-Iranian platform from Infracambrian until

TABLE 1

HYPOCENTRAL PARAMETERS OF THE IRANIAN EARTHQUAKES STUDIED AS DETERMINED BY NEIS

Origin Time Latitude Longitude

Focal

No. Date (oN) (oE) Depth M. h m (km)

1 16 Sep. 1978 15 35 56.6 33.386 57.434 33 7.4 2 14 Nov. 1979 02 21 22.1 33.918 59.741 33 6.6 3 27 Nov. 1979 17 10 32.9 34.134 59.877 10 7.1

Triassic (see also Shearman, 1976). His assertion would imply that the boundary of the Gondwana and Laurasia landmasses, thought to lie along the Zagros thrust line by many (Dewey et al., 1973; Stonely, 1974; Laubscher and Bernoulli, 1977, to mention only a few) should be sought further north, and that east-central Iran must be considered originally as part of the Gondwana landmass.

Other structures bounding this region include Alborz (AL) and Kopeh Dagh (KD) to the north, Makran ranges to the south, and the east Iran-Harirud fault system to the east. The pre-Jurassic Harirud fault (HF) system, which runs parallel to the eastern border of Iran with neighboring Afghanistan, extends northward across the Turan platform of Soviet central Asia. While no significant seismicity is directly attributed to the Harirud fault, it serves as an important seismotectonic boundary at the present time (Shoja-Taheri and Niazi, 1981). The east-central Iran region is divided by the north-south trending Nayband (NF) fault. The Lut block, to the east of this line, is a north-south elongated rigid block with little Alpine deformation (Stocklin, 1974). The western half is formed by a complex succession of arcuate horst and graben structures which have existed since the Triassic time. Most of the sediments which were deposited in the grabens during Jurassic and Cretaceous epochs are strongly folded. The Tabas earthquake hypocenter is located on one of the old structures controlling one such graben.

SOURCE PARAMETERS OF EARTHQUAKES FROM SURFACE WAVES 1203

..

38° +

CASPIAN SEA

37° +

32° +

/

..

+ 57°

J20

+ 58°

I I

'

I (

I

I

I .. 61°

+

+

+

+

+ 62°

FIG. 1. Recent surface faulting associated with the 1978 and 1979 earthquakes in northeastern Iran in relation to the general faulting pattern in east and central Iran as shown by Stiicklin (1974). The distribution of ophiolitic rocks in the region is shown by the shaded area. The Zagros Main Thrust line (ZMT) near the southwest corner of the map coincides with the suture zone between the Arabian platform and Central Iran. The earthquake focal mechanisms as well as the sense of recent fault movements are consistent with horizontal compression in the NE-SW direction.

REGIONAL SEISMICITY

The seismicity of east-central Iran is characterized by a diffuse pattern which dies out abruptly across the Harirud fault. Beginning in August 1968, there has been a significant increase in the level of activity (Shoja-Taheri and Niazi, 1981) over the

40. 0

39. c

38. c

37. c

3S. C

1204 MANSOUR NIAZI AND HIROO KANAMORI

reported seismicity in the previous decades. Nowroozi (1971) and McKenzie (1972) have discussed the relation of the recent seismic activity to the structural trends and neotectonic features of the region. Ambraseys and Melville (1977) and Berberian (1979) have compiled the historical seismicity of the area as far back as the ninth century A.D. While large gaps exist in the historical record, the data in general coincide with the distribution of the instrumentally determined epicenters of this century (Niazi, 1981).

0

D

IJ D

ff

1·J n

r:J

[] '

MRGNITUDE

[]

D []

[]

35. c 0

~~ D )-',_ -;, --fr :--nD~f]-y- --i '.1 _____ [ ___ '.] ________ ~

I .1

1979 ti i D

34 0

IJ

33. c +

32. 0 +

D

~' 0 •

I]

,1978 ~~. er 6/17/74. O f:T~U } .

IJ [] D /'-.~ I 0 . 7. I~

0 • IJ

I ,

\ \ I \'

I l......,l

[J

!J

D

D

D

D

0 [J ) 1·

SEPT. 16, I 978 - DEC. 31, 1979

D

oO 0 fi 0 I

I

5.(.0 S3.C :i4C SSJ ~)G.C sir: Jee 5q(] 000 02C

TELESEISMIC PL0T OF EARTHQUAKES JN'-!. I, 1960-SEPT.15, 1978

Frc. 2. Regional seismicity from the beginning of 1960 to the end of 1979 as has been compiled on NOAA file. The total regional activity during the last 470 days (16 September 1978 to 31December1979) is confined to those events within the frame which has been transplanted from its original location 7° eastward outside of the map. The recent activity includes the September 1978 and November 1979 sequences, the correct locations of which are shaded on the map. The 1979 activity occurred to the east of the epicentral region of the 1968 Dasht-e Bayaz sequence. The dates of three moderate earthquakes located within the epicentral region of the 1978 Tabas sequence are also shown.

The epicentral plot of the earthquakes from 1960 through 1979 (20-yr period) is shown in Figure 2. The location of the epicenters prior to 1960 is less accurate. Since 1968 nearly ten earthquakes of magnitude 6 or more have occurred in east and central Iran, including three major events of magnitude 7 plus. Another notable feature is the relatively isolated activity from 5 May 1973 to 17 June 1974 in the aftershock area of the 1978 event which appears to have been generally quiet before the 1978 event. Berberian (1979) reports another earthquake dated 26 September

SOURCE PARAMETERS OF EARTHQUAKES FROM SURFACE WAVES 1205

1977 near Tabas from the newspaper report. The magnitude of this event is estimated by Mashad University to be less than 4 (Shoja-Taheri, personal commu­nication). Most of the recent seismicity is confined to an elongated zone of about 300-km length. The latest sequence occurred in November 1979 about 200 km east­northeast of Tabas (Haghipour and Amidi, 1980). In order to show more clearly the seismicity pattern in the last 15 months of the 20-yr period, we have plotted all the teleseismically located epicenters after 15 September 1978 in a separate frame. They consist mainly of the Ta bas and Qainat sequence of September 1978 and November 1979. No other earthquake for the whole region was reported during this 15 months. Most of the strong earthquakes of the region in recent years have been associated with surface faulting, often in the form of reactivation of older Quaternary faults. The prevailing regional stresses as derived from earthquake source mechanism studies agree with a NE-SW trending compression.

IRRN 78 ~------ --~-----·------ ~------- ·---------

I

FIG. 3. The distribution of the IDA recording stations around the epicenter on an azimuthal equidis­tant projection.

SOURCE PARAMETERS OF THE 1978 T AHAS EARTHQUAKE

Berberian et al. (1979) determined the focal mechanism of this earthquake from the polarity distribution of the P-wave first motion at teleseismic distances to be a

1206 MANSOUR NIAZI AND HIROO KANAMORI

thrust with the following focal planes

<f>1 = 127°, 01 = 62° SW and cf>2 = 152°, 02 = 31 °NE

(cf>= strike azimuth; o = dip angle).

From the hypocentral distribution of a limited number of selected aftershocks, and field observations, they picked the latter as the fault plane. As seen in Figure 3 of their paper, the southwest dipping auxiliary plane is firmly constrained by the data. The data, however, have relatively weak constraint on the northeast dipping fault plane, which can accommodate strikes ranging from N5°W to N65°W. Because of the engineering importance of the near-field accelerations reported for the Tabas

40

50

60

HOURS

FIG. 4. The record section of the 1978 Tabas earthquake across the IDA stations. Note that the record at CMO stations shows a strong asymmetric radiation pattern (i.e., R 2n + 1 > R,n). Also note that the asymmetries at PFO and KIP are opposite to that at CMO station. PFO and KIP stations bracket CMO station in azimuth and are within an azimuthal range of 43°.

earthquake, we investigated the source parameters of this earthquake using an independent set of data. The data consist of the digitally recorded R2 and Ra phases at eight stations of the IDA network (Agnew et al., 1976). The stations used in this study are shown in Figure 3. The record section of the 1978 Tabas earthquake across these stations is shown in Figure 4. These Rayleigh-wave trains were inverted to obtain source parameters by using the method described by Kanamori and Given (1981).

SOURCE PARAMETERS OF EARTHQUAKES FROM SURFACE WAVES 1207

Using surface-wave data at a period of 256 sec, inversion was performed twice using a. moment tensor source and a dislocation fault model. Since as discussed by Kanamori and Given (1981) the inversion becomes ill-conditioned for a very shallow earthquake such as this one, we performed the inversion with the following con­straints, and tried several different models.

First, we removed the ill-conditioning by requiring that the fault mechanism be either vertical strike slip or a pure dip slip on a 45° dipping fault (i.e., Mzx = Mzy =

O; see Kanamori and Given, 1981). Although the solution so constrained is somewhat restrictive, a good first approximation can be obtained for the fault type, the strike direction, and the seismic moment.

TABLE 2

SOURCE PARAMETERS OF THE 16 SEPTEMBER 1978 EARTHQUAKE COMPUTED FROM MOMENT TENSOR

INVERSION (1 AND 3) AND FROM FAULT MODEL SOLUTION (2 AND 4) AT THE PERIOD OF 256 SEC

Mxy Myy -Mxx MYY + Mxx

Model

T (sec)

Mo* 81

Major Double Couple i/;1 82 i/;2

Minor Double Couple rms

Model

T (sec)

Mo 81 (constrained) i/;1 (constrained) ,\!

8, i/;2 rms

0.54 ± 0.21 -0.35 ± 0.43 -1.0 ± 0.2

1.1 45°

234° 45° 54°

3.8% 0.928

1.3 ± 0.2 62°

217° 74° ± 10° 32° 68° 0.915

33

0.67 ± 0.13 -0.17 ± 0.27 -1.3 ± 0.1

1.3 45°

229° 45° 49°

3.5% 0.591

33

1.5 ± 0.1 62°

217° 79° ± 6° 30° 60° 0.587

* The units of the moment tensor Mv and the scalar moment Mo are 1027 dyne-cm. 8; and if;; are the dip angles and the dip directions, respectively. ,\1 in models 2 and 4 is the slip angle. The moment tensor is decomposed into major double couple and minor double couple.

Second, since one of the nodal planes is constrained very well by the first-motion data (Berberian et al., 1979), we fixed this plane and determined the other fault parameters by inversion using a fault model.

In each case, we tried the inversion both with and without the source process time T (see Kanamori and Given, 1981) increasing the total number of solutions to four. A value of T = 33 sec is considered appropriate for an earthquake with Ms = 7.5 and is used in solutions 3 and 4. The results of the inversion are shown in Table 2.

As seen by the rms values, the results obtained with T = 33 sec under both sets of constraints have much smaller standard errors than those without it, indicating that this amount of source process time is reasonable for this event. The solution with Mzx = Mzy = 0 (models 1 and 3) indicates a strike direction which is consistent both

1208 MANSOUR NIAZI AND HIROO KANAMORI

with first-motion data and geologic information (Berberian et al., 1979). Models 2 and 4, obtained with one nodal plane constrained (to be consistent with the first­motion data), are almost pure dip slip fault. This mechanism is very close to the one obtained from first-motion studies by Berberian et al. (1979). The estimated seismic moments range from 1.1to1.5 X 1027 dyne-cm. We prefer model 4, Figure 5b, for the mechanism of this earthquake with Mo= 1.5 X 1027

, which corresponds to Mw = 7.4. The calculated mean value of the source moment is in general agreement with estimations from the field data and aftershock distribution (Berberian et al., 1979).

The seismic moment determined from surface waves represents the overall size of the earthquake. It has been known that the seismic moment determined from surface waves is often considerably larger than that determined from body waves. This is especially true for large events, particularly for those from subduction zones (e.g., Lay and Kanamori, 1980). Since relatively few data are available for large

/

N

M = 1.3 x I 027 dyne-cm 0

(a) Model 3

N49E /

<f!= 139° 8 = 45°

/

N46E

.. p

'1'2 = 150°

Ii 2 = 300

M = 1.5 x I 027

dyne-cm 0

(b) Model 4

'1'1 = 127° (fixed)

81 = 62°

FrG. 5. Focal mechanism of the 1978 Tabas earthquake obtained from long-period surface waves. (a) is obtained from moment tensor inversion with M,, and Mzy set equal to zero (Kanamori and Given

· 1981). (b) is obtained from fault 11!-odel solutioi;i by assuming one of t~e no~al planes to be constrained by the P-wave first-mot10n observat10ns (Berberian et al., 1979). Both mvers10ns are made at the period of 256 sec. The source process time in both cases is assumed to be 33 sec. Both (a) and (b) are lower hemisphere equal area projection.

intracontinental events, we analyzed the P waves from this earthquake to compare the seismic moment determined at two different ptriods.

In this comparison we have made use of the vertical component of the P wave recorded at six of the WWSSN stations at which the seismograms remained on scale while recording P phase. Assuming a trapezoidal source time function of 12-sec duration and 10-km focal depth in a simple half-space model with a= 6.50 km/sec, /3 = 3.75 km/sec, and p = 3 gm/cm3

, we tried to match the amplitude of the synthetic and observed records for the six stations, taking into account the contributions of pP and sP as well. The results of this comparison are summarized in Table 3. The estimated moments from the P waves recorded at these six stations show a scatter

SOURCE PARAMETERS OF EARTHQUAKES FROM SURFACE WAVES 1209

between 2.8 and 9.3 X 1026 dyne-cm. The average of all the stations is 5. 7 x 1026

dyne~cm, and that of stations at 30° < ii ~ 90° is 8.2 X 1026 dyne-cm. Since the body waves at ii > 90° may be affected by the core, the average 8.2 X 1026 dyne-cm is considered more appropriate. This value is still smaller than the surface-wave estimate by about a factor of 2. Similarly, a low value of the seismic moment, 4.41

TABLE 3

SEISMIC SOURCE MOMENT ESTIMATE OF THE 16 SEPTEMBER 1978 EARTHQUAKE FROM THE VERTICAL COMPONENT OF P-WAVES RECORDED AT Six WWSSN STATIONS

Station Distance Azimuth Observed Amplitude* Mn X 102

h dyne-cm

n (0) (cm)

STU 38.80 308.2 8.2 6.4 WES 91.27 324.5 3.3 4.1 OGD 93.86 325.7 2.3 2.8 KTG 55.56 336.3 13.8 9.3 MSO 99.78 354.0 2.3 2.8 ANP 55.83 80.3 14.0 9.0

Mean 5.7

*Peak-to-peak amplitude normalized to 1500 magnification.

x 1026 dyne-cm, for this earthquake was obtained from the P-wave analysis by Souffleris and Berberian (personal communication, 1980). The simplest explanation for the moment difference derived from body and surface waves is that the stress

T ABAS (IRAN) 1978

c .<:'. v 2 0 uI VJ <( I 0..

CMO Rz

-3

5

4 ~ c~o R3 V)

E 3 u

0..: 2 ::2: <(

0 TWO R~ ESK R3

0 60 120 180 240 300 360

AZIMUTH (DEGREE)

FIG. 6. Comparison of ~he observed p?ases and amplitudes of the 1978 Ta bas earthquake as recorded across the IDA network with the theoretical values computed for model 4. Theoretical phases are nearly zero. Note the clear antisymmetry of CMO and KIP observations.

1210 MANSOUR NIAZI AND HIROO KANAMORI

drop on the fault plane is heterogeneous and the body-wave radiation mainly came from a relatively small high-stress spot (asperity).

The amplitude and the phase spectra of the surface-wave data at a period of 256 sec are shown in Figure 6 as a function of azimuth. Although the overall fit is good, the asymmetry of the amplitude at CMO, KIP, and PFO is quite obvious. Since the source dimension of this earthquake is about 80 km, the observed asymmetry at the period of 256 sec cannot be explained by the source finiteness.

Furthermore, as shown by Figure 7, the pattern of asymmetry at CMO is opposite to that at KIP and PFO. Rayleigh waves with odd order numbers are larger than

80

90

JOO

w w

CMO

::c::::;;'.~N;:---~:=-=-:::-~_ :c:::i::; r : = -: ===- :::::: _::: __ :;;:::;:-:=:::)J

L _l'::::-:-:-:::i~==-==-=::: = =-=-=== == : I

G 110 PFO w ~~~·· OBS

SYN 9 <I

120 KIP

140

ISO

\('-~~--~~~~~----- ------v-v\/'-------~------,.,-~ OBS

'V\V------- ~~~vv~---- SYN

1v---~~~~~~---~--~ ---------~V\;'--'-~ , _ ___,,--..,~"\J\;tv'----------- OBS

-~~1J\fc----~'Jc--- SYN

OBS \,'"V\~ -- --- ------------- ---------- ----------

0 10 20 30

MINIJTFS

SYN

FIG. 7. Comparison of the synthetic (for model 4) and observed seismograms at several IDA stations. Both observed and synthetic records are band-pass filtered between 150 and 1500 sec. Notice that the observed Rayleigh waves with odd order numbers are larger than synthetic phases at CMO and are smaller at KIP and PFO. The even order phases show an opposite pattern.

synthetics at CMO and smaller at KIP and PFO while those with even order numbers show an opposite pattern. The difference in the azimuth between these stations is relatively small, and furthermore, CMO lies between KIP and PFO. It would be extremely difficult to explain this pattern by any finite fault. This asymmetry is probably caused by regional structural heterogeneities near the source or along propagation paths, or possible departure of the fault plane from simple planar geometry although we do not have a model to explain it at present. Expla­nation of this strong asymmetry would require further studies.

SOURCE PARAMETERS OF EARTHQUAKES FROM SURFACE WAVES 1211

GENERAL REMARKS ABOUT THE NOVEMBER 1979 QAINAT EARTHQUAKES

A preliminary account of field observations of the two earthquakes of 14 and 27 November 1979, including the mapped trace of the faulting which accompanied them, is given by Haghipour and Amidi (1980). They reported two fault traces nearly perpendicular to each other. The smaller earthquake of 14 November had a right lateral mechanism on a nearly north-south striking arcuate fault trace which is convexing westward. The faulting associated with the 27 November event is left lateral on a nearly east-west trace at the eastern extension of the Dasht-e Bayaz fault activated during the 31 August 1968 earthquake with the same mechanism (Niazi, 1969). Both faults are nearly vertical. The spatial relation of the November

IRAN NOV. 1979 BKS ULP Z

oBsrnvrn ..,.---'V· r""'v-/\ f' r-Vr'\' /' NOV.27 IJ V V v

SYNTHETIC 26

M0 = 8 x 10 dyne-cm

l • rn

OBSERVED~~~~-------­NOV. 14

!RAN NOV. 1979 BKS ULP

OBSERVED NOV. 27 Vy/'\

SYNTHETIC 26

M0

= 8 x 10 dyne-cm

OBSERVED NOV. 14

NE-SW

FIG. 8. Comparison of the synthetic and observed R2 and G2 phases written by Berkeley ultra-long­period instruments.

1979 sequence with respect to the 1968 sequence and the Dasht-e Bayaz fault is shown in Figure 1. Here we have used the mapped surface traces of the 1979 conjugate faults as the starting models and computed the R2 and G2 phases at Pasadena and Berkeley. The synthetic seismograms for Berkeley are compared with the observed ones in Figure 8. The final source parameters for which the synthetics are computed are

14 November 1979 27 November 1979

Original time 02h21 m19.3s 17hl0m32.98

vertical right-lateral vertical left-lateral Strike N 13°E N 73°E Mo 1.0 X 1026 dyne-cm 8.0 x 1026 dyne-cm Mw 6.6 7.2.

1212 MANSOUR NIAZI AND HIROO KANAMORI

The 27 November earthquake is in many respects similar to the 1968 earthquake. They are about the same size, have the same mechanism, and are associated with the same structure, namely the Dasht-e Bayaz fault. The casualty and damage reports of the recent two earthquakes are very scanty. The unofficial report issued after the 14 November event indicated a loss of nearly 500 lives. No report has been issued for the second and larger earthquake of 27 November.

CONCLUSIONS

(1) The source mechanism derived for the Tabas earthquake of 16 September 1978 from inversion oflong-period surface waves is in close agreement with the first­motion and field data in assigning a nearly pure thrust mechanism with a NW-SE strike to this earthquake. Inclusion of a 33-sec source process time would result in the improvement of the fit to the observations.

(2) The estimations of source moment from long-period surface waves range between 1.1to1.5 X 1027 dyne-cm with a preferred value of 1.5 X 1027 dyne-cm (Mw = 7.4) while the values obtained from the observed P-wave amplitudes are nearly two times smaller.

(3) The observed asymmetery of the Rz and R3 amplitudes of the 1978 earthquake at IDA stations cannot be explained by simple directivity of the source. The regional heterogeneities with dimensions of several hundred kilometers, especially in the source region, are prime suspects. The deviation of the fault surface from planar geometry and anisotropic features along the propagation paths, however, may not completely be ruled out.

(4) Taking into account the pattern of faulting associated with the 14 and 27 November 1979 earthquakes observed in the field, we used the Rayleigh- and Love­wave amplitudes at Pasadena and Berkeley to estimate seismic moments of 1.0 X

1Q26 and 8.0 X 1026 dyne-cm for these earthquakes, respectively. The corresponding Mw for the two earthquakes are 6.6 and 7.2.

ACKNOWLEDGMENTS

The IDA records used for the present study were provided by the IDA project team at the Institute of Geophysics and Planetary Physics, University of California, San Diego. The ultra-long-period records at Berkeley were provided by the Seismographic Station, University of California, Berkeley. We also thank an anonymous reviewer for several helpful comments. This work was partially supported by Earth Sciences Section, National Science Foundation Grant EAR 78-11973 and U.S. Geological Survey Contracts 14-08-0001-19265 and 14-08-0001-19755.

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Ambraseys, N. N. and C. P. Melville (1977). The seismicity of Kuhistan, Iran, Geograph. J. 143, 179-199. Berberian, M. (1979). Earthquake faulting and bedding thrust associated with the Tabas-e Golshan (Iran)

earthquake of September 16, 1978, Bull. Seism. Soc. Am. 69, 1861-1887. Berberian, M., I. Asudeh, R. G. Bilham, C.H. Scholz, and C. Soufleris (1979). Mechanism of the main

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SOURCE PARAMETERS OF EARTHQUAKES FROM SURFACE WAVES 1213

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central Iran: a preliminary field report, Geophys. Res. Letters 6, 689-692. Niazi, M. (1969). Source dynamics of the Dasht-e Bayaz earthquake of August 31, 1968, Bull. Seism. Soc.

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Maurice Ewing Series, AGU, vol. 4, D. L. Simpson and P. Richards, Editors. Nowroozi, A. A. (1971). Seismotectonics of Persian plateau, eastern Turkey, Caucasus and Hindukush

region, Bull. Seism. Soc. Am. 61, 317-341. Shearman, D. J. (1976). The geological evolution of southern Iran: the report of the Iranian Makran

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TERA CORPORATION 2150 SHATTUCK AVENUE BERKELEY, CALIFORNIA 94704 (M.N.)

Manuscript received January 22, 1981

SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA 91125 (H.K.) CONTRIBUTION No. 3551