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Post-earthquake observations at Dasht-e Bayāz, Iran
Tectonophysics, 26 (1975) 267-279 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
POST-EARTHQUAKE
T. V. MeEVILLY*
OBSERVATIONS
AT DASHT-E
BAYAZ, IRAN
and M. NIAZI
Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Calif. (U.S.A.) University of Mashad, Mashad (Iran) (Accepted
for publication
November 24, 1973)
ABSTRACT McEvilly, T.V. and Niazi, M., 1975. Post-earthquake Tectonophysics, 26: 267-279.
observations at Dasht-e Bay%, Iran.
A program of field observations of post-earthquake characteristics has been established at the site of the devastating 1968 Dasht-e Bayzz earthquake in northeastern Iran. Data for the period 2-4s years following the main shock have been analyzed. Aftershocks are still occurring, about seven per day of magnitude over zero, and their rate of occurrence is decaying at approximately t-l.l. The are confined the 1968 zone, concentrated the ends a conspicuous region near centrally situated focus. The define a dipping fault by their this plane consistent with 1968 surface and fault-plane as well a composite solution of aftershocks. Left-lateral creep of mm/year seems be occurring, evidenced by monument arrays creepmeters installed the study.
The destructive Bay% earthquake August 31,1968, its effects been described detail by (1968,1969), Ambraseys al. (1969), and Tchalenko and Crampin The magnitude earthquake occurred a region previous large though it to have the largest 800 years 1969), and accompanied by surface faulting some 60-80 This investigation, in late has been to monitor post-earthquake characteristic the region to search any time in geophysical associated with phase of regional behaviour.
Authors names
alphabetically.
268
The study, continuing at present, has several facets: (1) air-photo interpre~tion for evidence on the existing dist~bution of faulting in the region; (2) installation of a permanent, high-gain seismographic station in the area; (3) operation of an array of portable seismographic stations for one to two weeks on a regular basis, at least yearly; (4) ins~llation and regular surveys of four monument quadrilate~~s across the 1968 break; (5) emplacement of two recording creepmeters across the 1968 break and an apparently active fault trace nearby. This report presents results of the study through 1972, with discussion of the various aspects of the observational program. FAULTING
Figure 1 is a fault map of the Dasht-e Bay% area, showing faults evident from the aerial photographs taken shortly after the 1968 earthquake. The 1968 break is shown with locations of the creepmeters and qua~ila~~s ~t~an~lation sites). The region appears to be heavily faulted. The 1968 break is at the northern limit of an area of relatively deep alluvial deposits, partially making possible fault evidence there. The 1968 fault appears to show north dip of 73” from hypoeenter distributions. Many small shocks in the ongoing activity seem to be located in the shattered zone of the 1968 break, Detailed geology of the region is not well known. Local topography, reaching elevation of over 2000 m, is formed principally by fossiliferous limestone beds of Cretaceous age overlying unconformably light to moderately altered Jurassic green shale and sandstone beds which crop extensively out to the north of the recent rupture zone and are often i~terbedded with limestone layers ~Eftekh~-Nejad et al., 1968; Behzadi, 1972). The basement rocks exposed some 20 km north of the recent fault trace on either side of the main north--south road {Fig. 1) consist primarily of po~~~tic micro~anite and ~anodiorite (Behzadi, 1972).
An array of three portable micro-earthquake systems was installed and operated for two days in November, 1970, as a check on the level of seismicity in the area. Many earthquakes were recorded, with a wide range of S--P intervals, indicating a relatively high level of seismicity along the fault zone. In January and March, 1971, and in June, 1972, recording periods of approximately one week with three stations, were established. From several hundred earthquakes recorded during these periods, 59 were selected as representative, with clear onsets, and located using P and S arrivals with a
269
1968 FAULT BREAK OLD FAULTS _ ROAD
+
3&i
-2.
----
.
---^c=_\
oi
/
--..... ......
FAULT .WAP
OF DASHT
E
BAVAZ
AREA.
A
IRAN
Fig. 1. A. Fault map, from aerial-photo study, of region around 1968 break. B. Situation map.
modified version of the computer program in use at Berkeley (Bolt and Turcotte, 1963). Figure 2 shows the distribution of the epicenters in the region. The data are listed in Table I. The velocity model used was determined from quarry blasts near Mashad, Iran, where a profile to 30 km distance has been recorded (50 km is approximately the maximum distance used in the
270
TABLE1 Earthquakehypocenters
Date
Time(GMT)
Latitude (N)
Longitude (W
Depth (km)
25 Jan,1971 25 26 26 26 36 27 27 27 28 28 29 29 29 29 29 29 29 29 29 29 29 30 30 31 31March,1971 31 31 31 31 1 April, 1971 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 3
17 36 45.3 1957 32.2 04 25 30.8 06 02 36.1 17 38 31.1 19 23 33.5 12 39 53.5 12 43 49.0 19 3111.0 02 38 33.7 2022 50.0 02 27 54.0 02 43 22.0 04 36 24.1 07 08 01.1 17 46 17.4 18 09 12.5 18 26 53.8 20 47 03.6 2109 14.4 23 08 24.8 23 35 50.4 00 0236.5 00 02 59.4 15 58 20.9 18 56 07.0 19 49 32.8 19 56 41.5 20 18 42.7 22 27 31.5 18 16 31.5 18 58 05.2 21 49 45.6 22 18 58.4 22 45 25.4 03 40 21.1 03 4136.0 03 59 36.2 05 22 06.0 05 43 14.2 05 46 04.2 13 23 49.3 13 53 46.6 14 37 22.7 15 11 37.0 2214 52.3 0117 07.0
34 05.3 34 07.2 34 10.8 34 12.5 34 07.7 34 05.5 34 02.7 34 11.3 34 03.3 34 02.7 34 13.1 34 12.0 34 07.2 34 09.8 34 02.8 34 03.7 34 05.4 34 08.3 34 12.3 34 10.0 34 05.6 34 04.3 33 59.6 33 58.7 34 04.0 34 02.6 34 09.2 34 05.2 34 00.2 33 59.6 34 04.7 34 07.1 34 07.9 34 06.9 34 06.9 34 08.5 34 04.7 34 02.8 34 04.0 34 04.3 34 00.6 34 06.5 34 02.9 34 05.4 34 06.9 34 05.3 34 03.6
58 48.2 59 01.5 59 10.3 59 24.0 58 45.9 58 39.3 59 05.5 59 11.9 58 46.5 58 38.8 59 15.3 59 17.1 59 00.9 58 34.4 58 41.3 58 44.5 58 44.3 59 13.7 59 16.9 59 12.4 58 37.2 58 21.8 58 24.7 58 25.3 58 38.1 58 55.8 59 06.3 58 51.6 59 43.1 59 18.9 5914.5 59 17.6 59 09.6 5930.1 5930.1 5917.2 5916.5 59 13.2 59 21.5 58 50.2 59 10.5 59 32.4 59 19.6 58 52.4 59 07.0 59 13.5 5918.4
16.4 12.2 35.8 12 (R)* 28.4 8.5 20.5 30.3 2.4 4.9 12 (R) 1.4 12.5 12 (R) 12 (R) 6.0 14.4 17.1 12 (R) 7.8 12 (R) 12 (R) 12 (R) Xis3(R) 3.9 3.8 20.3 12 tR) 12 (R) 1.6 11.9 12 (R) 15.1 15.1 12 (R) 11.2 13.0 14.9 12.0 12.5 12 (R) 10.3 10.5 6.6 7.2 1.2 (R)
271 TABLE I (continued) Time(~~T)
Date ____3 3 3 3 3 9 June, 1972** 9 10 11 14 14 14
01 42 01 57 02 28 02 44 06 48 2102 21 04 1147 20 36 03 11 15 20 17 55
08.5 15.8 34.5 43.2 04.3
Latitude W) -__34 34 34 34 34 34 34 34 34 34 34 34
Longitude (E)
Depth (km) ----
59 58 59 59 59 59 59 59 59 59 59 58
12 24.2 3.9 5.2 12 12 12 12 12 12 12 12
06.2 07.8 07.3 05.8 04.7 10 09 04 10 09 07 12
18.7 34.2 03.3 01.5 18.2 09 08 16 09 08 16 51
(R)
(R) (R) (R) (R) (R) (R) (R) (R)
* (R) = depth restricted in locating. ** Last seven events located graphically.
hypocenter locations). The model consists of 1.3 km of cr = 3.5 kmlsec, /3 = 2.0 km/see material overlying a: = 5.75 and /3 = 3.3 km to a total crustal thickness of 31.3 km. Mantle velocities used were 8.0 and 4.6 km/set. This relatively slow model was consistent with the quarry blast profile and also was indicated by numerical experiments in which clusters of earthquake locations were found to be most independent of the station positions in the various recording periods when the above model was used.
. l l
i
io
Fig. 2. Epicenters determined with symbols keyed to recording period.
272
Other numerical experiments with reasonable variations on the above model showed that hypocenters within about l--,2 diameters of the station network would vary no more than 2----3 km in position. From these experiments we feel that the hypocenters near the triangle of stations are accurate to better than 5 km in location error. The distribution of epicenters in the figure shows very clear association with the 1968 break. This is not an effect of station distribution, since stations were located near both ends of the zone and would have recorded earthquakes outside the 1968 zone. Thus it appears that the present seismicity of the region is, in fact, aftershock activity associated with the 1968 main shock. Depths indicate crustal foci, the greatest depth obtained being 35.8 km. A clear delineation of what appears to be the fault surface is possible through a fortuitous cluster of earthquakes which occurred in the January, 1971, recording period. Excellent locations were obtained for five shocks (north of Dasht-e BayGz in Fig. 2). In Fig. 3 these hypocenters are projected onto a north---south plane through Dasht-e Bay% A fault plane with apparent dip of 73” north, intersecting the surface very close to the 1968 break, is indicated. Errors in the velocity model, or lateral velocity variation across the fault zone, can, of course, introduce apparent dip by systematic bias of hypocenters. However, independent evidence of north dip exists in the fault-plane solution of Niazi (1969) where 80” north dip was estimated, as well as the composite fault-plane solution constructed from the data of this study.
Daiht
0
’
A
BaYi2
N
E EtC,F Bvjuk
10 2 Y
0
I = 20
a E
\ 0
30
DIP
40
I 34O
Fig. 3. Projection onto northsouth BayZiz.
\
73=‘N I 34015’
plane of well-located earthquakes near Dasht-e
273 UPPER
EQUAL _-
FOCAL,
HEMISPHERE
AREA Strike DIP Left
E-W 70*N Lateral
Fig. 4. Composite fault-plane solution based on first motions from 43 shocks,
All clearly discernable first motions were read for the earthquakes located in this investigation and a composite upper focal hemisphere projection of some 75 first motions from 43 shocks is presented in Fig. 4. With exception of the group of compressions in the southwestern quadrant, the data are well represented by a left-lateral transcurrent fault striking east---west and dipping northerly at 70”. The anomalous compressions to the southwest are all from shocks of January, 1971, in the northeastern part of the area, recorded at station A, up to 50 km distant. The great majority of the useable first motion data are consistent with left-lateral motion on the 1968 fault surface. We see no indication of right-lateral motion as inferred by Crampin (1969) for early aftershocks and suspect that errors in locations may have been involved. In April, 1971, one of the portable instruments was installed near the village of Kakhk, northwest of Da&t-e Bay%, as a permanent monitor of area seismicity. During the previous January recording session there had been an instrument near this site. Recording with a magnification of about 750,000 at 10 Hz, the instrument records clearly earthquakes of magnitude (ML equivalent) greater than zero at 20 km distance. This magnitude threshold is based on the local magnitude scale, extended to micro-earthquakes, as used at Berkeley. For the range of epicentral distances
214
150
50
0
1
2
3
4
5
S-P
Fig. 5. Histogram
6
7
6
9
10
TIME bed
of 1164 S-P
intervals
in approximately
4 months
during
1971.
from Kakhk represented by the ongoing seismicity in the area, a given record amplitude implies a range of about one half unit in magnitude, if produced by shocks over the observed distance range. A 4-month data sample was selected for study of the distribution of S-P times for all events recorded at the Kakhk station. With the exception of a very few distant e~hquakes (S-P of 20 sec. or more), virtually all the events seen had S-P times of less than 11 sec. A histogram of the S-P values for 1,164 shocks is presented as Fig. 5. The high incidence of S--P values less than 3 set represents a concentration of earthquakes of ML > 0 within about 20 km of Kakhk. The minimum at values of 5-6 set represents a region of low seismicity in the vicinity of the 1968 main shock, while the second concentration around 8 set S-P time represents earthquakes of about M > 1 at some 50 km distance. This distribution fits very well the pattern of epicenters shown in Fig. 2. The long-term occurrence characteristics thus indicate a fairly stable spatial distribution of activity confined to the zone of observed surface faulting in 1968, with strongest seismicity near the ends of the zone and a distinctly lower rate of activity near the 1968 epicenter at the center of the zone. This is further evidence that the present seismicity represents continuing aftershock activity associated with the 1968 earthquake, which would appear to have been a bilateral rupture. The stability of this spatial pattern seems to hold throughout the time spanned by this study. Next, an attempt was made to measure the decay characteristics of the occurrence data, assuming the earthquakes do in fact represent an aftershock sequence viewed some 2.5-4.5 years after its onset. In Fig. 6 are plotted the daily occurrence rates, based in 3-day averages UntilDecember, 1971, then
monthly averages, as observed at the Kakhk station. There is clear indication of a general decline in activity over the period of observation. We now attempt to deduce the decay rate from 1968 through 1972 by incorporating the data of Crampin (1969). In a field study immediately
100000
10
I
I
I
10
100
, 1000
\, loo00
O&YS
TIME
Fig. 7. Decay
curve
for aftershocks
with ML greater
than about
zero.
following the earthquake, an instrument was operated about 15 km from our Kakhk site, obtaining aftershock data in intervals from 15 to 40 days following the main shock. Figs. 5 and 6 of Crampin (1969) present data which imply a threshold (assuming constant “b” value to ML = 0) of ML about 0.5 for events within 20 km. On this basis, the occurrence data of Crampin at his station AA were converted to equivalent Kakhk station data and combined with our data. The resulting time decay is presented in Fig. 7 on a logarithmic plot spanning over 4 years. The combined data can be represented closely by the inverse power law N(t) = 30000t-i .I. The value of the exponent is typical of those found for decay rates of aftershock sequences at much larger threshold magnitudes (Utsu, 1961) and compares to the value, also 1.1, found by Crampin (1969) for the 15-40 day time interval in 1968. The value 30,000 represents the rate of occurrence (earthquakes per day) for ML greater than about zero one day after the main shock, assuming the relation holds to that early time. FAULT CREEP
Monument arrays for monitoring possible fault slip have been installed at four sites along the 1968 fault break. Locations of these are shown in Fig. 1, and described in Table II. The sites were selected on the basis of a clearly evident 1968 scarp in an area of minimal topographic relief. Typically, two or three concrete monuments were set 0.5~--1 meter into the ground on each side of the fault trace. Spacings between adjacent or opposite monuments were typically 30-50 m. Repeated surveys (direct measurement by metal tape) showed an uncertainty in a given measurement of some 5 mm, probably a combination of temperature differences and measuring errors. This seemed satisfactory as observations were planned for several years and creep rates as low as l--2 mm per year should become evident over this time. As of late 1972, with about two years of observations, the survey networks indicate a strong possibility of left-lateral fault slip, amounting to
TABLE II Fault monument array
Location
No. Elements
Installed
Resurveyed
M - Boskabad H - East of Dasht-e Bay% W - East of Dasht-e Bay% N - Moghri
5 6 5 5
Feb. Dec. Feb. July
Feb. 1971, July 1971, Dec. 1972 July 1971, July 1972, Dec. 1972 Nov. 1970, July 1971
1970 1970 1970 1972
about 5 mm per year, with an uncertainty of the same order, i.e., the noise levels have been just barely surpassed. In order to monitor with more precision this possible fault slippage, recording creepmeters were fabricated for installation on the fault trace. The instruments are quite simple, utilizing invar rod some 7.5 m in length, rigidly attached at one end while the free end forms the core of a linear variable differential transformer displacement sensor and terminates against the sensing shaft of a 25 mm range dial indicator with 1 mm per dial revolution, easily resolving 0.01 mm. A slow speed recorder, at a sensitivity of 1 mm full scale, resolution of about 0.02 mm, operates at the site powered by batteries. The first creepmeter was installed at Boskabad in July, 1971. At this site the 1968 offset was about 1 m left-lateral, and it is near the western end of the zone of surface rupture. The ins~ment has been read every l-2 months and is showing clear accumulation of offset in a left-lateral sense. Total motion indicated between installation in July, 1971, and our last reading in February, 1973, is some 6 mm. Readings were erratic in the first months but for the last 6 months of operation creep has been relatively steady with something over 1 mm for the period. The nature of the creep is fairly steady with occasional small step-like events. In September, 1971, a second creepmeter was installed in a remote region near Moghri, toward the eastern end of the 1968 surface break. Because of local terrain conditions, the site selected was on a hillside. For about 9 months very erratic readings were obtained, which we attribute to possible slumping and sliding of the hillside. With no indication of stabilizing, the meter was then removed to another site on the north--south trending fracture system near which many small events were recorded during the occupation of site M in June, 1972. The third site, near Hatamabad, east of Moghri, is located on a prominent northsouth trending fault which was discovered during field work in the area, can be seen clearly on the air-photos, and appears to be associated with part of the seismicity around the east end of the 1968 break. Installed in September, 1972, this creepmeter has shown rather uniform right-lateral creep during the first four months of operation, with total offset of 1 mm. TABLE III Creepmeters
Location
Installed
Results to Feb. 1973
A - Boskabad B - Moghri C - Hatamabad
July 1971 Sept. 1971 August 1972
6 mm total offset, left-lateral erratic - removed after 9 months 1 mm total offset, right-lateral. on subsidiary fault
278
Field observations suggest possibility of cummulative right-lateral offset of up to 3 km on this north-south trending fault (Behzadi, 1972). Creepmeter sites are shown in Fig. 1 and summarized in Table III. DISCUSSION
A program of field observations of the post-earthquake characteristics has been undertaken, beginning about two years after the disastrous magnitude 7.3 main shock, in the Dasht-e Bayaz area of Khorassan, in the northeast of Iran. Data taken in the interval 2 -4% years after the earthquake show the following features of the post-earthquake regime: (1) The aftershock sequence continues through 1972, with some seven shocks per day of magnitude zero and greater. The aftershock rate is still measurably decaying with time, apparently at an inverse power rate proportional to t - l .l, very similar to early aftershock observations at Dasht-e Bayaz and larger magnitude aftershock behavior in other regions. (2) Aftershocks are concentrated almost exclusively along the zone of surface rupture in 1968, with a relatively aseismic gap in the center of the zone, near the 1968 epicenter. (3) A composite fault-plane solution for 43 shocks is consistent with left-lateral motion on a plane dipping steeply to the north. (4) Aftershocks occur to depths around 30 km, and define a steeply dipping plane, presumably the fault plane in 1968, which intersects the surface near the 1968 surface rupture and dips northerly at about 70”) in agreement with previous studies of the 1968 focal mechanism and with the composite fault-plane solution for the aftershocks. (5) Survey monuments and creepmeters installed along the 1968 trace have begun to provide clear indications of continuing left-lateral creep of several mm/yr along the zone, with possible right-lateral creep on a prominent north--south trending subsidiary fault. Uncertainties will decrease with time in this phase of the program. The program of post earthquake observations will continue. A permanent seismographic station is operating in the area at Kakhk and regular surveying of the monuments and creepmeters will continue. The aim of the effort is to follow the activity over the years until a steady-state situation seems to be reached. Hopefully the accumulated data on post-earthquake behavior will improve our basic knowledge on the characteristics of major earthquakes in Iran and similar regions. ACKNOWLEDGEMENTS
The authors are indebted to the National Academy of Sciences for support of this research through a grant from the Arthur L. Day Bequest and to Carnegie Institution of Washington for the support through a Harry Oscar Wood Award to M. Niazi. The investigation could not have been made
279
without their support. The fault map of the area (Fig. 1) was compiled by Mr. Hassan Behzadi of Mashad University from 1 : 20,000 aerial photographs and local field studies. Mashad University provided local transportation and other logistics for which we are very appreciative. REFERENCES Ambraseys, N.N. and Tchalenko, J.S., 1969. The Dasht-e Bayaz (Iran) earthquake of August 31, 1968. Field Rep., Bull. Seismol. Sot. Am., 59: 1’751-1792. Ambraseys, N.N., Anderson, G., Bubnov, S., Crampin, S., Shahidi, M., Tassios, T.P. and Tchalenko, J.S., 1969. Dasht-e Bayaz earthquake of August 31, 1968. UNESCO Rep. Ser. No. 1214/BMSRD/SCE, Paris. Behzadi, H., 1972. Geologic environment of the 1968 Dasht-e Bayaz rupture zone (in Persian). Publ. Fat. Sci., Univ. Mashad, Iran. Bolt, B.A. and Turcotte, F.T., 1963. Computer location of local earthquakes within the Berkeley seismographic network. Comput. Miner. Ind., Publ. Geol. Sot., Stanford Univ., 9: 561-576. Crampin, S., 1969. Aftershocks of the Dasht-e Bay?iz, Iran, earthquake of August, 1968. Bull. Seismol. Sot. Am., 59: 1823~-1841. Eftekhar-Nejad, J., Haqui-poor, A.A. and Davood-Zadeh, M., 1968. Report of the investigation of the Khorassan earthquake of August, 31, 1968. Geol. Surv. Iran Publ. (in Persian). Niazi, M., 1968. Fault rupture in the Iranian (Dasht-e Bayaz) earthquake of August, 1968. Nature, 220 (5167): 569-570. Niazi, M., 1969. Source dynamics of the Dasht-e Bayaz earthquake of August 31, 1968. Bull. Seismol. Sot. Am., 59: 1843-1861. Utsu, T., 1961. A statistical study on the occurrence of aftershocks. Geophys. Msg., 30: 521-605.
POST-EARTHQUAKE
T. V. MeEVILLY*
OBSERVATIONS
AT DASHT-E
BAYAZ, IRAN
and M. NIAZI
Seismographic Station, Department of Geology and Geophysics, University of California, Berkeley, Calif. (U.S.A.) University of Mashad, Mashad (Iran) (Accepted
for publication
November 24, 1973)
ABSTRACT McEvilly, T.V. and Niazi, M., 1975. Post-earthquake Tectonophysics, 26: 267-279.
observations at Dasht-e Bay%, Iran.
A program of field observations of post-earthquake characteristics has been established at the site of the devastating 1968 Dasht-e Bayzz earthquake in northeastern Iran. Data for the period 2-4s years following the main shock have been analyzed. Aftershocks are still occurring, about seven per day of magnitude over zero, and their rate of occurrence is decaying at approximately t-l.l. The are confined the 1968 zone, concentrated the ends a conspicuous region near centrally situated focus. The define a dipping fault by their this plane consistent with 1968 surface and fault-plane as well a composite solution of aftershocks. Left-lateral creep of mm/year seems be occurring, evidenced by monument arrays creepmeters installed the study.
The destructive Bay% earthquake August 31,1968, its effects been described detail by (1968,1969), Ambraseys al. (1969), and Tchalenko and Crampin The magnitude earthquake occurred a region previous large though it to have the largest 800 years 1969), and accompanied by surface faulting some 60-80 This investigation, in late has been to monitor post-earthquake characteristic the region to search any time in geophysical associated with phase of regional behaviour.
Authors names
alphabetically.
268
The study, continuing at present, has several facets: (1) air-photo interpre~tion for evidence on the existing dist~bution of faulting in the region; (2) installation of a permanent, high-gain seismographic station in the area; (3) operation of an array of portable seismographic stations for one to two weeks on a regular basis, at least yearly; (4) ins~llation and regular surveys of four monument quadrilate~~s across the 1968 break; (5) emplacement of two recording creepmeters across the 1968 break and an apparently active fault trace nearby. This report presents results of the study through 1972, with discussion of the various aspects of the observational program. FAULTING
Figure 1 is a fault map of the Dasht-e Bay% area, showing faults evident from the aerial photographs taken shortly after the 1968 earthquake. The 1968 break is shown with locations of the creepmeters and qua~ila~~s ~t~an~lation sites). The region appears to be heavily faulted. The 1968 break is at the northern limit of an area of relatively deep alluvial deposits, partially making possible fault evidence there. The 1968 fault appears to show north dip of 73” from hypoeenter distributions. Many small shocks in the ongoing activity seem to be located in the shattered zone of the 1968 break, Detailed geology of the region is not well known. Local topography, reaching elevation of over 2000 m, is formed principally by fossiliferous limestone beds of Cretaceous age overlying unconformably light to moderately altered Jurassic green shale and sandstone beds which crop extensively out to the north of the recent rupture zone and are often i~terbedded with limestone layers ~Eftekh~-Nejad et al., 1968; Behzadi, 1972). The basement rocks exposed some 20 km north of the recent fault trace on either side of the main north--south road {Fig. 1) consist primarily of po~~~tic micro~anite and ~anodiorite (Behzadi, 1972).
An array of three portable micro-earthquake systems was installed and operated for two days in November, 1970, as a check on the level of seismicity in the area. Many earthquakes were recorded, with a wide range of S--P intervals, indicating a relatively high level of seismicity along the fault zone. In January and March, 1971, and in June, 1972, recording periods of approximately one week with three stations, were established. From several hundred earthquakes recorded during these periods, 59 were selected as representative, with clear onsets, and located using P and S arrivals with a
269
1968 FAULT BREAK OLD FAULTS _ ROAD
+
3&i
-2.
----
.
---^c=_\
oi
/
--..... ......
FAULT .WAP
OF DASHT
E
BAVAZ
AREA.
A
IRAN
Fig. 1. A. Fault map, from aerial-photo study, of region around 1968 break. B. Situation map.
modified version of the computer program in use at Berkeley (Bolt and Turcotte, 1963). Figure 2 shows the distribution of the epicenters in the region. The data are listed in Table I. The velocity model used was determined from quarry blasts near Mashad, Iran, where a profile to 30 km distance has been recorded (50 km is approximately the maximum distance used in the
270
TABLE1 Earthquakehypocenters
Date
Time(GMT)
Latitude (N)
Longitude (W
Depth (km)
25 Jan,1971 25 26 26 26 36 27 27 27 28 28 29 29 29 29 29 29 29 29 29 29 29 30 30 31 31March,1971 31 31 31 31 1 April, 1971 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 3
17 36 45.3 1957 32.2 04 25 30.8 06 02 36.1 17 38 31.1 19 23 33.5 12 39 53.5 12 43 49.0 19 3111.0 02 38 33.7 2022 50.0 02 27 54.0 02 43 22.0 04 36 24.1 07 08 01.1 17 46 17.4 18 09 12.5 18 26 53.8 20 47 03.6 2109 14.4 23 08 24.8 23 35 50.4 00 0236.5 00 02 59.4 15 58 20.9 18 56 07.0 19 49 32.8 19 56 41.5 20 18 42.7 22 27 31.5 18 16 31.5 18 58 05.2 21 49 45.6 22 18 58.4 22 45 25.4 03 40 21.1 03 4136.0 03 59 36.2 05 22 06.0 05 43 14.2 05 46 04.2 13 23 49.3 13 53 46.6 14 37 22.7 15 11 37.0 2214 52.3 0117 07.0
34 05.3 34 07.2 34 10.8 34 12.5 34 07.7 34 05.5 34 02.7 34 11.3 34 03.3 34 02.7 34 13.1 34 12.0 34 07.2 34 09.8 34 02.8 34 03.7 34 05.4 34 08.3 34 12.3 34 10.0 34 05.6 34 04.3 33 59.6 33 58.7 34 04.0 34 02.6 34 09.2 34 05.2 34 00.2 33 59.6 34 04.7 34 07.1 34 07.9 34 06.9 34 06.9 34 08.5 34 04.7 34 02.8 34 04.0 34 04.3 34 00.6 34 06.5 34 02.9 34 05.4 34 06.9 34 05.3 34 03.6
58 48.2 59 01.5 59 10.3 59 24.0 58 45.9 58 39.3 59 05.5 59 11.9 58 46.5 58 38.8 59 15.3 59 17.1 59 00.9 58 34.4 58 41.3 58 44.5 58 44.3 59 13.7 59 16.9 59 12.4 58 37.2 58 21.8 58 24.7 58 25.3 58 38.1 58 55.8 59 06.3 58 51.6 59 43.1 59 18.9 5914.5 59 17.6 59 09.6 5930.1 5930.1 5917.2 5916.5 59 13.2 59 21.5 58 50.2 59 10.5 59 32.4 59 19.6 58 52.4 59 07.0 59 13.5 5918.4
16.4 12.2 35.8 12 (R)* 28.4 8.5 20.5 30.3 2.4 4.9 12 (R) 1.4 12.5 12 (R) 12 (R) 6.0 14.4 17.1 12 (R) 7.8 12 (R) 12 (R) 12 (R) Xis3(R) 3.9 3.8 20.3 12 tR) 12 (R) 1.6 11.9 12 (R) 15.1 15.1 12 (R) 11.2 13.0 14.9 12.0 12.5 12 (R) 10.3 10.5 6.6 7.2 1.2 (R)
271 TABLE I (continued) Time(~~T)
Date ____3 3 3 3 3 9 June, 1972** 9 10 11 14 14 14
01 42 01 57 02 28 02 44 06 48 2102 21 04 1147 20 36 03 11 15 20 17 55
08.5 15.8 34.5 43.2 04.3
Latitude W) -__34 34 34 34 34 34 34 34 34 34 34 34
Longitude (E)
Depth (km) ----
59 58 59 59 59 59 59 59 59 59 59 58
12 24.2 3.9 5.2 12 12 12 12 12 12 12 12
06.2 07.8 07.3 05.8 04.7 10 09 04 10 09 07 12
18.7 34.2 03.3 01.5 18.2 09 08 16 09 08 16 51
(R)
(R) (R) (R) (R) (R) (R) (R) (R)
* (R) = depth restricted in locating. ** Last seven events located graphically.
hypocenter locations). The model consists of 1.3 km of cr = 3.5 kmlsec, /3 = 2.0 km/see material overlying a: = 5.75 and /3 = 3.3 km to a total crustal thickness of 31.3 km. Mantle velocities used were 8.0 and 4.6 km/set. This relatively slow model was consistent with the quarry blast profile and also was indicated by numerical experiments in which clusters of earthquake locations were found to be most independent of the station positions in the various recording periods when the above model was used.
. l l
i
io
Fig. 2. Epicenters determined with symbols keyed to recording period.
272
Other numerical experiments with reasonable variations on the above model showed that hypocenters within about l--,2 diameters of the station network would vary no more than 2----3 km in position. From these experiments we feel that the hypocenters near the triangle of stations are accurate to better than 5 km in location error. The distribution of epicenters in the figure shows very clear association with the 1968 break. This is not an effect of station distribution, since stations were located near both ends of the zone and would have recorded earthquakes outside the 1968 zone. Thus it appears that the present seismicity of the region is, in fact, aftershock activity associated with the 1968 main shock. Depths indicate crustal foci, the greatest depth obtained being 35.8 km. A clear delineation of what appears to be the fault surface is possible through a fortuitous cluster of earthquakes which occurred in the January, 1971, recording period. Excellent locations were obtained for five shocks (north of Dasht-e BayGz in Fig. 2). In Fig. 3 these hypocenters are projected onto a north---south plane through Dasht-e Bay% A fault plane with apparent dip of 73” north, intersecting the surface very close to the 1968 break, is indicated. Errors in the velocity model, or lateral velocity variation across the fault zone, can, of course, introduce apparent dip by systematic bias of hypocenters. However, independent evidence of north dip exists in the fault-plane solution of Niazi (1969) where 80” north dip was estimated, as well as the composite fault-plane solution constructed from the data of this study.
Daiht
0
’
A
BaYi2
N
E EtC,F Bvjuk
10 2 Y
0
I = 20
a E
\ 0
30
DIP
40
I 34O
Fig. 3. Projection onto northsouth BayZiz.
\
73=‘N I 34015’
plane of well-located earthquakes near Dasht-e
273 UPPER
EQUAL _-
FOCAL,
HEMISPHERE
AREA Strike DIP Left
E-W 70*N Lateral
Fig. 4. Composite fault-plane solution based on first motions from 43 shocks,
All clearly discernable first motions were read for the earthquakes located in this investigation and a composite upper focal hemisphere projection of some 75 first motions from 43 shocks is presented in Fig. 4. With exception of the group of compressions in the southwestern quadrant, the data are well represented by a left-lateral transcurrent fault striking east---west and dipping northerly at 70”. The anomalous compressions to the southwest are all from shocks of January, 1971, in the northeastern part of the area, recorded at station A, up to 50 km distant. The great majority of the useable first motion data are consistent with left-lateral motion on the 1968 fault surface. We see no indication of right-lateral motion as inferred by Crampin (1969) for early aftershocks and suspect that errors in locations may have been involved. In April, 1971, one of the portable instruments was installed near the village of Kakhk, northwest of Da&t-e Bay%, as a permanent monitor of area seismicity. During the previous January recording session there had been an instrument near this site. Recording with a magnification of about 750,000 at 10 Hz, the instrument records clearly earthquakes of magnitude (ML equivalent) greater than zero at 20 km distance. This magnitude threshold is based on the local magnitude scale, extended to micro-earthquakes, as used at Berkeley. For the range of epicentral distances
214
150
50
0
1
2
3
4
5
S-P
Fig. 5. Histogram
6
7
6
9
10
TIME bed
of 1164 S-P
intervals
in approximately
4 months
during
1971.
from Kakhk represented by the ongoing seismicity in the area, a given record amplitude implies a range of about one half unit in magnitude, if produced by shocks over the observed distance range. A 4-month data sample was selected for study of the distribution of S-P times for all events recorded at the Kakhk station. With the exception of a very few distant e~hquakes (S-P of 20 sec. or more), virtually all the events seen had S-P times of less than 11 sec. A histogram of the S-P values for 1,164 shocks is presented as Fig. 5. The high incidence of S--P values less than 3 set represents a concentration of earthquakes of ML > 0 within about 20 km of Kakhk. The minimum at values of 5-6 set represents a region of low seismicity in the vicinity of the 1968 main shock, while the second concentration around 8 set S-P time represents earthquakes of about M > 1 at some 50 km distance. This distribution fits very well the pattern of epicenters shown in Fig. 2. The long-term occurrence characteristics thus indicate a fairly stable spatial distribution of activity confined to the zone of observed surface faulting in 1968, with strongest seismicity near the ends of the zone and a distinctly lower rate of activity near the 1968 epicenter at the center of the zone. This is further evidence that the present seismicity represents continuing aftershock activity associated with the 1968 earthquake, which would appear to have been a bilateral rupture. The stability of this spatial pattern seems to hold throughout the time spanned by this study. Next, an attempt was made to measure the decay characteristics of the occurrence data, assuming the earthquakes do in fact represent an aftershock sequence viewed some 2.5-4.5 years after its onset. In Fig. 6 are plotted the daily occurrence rates, based in 3-day averages UntilDecember, 1971, then
monthly averages, as observed at the Kakhk station. There is clear indication of a general decline in activity over the period of observation. We now attempt to deduce the decay rate from 1968 through 1972 by incorporating the data of Crampin (1969). In a field study immediately
100000
10
I
I
I
10
100
, 1000
\, loo00
O&YS
TIME
Fig. 7. Decay
curve
for aftershocks
with ML greater
than about
zero.
following the earthquake, an instrument was operated about 15 km from our Kakhk site, obtaining aftershock data in intervals from 15 to 40 days following the main shock. Figs. 5 and 6 of Crampin (1969) present data which imply a threshold (assuming constant “b” value to ML = 0) of ML about 0.5 for events within 20 km. On this basis, the occurrence data of Crampin at his station AA were converted to equivalent Kakhk station data and combined with our data. The resulting time decay is presented in Fig. 7 on a logarithmic plot spanning over 4 years. The combined data can be represented closely by the inverse power law N(t) = 30000t-i .I. The value of the exponent is typical of those found for decay rates of aftershock sequences at much larger threshold magnitudes (Utsu, 1961) and compares to the value, also 1.1, found by Crampin (1969) for the 15-40 day time interval in 1968. The value 30,000 represents the rate of occurrence (earthquakes per day) for ML greater than about zero one day after the main shock, assuming the relation holds to that early time. FAULT CREEP
Monument arrays for monitoring possible fault slip have been installed at four sites along the 1968 fault break. Locations of these are shown in Fig. 1, and described in Table II. The sites were selected on the basis of a clearly evident 1968 scarp in an area of minimal topographic relief. Typically, two or three concrete monuments were set 0.5~--1 meter into the ground on each side of the fault trace. Spacings between adjacent or opposite monuments were typically 30-50 m. Repeated surveys (direct measurement by metal tape) showed an uncertainty in a given measurement of some 5 mm, probably a combination of temperature differences and measuring errors. This seemed satisfactory as observations were planned for several years and creep rates as low as l--2 mm per year should become evident over this time. As of late 1972, with about two years of observations, the survey networks indicate a strong possibility of left-lateral fault slip, amounting to
TABLE II Fault monument array
Location
No. Elements
Installed
Resurveyed
M - Boskabad H - East of Dasht-e Bay% W - East of Dasht-e Bay% N - Moghri
5 6 5 5
Feb. Dec. Feb. July
Feb. 1971, July 1971, Dec. 1972 July 1971, July 1972, Dec. 1972 Nov. 1970, July 1971
1970 1970 1970 1972
about 5 mm per year, with an uncertainty of the same order, i.e., the noise levels have been just barely surpassed. In order to monitor with more precision this possible fault slippage, recording creepmeters were fabricated for installation on the fault trace. The instruments are quite simple, utilizing invar rod some 7.5 m in length, rigidly attached at one end while the free end forms the core of a linear variable differential transformer displacement sensor and terminates against the sensing shaft of a 25 mm range dial indicator with 1 mm per dial revolution, easily resolving 0.01 mm. A slow speed recorder, at a sensitivity of 1 mm full scale, resolution of about 0.02 mm, operates at the site powered by batteries. The first creepmeter was installed at Boskabad in July, 1971. At this site the 1968 offset was about 1 m left-lateral, and it is near the western end of the zone of surface rupture. The ins~ment has been read every l-2 months and is showing clear accumulation of offset in a left-lateral sense. Total motion indicated between installation in July, 1971, and our last reading in February, 1973, is some 6 mm. Readings were erratic in the first months but for the last 6 months of operation creep has been relatively steady with something over 1 mm for the period. The nature of the creep is fairly steady with occasional small step-like events. In September, 1971, a second creepmeter was installed in a remote region near Moghri, toward the eastern end of the 1968 surface break. Because of local terrain conditions, the site selected was on a hillside. For about 9 months very erratic readings were obtained, which we attribute to possible slumping and sliding of the hillside. With no indication of stabilizing, the meter was then removed to another site on the north--south trending fracture system near which many small events were recorded during the occupation of site M in June, 1972. The third site, near Hatamabad, east of Moghri, is located on a prominent northsouth trending fault which was discovered during field work in the area, can be seen clearly on the air-photos, and appears to be associated with part of the seismicity around the east end of the 1968 break. Installed in September, 1972, this creepmeter has shown rather uniform right-lateral creep during the first four months of operation, with total offset of 1 mm. TABLE III Creepmeters
Location
Installed
Results to Feb. 1973
A - Boskabad B - Moghri C - Hatamabad
July 1971 Sept. 1971 August 1972
6 mm total offset, left-lateral erratic - removed after 9 months 1 mm total offset, right-lateral. on subsidiary fault
278
Field observations suggest possibility of cummulative right-lateral offset of up to 3 km on this north-south trending fault (Behzadi, 1972). Creepmeter sites are shown in Fig. 1 and summarized in Table III. DISCUSSION
A program of field observations of the post-earthquake characteristics has been undertaken, beginning about two years after the disastrous magnitude 7.3 main shock, in the Dasht-e Bayaz area of Khorassan, in the northeast of Iran. Data taken in the interval 2 -4% years after the earthquake show the following features of the post-earthquake regime: (1) The aftershock sequence continues through 1972, with some seven shocks per day of magnitude zero and greater. The aftershock rate is still measurably decaying with time, apparently at an inverse power rate proportional to t - l .l, very similar to early aftershock observations at Dasht-e Bayaz and larger magnitude aftershock behavior in other regions. (2) Aftershocks are concentrated almost exclusively along the zone of surface rupture in 1968, with a relatively aseismic gap in the center of the zone, near the 1968 epicenter. (3) A composite fault-plane solution for 43 shocks is consistent with left-lateral motion on a plane dipping steeply to the north. (4) Aftershocks occur to depths around 30 km, and define a steeply dipping plane, presumably the fault plane in 1968, which intersects the surface near the 1968 surface rupture and dips northerly at about 70”) in agreement with previous studies of the 1968 focal mechanism and with the composite fault-plane solution for the aftershocks. (5) Survey monuments and creepmeters installed along the 1968 trace have begun to provide clear indications of continuing left-lateral creep of several mm/yr along the zone, with possible right-lateral creep on a prominent north--south trending subsidiary fault. Uncertainties will decrease with time in this phase of the program. The program of post earthquake observations will continue. A permanent seismographic station is operating in the area at Kakhk and regular surveying of the monuments and creepmeters will continue. The aim of the effort is to follow the activity over the years until a steady-state situation seems to be reached. Hopefully the accumulated data on post-earthquake behavior will improve our basic knowledge on the characteristics of major earthquakes in Iran and similar regions. ACKNOWLEDGEMENTS
The authors are indebted to the National Academy of Sciences for support of this research through a grant from the Arthur L. Day Bequest and to Carnegie Institution of Washington for the support through a Harry Oscar Wood Award to M. Niazi. The investigation could not have been made
279
without their support. The fault map of the area (Fig. 1) was compiled by Mr. Hassan Behzadi of Mashad University from 1 : 20,000 aerial photographs and local field studies. Mashad University provided local transportation and other logistics for which we are very appreciative. REFERENCES Ambraseys, N.N. and Tchalenko, J.S., 1969. The Dasht-e Bayaz (Iran) earthquake of August 31, 1968. Field Rep., Bull. Seismol. Sot. Am., 59: 1’751-1792. Ambraseys, N.N., Anderson, G., Bubnov, S., Crampin, S., Shahidi, M., Tassios, T.P. and Tchalenko, J.S., 1969. Dasht-e Bayaz earthquake of August 31, 1968. UNESCO Rep. Ser. No. 1214/BMSRD/SCE, Paris. Behzadi, H., 1972. Geologic environment of the 1968 Dasht-e Bayaz rupture zone (in Persian). Publ. Fat. Sci., Univ. Mashad, Iran. Bolt, B.A. and Turcotte, F.T., 1963. Computer location of local earthquakes within the Berkeley seismographic network. Comput. Miner. Ind., Publ. Geol. Sot., Stanford Univ., 9: 561-576. Crampin, S., 1969. Aftershocks of the Dasht-e Bay?iz, Iran, earthquake of August, 1968. Bull. Seismol. Sot. Am., 59: 1823~-1841. Eftekhar-Nejad, J., Haqui-poor, A.A. and Davood-Zadeh, M., 1968. Report of the investigation of the Khorassan earthquake of August, 31, 1968. Geol. Surv. Iran Publ. (in Persian). Niazi, M., 1968. Fault rupture in the Iranian (Dasht-e Bayaz) earthquake of August, 1968. Nature, 220 (5167): 569-570. Niazi, M., 1969. Source dynamics of the Dasht-e Bayaz earthquake of August 31, 1968. Bull. Seismol. Sot. Am., 59: 1843-1861. Utsu, T., 1961. A statistical study on the occurrence of aftershocks. Geophys. Msg., 30: 521-605.