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braseys and Melville, 1982; Jackson and McKenzie,. * Corresponding author. ..... where some faults termi- nate and are buried under the apparently unmodified.
TECTONOPHYSICS ELSEVIER

Tectonophysics

287

( 1998) 187- I99

A geologic contribution to the evaluation of the seismic potential of the Kahrizak fault (Tehran, Iran) P.M. De Martini ‘q*,K. Hessami b, D. Pantosti ‘, G. D’Addezio ‘, H. Alinaghi b, M. Ghafory-Ashtiani b ” Istiruro

Nu:ionalr

” International

di Geofisica,

Institute

Received

Via di Vigna Muratu

of‘ Earthquake I4 October

Engineering

605, 00143 Rome, Ita!\

and Seismulog~,

Tehran, Iran

1996: accepted 23 September 1997

Abstract In this paper we present the results of preliminary geomorphic and trenching investigations along the Kahrizak fault. This fault is located south of the highly populated metropolis of Tehran and represents one of the main structures in the area containing important seismic potential. The Kahrizak fault has a very clear expression at the surface where it forms a prominent 35-km-long, 15-m-high scarp on Holocene alluvial deposits. The fault strikes N70”-80”W and dips to the north. Movement is prevalently right-lateral with the northern side of the fault up. Trench excavations exposed a sequence of weathered, massive, alluvial deposits which are dated, by means of radiometric methods, to the Holocene. In the trenches the sequence is intensely deformed by north-dipping, high- and low-angle faults within a 30-m-wide zone. On the basis of stratigraphic and structural relations, some evidence for individual Holocene earthquakes is found; however, we were not able to reconstruct the seismic history of the fault nor to evaluate the size of deformation produced by each event. Because of the possible -10 m offset of ancient linear hydraulic artifacts (yund~~), that cross the fault, we hypothesize that the most recent event may have occurred in historical times (more recent than 5000 yr B.P.) and it may be one of those reported in this area by the current catalogues of seismicity. Based on these preliminary investigations we estimate an elapsed time between 5000 and 800 years, a maximum slip per event d,,,, of -10 m, a minimum Holocene vertical slip rate of -I mm/yr versus a horizontal slip rate of -3.5 mm/yr, a maximum of -3000 years for the average recurrence time, and an expected M, = 7.0 to 7.4. These can be considered as a first-hand reference for the activity on this fault. 0 1998 Elsevier Science B.V. All rights reserved. Kewords:

Iran; paleoseismology;

geomorphology;

seismic hazard assessment

1. Introduction Iran is one of the most seismically active regions of the globe (Shoja-Taheri and Niazi, 1981; Ambraseys and Melville, 1982; Jackson and McKenzie, * Corresponding author. [email protected]

Fax:

+39

6504-I 181;

E-mail:

0040- 195I /98/$19.00 0 I998 Elsevier Science B.V. All rights reserved PII SOO40-

195

I (97)00246-

I

1984; Berberian, 1994; Priestley et al., 1994). Seismicity here is the direct evidence of the continental convergence between the Arabian and the Eurasian plates that produces, according to global plate motion models such as NUVEL-1, a N-S convergence of about 35 mm/yr at the longitude of central Iran (De Mets et al., 1990). Most of the deformation within the region is not distributed but concentrated

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287 (1998) 187-199

S/. % e,

;\:

312-280 B.C

Fig. I. Simplified map of the Tehran region with the main active faults (dashed when inferred, modified from Berberian et al., 1983). Circles indicate historical earthquakes (from Ambraseys and Melville, 1982 and Berberian, 1994). The dotted area refers to Fig. 4. The small inset in the upper right shows the distribution of seismicity (M z 4) in the region (from I.S.C. 1964-1989).

along main deformation belts: the Zagros range to the southwest, the Alborz range to the north, and the Kopet Dag range to the northeast (Jackson et al., 1995) (Fig. 1). These belts of active deformation are clearly underlined by instrumental and historical seismicity (Ambraseys and Melville, 1982; Jackson and McKenzie, 1984; Berberian, 1994). The extremely high density of population concentrated in the Tehran metropolis (with 12 million inhabitants) coupled with the fragility of houses and life-lines, highlight the urgency of a reliable assessment of seismic hazard based on the deterministic

knowledge of the seismogenic structures in this region. To produce a model of seismicity of this area for seismic hazard determination, information can be derived from historical and instrumental records. However, even if current historical catalogues include observations from about 2500 yr B.C., their true interval of completeness is much shorter. Thus, with respect to average repeat times typical of continental regions 2 1000 yr, historical and instrumental datasets may not be representative of the activity of all seismogenic structures in Iran. In this context, the geological approach may provide a longer perspec-

PM. De Marfini et al. /Tectonoph~sics

tive of the seismic history of the area and, at the same time, may contribute to relate historical events to their causative faults. This is particularly effective in hazard calculation because it overcomes the intrinsic uncertainties related to the location of macroseismic epicentres (Ambraseys and Melville, 1982). Thus, taking into account the potential seismic risk of the Tehran area and the significant contribution of geological studies in the elaboration of models for seismic hazard calculation a collaboration was started between the International Institute of Earthquake Engineering and Seismology of Tehran and the Istituto Nazionale di Geofisica of Rome. In this paper we present the preliminary results of the geomorphic and paleoseismologic investigation of the Kahrizak fault located south of Tehran (Fig. 1). It was the first time that scientific paleoseismological trenching was carried out in Iran and this work is intended as a first contribution to a modern assessment of the seismic potential in the area of Tehran based on geologic input. 2. The Tehran region The metropolis of Tehran is located at the southern foothills of the Alborz Mountains that are the northern branch of the Alpine-Himalayan orogen in Iran (Fig. 1). Due to the compressional motion between Arabia and Eurasia an early Alborz range began to form in Late Cretaceous-Early Paleocene (65 Ma); the most important erogenic paroxysm occurred in the Oligocene and was accompanied by significant uplift that made the range acting as a barrier separating the Caspian basin from the Neogene basins of central Iran (Stocklin, 1968; Berberian and King, 1981). Because of this, different sedimentation and tectonic regimes developed at the northern and southern sides of this barrier (Tchalenko et al., 1972). The present tectonic activity is part of a compressional phase started in the Late Pliocene and that is responsible for the deformation of the Pliocene-Pleistocene molasse sediments at the periphery of the Alborz range axis. The belt consists of a broad arch of parallel folds, reverse faults and nappes forming imbricate structures, verging both towards and away from the Caspian basin (Berberian, 1983). Previous studies of focal mechanisms and active faulting had emphasised that reverse faulting

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and folding are undoubtedly the main processes that build up the Alborz Mountains, though there was evidence of an active significant horizontal component during the surface faulting earthquakes both of Buyin Zara, 1962, and of Rudbar-Tarom, 1990 (Ambraseys, 1963; Berberian et al., 1992). Priestley et al. (1994) interpret this as evidence of slip partitioning; thus, in the Alborz Mountains, where the strains are largest, the deformation is mainly partitioned in pure strike-slip and thrust structures whereas to the south, toward the border of the central Iranian plateau where strains are presumably lower, oblique slip seems to prevail. The present seismicity appears to follow the arcuate structural system of the Alborz range, turning from a predominant NW-SE strike of the main faults in the west to a NE-SW trend in the east. Large earthquakes are reported in the historical catalogues too although they appear less frequently than in other active regions of Iran. It is notable that the hinge of this arcuate system coincides with the location of a late Quatemary volcano up to 5670 m high, the Damavand Volcano. Two main active fault zones occur in the Tehran area: the North Tehran-Musha fault zone to the north, and the Rey-Kahrizak fault zone to the south (Fig. 1). Both fault zones show geologic and geomorphic evidence for late Quatemary activity (Pedrami, 1981) with the sense of movement commonly interpreted as thrust (Berberian et al., 1983). However, most of the exposures of the North-Tehran and Rey faults in Late Pleistocene-Holocene deposits are completely obliterated, either because of significant human modification on the fault strands closer to the metropolis, or possibly because no deformation occurred in Holocene time. On the contrary, the Kahrizak fault, intercepting the wide flat alluvial plane at the base of the Alborz range, produces an exceptional surface expression, even surviving the intense agricultural works (Figs. 24a). For this reason we concentrated our first set of investigations mainly on the Kahrizak fault, with a plan to carry out a detailed study of the other structures in future. 3. The Kahrizak fault The Kahrizak fault forms an impressive scarp at the surface up to 1.5 m high on Holocene alluvial deposits and results in the subsidence of the southern

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Fig. 2. View of the trench site (location

in Figs. 3 and 4). At this site the fault scarp (6 to 8 m high) offsets Holocene

block (Figs. 1 and 2). Although the scarp prominence suggests significant recent activity on this fault, no relevant topography was produced in the long term. The scarp extends for about 35 km and shows a variable strike that averages N70”-80”W. Berberian et al. (1983) interpreted the Kahrizak fault as a south-verging thrust. On the basis of detailed aerial photos survey (scale 1 : 20,000) and field work, the central part of the Kahrizak scarp was mapped in detail (Fig. 4a). The fault zone appears quite complicated and formed by en-echelon N60”W structures connected by a main NSO”W-trending structure. The en-echelon structures are very subtle but evidenced at the surface: in general they control the presence of bends along the main scarp and small basins. As a direct consequence of the relative difference in elevation at the two sides of the fault, incipient drainage with several deep incised small creeks characterized the northern block, whereas only a few large rivers exist in the southern sector. Locally some small creeks show evidence for dextral movement (up to 50 m), and some qana’ts suggest abandonment following an event of lateral offset (Fig. 4b).

alluvial depl

Qana’ts are artificial linear wells, excavated in this area since 5000 years ago, in order to join the water table for captures and to induce the formation of a sub-surface channel; here they are usually dug perpendicular to the fault scarp. A possible figure of - 10 m for the horizontal offset produced by the most recent faulting event is suggested from examples G and possibly E of Fig. 4b. Example G shows the abandonment of an offset qana’t and its successive reconstruction in a section of -500 m across the fault scarp. In this section new lines of wells were drilled in order to recapture the underground channel offset by an event of instantaneous deformation (i.e., a paleoearthquake) that would have interrupted the regular flow of water in the qana’ts. The overall geometry and geomorphology of the surface expression of the Kahrizak fault scat-p suggest that an important component of dextral movement characterizes this structure. In this context, the abrupt double bend of about 90” of the Jaj River (Fig. 1) could be related to the long-term activity of the Kahrizak fault both because the river shows a deviation consistent with the sense of movement expected on the fault and because the anomaly in the

P.M. De Martiniet al./Tectonophysics287 (1998) 187-199

_

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360

400

Distance along profile (m)

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500

600

N

Fig. 3. Topographic profiles across the Kahrizak scarp at and next to the trench site (numbers in the inset indicate the profiles); the net vertical displacement is estimated by reconstructing the original surface of the fan far from the fault scarp. Notice in profile 3 the deep incised creeks on the upthrown side of the fault.

natural flow of the Jaj River is right on the hypothetical eastward prosecution of the fault scarp. This indirect geomorphic evidence propounds a possible total length of -55 km for the Kahrizak fault. 4. Trenching 4.1. The site We opened two trenches for paleoseismologic investigations across the central part of the Kahrizak fault (Figs. 2-4a). We selected a site where no important agricultural modification occurred and which was close to the narrow incision of an ephemeral creek, in an attempt to intercept very young alluvial

deposits. At this site topographic profiles across the scarp show a vertical displacement, measured on the fan surface, not exceeding 8 m (Fig. 3). The trenches were both excavated across the scarp in its lower part: trench Kl was 60 m long and its excavation was interrupted at the base of the scarp because of the presence of a calcrete hardpan layer; trench K2 was 20 m long and was excavated about 2 m west of Kl, with 1 m overlap between the two trenches (Figs. 3 and 5). 4.2. Stratigraphy and structures The trenches exposed an intensely weathered sequence of massive, clay-rich alluvial deposits and

P,M. De Martini et al. I Tectonophysics 287 (1998) 187-199

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287 (1998) 187-199

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P.M. De Martini et al./Tectonophysics 287 (1998) 187-199

Fig. 6. Detail of a fault in the deepest part of trench K2. Striations

at m 12, but the top of the unit seems undisturbed. One explanation is that after the deformation event some erosion of the top of unit 2b occurred and was followed by deposition of unit 2a which represents a post-event deposit with typical onlap geometry against the scarp. (D) Base of unit 2b in trench K2, because this unit appears very similar to unit 2a in terms of position, geometry and nature of deposit and thus may represent a post-event sediment. Whether paleoearthquake horizons are related to the same deformation events is not obvious. However, assuming that our tentative interpretation of the

clearly show that movement

on the fault is prevalently

horizontal.

event horizons is correct, we may envision at least two possibilities. On one hand, each piece of evidence may be related to an individual earthquake, but we see this as unlikely. On the other hand, evidence (A) may be related to an old event that may be also responsible for evidence (D), while evidences (B) and (C) may be related to the most recent earthquake. It is clear anyway that both of these hypotheses are not sufficient to explain the whole deformation observed at the trench site, especially considering that the vertical component of movement is secondary with respect to the horizontal one.

P.M. De Martini et al./Tectonophysics

5. Evaluation of the main seismic parameters the Kahrizak fault

of

The results we obtained from the geomorphic and trenching investigation on the Kahrizak fault scarp indicate that slip on this structure occurs by successive earthquakes and thus that this fault contains a seismogenic potential. The fault geometry is directly derived from field observations indicating: (1) an average strike of N70“-8O”W, (2) a dip of 70”-80”N, and (3) a rupture length at the surface of -35 km that is derived from the surface expression of the main scarp, and that may reach -55 km because of the geomorphic considerations discussed above. Radiometric, historical and archaeological dating allow also the estimation of those seismic fault parameters that describe the fault behaviour such as elapsed time, slip per event, slip rate, average recurrence time, and M,.,. 5.1. Elapsed time

The age of the most recent event derived from geologic considerations and radiometric dating of sediments from the trenches is scarcely constrained to an undefined Holocene time and, in particular, in a range of 7000 years to present (Fig. 5). On the other hand, archaeological information, especially the maximum age of qandts, allows us to constrain the age of the most recent event in the interval of 5000 yr B.P. to present. Historical catalogues report four large earthquakes that, based on their macroseismic location, may have occurred on the Kahrizak fault: the events of 3 12-280 B.C., 855 A.D., 958 A.D., and 1177 A.D. (Fig. 1). However, considering the uncertainties related to the location of the epicentres based on the sole macroseismic observation that even in this century may lead to mislocations up to 40 km as described by Ambraseys and Melville (1982) for the 1927, July 22, Dasht-i Kavir earthquake, we suspect that any of the events reported in the area by the current historical seismicity catalogues (Ambraseys and Melville, 1982; Berberian, 1994) may have occurred on the Kahrizak fault. Moreover, the area is dissected by several other active faults (Fig. 1) that may have produced the historical earthquakes mentioned above. So, the

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possibility that the Kahrizak fault was silent during the entire period covered by the historical seismicity catalogues should also be envisioned. Based on the maximum age of qandts and the last historical event possibly related to the Kahrizak structure, the time elapsed since the most recent earthquake on this fault is in the range of 5000-800 years. Because some qandts appear to record repeated offset events, as shown in examples F and possibly E of Fig. 4b, the lower part of this range is preferred. 5.2. Slip per event The possible -10 m lateral offset of one of the qana’ts that are built across the fault is the only

direct observation available to estimate the typical slip per event associated with this fault. This slip is assumed here to be the effect of one individual paleoearthquake. However, comparing this offset with the fault length of 35-55 km, on the basis of the empirical relation between displacement and surface rupture length as proposed by Wells and Coppersmith (1994), we would expect a much longer surface rupture. Because of this we consider the -10 m offset as the maximum displacement (d,,,,,) for the Kahrizak fault. 5.3. Slip rate For estimating the slip rate of this fault we can use and combine the minimum 8 m long-term vertical displacement of the Holocene fan surface, obtained from topographic profiling, and the -l/3.5 relation between vertical and horizontal slip component on the fault deduced from slip vectors, directly measured on fault planes exposed in the trenches. This yields a minimum Holocene vertical and horizontal slip rate of -1 mm/yr and -3.5 mm/yr, respectively. 5.4. Average recurrence time On the basis of the minimum Holocene horizontal slip rate (3.5 mm/yr) and the maximum lateral slip per event (10 m), assuming a periodic strain release, we can obtain a maximum estimate of the average recurrence time for this seismogenic fault of -3000 years.

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5.5. Maximum expected magnitude

The fault rupture parameters obtained by field geology provide the basis to estimate the moment magnitude M,,, expected for the Kahrizak fault through the calculation of the Seismic Moment MO. According to Hanks and Kanamori (1979): l&l, = 0.666 x log Ma - 10.7 where the seismic moment MOis: MO = /.Lx day x A For the shear modulus p we used a value of 3 x 10” dyne/cm2, usually taken for crustal faults (Jackson and McKenzie, 1988); as average displacement d,, we adopted the range 0.2 x d,,, to 0.8 x d,,, from the empirical relation proposed by Wells and Coppersmith (1994) where ford,,, the 10 m offset is taken; to calculate the area A of the fault surface that is expected to rupture we used a width of 15 km, that is the thickness of the seismogenic layer in the study area according to Jackson and McKenzie (1988), and a subsurface rupture length that is the observed surface rupture length for which we considered the 35km-long main scat-p, plus its 25%, based on the empirical relation of Wells and Coppersmith (1994). This yields a seismic moment MO ranging between 3.94 and 15.75 x 1O26dynecm that can be converted to M, = 7.0 to 7.4 for the Kahrizak fault paleoearthquakes. Considering that there is no systematic difference between MS and M, in the magnitude range of 5.7 to 8.0 (Wells and Coppersmith, 1994) we find a consistency between the expected magnitude range for the Kahrizak fault and that estimated for the historical events of 855 A.D. and 1177 A.D. (Fig. 1). Nevertheless, considering that we used the 35-km-length of the main scarp for this calculation and that we envisioned above the possibility of a 55-km-long fault, we suggest to consider our M, estimate as a minimum. 6. Conclusions Geomorphic and trenching investigations along the Kahrizak fault provide some new preliminary constraints on the activity of this structure and on its past seismic history. The fault is located south of Tehran and represents one of the major local structures that may contain important seismic potential

287 (1998) 187-199

for this highly populated metropolis. The Kahrizak fault is a very distinct feature at the surface, where it forms a clear 35-km-long fault scarp up to 15 m high on Holocene deposits, and has an overall geomorphic expression up to -55 km long. On the basis of geomorphic evidence and direct measurements on the trench walls, the fault strikes N70”-80”W, dips 70”-80” to the north and shows prevalent right-lateral movement, in agreement with the N-S regional direction of convergence (Jackson et al., 1995). The absence of relevant topography could be explained by the predominant horizontal slip. We opened two trenches across the scarp and although we found possible evidence for individual events, the earthquake history is not well defined. At least two paleoearthquakes occurred during the Holocene, but, given the surface offset of Holocene deposits and considering that also man-made qandts (maximum age -5000 years) appear to be significantly offset by the fault, we would expect a greater number of events in Holocene time. Trenches, the geomorphology associated to the fault scarp, and historical considerations provide a preliminary framework for evaluating seismic fault parameters. We estimated a maximum slip per event, dmax, of -10 m and minimum Holocene vertical and horizontal slip rates of -1 mrn/yr and -3.5 mm/yr, respectively. Thus, assuming a periodic strain release we extrapolated a maximum of -3000 years for the average recurrence time. Archaeological and historical information yield an elapsed time of 5000800 years for the Kahrizak fault. Consequently, if the true elapsed time falls in the upper part of this range and given the maximum average recurrence time, the Kahrizak fault should be considered as a fault with potential for generating large earthquakes in the near future. Finally, even if there are objective important uncertainties in this estimate, we suggest a M, value of 7.0-7.4 for the Kahrizak fault paleoearthquakes; it should be noted that this value shows a better fit with the 855 A.D. and 1177 A.D. historical earthquakes than the 312-280 B.C. and 958 A.D. events. New radiometric dating, further detailed geomorphic investigations and excavations of new trenches parallel to and across the scarp are planned in the near future and hopefully will contribute to better evaluate the seismic potential of one of the main seismogenic faults in the Tehran area.

P.M. De Martini

et al./Tectonophyics

Acknowledgements We wish to thank Prof. E. Boschi and Prof. D. Giardini for their encouragement and enthusiastic support, Prof. M. Pedrami for his precious co-operation during field investigations, M. Voltaggio for his help with the dating, S. Pondrelli for helping with seismicity analysis and the IIEES (International Institute of Earthquake Engineering and Seismology) for their fundamental support and patience. We are grateful to M. Stucchi for providing a precious copy of the English version of Ambraseys and Melville’s catalogue and C. Baker for his thoughtful revision and constructive comments that helped substantially to improve this paper. References Ambraseys, N.N., 1963. The Buyin-Zara (Iran) earthquake of September 1, 1962: a field report. Bull. Seismol. Sot. Am. 53, 705-140. Ambraseys, N.N., Melville, C.P., 1982. A History of Persian Earthquakes. Cambridge University Press, Cambridge, 2 19 pp. Berberian, M., 1983. The Southern Caspian: a compressional depression floored by a trapped, modified oceanic crust. Can. J. Earth Sci. 20, 163-183. Berberian, M., 1994. Natural Hazards and the First Earthquake Catalogue of Iran, Vol. I. Historical Hazards in Iran Prior to 1900. International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, 603 pp. Berberian, M., King, G.C.P., 1981. Towards a paleogeography and tectonic evolution of Iran. Can. J. Earth Sci. 18, 210-265. Berberian, M., Qorashi, M., Arzhang-Ravesh, B., Mohajer-Ashjai, A., 1983. Recent tectonics, seismotectonics and earthquake-fault hazard study in the Greater Tehran region, Geol. Surv. Iran Rep. 56, 130 pp. (in Farsi).

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Berberian, M., Qorashi, M., Jackson, J.A., Priestley, K., Wallace, T., 1992. The Rudbar-Tarom earthquake of 20 June 1990 in NW Persia: preliminary field and seismological observations, and its tectonic signiticance. Bull. Seismol. Sot. Am. 82. 1726-1755. De Mets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophys. J. Int. 101, 425-478. Goudie, AS., 1982. Calcrete. In: Goudie, A.S., Pye, K. (Eds.), Chemical Sediments and Geomorphology, Precipitates and Residua in the Near-Surface Environment. Academic Press, London, pp. 93- I3 I. Hanks, T.C., Kanamori, H., 1979. A moment-magnitude scale. J. Geophys. Res. 84.2348-2350. Jackson, J.A., McKenzie, D.P., 1984. Active tectonics of the Alpine-Himalayan belt between western Turkey and Pakistan. Geophys. J.R. Astron. Sot. 77, 185-264. Jackson, J.A., Haines, J., Holt, W., 1995. The accommodation of Arabia-Eurasia plate convergence in Iran. J. Geophys. Res. 100, 15205-15219. Pedrami, M., 1981. Pasadenian Orogeny and the geology of 700,000 years ago in Iran. Internal Report Geological Survey of Iran, 355 (in Farsi). Priestley, K., Baker, C., Jackson, J.A., 1994. Implications of earthquake focal mechanism data for the active tectonics of the south Caspian Basin and surrounding regions. Geophys. J. Int. 118, 111-141. Shoja-Taheri, J., Niazi, M., 1981. Seismicity of the Iranian plateau and bordering regions. Bull. Seismol. Sot. Am. 71, 477-489. Stocklin, J., 1968. Structural history and tectonics of Iran: a review. Bull. Am. Assoc. Pet. Geol. 7, 1229-1258. Tchalenko, J.S., Iranmanesh, M.H., Mohajer-Ashjai, A., 1972. The Babol-Kenar (197 I) earthquake and the seismotectonics of the Central Alborz (Iran). Ann. Geofis. 25, 27-36. Wells, D.L., Coppersmith, K.J., 1994. New empirical relationships among Magnitude, rupture length, rupture width, rupture area, and surface displacement. Bull. Seismol. Sot. Am. 84, 974- 1002.