articolo hessami.pdf Key words active tectonics – paleoseismology – Iran Tabriz – earthquake 1. Introduction NW Iran is a region of intense deformation and seismicity situated between two thrust belts of the Caucasus to the north and the Zagros Mountains to the south (fig. 1). Earthquake focal mechanisms suggest that the convergence be- tween Arabia and Eurasia has been accommo- dated mainly through WNW-trending right-lat- eral strike-slip faults in this region (fig. 1; Jackson, 1992). These strike-slip faults appear to be the southeastern continuation into NW Iran of the North Anatolian Fault and other right-lateral faults in SE Turkey (Westaway, 1990, 1994; Jackson, 1992). However, right-lat- eral faulting in the SE Turkey-NW Iran region is not continuous but consists of several discontin- uous fault segments (figs. 1 and 2a). Three of these segments ruptured during earthquakes in 1930, 1966 and 1976 (McKenzie, 1972; Tok- söz et al., 1977; Jackson and McKenzie, 1984; Westaway, 1990; Jackson, 1992). The North Tabriz Fault Segment, however, was seismically inactive during the last two centuries. Among the many historical earthquakes that have occurred in the Tabriz region (e.g., the 858, 1042, 1273, 1304, 1550, 1641, 1717, 1721, 1780 and 1786 earth- quakes), the destructive earthquakes of 1042 903 ANNALS OF GEOPHYSICS, VOL. 46, N. 5, October 2003 Mailing address: Dr. Khaled Hessami, International Institute of Earthquake Engineering and Seismology (IIEES), P.O. Box 19395/3913, Tehran, Iran; e-mail: khaled@dena.iiees.ac.ir Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results Khaled Hessami (1), Daniela Pantosti (2), Hadi Tabassi (1) (3), Esmael Shabanian (1), Mohammad R. Abbassi (1), Khalil Feghhi (1) and Shahryar Solaymani (1) (1) International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran (2) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma 1 - Sismologia e Tettonofisica, Roma, Italy (3) Department of Geology, Islamic Azad University, Ashtian, Tehran, Iran Abstract The North Tabriz Fault is a major seismogenic fault in NW Iran. The last damaging earthquakes on this fault occurred in 1721, rupturing the southeastern fault segment, and in 1780, rupturing the northwestern one. The understanding of the seismic behavior of this fault is critical for assessing the hazard in Tabriz, one of the major cities of Iran; the city suffered major damage in both the 1721 and 1780 events. Our study area is located on the northwestern fault segment, west of the city of Tabriz. We performed geomorphic and trenching investigations, which allowed us to recognize evidence for repeated faulting events since the Late Pleistocene. From the trenches, we found evidence for at least four events during the past 3.6 ka, the most recent one being the 1780 earthquake. On the basis of different approaches, horizontal slip per event and slip rates are found in the ranges of 4 ± 0.5 m and 3.1-6.4 mm/yr, respectively. We also attempted an estimate of the average recurrence intervals which appears to be in the range 350-1430 years, with a mean recurrence interval of 821 ± 176 years. On the basis of these re- sults, the northwestern segment of the North Tabriz Fault does not appear to present a major seismic potential for the near future, however, not enough is known about the southeastern segment of the fault to make a comparable conclusion. (Ms 7.3), 1721 (Ms 7.3) and 1780 (Ms 7.4) were accompanied by coseismic surface faulting (Ambraseys and Melville, 1982; Berberian and Yeats, 1999). The 1721 and 1780 surface rup- tures extended for at least 50 and 60 km long, respectively (fig. 2b; Ambraseys and Melville, 1982) and occurred 60 years apart in time on adjacent fault sections suggesting that large earthquakes along the North Tabriz Fault are Khaled Hessami, Daniela Pantosti, Hadi Tabassi, Esmael Shabanian, Mohammad R. Abbassi, Khalil Feghhi and Shahryar Solaymani Arabia Turkey Black Sea Eurasia Iran 40.00 N 31.00 N 40.00 E 50.00 E C aspian S ea Zagros M ts M RF NAF EA F NTF 1939.12.26 1966.8.20 1966.3.7 1976.11.24 1977.5.26 1970.3.14 1970.10.25 1963.3.24 2 5 m m /yr 47.00 N Fig. 1. Location map of the NW Iran-Eastern Turkey, adapted from Jackson (1992), with focal mecha- nisms of some of the large earthquakes (mb > 5.3) in the Tabriz-Chaldiran seismogenic zone (Jackson and McKenzie, 1984). North Tabriz Fault (NTF), East Anatolian Fault (EAF), Main Recent Fault (MRF), North Anatolian Fault (NAF). Fig. 2a-c. a) Small-scale regional map of active faults in NW Iran-Eastern Turkey (after Jackson and McKenzie, 1984). The rectangle encloses fig. 2b. b) Simplified map of the NTF and location of historical earthquakes, modified after Berberian (1997). Rectangle encloses fig. 2c. c) NW section of the North Tabriz Fault (after Karakhanian et al., 2001). Rectangles enclose figs. 4a,b and 6. 904 c a b clustered in time and are inter-related in space (Berberian and Yeats, 1999). The 1976 Chal- diran earthquake and its 55 km long surface rup- ture in Turkey (fig. 2a,b; Toksöz et al., 1977) sug- gest that there may be a typical surface rupture length for the most recent historical events in the Chaldiran-North Tabriz Fault System. Total seismic moment release over 80 years implies that the coseismic deformation rate of about 20 mm/yr (Jackson, 1992; fig. 1), con- tributed by strike-slip faults in the SE Turkey- NW Iran region, accounts for almost all the strike-slip component of 22 mm/yr predicted by NUVEL 1 (DeMets et al., 1990). This study describes the main geomorphic features and results of the first trench investiga- tions conducted on a portion of the North Tabriz Fault that is supposed to have ruptured during the 1780 Tabriz earthquake (fig. 2b,c). Based on the results of radiometric dating and fault dis- placements during the late Holocene, the age of previous large events is estimated together with the possible slip per event and slip rates. 2. The North Tabriz Fault The North Tabriz Fault (NTF) is one of the active faults in NW Iran that has a clear surface expression. It has an average strike of NW-SE over a length of about 150 km and appears to be generally close to vertical in dip. Right-lat- eral movement along this fault, documented by Berberian and Arshadi (1976) from study of aerial photographs, can also be seen clearly in the field (Karakhanian et al., 2001). Reporting right-lateral displacement of gullies for about 8 m during late Holocene, Karakhanian et al. (2001) suggest that the minimum slip rate along the NTF is about 2 mm/yr. The 1780 earthquake ruptured the northwest part of the NTF, whereas the 1721 event rup- tured the southeast part of the fault (fig. 2b; Ambraseys and Melville, 1982; Berberian and Yeats, 1999). We investigated the northwest part of the fault within the rupture zone of the 1780 earthquake, with particular attention to the fault in the vicinity of the city of Tabriz. Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results B A 11.711.7 +_ 0.5 m0.5 m Old C anal Recent Ca nal Qanat Shaft Fig. 3. Lateral offsets of the old canal across the North Tabriz Fault (looking north). The distance between old canal and its original position (i.e. the Qanat shaft) is 11.7 m. The new canal is built where the old canal used to be. 905 Khaled Hessami, Daniela Pantosti, Hadi Tabassi, Esmael Shabanian, Mohammad R. Abbassi, Khalil Feghhi and Shahryar Solaymani Fig. 4. Offsets of stream beds along the North Tabriz Fault. A and B show aerial photographs (1:10.000) of cumulative right-lateral displacements along the NW section of the North Tabriz Fault. Top photographs in A and B represent present-day configuration of rivers and stream beds. Lateral offsets of the same geomorphic lev- els are recognized by reconstructing previous steps of deformation during the Late Pleistocene (top to bottom). 906 Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results The city is located in a pull-apart basin oc- curring between two major fault sections of the NTF arranged in an en échelon pattern, stepped right (fig. 2b,c; Karakhanian et al., 2001). The fault section to the northwest shows clear geomorphic evidence for repeated surface faulting events (fig. 2c). This, and the fact that the fault crosses Late Pleistocene deposits, led us to start our investigation in this part of the fault. Here stream beds and gullies are system- atically offset along the fault trace. The most spectacular feature among them is a line of Qanat (underground water tunnels, marked by lines of access shafts) displaced right-laterally (figs. 3 and 6). At this locality, the new canal (A in fig. 3) was dug after the older canal (B in fig. 3) was offset right-laterally for about 11.7 m ± 0.5 (see fig. 6 for location). Offsets of several stream beds elsewhere along this section of the fault contain evidence for cumulative dis- placements by several individual offset events, however, these offsets are difficult to interpret. A maximum 256 m dextral offset can be measured by matching drainage sys- tems (fig. 4). The vertical component of dis- placement varies from place to place between 2 and 11 m, but in any case, the dip slip com- ponent is subsidiary to the main right-lateral strike-slip movement, or it may represent sep- aration rather than slip. In fact, although it is the southern block which is downthrown rela- tive to the northern side, the strike-slip move- ment along the fault has created scarps facing both north and south by shifting topography laterally in this relatively high-relief terrain. 3. Trenches: stratigraphy, structure and paleoearthquakes In order to investigate the paleoearthquakes which occurred on this fault, we excavated two trenches 100 m apart perpendicular to the trace of that part of the fault that is supposed to have ruptured during the 1780 Tabriz earthquake. Trenches were also excavated parallel to the fault on either side at both locations (see figs. 2c and 6 for location). 3.1. Trench 1 Location of trench 1 was selected where an upstream reach was dammed by a shutter ridge across the fault zone. At this location, two apparently recent scarplets were also recog- nized (fig. 5). Because of logistical problems, the trench exposed the shorter of these fault scarps as a zone of shearing a few meters wide (figs. 5 and 7a). The lower Pleistocene depos- its forming the shutter ridge to the south (unit 100) are juxtaposed across this fault zone against the upper Pleistocene-Holocene deposits (Hos- sein-Khan-Nazer, 1999) of fluvial and debris- flow origin (units 107 to 118 in fig. 7a,c). The record of repeated dextral offset during multi- ple faulting events along this fault section is preserved in a sequence of scarp-derived deposits trapped into coseismically-formed depressions, referred to here as colluvial wedges. These colluvial wedges are mainly composed of coarse-grained sand, gravel, and angular pebbles within a sandy clay matrix. Individual colluvial wedges interpreted as evi- dence for individual earthquakes have been discriminated on the basis of their sedimentol- ogy and by using structural relations (fig. 7a,c). The bottom unit (101) of this sequence is se- verely deformed so that pebbles are sheared along fractures and are compacted in their matrix. A few fractures appear to be buried under this unit. The second unit (102) is com- pacted but is less deformed, with pebbles reori- ented only within 10-20 cm of the fault plane. The third colluvial unit (103) is cut by only a few joints, but it clearly buries a fault. Unit 104 is mainly alluvium deposited over the youngest wedge of colluvium. All these units are bound- ed on their northern side by the fault zone which is overlain by the youngest stratigraphic unit (106) of fine-grained alluvial sand and silt. The presence of these alluvial deposits sug- gests that some erosion may have possibly occurred in the upper part of the section. On the basis of this stratigraphic interpreta- tion of colluvial wedges, we could speculate the occurrence of four events; however, for two of them we have also other independent evidence and thus, to be conservative, we consider only these of high reliability. Evidence for the oldest 907 908 Khaled Hessami, Daniela Pantosti, Hadi Tabassi, Esmael Shabanian, Mohammad R. Abbassi, Khalil Feghhi and Shahryar Solaymani Fig. 5. Topographic profile across the fault trace near trench sites. This profile indicates a total vertical dis- placement of 9.8 ± 0.3 m across two fault scarps. Trench 1 exposed the shorter of these fault scarps (4 m high) as a zone of shearing a few meters wide. Table I. Measured and dendrochronologically corrected ages of samples collected in the trenches 1 and 2. Measured ages have been corrected for C12/C13 in the laboratory and for C12/C14 by using the calibration pro- gram Oxcal v3.5 (Bronk Ramsey, 2000). For each sample a probability density is obtained. We report the 2 interval with the associated probabilities. For our study we took under consideration only the age intervals with probability above 10%. Sample # Unit Measured age BP Cal age 2 Probability % T2N02 T2-? 965 ± 40 990-1190 A.D. 95.4 T2N03 T2-204 1275 ± 40 660-870 A.D. 95.4 T2N01 T2-? 1020 ± 40 890-830 A.D. 5.2 950-1070 A.D. 74.8 1080-1160 A.D. 15.4 T1N02 T1-102 1915 ± 40 0-220 A.D. 95.4 T1N03LOW T1-103 1510 ± 40 430-640 A.D. 95.4 T1N03UP T1-103 1230 ± 70 660-970 A.D. 95.4 T1N01 T1-101 3240 ± 40 1620-1410 B.C. 95.4 T1N04 T1-104 740 ± 50 1190-1320 A.D. 86.7 1359-1390 A.D. 8.7 event (event T1-D) is marked by the oldest of these colluvial wedges (unit 101) which accu- mulated in the area that sagged against the fault and overlies some fractures and faults in unit 101. The second event (uncertain T1-M?) could be located at the base of unit 102. The third event is well constrained (event T1-A) as it is marked by the upward termination of the south branch of the main fault at the base of unit 103 and by the deposition of unit 103 itself (fig. 7a,c). Unit 104 does not seem to record a dis- tinct event of deformation but it may have oc- curred in a depression formed by a more recent event (event T1-L?). Finally, the main fault branch appears clearly capped by the alluvium of unit 106. This contact is clearly an erosional one, but it may represent the most recent event hori- zon in this trench (event T1-C). The lack of direct stratigraphic relations prevents us from under- standing whether event T1-C coincides with T1- L? or whether they are two independent events. Further evidence of older events in the trench sec- tion includes event T1-B at the top of unit 107. The timing of these paleoearthquakes is poorly constrained by few radiocarbon dates on organic rich deposits as no charcoal was found (see table I and fig. 7a for sample age and loca- tion). At least four surface faulting events have occurred since 3.6 ka, the most recent one being younger than 1190 A.D. A summary of the age of the events is in table II. 3.2. Trench 2 Trench 2 was excavated across the fault scarp near a pond area on the side of a small erosional gully (figs. 6 and 7b). The trench exposes recent fluvial deposits which are mainly composed of fine-grained sand and silt within a clay matrix (units 200 to 221 in fig. 7b). Deposits exposed in trench 2 have been intensely disrupted by north- and south-dip- ping, high-angle faults over a 15 m wide zone within which we distinguished two major zones of deformation, indicated as northern fault zone and southern fault zone (fig. 7b). The northern fault zone in trench 2 can be traced to the fault zone in trench 1 (fig. 6). Evidence for some small scarplets at the surface suggests Late Ho- locene faulting at this site (fig. 7b,d). However, all these scarplets could not be traced to the trench walls, probably due to bioturbation. Truncation of fault splays by sedimentary units at different stratigraphical levels as well as scarp-derived colluvium is interpreted as evi- dence for the occurrence of at least four individ- ual paleoearthquakes. The event horizons (i.e. the ground surface at the time of the faulting) for these paleoearthquakes are located in the follow- ing stratigraphic positions: a) Event T2-X, top of unit 204 or middle por- tion of unit 208 as suggested by some open fis- sures in-filled with unit 208 and fault splays 909 Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results Table II. Ages of paleoearthquakes as constrained in the trenches. Trench 1 Event # Max age Min age Samples used T1-C 1190 A.D. 1780 A.D. T1N04, historical considerations T1-L? 660 A.D. 1320 A.D. T1N03UP, T1N04 T1-A 0 A.D. 640 A.D. T1N03DOWN, T1N02 T1-M? 1620 B.C. 220 A.D. T1N02, T1N01 T1-D Not available 1410 B.C. T1N01 T1-B No age constraints - - Trench 2 Event # Max age Min age Samples used TR2-X 990 A.D. 1780 A.D. T2N04, historical considerations TR2-W 660 A.D.? 1160 A.D. T2N01, T2N03 assuming 204 was offset also by this event TR2-Z Not available Not available - TR2-K Not available Not available - apparently truncated by the upper part of unit 208 (if this latter is not an effect of bioturbation). The vertical displacement of unit 204 could be attrib- uted to horizontal juxtaposition of this unit with irregular thickness along the strike-slip fault, but it can also suggest evidence for further events. b) Event T2-W, top of unit 210, where fault splays are truncated by unit 211 (at 5 m). Evi- dence for this event possibly exists also between 0 and 1 m where some of the open fissures, in-fill- ed by unit 206, are not aligned with the most re- cent ones. This suggests the occurrence of surface faulting just after or during deposition of unit 206. c) Event T2-Z, base of unit 213, where fault splays terminate and are buried by unit 213 at 10- 11 m and 15-16 m. This event may coincide with event T2-W, however, the lack of direct strati- graphic relations does not allow us to confirm this hypothesis. d) Event(s) T2-K, at the top of unit 200, three channel fills are stacked onto the southern fault zone (at 10-16 m) and cap several fault traces. The timing of only two of these paleo- earthquakes is constrained by radiocarbon dates on organic rich deposits as no charcoal was found elsewhere (see table I and fig. 7b for sample age and location). The most recent event is younger than 990 A.D., whereas the penultimate event may be constrained between 660 and 1160 A.D. A summary of the age of the events is in table II. Khaled Hessami, Daniela Pantosti, Hadi Tabassi, Esmael Shabanian, Mohammad R. Abbassi, Khalil Feghhi and Shahryar Solaymani Fig. 6. Large scale topography map (1 m contour line) of the study area and location of trenches (T1 and T2) based on 1075 measured points using Total Station equipment. Displacement of Qanat shaft can be seen on either side of the fault trace. A-A shows location of topographic profile in fig. 5. Double lines show trench locations. 910 911 Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results south 0 5 north event T2-X 10 15 20 m m T2N03 660-870 A.D. T2N02 990-1190 A.D. T2N01 950-1160 A.D. event T2-Z event T2-K event T2-W event T2-W? northern fault zone southern fault zone 200 221 218 219 220 209 213 216 217 215 214 209 213210 211 212 205 207 204 201 203 206 208 209 202 117 113 114 118 108 104 105 103 101 north m m Trench 1 7 4 0 south T1N04 1190-1320 A.D. T1N03UP 660-970 A.D. T1N03DOWN 430-640 A.D. T1N02 0-220 A.D. T1N01 1620-1410 B.C. event T1-A event T1-M? event T1-L? event T1-C event T1-D event T1-B 107 100 111 112 109 110 106 116 115 102 Sedimentary contacts Fault Fracture C14 sample location Unit 211Unit 211 Unit 209Unit 209 Unit 212Unit 212 Fig. 7a-d. Trench logs along the North Tabriz Fault west of Tabriz city. a) Simplified log of trench 1 based on 1:20 field mapping. Trench 1 exposed Up- per Pleistocene-Holocene deposits of fluvial (units 107-118) and debris flow-colluvial (units 101-104) origin. These are mainly composed of coarse-grained sand, gravel, and angular pebbles within a sandy clay matrix. b) Simplified log of trench 2 based on 1:20 field mapping. Trench 2 exposed Upper Pleistocene- Holocene deposits of fluvial origin (all units aside from units 211 and 212) which are mainly composed of fine-grained sand and silt within a clay matrix. Units 211 and 212 are mainly of debris flow origin. c) Detail of the fault zone of trench 1, east wall, look- ing south. d) View of the northern fault zone of trench 2. Notice the small scarplets at the surface on top of the fault zone. Trench 2 Trench 1 c d a b 4. Coseismic slip, slip rates and recurrence intervals 4.1. Slip per event Unfortunately no direct information about the horizontal movement during each of the surface faulting events recognized in trenches 1 and 2 could be retrieved from the fault-parallel excava- tions (fig. 6). From the trenches, only rough esti- mates of the amount of individual vertical dis- placements can be obtained. These are based only on the thickness of the deposits interpreted as colluvial wedges, assuming that the vertical slip at the trench site reflects true slip on the fault and not lateral topographic juxtaposition. Sele- cting a crest of an interfluve on either side of the fault (marked AA on fig. 6), we avoided juxtapo- sition of topography along the topographic profile in fig. 5. Even taking into consideration these large uncertainties, we can obtain from trench 1 a thick- ness of colluvium that varies between 0.3 and 0.9 m that directly translate to minimum vertical co- seismic slip. An estimate of the horizontal coseismic slip can be obtained by measuring the right-lateral off- set of modern stream channels measured along the strike of the North Tabriz Fault in the study area. A frequency histogram of lateral offsets over 2.5 km distance along the fault trace is provided in fig. 8 to identify peaks, which could represent the amount of offset related to one or more individual pale- oearthquakes. Figure 8 shows several peaks indi- cating that the cumulative slip of offset channels could be associated with individual increments of 3.5-4.5 m. This figure could thus represent the coseismic offset produced by the most recent earthquake and also the typical offset for this sec- tion of the North Tabriz Fault. However, even if we consider it dubious, we cannot exclude the possi- bility that the 4 ± 0.5 m offset increments may have been produced by more than one earthquake. 912 Khaled Hessami, Daniela Pantosti, Hadi Tabassi, Esmael Shabanian, Mohammad R. Abbassi, Khalil Feghhi and Shahryar Solaymani 0 20 40 60 80 100 120 140 0 1 2 3 4 N u m b e r o f d is p la ce d f e a tu re s Displacement (m) ∼ 4 m ∼ 50 m ∼ 98 m ∼ 136 m Fig. 8. Frequency histogram of lateral offsets (represented as Gaussian probability density functions) over 2.5 km distance along northwestern section of the North Tabriz Fault. Several peaks indicate the cumulative slip of offset channels could be associated with individual increments of 3.5-4.5 m. 4.2. Slip rate The only direct estimate of the horizontal slip rate along this section of the North Tabriz Fault is based on the offset Qanats (figs. 3 and 6). The maximum age of the Qanat line is 3000 years (Forbes, 1964; Goblot, 1979; Kamiar, 1983; Potts, 1990); because it is horizontally offset for 11.7 ± 0.5 m, it implies a minimum horizontal slip rate of 3.7-4.0 mm/yr. The ages of the offset drainages are not known, but by making some assumptions on the maximum age of the incised surfaces, we can compare long-term slip rates to the slip rate obtain- ed for the past 3 ka. Channels incised within the Upper Pleistocene (maximum 125 ka) sediments (Hossein-Khan-Nazer, 1999) are right-laterally of- fset for about 250 m, indicating a minimum slip rate of about 2 mm/yr. On the other hand, young alluvial fans, in- cised by drainages that show a 67 ± 10 m hori- zontal offset (fig. 4) and a 9.8 ± 0.3 m vertical dis- placement (fig. 5) could be attributed to the last important post-glacial deposition following the last glacial peak (18 to 10 ka; Pedrami, 1987). If this assumption is correct, the minimum hori- zontal slip rate is 3.1-6.4 mm/yr, consistent with that obtained from the Qanats. On the same age assumption, the total vertical displacement of the same surface indicates a vertical slip rate of 0.5- 0.8 mm/yr. 4.3. Recurrence intervals Based on the chronological constraints in each trench for the occurrence of the paleoearthquakes, we attempted a correlation of events between the two trenches (fig. 9). At least four earthquakes oc- curred during the past 3.6 ka. The most recent event is younger than 990 A.D. in both trenches. Since the historical reports indicate that the 1780 earthquake ruptured this section of the fault (Ambraseys and Melville, 1982; Berberian and Yeats, 1999), we assume that the most recent e- vent recorded in the trenches is the 1780 earth- quake. The ages of the other events fall in very wide ranges because of the limited amount of dateable material found in the trenches. Even with these high uncertainties we can infer that the penultimate event occurred 910 ± 250 A.D., the third event back 320 ± 320 A.D., and the oldest one 700 ± 920 B.C. Comparing these ages with the historical record, the 1042 earthquake falls in 913 penultimate most recent 0 A.D. 2000 A.D.1000 T1-A T1-C Tabriz trenches - Age of events 1 7 8 0 A .D . T1N03U T1N01 T1N04 T1N02 T1N03D T1N02 T2N02 T2N03 T1N04T1-L? T1-M? T1-D T2-X T2N01 T2-W 1500 B.C. T1N01 Fig. 9. Attempt of correlation of paleoearthquakes between the two trenches. Black bars are the age ranges of the samples used to constrain the age of occurrence of paleoearthquakes which are reported as thick gray bars. Sample names and event names are shown too. Gray boxes show the correlated age of the most recent and penul- timate events. On the basis of historical considerations we assume the most recent earthquake be the 1780 one. Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results the age interval of the penultimate surface faulting event found in the trenches. Although information on this event is mostly derived from the city of Tabriz, and thus it is hard to define the earthquake source only on the basis of felt reports, surface faulting is reported and certainly support the hypothesis that the 1042 earthquake ruptured at our trench site. No evidence for the other events which produced damage in Tabriz is found in the trenches. This may be due to different reasons: a) events were too small to produce surface faulting at the trench site; b) events occurred on different faults but this cannot be verified because of the limited historical information on the damaged area, c) mimicking of the geological evidence for these events occurred by more recent and larger earthquakes. On the basis of these ages an average recurrence interval for surface faulting events of ca. 350 to 1430 years and also a mean recurrence interval of 821 ± 176 years can be estimated. Average recurrence interval estimates can also be obtained based on the number of events that contributed to displace a given anthropic/ geomorphic elements and their ages. By using the maximum 3000 years old Qanat (Forbes, 1964; Goblot, 1979; Kamiar, 1983; Potts, 1990) displaced 11.7 ± 0.5 m and the 4 ± 0.5 m slip per event, we can infer that at least 3 events con- tributed to its displacement. This would convert to an average recurrence interval of ca. 1000 years. On the other hand, comparing the individ- ual offset of 4 ± 0.5 m with the 67 ± 10 m long term displacements recorded by the 12-18 ka sur- face, we can infer that 12 to 22 events contributed to the lateral offset of the drainages. This trans- lates to average recurrence intervals ranging between 545 and 1800 years, in good agreement with the observations derived from the Qanat and trenches. 5. Conclusions Geomorphic features and trenches opened west of Tabriz city show evidence for repeated surface faulting along this section of the North Tabriz Fault. At least four surface faulting e- vents have been recognized during the past 3.6 ka. Because of the lack of datable materials, ages of events are poorly constrained, and cor- relation between the two trenches is difficult. However, taking into consideration these intrin- sic uncertainties, we defined as much as we could the age of these events as 1780 A.D., 910 ± 250 A.D., 320 ± 320 A.D., and 700 ± 920 B.C. (see table II and fig. 9). Slip per event is estimated as 4 ± 0.5 m horizontal and minimum 0.6 ± 0.3 m vertical although this latter may not be truly representative of slip on the fault. Horizontal slip rates of 3.7-4.0 and 3.1-6.4 mm/yr are obtained on the basis of faulted anthropic fea- tures and on offset drainages, respectively. On a similar basis, vertical slip rates may be of the order of 0.5-0.8 mm/yr. Finally, average recur- rence intervals calculated following different reasoning lines are as small as 350 years and as large as 1430 years. Our results show that large earthquakes (M > 7) on the NW segment of the North Tabriz Fault occur each 821 years (mean time) which is consistent with historical data. Since this section of the North Tabriz Fault has not produced any major destructive earth- quakes with surface faulting since 1780 (the past 220 years), assuming a constant slip rate accumulation and a characteristic slip per event at least 0.7-1.4 m of strain must have been accu- mulated along the northwest part of the NTF. This would suggest the potential of this part of the fault is not critical for the next few cen- turies, however, the possibility of smaller rup- tures and clustering with periods of more fre- quent moderate earthquakes along the fault, should be considered. Although this paper contains a preliminary understanding of the seismic behavior of this fault, important issues that have a critical im- portance for the highly populated city of Tabriz remain open such as the segmentation of the North Tabriz Fault Zone, the possibility that the fault ruptures also during earthquakes smaller than the 1780 one, the significance of the histori- cal earthquakes not found in the trenches, and the existence of other sources in the area. Acknowledgements We thank Dr. C.J. Talbot for constructive comments and suggestions for improvements on an early draft of this work. We wish to thank 914 Khaled Hessami, Daniela Pantosti, Hadi Tabassi, Esmael Shabanian, Mohammad R. Abbassi, Khalil Feghhi and Shahryar Solaymani Drs. R.S. Yeats and M. Berberian for their con- structive reviews that substantially improved the original manuscript. We also thank M.J. Bo- lourchi and F. Ansari-Moghadam for help in log- ging the trenches. 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Res., 99, 12071-12090. 915 Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results òlahdflòkasdfòklaklòjadf