Patterns of Historical Earthquake Rupture in the Iranian Plateau

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Patterns of Historical Earthquake Rupture in the Iranian Plateau 1351988; Jackson, 1992). Only about 15% of the long-term convergencerate across the Zagros Mountains, estimated at 20mm/yr, can be attributed to seismic moment release duringthis period. Seismic moment release can account for morethan 50% of the shortening in northeast Iran, but only about10% in the Alborz Mountains (Jackson and McKenzie,1984; Jackson et al., 1995). In northwest Iran, twentiethcenturyearthquakes account for all of the expected fightlateralcomponent of convergence but less than 25% of theexpected shortening by dip slip (Jackson, 1992). Unfortunately,there are no geodetic estimates, either space based orterrestrial, of active deformation rates within the Iranian plateau.Accordingly, the question about whether the historicalslip deficit is due to aseismic slip or to an inadequate samplingof the seismic moment release rate in the historicalperiod cannot be answered. We return to this question belowwhen we consider the seismic hazard of long, straight strikeslipfaults with geological evidence of Holocene deformationthat have not sustained large-magnitude earthquakes in thehistorical period.Temporal Clustering and Loading of Adjacent FaultsAlthough earthquakes release accumulated elasticstrain, they also load faults in nearby and even more distantregions that will be the location of future events (Stein etal., 1992; Stein and Lisowski, 1983; Hudnut et al., 1989;Reasenberg and Simpson, 1992; Du and Aydin, 1993; Kinget al., 1994; Bennett et al., 1995; Harris and Simpson, 1996;Jacques et al., 1996). The geometry of interactions betweengroups of active faults is critical for characterizing seismicsources of large earthquakes.The earthquake cluster in the Dasht-e-Bayaz area ineastern Iran (Figs. 5, 6, and 7) consisted of 11 earthquakesofM > 6.0 in 61 yr (1936 to 1997) in an area of 160 by 110km, approximately the same size as the Los Angeles metropolitanarea. The Dasht-e-Bayaz/Abiz earthquake sequenceruptured left-lateral and fight-lateral faults at approximatelyfight angles to one another, consistent withCoulomb stress changes predicted for the 1992 Landers,California, earthquake, although in detail, some of the earthquakesmay not fit this simple pattern. This shows that interactingsmaller faults may contribute a large part of thehazard in Iran and may make up the slip deficit in a limitedregion in a short period of time.The Tehran and Tabriz sequences illustrate loading offault tips by earthquakes, in which the most likely site forthe next earthquake would be the fault segment just beyondthe segment that last ruptured. At Tabriz, the sequence propagatedfrom east to west in a sequence of four earthquakesfrom 1721 to 1786. If an earthquake farther west in 1807 ispart of this sequence, it left a gap. The Tehran sequenceruptured from west to east from 958 to 1830, but the westand east ends of the Mosha fault ruptured before the centralsection did. Also, the Qazvin earthquake of 1119 is west ofthe 958 rupture, leaving an unruptured gap. These relationssuggest that although fault loading is important, strength ofindividual fault segments may allow an earthquake to skipa segment and fill it in later.The Sangavar earthquakes of 1863 and 1896 rupturedadjacent segments, but the Pambukh and Badalan earthquakesleft unruptured segments, as did the Gowk and Sirchearthquakes in southeastern Iran.Slip Rates and Recurrence IntervalsIntraplate deformation of the Iranian plateau, which isa result of the collision of the Arabian and Eurasian plates,is nonuniformly distributed and is concentrated along severalbelts that surround more stable, relatively aseismicblocks. The rates of shortening and uplift are highest in theZagros belt, close to the Arabian plate boundary, where theNE-SW shortening between Arabia and central Iran is accommodatedpredominantly by blind reverse faults with slipvectors oriented in a direction similar to that of the plateconvergence vector, and decreases northeastward. The Zagrosis characterized by rapid uplift, high seismicity, andmoderate-magnitude earthquakes on several blind reversefaults with historical recurrence intervals of 300 to 500 yrin the Lar and Qir region and medium- to large-magnitudeearthquakes on strike-slip faults around the edges of the Zagros,especially the Zagros Main Recent fault (Berberian,1995a).In contrast, lower slip rates are characteristic of the Iranianplateau away from the plate boundary. Areas such asthe Alborz Mountains and Kopeh Dagh are characterized bylarge-magnitude, shallow earthquakes accompanied by surfacefaulting separated by quiescent periods of 3000 to 5000yr on frontal reverse faults and some strike-slip faults, basedon historical recurrence intervals on the Ipak fault and theRudbar fault (Baklor-Kabateh-Zard Geli fault segments)southwest of the Caspian Sea (Berberian and Yeats, 1999).Active faults have reactivated ancient collisional zones,and the intervening blocks are more stable. Based on offsetqanats and Holocene offsets of streams, a minimum slip rateof 2.5 mm/yr and a recurrence interval of 1000 yr were estimatedfor the Dasht-e-Bayaz fault.Within central Iran, there are temporal variations in recurrenceintervals of large-magnitude earthquakes. The Tabrizearthquakes of 1273 to 1304 and 1721 to 1786, the 1209to 1405 Neyshabur-Binalud earthquakes, and the 1968 to1997 Dasht-e-Bayaz/Abiz earthquakes were temporal clustersof interactive earthquakes, although pretwentieth centuryearthquakes at Dasht-e-Bayaz/Abiz show no evidenceof clustering. However, the Sabzevar, Esfarayen, Kumes,Ipak, Nauzad, and Nosratabad earthquakes were not interactive.The presence of clustering has been demonstrated inseveral parts of the Iranian plateau, but the variability inearthquake recurrence patterns is greater than had been assumed.Could an M 8 Earthquake Strike the Iranian Plateau?Most large earthquakes in Iran for which magnitude estimatescan be made are in the range M 6.6 to M 7.2, with
136 M. Berberian and R. S. Yeatsthe largest historical earthquake M 7.7. Given a historicalrecord of several thousand years, one might conclude thatno earthquakes larger than M 7.7 could be expected. Onereason for this could be the structural complexity of thoseareas with large historical earthquakes, preventing meizoseismalzones to be longer than 75 to 125 km. Where thereare earthquake sequences, earthquake segment boundariescommonly (but not always) correspond to geometric discontinuities.For example, the Tabriz sequence formed in threeearthquakes rather than one large one, and the two Dasht-e-Bayaz, the two Gowk, and the two Sangavar earthquakesalso did not rupture in single events. The strain buildup waslarge enough for earthquakes to rupture a segment but notlarge enough for earthquakes to rupture across segmentboundaries.Could a larger event strike the Iranian plateau? Analogsfrom Mongolia (Baljinnyam et al., 1993; Bayarsayhan et al.,1996; C. Prentice, personal comm., 1997) suggest that thisis possible. Slip rates on Mongolian strike-slip faults arerelatively low. Earthquakes that occurred on relatively shortfaults were of M < 7.3, but three great earthquakes of M _-->8 ruptured long, continuous strike-slip faults: Bulnay in1911, Fuyun (across the border in China) in 1933, and Gobi-Altai on the Bogd fault in 1957. Preliminary work fromtrench excavations on the Bogd fault suggests that the earthquakerecurrence interval on the Bogd fault may be severalthousand years.The Nayband-Gowk fault (Fig. 6) has not had a largehistorical earthquake on the northern 250 km of its trace,which is also structurally the simplest part of the fault. The1981 Sirch earthquake, M 7.1, occurred at the southern endof the straight trace of the Nayband-Gowk fault where itintersects a set of northwest-striking reverse faults. East ofthe Nayband-Gowk fault, the East Neh, West Neh, and Kahurakfaults have not produced a large historical earthquakealong 360 km of the fault system north of the Nosratabadearthquake (M 7.0) of 1838 (Fig. 6). The longest strike-slipfault in the Iranian plateau is the broadly curved Dorunehfault, 600 km long, which has not produced a large historicalearthquake, although nearby shorter faults to the north andsouth have sustained large-magnitude earthquakes (Neyshabur,Jangal, Dasht-e-Bayaz; Fig. 1).If earthquake recurrence intervals on these long faultsare several thousand years long, then we conclude that allof them have been accumulating strain because the platetectonicslip deficit has not been relieved by historical earthquakesin this region. Paleoseismological investigations willbe necessary to determine the Holocene earthquake historyof these faults, but until that is accomplished, Iran shouldplan on the possibility of M 8 earthquakes on these longfaults.Implications for Southern CaliforniaLike Iran, southern California is cut by many activestrike-slip and reverse-slip faults, some of which have resuitedin interactive earthquakes. The main difference is thethroughgoing San Andreas fault, which has generated earthquakesof M -- 8. As in Iran, there are historical earthquakesoff the San Andreas fault with M > 7, including the 1992Landers earthquake, the 1952 Arvin-Tehachapi earthquake,and the poorly located 1927 Lompoc earthquake at the farwestern end of the Transverse range.However, the active faults in and adjacentto the heavilypopulated regions of the Transverse ranges, northernmostPeninsular ranges, and Los Angeles basin have generatedonly three moderate-magnitude earthquakes, all with M 7 are to be expected in the Los Angelesmetropolitan region, like those that have occurred in northwestNelson (New Zealand), northwest Argentina, and Japan.Paleoseismic excavations on the Sierra Madre faultnorth of Los Angeles revealed evidence for two events witha total displacement of 10 m, consistent with earthquakes ofM > 7 (Rubin et al., 1998).Conclusions"As the evil spirit rushed in, the earth shook and thesubstance of mountains was created in the earth. FirstMount Alborz arose; afterwards, the other ranges ofmountains of the middle earth; for as Alborz grew forthall the mountains remained in motion, for they have allgrown forth from the root of Alborz..." (Bundahishn,VIII/1:29).The long historical and archeological record of earthquakesin Iran allows unique insights into earthquake magnitudes,recurrences, segmentation, and clustering. The Neyshabursequence of four earthquakes between 1209 and 1405respected the geometric segment boundary between the Neyshaburand Binalud fault systems and may consist of twopairs of earthquakes comparable to the 1971 and 1994 SanFernando Valley, California, earthquakes. The Dasht-e-Bayaz sequence consisted of 11 earthquakes between 1936and 1997 that ruptured both strike-slip and reverse-slipfaults; pretwentieth century (historic) earthquakes in this regiondid not occur in clusters. The North Tabriz fault systemruptured from southeast to northwest in three earthquakesfrom 1721 to 1786; a previous earthquake cluster may haveoccurred in the same region (although not necessarily on thesame faults) in 855 to 856 to 958. The fault system north ofTehran ruptured in earthquakes in 958, 1665, and 1830. Pairsof earthquakes include a strike-slip reverse-slip pair in theTalesh Mountains near the Caspian Sea in 1863 and 1896,a pair of earthquakes near the Turkish border in 1840 and1843, and two earthquakes at the southern end of the Nayband-Gowkstrike-slip fault in 1981, one strike-slip and onereverse-slip.
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Patterns of Historical Earthquake Rupture in the Iranian Plateau

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