doi: 10.1111/j.1365-3121.2008.00797.x

Source model for the Mw 6.1, 31 March 2006, Chalan-Chulan Earthquake (Iran) from InSAR M. Peyret,1,2 F. Rolandone,3,4 S. Dominguez,1,2 Y. Djamour5 and B. Meyer3,4 1

Universite´ Montpellier 2, Ge´osciences Montpellier, 34095 Montpellier, France; 2CNRS, Ge´osciences Montpellier, CNRS, 34095 Montpellier, France; 3Universite´ Pierre et Marie Curie – Paris 06, UMR 7072, F-75005 Paris, France; 4CNRS, UMR 7072, F-75005 Paris, France; 5 National Cartographic Center, Tehran, Iran

ABSTRACT We use InSAR to measure deformation and kinematics of the Mw = 4.9 Borujerd (2005 ⁄ 05 ⁄ 03) and Mw = 6.1 Chalan-Chulan (2006 ⁄ 03 ⁄ 31) earthquakes that occurred in the Zagros fold-andthrust belt. The focal mechanism of the 2006 event is consistent with right lateral strike-slip motion and the event ruptured the Dorud-Borujerd segment of the Main Recent Fault. An Envisat interferogram spanning the 2006 event shows peak ground deformation of 9 cm in the satellite line-of-sight along a 10 km long fault portion. The interferogram spanning the 2005

Introduction The Zagros range extends for 1500 km from southeastern Turkey to Hormuz Strait and Persian Gulf (Fig. 1). This active fold and thrust belt is composed of deformed sediments of the Arabian margin and has grown since Early-Middle Eocene in response to convergence and ongoing collision between Arabia and Eurasia plates (e.g. Tchalenko and Braud, 1974; Berberian, 1981; Agard et al., 2005; Ghasemi and Talbot, 2006). Crustal shortening in the Zagros is expressed by active folding and thrusting associated with a widely distributed shallow seismicity (depth < 20 km) (e.g. Berberian, 1995; Maggi et al., 2000; Talebian and Jackson, 2004; Engdahl et al., 2006). The Arabia-Eurasia suture, delimiting the northern extension of the Zagros range, is marked by the Main Zagros Reverse Fault (MZRF), a north-dipping thrust that separates the Sanandaj-Sirjan metamorphic belt from the Zagros fold-and-thrust belt (e.g. Berberian, 1995; Agard et al., 2005, 2006; Ghasemi and Talbot, 2006). In the northwestern Zagros, the MZRF is cut by the Main Recent Correspondence: Michel Peyret, Universite´ Montpellier 2, Ge´osciences Montpellier, 34095 Montpellier, France. Tel.: 33 467 14 36 69; fax: 33 467 14 36 42; e-mail: [email protected] 126

earthquake is rather related to atmospheric artefact than to ground deformation. Dislocation models of the 2006 ChalanChulan event indicate dextral slip amounting to a maximum of 90 cm at a depth of 4 km. The predicted vertical displacements are in good agreement with differential levelling data. The 2006 event filled only a small part of the seismic gap located between large M = 7 events that occurred in 1909 and 1957.

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Fault (MRF), a 650-km long segmented right-lateral strike-slip fault almost parallel to the MZRF (Fig. 1, Wellman, 1966; Tchalenko and Braud, 1974; Talebian and Jackson, 2002).

According to recent studies, the MRF has accommodated 50–70 km of right-lateral offset (Talebian and Jackson, 2002), but the uncertainty on the onset of motion does not provide a reliable geological slip-rate (J. Jack-

Fig. 1 Seismotectonic map of northwest Zagros (Iran). Major active faults are shown as black lines. Historical and instrumental seismicity is from IIEES earthquake catalogue. Yellow dots refer to earthquakes occurring after 2006 ⁄ 03 ⁄ 30. Meizoseismal areas of the 1909 Silakhor earthquake, the 1958 Nahavand earthquake and the 2006 Chalan-Chulan earthquake are, respectively, reported from Ambraseys and Melville (1982), Tchalenko and Braud (1974) and National Geoscience Database of Iran (NGDIR 2006). Boxes indicate the studied area. In the general location map (upper right corner), GPS vectors with respect to stable Eurasia frame are from Hessami et al. (2006) and Vernant et al. (2004).  2008 Blackwell Publishing Ltd

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............................................................................................................................................................. Recently, the area northwest of the 1909 rupture has been shaken by the 2005 ⁄ 05 ⁄ 03 Mw = 4.9 Borujerd and the 2006 ⁄ 03 ⁄ 31 Mw = 6.1 ChalanChulan earthquakes (Fig. 1). This last event induced severe damage in the Chalan-Chulan District with 68 fatalities and more than 1400 people injured (NGDIR 2006, http://www.ngdir.com/GeoPortalInfo/ subjectInfoDetail.asp?PID=561).

The 2005 and 2006 events: seismicity and InSAR data

Fig. 2 Simplified seismotectonic map of the Main Recent Fault and adjacent structures adapted from Geological Survey of Iran (GSI) geological maps and Background DEM image from SRTM data (http://www2.jpl.nasa.gov/srtm). Seismicity since 2000 is from IIEES with stars denoting the 2005 and 2006 events. Focal mechanisms of the 2006 event from body wave modelling (courtesy of J. Jackson) and from Harvard CMT solution are indicated. The simplified cross-section across NW Zagros is modified from McQuarrie (2004).

son, personal communication; Meyer et al., 2006; Meyer and Le Dortz, 2007). Neither the short-term geological slip-rate nor the geodetic interseismic strain accumulation has yet been documented. Several studies (Vernant et al., 2004; Authemayou et al., 2006; Reilinger et al., 2006) have used the available GPS data for estimating a slip-rate of 2–7 mm yr)1. Whatever its current rate, the fault is known to have produced several destructive earthquakes. In the last 100 years, four destructive earthquakes (1909 Ms = 7.4 Silakhor, 1957 Ms = 7.0 Farsineh, 1958 Ms = 6.6 Nevahand and 1963 Ms = 5.8 Karkhaneh earthquakes) have occurred on distinctive segments of the fault. The most destructive was the 1909 earthquake that occurred in the Silakhor valley, southeast of Dorud (Fig. 1). It was  2008 Blackwell Publishing Ltd

strongly felt in a large area including Borujerd, Kirmanshah (3418¢N– 4704¢E) and Hamadan, where 128 villages were damaged and more than 6000 fatalities reported (Ambraseys and Melville, 1982). This earthquake occurred on a 45-km-long fault segment of the MRF running from east of Dar Astaneh to 25 km northwest of Dorud. Discontinuous surface breaks have been documented, but the tectonic break remains poorly described. Down-to-the-north, 1–2 m high scarps offsets have been reported without compelling evidence for strike-slip motion (Ambraseys and Monifar, 1973; Ambraseys and Melville, 1982). Nevertheless, the mechanism of this event is quoted as quasi pure right-lateral strike slip (EMMA database of European-Mediterranean Seismological Center).

According to the International Institute of Earthquake Engineering and Seismology (IIEES), the epicenter of the 2006 ⁄ 03 ⁄ 31 Mw = 6.1 earthquake locates at 48.9E and 33.62N in the central part of the Silakhor basin, close to Chalan-Chulan village (Fig. 2). In agreement with the distribution of aftershocks and the strikeslip focal mechanism, consistent with right-lateral faulting (strike = 314, dip = 54, slip = )180 NEIC; strike = 313, dip = 78, slip = )174 Harvard CMT), the event occurred along the MRF. The hypocenter depth, poorly constrained, is reported between 12 and 27 km (Harvard, NEIC, IIEES). The best fitting body wave model is displayed in Fig. 3 (courtesy of J. Jackson) with corresponding parameters listed in Table 2. The good distribution of P and SH waveforms further constrains the source parameters. The improved fault plane solution (strike = 318, dip = 63, slip = 174) is similar to the Havard CMT solution, with a better constrained centroid depth of 6 km. The seismic moment is 1.52 · 1018 Nm. According to IIEES, two foreshocks with magnitude Ml = 4.6 and Ml = 5.2 preceded the earthquake and several Ml > 4 aftershocks were also recorded (Ml = 4.9 and Ml = 5.3, 2006 ⁄ 03 ⁄ 31, Ml = 4.1, 2006 ⁄ 04 ⁄ 01). Although rockfalls and old landslides were reactivated close to the MRF near Chalan-Chulan (NGDIR report, 2006), no clear tectonic surface break was reported along the Borujerd or Dorud segments of the fault. Radar interferometry (InSAR) estimates ground surface deformation along the satellite line-of-sight from phase difference between two repeatpass radar images (e.g. Massonnet and 127

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Fig. 3 Minimum misfit solution for the 31 March 2006 Chalan-Chulan earthquake, calculated by inverting P and SH body waves (courtesy of J. Jackson). The focal spheres show P (top) and SH (bottom) nodal planes in lower hemisphere projections; closed and open circles represent the P- and T-axes respectively. Fault plane orientation is defined by strike = 318, dip = 63 and rake = 174. Centroid depth is 6 km. Seismic moment is 1.52 · 1018 Nm. Observed (solid) and synthetic (dashed) waveforms are plotted around the focal spheres; the inversion window is indicated by vertical ticks, station codes are written vertically and station positions denoted by capital letters. The STF is the source-time function, and the scale bar below it (in s) is that of the waveforms. Table 1 List of processed interferograms with associated time interval and perpendicular baseline. Time interval 31 11 25 20

January–11 April 2005 April–25 July 2005 July–29 August 2005 February–1 May 2006

Envisat ASAR images have been acquired in descending pass (track = 192, swath 2 with mean elevation angle of 23).

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Table 2 Source parameters of the 2006 Chalan-Chulan earthquake from Harvard CMT solution, body wave solution (courtesy of J. Jackson), and from inversion of interferometric data with preferred uniform and variable slip models.

B^ (m) 150 365 145 540

Feigl, 1998; Bu¨rgmann et al., 2000; Lohman and Simons, 2005). The measurement accuracy is a fraction of the radar wavelength (k = 5.66 cm for the Envisat ASAR instrument). To analyse the 2006 Mw = 6.1 and 2005 Mw = 4.9 events, we processed four interferograms (Table 1) with DIAPASON software (CNES, 1996). The interferograms combine Envisat ASAR images acquired in descending orbits, swath 2 (mean elevation angle is 23). We processed Single Look Complex (SLC) images provided by ESA, with a 40 m resolution in azimuth and ground-range directions. Precise orbits estimated with the DORIS system help in removing the orbital phase signature. The 90-m SRTM digital elevation model (Farr et al., 2007) allows removing the topographic phase signature and georeferencing the interferograms. The two differential interferograms, one including the coseismic deformation of the 2006 event and the other spanning the 2005 event are given in Figs 4 and 5a respectively. The interferogram including the 2006 Mw = 6.1 event shows clear fringes within the Dorud Valley (Fig. 4). The fringes locate north of the MRF and delineate two distinctive N140E trending lobes that parallel the fault. A conspicuous lobe is made of three concentric fringes and locates southeast of the epicenter. Another lobe made of two poorly discernible circular fringes can also be distinguished northwest of the epicenter. For both lobes, fringe systems close up near the elevated topography that marks the MRF trace, corroborating the latter fault as the source of the earthquake. The fringe pattern indicates a ground-to-satellite range shortening for the southeast lobe (about 9 cm) and a ground-to-satellite lengthening for the northwest lobe (about

Model

Strike

Dip

Rake

USGS PDE Harvard CMT Body wave InSAR uniform slip InSAR variable slip

314 313 318 320 320

54 78 63 60 60

180 )174 +6 180 180

Centroid (km)

Length (km)

Width (km)

Average slip (m)

Moment (Nm)

1.07 0.40

1.0 · 1018 1.71 · 1018 1.52 · 1018 1.70 · 1018 1.58 · 1018

12 6 13.5 20

3.9 6

Rake is given according to Harvard CMT conventions.

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Fig. 4 Coseismic interferogram for 31 March 2006 earthquake, combining 20 February and 1 May 2006, Envisat ASAR images. Phase information has been masked where coherency is low, mainly over high-relief regions. Arrow is the radar look direction referring to the descending track used. The star locates the epicenter of the 2006 event.

(a)

5 cm), as expected from the horizontal and vertical components of the coseismic displacement field of a strike-slip fault. The lack of coherence on the high-relief zones prevents observing symmetrical lobes south of the fault but the fringe pattern is consistent with a primary right-lateral slip on a 10–20 km long MRF fault-portion. The loss of coherency has both temporal and spatial origins, because of high vegetation coverage and steep slopes respectively (Zebker and Villasenor, 1992). The 2005, Mw = 4.9 earthquake, has a Harvard CMT solution (strike = 316, dip = 74, slip = )161) similar to that of the 2006 event. The interferogram of Fig. 5a spans the 2005 event. Although it has little coherence, a faint one-fringe circular pattern is discernible. The fringe coincides with the area delineated by the conspicuous northern lobe of the 2006 event. This fringe reveals either ground deformation or changes in the tropospheric delay between the acquisitions. Because the

(b)

(c)

Fig. 5 (a) Interferogram spanning the 3 May 2005 earthquake by combining images acquired on 11 April and 25 July 2005. A faint fringe is observed and has a location and an elongated shape similar to the northern lobe seen in the 2006 interferogram. (b) Preseismic interferogram combining 31 January and 11 April acquisitions. (c) Post-seismic interferogram combining 25 July and 29 August acquisitions.

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interferograms preceding and following the event but sharing one scene with the ÔcoseismicÕ interferogram do not show a similar fringe pattern (Fig. 5b,c), one may favour a tectonic origin. However, the noise level of the interferogram (Fig. 5b) is very high. Such a small earthquake, whatever the hypocenter depth, would have probably induced ground deformation over a smaller zone than that suggested by the fringe. The fringe occurs 15 km south of the 2005 epicenter, as reported by IIEES, and we favour the hypothesis of an atmospheric artefact.

Modelling of the 2006 earthquake

Fig. 6 Model parameter tradeoffs. Results of the least square inversions, assuming a pure strike-slip mechanism, are plotted on a geodetic moment vs. interferometric phase rms graph. This graph displays the respective influence of smoothness constraint (symbols) and dip angle (curves). White region delimits the acceptable range of solutions. The best compromise between InSAR rms and geodetic moment is for a smoothness constraint of 10)6and for dip angles of 50–60.

Fig. 7 Along dip vs. along strike slip distribution for the 2006 Chalan-Chulan earthquake. Colours and contours show the magnitude of right-lateral slip.

We model the Chalan-Chulan coseismic interferogram using a direct stochastic method based on the Neighbourhood Algorithm (Sambridge, 1999a,b; Peyret et al., 2007), and a standard weighted linear leastsquare inversion method (Tarantola, 1987; Du et al., 1992; Peyret et al., 2007). Both assume a shear dislocation in a uniform elastic half space (Okada, 1985). The Young modulus and PoissonÕs ratio are set to 75 GPa and 0.25 respectively. We solve for the slip distribution on a-priori fault geometry by minimizing the misfit between measured and predicted coseismic interferometric phases. This incorporates a weighting matrix expressing our confidence in each interferometric phase value. We unwrap the interferogram with SNAPHU software (Chen and Zebker, 2002). As ground deformation spreads out over most of the coherent zone, we evenly subsample it to 400 m spatial resolu-

Fig. 8 Model fits to InSAR data. Observed (left), simulated (centre) and residual (right) interferograms of the deformation of the 2006 Chalan-Chulan earthquake in UTM coordinates. The dashed white box shows the extent of the north-dipping fault plane (Fig. 7). Residues are mainly located in the northern part of the Silhakor basin likely affected by atmospheric artefacts.

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(b)

Fig. 9 (a) Map of the vertical displacement as predicted from our preferred model. Numbers indicate the leveling benchmarks. Colours and contours show the displacement magnitude. (b) Vertical displacements along the Borujerd-Razan road. Differential leveling measurements (dashed line) are compared to the vertical deformation predicted by InSAR (continuous line).

tion. The analysis of the 2005 postseimic interferogram (highly coherent and devoid of deformation) gives an rms of about 2 cm in the satellite line-of-sight. This rms value provides a reasonable estimate of the noise level expected for the 2006 interferogram. We impose the strike and location of the fault plane so that it intersects the surface along the MRF (Fig. 4). The down-dip width of the fault is limited to 11 km in agreement with the shallow centroid depth as provided by body wave modelling (Fig. 3, Table 2). First, we tested the dip angle and searched for admissible distributed  2008 Blackwell Publishing Ltd

dislocation models varying the northeastward dip angles between 20 and 80. Each tested fault plane is 34 km long and 11 km wide, and has been discretized. We impose a pure strikeslip mechanism as stated by seismology and perform a direct least-square inversion for each value of the dip angle. We use various smoothness constraints upon the slip distribution, the smoothness being defined as the L2-norm of the Laplacian of the slip model. The rms misfit to the subsampled interferogram is estimated for every model. Decreasing the smoothness leads to solutions with higher

geodetic moment and lower rms (Fig. 6). However, slip models with smoothness lower than some threshold do not significantly improve the fit to the data, and result in unrealistic slip gradients (Peyret et al., 2007). The solutions are plotted on a geodetic moment vs. rms graph (Fig. 6). The tradeoff between these parameters becomes better when dip angle increases from 20 to 50–60, and then gets poorer as dip angle increases from 50–60 to 80. Thus, in agreement with seismology, we consider that the likeliest dip is about 60. Such a value may be explained by the re-activation of a former Zagros inner-thrust by the MRF (Fig. 2). Retaining this dip, we searched for a simpler model with uniform slip on a rectangular fault patch. We used a stochastic approach based on the Neighbourhood Algorithm, and solved for seven parameters (centroid coordinates, length and width of the rectangular patch, dip-slip and strikeslip components). This exploration of the model space converges unambiguously towards a pure strike-slip solution (dextral strike-slip = 1.07 m, dip-slip = 0.01 m). The centre of the patch (UTM coordinates are X = 303 km and Y = 3727 km) locates at a depth of 4.8 km. The dimensions of the rectangular patch are 13.5 and 3.9 km, along the strike and dip directions respectively. The geodetic moment is 1.7 · 1018 Nm. All these parameters are consistent with the body wave focal solution (Table 2). The rms phase misfit is 1.95 cm along the satellite line-ofsight. The only significant residue locates in the northern part of the Dorud valley. It is a one-fringe roundshaped pattern that can be attributed either to a local un-modelled ground deformation, or most likely to an atmospheric artefact. The uniform slip analysis reinforces the assumption of a pure strike-slip dislocation for the variable slip model. The slip distribution for our preferred variable slip model (dip angle 60, smoothness constraint 10)6, and geodetic moment 1.58 · 1018 Nm, equivalent to Mw 6.1) is presented on Fig. 7. It shows a simple elliptical pattern of slip reaching a maximum of 1.1 m at a depth of 4 km. Most of the slip (>20 cm) occurs on a 20-km long and 6-km wide portion of the 131

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............................................................................................................................................................. fault (Table 2). As for the uniform model, the phase residue consists of a one-fringe round-shaped pattern of atmospheric origin, located in the northern part of the Dorud Valley.

with magnitude up to 7 is still pending along the intervening segment, whose seismic behaviour remains poorly constrained.

Acknowledgements Interpretation and conclusion Our preferred model has a pure strike-slip mechanism and excludes any significant rupture close to the surface in agreement with the lack of reported surface break. The simulated and residual interferograms are presented in Fig. 8. The rms misfit equals 1.5 cm and is close to the estimated noise level (1.2 cm in the far-field). The overall fit to the data is good, with a few significant residuals located only west of Borujerd in the area probably affected by atmospheric artefacts. We used the dislocation model in Fig. 7 to predict the horizontal and vertical displacements at the surface. The maximum predicted horizontal displacement is 12 cm close to the village of Chalan-Chulan. The maximum predicted vertical displacement amounts to 10 cm (Fig. 9a) and can be compared with the result of differential levelling surveys. A first campaign was carried out in 2004 along the Borujerd-Razan road. The levelling benchmarks, shown in Fig. 9a were surveyed by the National Cartographic Center (NCC) in April 2006, immediately after the Chalan-Chulan earthquake. The resulting vertical deformation (Fig. 9b) expresses mostly the deformation induced by both the 2005 and 2006 earthquakes. Point 9 is dismissed because it is affected by measurement error as post-seismic field investigations in the area pointed out the absence of deformation at that very location. Overall, the profile reveals a long wavelength zone of subsidence (2 cm) along its northwestern part, and a zone of uplift (6 cm) along its southeastern part. The profile is consistent with the vertical deformation predicted by our best InSAR model (rms = 0.7 cm; Fig. 9a,b) giving some confidence in the results of the inversion. The Chalan-Chulan (2006) earthquake is the most recent event that took place along the MRF. It has filled a small portion of the large seismic gap between the 1909 and 1957 earthquakes. An earthquake 132

We are grateful to J. Jackson for providing us his seismological study of the 2006 event and a helpful review. We thank J. Biggs and an anonymous reviewer for constructive comments. We acknowledge support from European Space Agency for Envisat images.

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