Subevent analysis for the Tabas earthquake of September 16, 1978 ...

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4 I. Sarkar et al. / Physics of the Earth and Planetary Interiors xxx (2005) xxx–xxx Fig. 1. (Continued ).
I. Sarkar et al. / Physics of the Earth and Planetary Interiors xxx (2005) xxx–xxx 5 M 0 = 0.92 × 10 19 ) Bam earthquake and (viii) the September 16, 1978 (M S = 7.7) Tabas earthquake, that we investigate in this study here (Fig. 1a). Tabas earthquake is associated with a concealed Quaternary thrust fault located at the base of a series of low hills, made up of Neogene clay deposits, separating the Shotori fold-thrust mountain belt (to the east) from the Tabas playa (to the west) (Fig. 1a). Major strike slip faults, ending in thrust faults whose displacement die away towards the ends, are widespread within east-central Iran. Such faults are a manifest of the style of shear deformation caused in the east Iranian block by the active convergence process of the Arabian and Eurasian plates and the Tabas earthquake is considered to have occurred on a listric thrust fault system (Berbarian, 1982; Walker et al., 2003). Various estimates of the Tabas hypocentral locations, based on regional (Atomic Energy Organization of Iran (AEOI); Bulletin of the Seismographic Network of Mashad University (BSNMU)) and teleseismic (National Earthquake Information Center (NEIC); International Seismological Centre (ISC); Harvard Centroid Moment Tensor (CMT) solution; Moscow Institute of Physics of the Earth(MOS); Centre of Seismologique, Euro-Mediterranean (CSEM); Berbarian, 1982 and Engdahl et al., 1998) recordings, are available. Of these, the location by Engdahl et al. (1998), that is obtained on the basis of arrival times of PKiKP, PKPdf, depth phases pP, pwP, sP in addition to regional and teleseismic P and S phases, is considered by us to be better constrained. The location errors for the estimated hypocenters may be as large as 10–15 km (Berbarian, 1979a). However, it is noteworthy that the estimated epicenters all lie within 20 km from each other, at the southeastern end of a geologically identified prominent surface break, believed to have been involved in the Tabas rupture process (Berbarian, 1982). The strike, dip and rake of the causative fault, estimated by different investigators from far-field data, are also varied. While the estimated strike, dip and slip of (i) Berbarian et al., (1979) viz. 332 ◦ N, 31 ◦ and 107 ◦ , respectively, from long period P-wave first motion data (ii) Niazi and Kanamori (1981) viz. 330 ◦ N, 30 ◦ , 110 ◦ , respectively, from moment tensor inversion of R 1 and R 2 phases from the IDA network and (iii) CMT solution viz. 328 ◦ N, 33 ◦ , 107 ◦ , respectively, are comparable, i.e. here the fault planes are striking in approximately WNW–ESE direction and having moderately high dips, (iv) the results of inversion of P and SH-waveforms available from far-field WWSSN network data viz. 355 ◦ N, 16 ◦ and 155 ◦ (Walker et al., 2003) are somewhat different viz. here the fault plane is striking in nearly N–S direction and having low dip. The former interpretation of the fault plane (strike of WNW–ESE) is more in line with the regional strike of the exposed faults in the region. Several investigations suggest that the Tabas earthquake had a complex faulting process. For example, (i) a prominent, generally NNW–SSE trending surface rupture, following the low anticlinal hills of the Tabas fold system lying to the west of the Shotori ranges, possibly associated with the co-seismic phase of the earthquake process, was identified from a detailed geological investigation (Berbarian, 1979b). This rupture, about 85 km in length, was splayed into 8–10 discontinuous and randomly distributed segments that extended over a zone of about 30 km width. (ii) Berbarian (1982) analyzed local recordings of one of the major aftershock sequences and opined that faulting had initiated at a depth greater than 8 km, at the southern end of the causative fault, near the basement, below the frontal sections of the Shotori fold-thrust belt and then propagated upwards, in a northwesterly direction along listric thrust planes to intersect the earth surface as a major rupture. As the propagating fault approached the surface, it splayed into several fault segments and ruptured the surface. (iii) Walker et al. (2003) viewed such a pattern of surface deformation as evidence of the occurrence of discrete earthquake events. These authors identified on digital topography and ASTER satellite imagery several indications of long-term and co-seismic growth within the Tabas fold system. From these they suggested that (a) such a fold growth had accommodated the major part of the complex fault thrust movement at depth and (b) the process of co-seismic rupture and long-term folding caused these apparently discontinuous, surface rupture segments. (iv) Shoja- Taheri and Anderson (1988) identified four sub-events on strong motion records of the Tabas earthquake which they inferred had occurred on the fault as the rupture propagated unilaterally in a northwesterly direction. (v) Hartzell and Mendoza (1991) analyzed strong motion records from three selected stations (Tabas, Bosroyeh and Deyhook) and also some selected WWSSN teleseismic P-waveforms. They found that the WWSSN stations located to the southeast of the earthquake
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