Bollettino di Geofisica Teorica ed Applicata Vol. 61, n. 4, pp. 469-486; December 2020 DOI 10.4430/bgta0314 Joint inversion approach to estimate earthquake source parameters using InSAR observations and strong ground motion data (2008 Qeshm earthquake in Iran) Z. GOLSHADI1, M.S. PAKDAMAN2 and M. REZAPOUR1 1 Institute of Geophysics, University of Tehran, Tehran, Iran 2 Department of Environment and Energy at Science and Research Branch, Islamic Azad University, Tehran, Iran (Received: 9 October 2018; accepted: 31 December 2019) ABSTRACT We define the geometric and kinematic characteristics of the fault activated during the 10 September 2008 Qeshm earthquake, from the joint inversion of three InSAR observation sets (L-band, C-band, and X-band) and permanent displacement obtained from strong ground motion data. Our best-fit solution for the mainshock is represented by a reverse fault with a left-lateral component, dipping 54°SE with a maximum slip of ~200 cm at the centre of fault. The more realistic surface movements in three directions are extracted from the modelled displacements too. The fault investigated in this study affected the sedimentary cover approximately at a depth of Hormoz salt while it does not reach the basement and surface. The Hormoz salt and Gurpi formations have a vital role as a barrier against slip. Coulomb stress changes suggest that this faulting originates the vertical growth of the Laft anticline with no effect on the Ramkan syncline. Our findings show a significant relationship between this earthquake and the annual continental convergence in NE direction. Key words: interferometry, strong ground motion, source parameter, slip distribution, stress transfer. 1. Introduction The 2008 Qeshm earthquake occurred at 14:30 IRST (11:00 UTC) on 10 September with the magnitude 6.0 on the moment magnitude scale, in Qeshm province, southern Iran. The reports on the mechanism of the mainshock of the Qeshm earthquake are shown in Table 1. Nissen et al. (2010) investigated the depth and geometry of reverse faulting and the influence of Hormoz salt as a barrier of slip propagation over four years with interferometric synthetic aperture radar (InSAR) and teleseismic data. The link between buried reverse faulting and surface folding was examined in Nissen et al. (2011), who suggested the detachment folding, not a forced one. Lohman and Barnhart (2010) also used the InSAR data and, in addition to source parameters of faulting, investigated the stress triggering between the November 2005 and September 2008 Qeshm earthquakes. The Global Centroid Moment Tensor (GCMT) solution provided by the Harvard group is consistent with a nearly pure reverse-slip motion. The U.S. Geological Survey (USGS) fault-plane solution is very similar to the GCMT solution, but a low seismic moment release is preferred. The locations of the mainshock and its 11 most © 2020 – OGS 469 Boll. Geof. Teor. Appl., 61, 469-486 Golshadi et al. Table 1 - Mechanism of the mainshock of the 2008 Qeshm earthquake for the best fault with uniform slip derived from seismic and geodetic observations adopted from various studies. The locations correspond to the epicentral coordinates for RHDRC and IGUT agencies except for GCMT, which corresponds to the centroid. Also, the location values reported by other authors correspond to the coordinates of fault-plane-centre. FM indicates the focal mechanism. Legend: RHDRC, Road, Housing and Development Research Center; IGUT, Institute of Geophysics, University of Tehran; GCMT, Global Centroid Moment Tensor. Reported by Width Length Depth Strike Dip Longitude Latitude Rake Slip MW MN ML FM (km) (km) (km) (°) (°) (°) (m) RHDRC - - 6 - - 55.58 26.95 - - 6 - 6.1 - IGUT - - 9.5 - - 55.829 27.002 - - - 6 - - 9 15 12 71 58 55.83 26.74 99 0.26 6.1 - - GCMT 9 15 12 234 33 55.83 26.74 76 0.26 6.1 - - Nissen 7.7 12.8 5.5 34 50 55.89 26.89 55 0.65 6 - - et al., 2010 Lohman and 6 10 6.2 30 55 55.94 26.88 67 0.8 6 - - Barnhart, 2010 significant aftershocks [obtained from Road, Housing and Development Research Center (RHDRC)] are shown in Fig. 1. Earthquake source parameters are a noteworthy database for synthesisers in successive seismological modelling. Even though we know the source parameters of the 2008 Qeshm earthquake using various studies, including the Earth’s surface deformation field, body wave modelling, microseismicity, and rupture characteristics [GCMT: Lohman and Barnhart (2010) and Nissen et al. (2010)], there is a significant difference in the reported locations and mechanisms relying on a single method alone (Fig. 1 and Table 1). Some of the applied techniques like teleseismic body-wave modelling, a way of modelling the source using waveform data of body wave phases (Langston and Helmberger, 1975), programmed by McCaffrey (1988), can constrain the centroid depth of the earthquake more appropriately than the routinely low-pass filtered solutions reported by the GCMT catalogue (Nissen et al., 2010). On the other hand, the presence of inhomogeneity in the real Earth, but not in the model, and complications from the Earth’s crust and outer core causes errors in modelling results. Thus, the distance range for the choice of recorded waveforms in teleseismic body-wave modelling is essential (Nissen et al., 2007). Also, modelling via seismic waveform cannot construct and model the high-frequency part of waveforms (Sokos and Zahradnik, 2008) and it is difficult to distinguish the real fault plane solution without data on surface faulting and ground deformation (He et al., 2016). SAR chain processing and modelling the surface deformation field, according to Okada (1985), have some advantages, compared with other geophysical methods, such as: covering the wide-area, quick monitoring, continuous measurement (Massonnet et al., 1993), acceptable high resolution, precision, and being inexpensive (Boerner et al., 1997). Factors that cause limitation in InSAR results are atmospheric perturbations (Zebker et al., 1997) and a direct path from the source to the receiver that leads to determine one component of displacement in the direction of the line of sight (LOS). Also, the InSAR data does not set constraints on the location where the slip initiated. 470 Earthquake source parameters by InSAR observations and strong ground motions Boll. Geof. Teor. Appl., 61, 469-486 Fig. 1 - Location of Qeshm Island and its earthquake sequence (IGUT) in Iran. The faults (Hessami et al., 2003) and the mainshock 2008 Qeshm earthquake reports and its aftershocks (retrieved from RHDRC) are also shown. The rectangles indicate the causative fault. Red star, pentagon, plus, diamond, and triangle are GCMT, RHDRC, Nissen et al. (2010), IGUT, and Lohman and Barnhart (2010) locations for the Qeshm earthquake, respectively. The plotted locations correspond to the epicentral coordinates for RHDRC and IGUT agencies except for GCMT, which corresponds to the centroid. Also, the plotted locations for other authors correspond to the coordinates of fault-plane-centre. The Ramkan and Laft structures are also shown. The aims of this paper are twofold. First, we document the source characteristics of the Qeshm mainshock using a comprehensive set of geodetic data that includes not only freely available InSAR but also local strong ground motion data, which are used as extra geodetic data. Second, we use the mainshock to investigate its effects as induced stress on surface structures. The strong focus of this paper is on the link between buried faults and the surface structure of Qeshm Island, the exact centroid point, slip distribution, and focal mechanism of this earthquake. Despite the absence of rupture on the surface, the main concern arising from the Qeshm 2008 earthquake is whether events of similar magnitude could affect the formation or deformation of anticlines and synclines on the ground. Due to the many differences (up to 20 km) obtained from reported locations of this earthquake and consequently the reported causative fault (Fig. 1 and Table 1) and to compensate the limitations of the methods mentioned above, permanent displacements obtained from the strong ground motion data, are used jointly with the displacements obtained from InSAR 471 Boll. Geof. Teor. Appl., 61, 469-486 Golshadi et al. observations. A joint approach is described for three couples of ascending and descending SAR data, i.e. L-band, C-band, and X-band interferograms and strong ground motion data, to precisely assess the geometry and location of the rupture process associated with this earthquake. It allows us to improve the source parameters and simultaneously modify the solution for the moment tensor. Lastly, the three components of displacement, and slip distribution are computed and its effects on surface structures, as induced stress, are examined. Meanwhile, single inversions are also performed on the L-band, C-band, and X-band observations, separately, and their differences are shown and discussed, too. 2. Geological and seismotectonic setting Qeshm is the biggest Persian Gulf island located parallel to Iran’s southern coastline, between the latitudes of 26°32΄N and 26°59΄N and longitudes of 55°15΄E and 56°17΄E. Iran lies within the zone of collision between the Eurasian and the Arabian plates (Talebian and Jackson, 2004). In the vicinity of Qeshm, the annual rate of convergence is about 25 mm (Vernant et al., 2004). Based on the existing correspondence between Qeshm Island’s big anticlines and Zagros’ anticlines, as well as the external sedimentological and tectonic similarities of the island with Zagros, Qeshm can be included in the southern part of the Zagros Mountains (Amrikazemi et al., 2004). Some of these anticlines and synclines are shown in Fig. 1. The stratigraphy of the formations in the Qeshm Island includes: Hormoz Series, Mishan formation dating back to the late Miocene, with an estimated thickness of 100 m, Aghajari formation dating back to the late Pliocene, Qeshm limestone unit with a thickness of 4-5 m dating back to 25 to 30 thousand years ago, Dulab conglomerate dating back to the early Holocene, Suza sandstone with a thickness varying from 3 to 4 m dating back to 4 to 5 thousand years ago, and the late Holocene sediments (Samadian, 1982; Nissen et al., 2010).