Modelling time-dependent stress changes of the earthquakes occured in the Lut Block (Iran)

Active tectonics of Iran is mainly determined by the oblique convergence of the Arabian Plate with respect to stable Eurasian plate. The deformation caused by the convergence is mainly accommodated through strike-slip faulting and laterally transported to the Caucasus, Talesh and Alborz thrust zones in eastern Türkiye and NW Iran. Nevertheless, the Zagros Thrust and Fold Zone (ZTFZ) extending in western and SW Iran play a key role in accommodating deformation in Central and Eastern Iran. Considerable amount of the deformation is also transferred beyond the ZTFZ within the Iranian High Plateau (IHP) to be accommodated by the thrusting and shorthening along the Alborz and Kopet Dagh mountain ranges. N-S running strike-slip faulting and block movements within the IHP are determined to convey the deformation to the mountain ranges in the Northern Iran. Lying parallel to the ZTFZ, The Main Recent Fault is another major tectonic property that accommodates boundary parallel component of the oblique convergence of the Arabian Plate along the Zagros Thrust Zone. The oblique convergence results in a kinematic deformation contrast between eastern and western Iran. This contrast is remedied by the strike-slip faults along the Lut Block that focused in the study. The Lut Block is bounded by Doruneh Fault in the North. Abiz, Neh, Nosratabad and Kahurak faults, all of which are dextral faults, constitute the eastern boundary. Tabas, Nayband, Golbaf-Sirch, Gowk and Bam faults extend along the western edge of the block. Excluding the Tabas Fault, the other western boundary faults are dextral. Sinistral Dasht-e Bayaz and thrusting Ferdows faults lie within the block. Geophysical studies such as seismic tomography, receiver function analysis and gravity data analysis have indicated that Moho Discontinuity lies in the depth range of 38-40 km and the brittle upper crust reaches a depth of 20 km across the Lut Block. In the present study, the time-dependent stress changes associated with the MW≥6.3 earthquakes within and around the Lut Block after the occurrence of the 31 August 1968 Dasht-e Bayaz (MW=7.1) earthquake are calculated to investigate earthquake stress interactions and future earthquake hazard. The Coulomb stress changes due to the coseismic ruptures, viscoelastic relexation of the lower crust and the upper mantle in the postseismic period and steady block motions are calculated. USGS-NEIC catalogue search has demonstrated that fourteen MW>=6.3 earthquakes occurred after 1968 including the 1968 Dasht-e Bayaz earthquake. The other earthquakes are 16 September 1978 Tabas-e Golshan (MW=7.1), 16 Ocak 1979 Boznabad, 14 Kasım 1979 Korizan, 27 Kasım 1979 Khuli-Buniabad, 11 Haziran 1981 Golbaf, 28 Temmuz 1981 Sirch, 24 Şubat 1994 Sefidabeh, 10 Mayıs 1997 Zirkuh-e Qa'enat, 14 Mart 1998 Fandoqa, 4 Mart 1999 Kerman, 26 Aralık 2003 Bam, 22 Şubat 2005 Dahuiyeh-Zarand and 20 Aralık 2010 Rigan earthquakes. Parameterizations of the earthquake ruptures have been based on the existed fault and surface ruptures mapping, source mechanism and finite-fault inversion studies. Firstly slip rates of the boundary faults are determined for interseismic stress changes calculations and and an Earth Rheology Model is constructed to be used for Postseismic (viscoelastic) stress changes. The slip rates are compiled from GPS and geological field studies. With exception of the viscosity values, the layers thicknesses, densities, seismic velocities and elastic parameters in the Earth Rheology Model are compiled from the available studies. There has been no study that gives viscosity values of the individual layers but rather an average value for a lithosphere thickness of 100 km has been computed in a study. In another study, in which postseismic stress changes were empirically estimated, viscosity value of the Earth has been approximated through judgment of the previous studies. Therefore, 6 Earth Rheology Models are tried in the study. The upper crust is purely elastic or brittle in the all models but viscosity values for the lower crust and the upper mantle are varied. The postseismic stress changes resulting from the different viscosity values are compared and discussed. Usage of different Earth Rheology Models has shown that the viscosity variations of the lower crust affect postseismic stress changes more than the upper mantle viscosity variations and, excluding the extraordinary model in which all layers are rigid, positive stress values estimated at the hypocenters of the 6 or 7 target earthquakes no matter which model in concern. The positive postseismic stress changes are calculated for the 1979 Boznabad, Korizan and Khuli-Buniabad earthquakes while the negative stress values are calculated for the 1978 Tabas-e Golshan and 2010 Rigan earthquakes from the all models. Although negative coseismic stresses are calculated for the 7 earthquakes including the 1979 Khuli-Buniabad and 1998 Fandoqa earthquakes. Nevertheless, the strong postseismic stress loads at the hypocentres of these earthquakes turned negative stresses into positive stresses. For the 1981 Sirch and 2003 Bam earthquakes, the positive coseismic stress values are exposed by the negative postseismic stresses, which are not sufficient to turn total stress into negative. These changing stress values exemplify how important time-dependent stress calculations are in reliable assessment of the earthquake hazard. From the calculated total stresses along the faults, the Nayband, Kahurak, Esfandiar, Cheshmeh, Bam, Gowk, Lakar kuh, Rafsanjan, Doruneh and Dasht-e Bayaz faults, wholly or partialy, are evaluated as the faults with higher earthquake hazard. The fact that there has been no earthquake in the eastern part of the Doruneh Fault, where the highest stress load is calculated, for 700 years and in the western part of the fault, which is under relatively moderate stress load, for thousands of years, has been interpreted as a high earthquake hazard, in consistence with the previous studies. The highly stresses Nayband Fault, which is described as a seismic gap in the literature, was lastly ruptured by an earthquake approximately 800 years ago with a slip of 3 m, as determined paleoseismologically. When the geodetically determined slip rate is also taken into account, it is interpreted that the probability of an earthquake in the order of MW=7.0 is high. As for the Kahurak Fault, which produced no earthquake since the 1838 Nosratabad Earthquake (M~7.0), it is considered that it has relatively lower earthquake hazard regarding its geodetically determined slip rate. The Ferdows Fault, which was last broken by a M=6.8 earthquake in 1948, is revealed to be under stress drop. As pointed out by the previous studies the reported historical and instrumental period earthquakes within and around the Lut Block is not sufficient to compensate for the deformations caused by the block movements. The relatively high stresses along the faults computed in the present study are assessed as compatible with the insufficient earthquakes. In this context, as the new information about the earth rheological structure, historical seismicity and fault slip rates become available, continuous updating of the time-dependent stress change estimations is considered imperative for reliable earthquake hazard interpretations.