Investigating the Impact of Gosht Earthquake on Saravan Fault System Using Coulomb Stress Transfer

Earthquake is one of the most perilous natural hazards which threatens the lives and imposes massive financial loss on societies. Due to its location in the Alpine-Himalayan belt, Iran is affected by this phenomenon. Damages caused by earthquakes are not limited to the main quake, and sometimes aftershocks also cause extensive destructions. One of the ways that can predict possible aftershock activities through the faults influenced around the main quake is the Coulomb stress transfer method. This method was first used by Stein in 1992 to investigate Landers earthquake aftershock activity in California. Bufe also employed this method in 2006 to study Denali fault earthquake in Alaska. In Iran, due to the weight of this issue, important studies have been done up to now. Jafari-Hajati and Agh-Atabai have used this method in 2012 for Silakhor and Qeshm earthquakes. In 2014, Sarkarinejad and Ansari utilized the method for Rudbar earthquake. Although studies have been conducted so far, many of Iran’s major earthquakes have not been considered in this regard, despite their importance. Among them, Gosht earthquake could be mentioned that occurred on April 16, 2013 with magnitude of 7.7 in southeast of Iran. In the present paper, using the Coulomb stress transfer method, the relationship between Gosht earthquake and the spatial distribution of the corresponding aftershock activity is investigated. It has also been attempted to study the impact of the earthquake on Saravan fault system and tried to predict the location of the aftershock events based on the obtained results. The study area is located in Sistan and Balouchestan province, between the coordinates 27°47ʹN and 28°20ʹN latitude and 61°57ʹE to 62°39ʹE longitude. The area is bounded on the north, west, and east by Khash, Iran-Shahr, and Saravan counties, respectively. Access to this area is the Khash – Saravan road, which crosses the Gosht city.
3. Material and Methods Following a library study, earthquake data gathered from the catalog of the International Institute of Seismology and Earthquake Engineering (IIEES). Earthquakes in the database were extracted from April 1973 to April 2015. Since earthquakes have been reported in different scales, the homogenization of extracted data is required. After homogenization, the magnitudes of completeness (MC) were calculated using Gutenberg-Richter power-low relation and ZMAP software package. All events smaller than MC, were excluded from the list in order to create a complete catalog and eventually 19 earthquakes remained in the desired spatio-temporal range. Okada’s analytical method and Coulomb 3.3 software package have been used to investigate the coseismic stress which was transferred by the Gosht temblor. Okada, in his method, considers the earth to be a homogeneous and elastic half-space. In this model, the faults are like rectangular discontinuities within the half-space. In this research, the focal mechanism proposed by Harvard University is used to determine the geometry of the reference fault and the corresponding rake angle. Given that Saravan fault is a reverse fault with a strike-slip component, the coefficient of friction is considered 0.6 in the calculations. Due to the tectonic setting of the area and the relatively high depth of the earthquake, it is not possible to measure the length of the failure of the reference fault, and therefore, the experimental relations provided by Wells and Coppersmith (1994) have been used. The study area is one of the most active parts of the Alpine-Himalayan orogenic belt. The active subduction of Makran as a part of northward convergence of the Arabian plate toward the Iranian micro-continent causes compressive tensions along North-South direction in this tectonic zone. Owing to such pressure, the crust have been cracked and thickened by reverse faulting and folding. A relatively large gap between trench and volcanic arc (more than 500 kms) and the wide width of the Makran accretionary prism can be attributed to the relatively low angle of the subduction slab. Since the subduction of the late Cretaceous is still ongoing, this tectonic zone can be considered seismically as one of the most active regions of Iran. Most of the seismic activity of this range can be attributed to trust faulting, which occur mainly at low to mid-depths. The lithosphere thickness in this area is estimated to be about 35 kms. Therefore, the occurrences of earthquakes up to depth of 35 kms are in correspondence with the lithospheric fault systems, while deeper events are linked with the subducting slab and Wadati-Benioff zone. Some researchers have considered Saravan fault as a right-lateral strike-slip fault. Near vertical transvers faults with a right-lateral shear sense have caused some displacements along the Saravan fault and alluvial deposits. In this regard, the activity of transverse faults can be considered more recent than Saravan fault. The most important of these transverse faults is the Gosht fault. Based on Coulomb stress transfer criterion, stress released during an earthquake can provoke the adjacent faults. The Coulomb criterion says earthquakes occur when the shear stress of the earth’s crust exceeds from the shear strength of rocks along the fault. At this time, the rock is surrendered and the stored tension energy is released at a relatively short time. Due to the lithospheric thickness of the study region, earthquakes occurred up to depth of 35 kms can be related with Saravan fault activities. Static stress transfer of Gosht earthquake can be investigated in two states: i) at depths less than 35 kms, and ii) 35-120 km. Due to the ambiguity in the focal mechanism solution and the impossibility of utilizing surface evidences to detect the earthquake fault owing to the relatively high focal depth, both nodal plains can be considered as the causing fault of Gosht earthquake. Therefore, the Coulomb stress transfer for each of the nodes is calculated separately. As it is expected, the aftershock activities occurred locally in accordance with the regions of increasing static tension. The aftershock sequence of Gosht earthquake consists of six events with magnitudes of more than 4.3, three of them have focal depths of more than 35 km and others have depths less than 35 km. Events with a focal depth of more than 35 km are related to the activity of the sub-ducting slab and events with less focal depth are associated with the activity of the Saravan fault system. The study of Gosht earthquake impact on Saravan fault confirms the significant transfer of Coulomb stress. In the case of strike-slip receiver fault the static stress change is much less with respect to a receiver fault with the transpressional geometry. It is important to note the location of three shallow afterschocks is consonant with the increasing stress area. In this case the second nodal plane has been considered as the reference fault and the Saravan fault is earmarked as the receiver. The Coulomb stress transfer is calculated by considering the receiver fault with an optimally oriented plane for depths of more than 35 kms. The reference fault with the second nodal plane geometry shows a good fit between the regions with increasing static stress and the location of focal points of three deep aftershock events. The results of this study indicate that Saravan fault system was influenced by Gosht earthquake. The occurrence of three aftershocks with low focal depths and the correlation of their epicenters with the regions of increasing static stress indicate Saravan fault was influenced by Gosht temblor. Considering relatively low Coulomb stress variations in the receiver fault model with strike-slip geometry, it can be deduced the transverse faults of the Saravan fault system are less affected by the temblor. The occurrence of three deep aftershocks is in correspondence through the second nodal plane geometry of reference fault with an optimum normal receiver fault. Also, in this study it was found that the nodal plane of 238/56/-102 is more probable as the causative fault of Gosht earthquake. However, it is not possible to determine the earthquake-causing fault by using the focal solution. In particular, because of Gosht earthquake hypocentral depth and tectonic setting of the area, morphological evidences cannot be used to determine the earthquake fault. Therefore, in this study, we are able to determine the earthquake causing fault from the nodal planes by estimating Coulomb stress transfer technique and spatial distribution pattern of aftershock events. It is worth noting that according to Anderson’s theory, the mentioned nodal plane can also be the causative fault of the earthquake.