توموگرافي دوبعدي سرعت امواج حجمي فشارشي در پوستة فوقاني با استفاده از زمين‌لرزه‌هاي محلي شمال باختر ايران

2D local earthquake tomography of P-wave velocity in the upper crust of NW Iran

در اين پژوهش با استفاده از زمان‌سير امواج حجمي P ثبت شده از زمين‌لرزه‌هاي محلي، توزيع دوبعدي ساختار سرعتي در پوسته بالايي شمال باختر ايران تعيين شده است. وجود خطاي زياد در تعيين عمق رويدادها سبب مي‌شود تا تنها تغييرات جانبي سرعت بررسي شود و از تغييرات عمقي آنها صرف‌نظر گردد. براي اين مطالعه، از چندين مجموعه داده شامل داده‌هاي ايستگاه‌هاي دائمي شبكه تبريز، وابسته به مؤسسه ژئوفيزيك دانشگاه تهران (1996 تا 2013 ميلادي)، داده‌هاي ايستگاه‌هاي موقت نصب شده توسط پژوهشگاه بين‌المللي زلزله‌شناسي و مهندسي زلزله (در بازه زماني آوريل تا جولاي 2004 ميلادي) و داده‌هاي ايستگاه‌هاي موقت نصب شده توسط دانشگاه تحصيلات تكميلي علوم پايه زنجان (2009 تا 2011 ميلادي) استفاده شده است. از كل زمين‌لرزه‌هاي ثبت شده، تنها 940 زمين­لرزه براي توموگرافي انتخاب شدند. در اين پژوهش، ابتدا يك مدل اوليه سرعت به‌همراه يك برآورد اوليه براي ضريبهاي زمين لرزه‌ها (شامل مكان و زمان وقوع زمين‌لرزه) تعيين و سپس مقدار تأخر و يا تقدم زماني فازهاي دريافتي توسط ايستگاه‌ها نسبت به‌مقدار پيش‌بيني شده توسط مدل اوليه محاسبه شده است. اين مقادير توسط برنامه SIMULPS14 به مدل سرعتي دوبعدي برگردانده شده و يك تفاوت آشكار در ساختار سرعتي در دو طرف گسل شمال تبريز را آشكار نموده است. علاوه بر آن، يك بي‌هنجاري كم‌سرعت در قسمت مركزي گسل شمال تبريز مشاهده مي‌شود كه با ضخامت زياد رسوبات در آن ناحيه متناسب است.

NW Iran is part of the complex tectonic system caused by the interaction between Arabian plate, Anatolia and Eurasia. The North Tabriz Fault (NTF) is one of the main structural features of the region and is considered to be the eastern termination of the Gailatu-Siah-Chesmeh-Khoy fault (Karakhanian et al., 2004), which merges with the Maku and the Nakhichevan dextral strike-slip faults and continues to move farther east. Part of the northward motion of Arabia is transferred to Anatolia by this complex system of faults and, the oblique orientation of the motion relative to the Zagros mountain range,results in the partitioning of the motion between shortening and thickening in the Caucasus and right-lateral strike-slip motion along the NTF. In this research, we investigated the laterally two-dimensional velocity structure in the upper crust of NW Iran (mainly around the NTF) using local earthquake P-waves tomography. Several data sets were utilized, including Pg phase pickings of the Tabriz Network permanent stations governed by Institute of Geophysics, University of Tehran (1996 to 2013), temporary seismic stations installed around the North Tabriz Fault by International Institute of Earthquake Engineering and Seismology (IIEES) (April to July 2004) and temporary seismic stations installed by Institute for Advanced Studies in Basic Sciences (IASBS) (2009 to 2011) that merged with our data set so as to improve ray coverage in the eastern parts of the study area. The merged data set, recorded by 72 stations, consisted of more than 20,000 local earthquakes out of which, only 940 earthquakes were good enough to be selected for the local earthquake tomography. The velocity structures were resolved via a simultaneous solution of the coupled hypocenter and velocity model programmed in SIMULPS14. The tomographic images obtained from the linearized inversion are dependent on the initial velocity models and hypocenter locations. We primarily calculated the initial velocity model through the use of the 940 earthquake selected datasets. The time difference between the observed phase arrival time and predicted arrival time was then calculated and called travel time residuals. The residuals were further used as inputs for SIMULPS14 simulator to be converted into velocity model, which would in turn be used to adjust earthquake location parameters. Following four iterations for our inversion process, we obtained a 2D velocity tomogram that clearly showed different velocity structures on the two sides of the NTF. The velocity contrast across the NTF might have been caused by existence of different kinds of rocks on the two side of the fault trace. The North Tabriz Fault is an active and steep strike-slip fault generating strong structural differences around its surficial trace. It is a WNW–ESE trending fault in which the motion is concentrated on the fault at a rate of 7 mm/year. Such a strong rate of sliding explains the clear structure difference on the two sides of the fault. An anomalous low velocity feature can be seen in the central part of NTF. Comparing the velocity tomogram with the geological map of the region, one can observe that there exists a thick sediment basin in the same area. The low velocity anomaly is probably related to the thick, low velocity sediments deposited in that area.