تعيين سرعت امواج لرزه‌اي و مدل سرعتي پوسته فوقاني در ناحيه شبكه لرزه‌نگاري شيراز

Determination of velocity of seismic waves and upper crustal velocity model in Shiraz seismological network

كشور ايران در ميان دو صفحه سنگ‌سپهري (ليتوسفري) اوراسيا و عربستان قرار دارد كه با آهنگي حدود mm/yr 25 هم‌گرا مي‌شوند. كوتاه‏شدگي ناشي از اين هم‌گرايي با فرورانش در مكران، چين‌خوردگي‌‌ و گسلش تراستي در كمربند كوهستاني زاگرس در جنوب، البرز و كپه‌داغ در شمال و نيز با لغزش در تعدادي گسل‌هاي امتداد‏‏لغز مهم (اغلب با روند شمال-جنوب) در ايران مركزي، آشكار مي‌شود. همين‌طور صفحه ايران يك ناحيه پهناور فشارشي در طول كمربند فعال كوه‌زايي آلپ-هيماليا است كه در بين صفحه عربي در جنوب غرب و سپر پايدار اوراسيا در شمال شرق قرار گرفته است. از اين‌رو ايران يكي از نواحي فعال لرزه‏خيز جهان محسوب مي‌شود. در اين ميان، ناحيه شيراز به‌سبب قرارگيري در نوار لرزه‏خيز جنوب ايران (ايالت لرزه‏زمين‌ساختي زاگرس) درخور توجه است. به‌منظور بررسي و تعيين نسبت سرعت امواج لرزه‏‏اي Vp/Vs و سرعت انتشار فازهاي متفاوت، داده‏هاي زمين‌لرزه‌‌‏هايي كه از 2002 تا 2009 در ايستگاه‏هاي شبكه لرزه‏نگاري شيراز ثبت شده بود، مورد بررسي قرار گرفت. در اين تحقيق با رسم منحني زمان-سير فاز‏هاي ثبت شده براي يك منطقه به مركزيت شيراز و شعاع تقريبا 150 كيلومتر سرعت فازهاي متفاوت Pg، Pn، Sg و Sn ‏به‌ترتيب برابر با 02/0 ± 16/6Vpg = ، 02/0 ± 71/3Vsg = ، 1/0±8/7Vpn = و04/0 ± 77/4Vsn = تعيين شد. همچنين با استفاده از زمان‌‌‌‌‌‌سير و زمان رسيد امواج لرزه‏اي ثبت شده در شبكه لرزه‌نگاري شيراز، متوسط نسبت سرعتي VP/VS برابر با 77/1 به‌دست آمد. درضمن در اين تحقيق 78 زمين‌لرزه‌‌ از مجموع زمين‌لرزه‌‌‏هاي ثبت شده در اين منطقه كه دقت مناسبي داشتند، انتخاب شد و با استفاده از داده‏هاي 78 زمين‌لرزه‌‌ موردنظر، با به‌كارگيري نرم‌افزار ولست، ساختار سرعتي در ناحيه مورد بررسي تعيين ‏شد. نتايج به‌دست آمده حاكي از وجود سه ناپيوستگي در عمق‌‌هاي 6، 10 و14 كيلومتر است، همچنين سرعت امواج تراكمي در لايه اول 68/5 كيلومتر بر ثانيه، لايه دوم 88/5 كيلومتر بر ثانيه لايه سوم 54/6 كيلومتر برثانيه و نيم فضا 66/6 كيلومتر بر ثانيه به‌دست آمد.

Iran is located between two lithospheric plates (Euroasia, Arabian) and these two plates converge toward each other with a velocity of 25 mm/yr rate. Shortening that produced from this convergence with Makran subductin shows faulting and folding in Zagros orogenic belt, Alborz and Kopedagh in north and sliding in sum strike slip faults with north-south trend in central Iran. The NW-SE trending Zagros fold and thrust belt, extends for about 1800 km from a southeast of the East Anatolian Fault in northeastern of Turkey to the Strait of Hormuz, where the north-south trending Zendan-Minab-Palami fault system separates the Zagros belt from the Makran accretionary prism. The Zagros range currently accommodates almost half of NS shortening between the Arabia and Eurasia. This active fold and thrust belt is subdivided into five morphotectonic units: the High Zagros Thrust Belt, the simple Fold Belt, the Zagros Fordeep, the Zagros Coastal Plain and the Persian Gulf-Mesopotamian lowland. The studied region in this paper located in Fars province, in the High Zagros Thrust Belt, north Eastern part of this region is Abarkuh desert that located in central Iran is less active than the other border region around studied region. The west and southeastern part of this region is located in Zagros seismotectonic province that a lot of earthquakes were seen. For study of velocity of seismic waves and upper crustal velocity model in Shiraz region the recorded data by Shiraz seismic network during 2002 to 2009 were used and for seismicity, International Seismological Centre (ISC) catalog, Harvard Centroid Moment Tensor (CMT) catalog and historical earthquakes catalog (Ambraseys and Melville, 1982) were used. Crustal structure of this region is a particular issue that is not yet resolved in the studied region. In order to study the crustal structure of studied region, we use Shiraz network that have 5 stations, To assesse the velocity model, subset of 78 events was selected that recorded by minimum of 4 stations, with an azimuthal gap less than 270?, residual RMS less than 0.3s and uncertainties in epicenter less than 6 km and depth less than 10 km. Consequently using these events, a crustal velocity model obtained with VELEST software for the upper crustal velocity model beneath studied region. The calculated velocity model for the studied region showed three discontinuities in 6, 10 and 14 kilometer depths. P wave velocity has been obtained 5.68 km/s, 5.88 km/s, 6.54 km/s and 6/66 for the first layer, second layer third layer and half space, respectively. Plotting Tsj-Tsi (S arrival time to stations i and j respectively for same event) versus Tpj-Tpi (P arrival time to stations I and j respectively for same event) for all events and all stations, Vp /Vs ratio was computed about 1.77 with 908 arrival times. Comparison of obtained VP/VS value with other research results shows that there is not a significant difference between them. Also, local velocity curves for Pg, Pn, Sg, and Sn phases are obtained in the study area by using the data base 2002 through 2009. The slopes of these curves give crustal P and S velocities of 6.16±0.02 and 3.71±0.02 kms-1, and Moho P and S velocities of 7.8±0.1 and 4.78±0.04 kms-1, respectively.