كليدواژه :
كاهندگي , امواج تراكمي , روش نرمال‌سازي كدا , سنندج- سيرجان , فارياب
چكيده فارسي :
گستره مورد مطالعه ناحيه فارياب، در جنوب شرق زون دگرگوني سنندج - سيرجان و در مجاورت گسل معكوس اصلي زاگرس MZRF قرار دارد. اين ناحيه جزو معدود نواحي در گستره سنندج - سيرجان است كه فعاليت لرزه اي بالايي از خود نشان ميدهد. زمين لرزه 9 اسفند 1384 با بزرگاي 0/6=Mw در اين ناحيه رخ داده است. به‌رغم بزرگي قابل توجه، آسيب قابل ملاحظه‌اي حتي به ساختمان‌هاي ضعيف خشت و گلي روستايي وارد نشد كه مي‌تواند نشانگر توان محيط در جذب بالاي انرژي امواج باشد. پس لرزه هاي اين رويداد جهت محاسبه كاهندگي امواج تراكمي مورد استفاده قرار گرفته است. جهت برآورد رابطه وابستگي فركانسي ضريب كيفيت امواج P به روش نرمال‌سازي كدا، از 431 پسلرزه به‌دقت تعيين محل شده، استفاده شده است. در اين روش ضريب كيفيت امواج تراكمي در 5 بازه فركانسي در منطقه فارياب تعيين شده است. رابطه وابستگي فركانسي براي امواج طولي در منطقه مورد مطالعه به صورت Qp=23 f 0.78 به دست آمده است. رابطه وابستگي فركانس براي پرتوهاي منتشر شده در دو امتداد شمال غرب - جنوب شرق و شمال شرق - جنوب غرب به‌ترتيب به صورت Qp=10 f 0.94 و Qp=25 f 0.75 است كه مقدار ضريب كيفيت در بسامد مرجع 0/1 هرتز كمتر از 200 ميباشد. بنابراين ميتوان نتيجه گرفت كه اين منطقه از لحاظ لرزه‌خيزي و زمين ساختي فعال ميباشد. كم بودن فاكتور كيفيت كه دلالت بر كاهندگي نسبتاً زياد دارد نشانگر فعاليت لرزهاي زياد اين ناحيه است و با انتظار ما از ساختار پوسته اين ناحيه كه زون دگرگوني است چندان انطباق ندارد. اين كاهندگي زياد احتمالاً ناشي از زون خورد شده در اثر زلزله هاي متعدد اين ناحيه است.
چكيده لاتين :
Seismology has an important role in identifying earth structure using seismic waves. The amplitude and frequency of these waves change when they pass into the earth due to anisotropy and heterogeneity. Seismic waves decay as they radiate away from their sources, partly for geometric reasons because their energy is distributed on an expanding wave front, and partly because
their energy is absorbed by the material they travel through. The energy absorption depends on the material properties.
The amplitude of seismic waves decreases with increasing distance from earthquake, explosion, and impact sources. How this amplitude decrease, occurs, how rapidly it occurs, and how it depends on the frequency of the seismic waves is fundamentally important to the efforts to describe Earth structure and seismic sources.
Attenuation of seismic waves is expressed with inverse quality factor (Q-1) and helps us to understand the physical laws governing the propagation of seismic waves in the lithosphere.
The observed seismic-wave amplitudes usually decay exponentially with increasing travel distance after the correction for geometrical spreading. These decay rates are proportional to the Q−1 values which characterize the spatial attenuation of seismic waves.
The study area is located in Fariab region, south-east of Sanandaj-Sirjan metamorphic zone and adjacent to the Main Zagros Reverse Fault (MZRF). This rejoin is among the rare regions located in Sanandaj-Sirjan area which has high seismic activity. Seismicity in this area has a north east- south west trend. Depths of events are relatively low, in the range of less than 40 km.
The 28 Feb. 2006 earthquake with a magnitude of Mw=6.0 has taken place in this region. Despite the remarkable magnitude of this earthquake, no significant damage was reported, even for low strength buildings such as clay buildings. This phenomenon may indicate a significant attenuation of the elastic wave in the area. Aftershocks of this event have been recorded by a temporary network. The well-located aftershocks, with hypocentral distances less than 100 km, have been used for estimation of the P-wave quality factor.
The attenuation of P-waves has been estimated using waveforms of 431 well-located aftershocks. The attenuation of P-wave (QP-1) has been estimated at 5 frequency bands (1.5, 3, 6,12,24 Hz) using extended coda normalization method. The estimated QP values show highly frequency dependency. The frequency dependent relation for longitudinal waves in the study area has been derived as Qp=23f-0.78. The QP values have been estimated by using waveforms with two major directions to investigate the probably existed attenuation anisotropy. The frequency dependent relation in two directions of north west-south east and north east-south west were calculated as Qp=10f-0.94 and Qp=25f-0.75, respectively.
It has been observed that the quality factor value at the reference frequency of 1 Hz is smaller than 200, which is reported for seismically active regions. Therefore it could be concluded that this region is active in terms of seismicity and tectonics.
The small values of the quality factor which demonstrates the relatively high attenuation indicate high seismic activity of this region. This is not so compatible with what we expect for the earth’s crust structure in this area with the metamorphic zone. This high attenuation is probably due to the crushed zone affected by various earthquakes in this region.
