Iranian Journal of Geophysics, 2019, Page 21 - 32 Estimation of kinematic source parameters and frequency independent shear wave quality factor around Bushehr Hoda Mahmoodi1, Habib Rahimi2, and Behzad Maleki1 1M. Sc., Institute of Geophysics, University of Tehran, Tehran, Iran 2Assistant Professor, Institute of Geophysics, University of Tehran, Tehran, Iran (Received: 25 December 2017, Accepted: 02 February 2019) Abstract In this paper, the shear wave quality factor and source parameters in the near field are estimated by analyzing the acceleration data in Zagros region. Accelerograms recorded by Building and Houses Research Center strong ground motion network have been used. The data have been considered with the magnitude of 4.7 to 6.3 collected from 1999 to 2014. In this approach, the theoretical S-wave displacement spectra conditioned by frequency independent Q was fitted with the observed displacement spectra. The source spectrum of an earthquake can be approximated by the omega-square ω2 model, which has ω2 decay for frequencies higher than the corner frequency. By following the mentioned approach, corner frequency, scalar moment, moment magnitude and frequency independent Q for each accelerogram were computed simultaneously, and the estimated error was given in the root-mean-square sense over the frequency range of interest. In this study, the generalized inversion method is used to estimate various source parameters as listed below. Thereby, it is estimated that the seismic moment range from 2.89E+23 to 1.21E+26 dyne- cm, average fault slip from 22 to 152 cm and average stress drop from 6 to 136 bars. The path average value Q are of the order Q=151-537. Keywords: Quality factor, Source parameters, Inversion method, Source spectrum Corresponding author: [email protected] 22 Mahmoodi et al. Iranian Journal of Geophysics, 2019 1 Introduction The source displacement spectrum can The main objective of this study is to be approximated from a displacement determine the kinematic source record after correcting with diminution parameters and the shear wave quality function, which accounts for geometrical factor. Kinematics is a branch of classical spreading and anelastic attenuation mechanics that describes the motion of (Boore and Atkinson, 1987; Joshi, 2006a, points, bodies, and systems of bodies 2006b; Zafarani et al., 2013). The seismic without considering the forces that hazard related to the future large caused the motion. The study of the earthquakes will be achieved by earthquake source rupture process specifying anelastic attenuation through the modeling of the observed characteristic of seismic waves released seismic radiation, in general, is done by during an earthquake. The anelastic adopting a kinematic approach to attenuations of seismic wave in any area describe the seismic source (Beroza and described by a dimensionless quantity Spudich, 1988; Zhang et al., 2003). The called quality factor Q (Knopoff, 1964; kinematic description of the earthquake Mahood, 2014). Up to now, a few source has the advantage of using a research have been devoted to the limited number of parameters to investigation of the issue of attenuation characterize the source, such as fault size, characteristics of the Iranian crust, on the final slip distribution on the fault, the which some researchers have worked rupture velocity, and the slip duration such as Nuthii (1980), Mitchel (1995), (rise time). and Rahimi and Hamzehloo (2008). In We chose to study the attenuation of the present work, the generalized S-waves based on the fact that S-waves inversion method was used to determine are more destructive compared to the P source parameters and quality factor of waves. The amplitudes of shear waves shear waves based on the proposed are about five times larger compared to techniques of Fletcher (1995) and Joshi the amplitudes of longitudinal waves and (2006a, 2006b). The acceleration data their periods are longer by at least a recorded by Building and Housing factor of 3. These facts arise from Research Center (BHRC) strong ground differences in wave propagation motion network was collected to reach velocities and the related differences in our goals. In this study, two main goals the corner frequencies of the P - and S - are attempted to be obtained: i) to wave source spectra. Thus, the study of estimate the kinematic source parameters the attenuation of shear waves has by using the acceleration data; ii) to important engineering implications. estimate the frequency independent shear In this study, the strong motions were wave quality factor in the Bushehr used for analyzing the source parameters region. of 14 occurred events since 1990. In this study, the source spectrum has been 2 Tectonic setting estimated based on the omega-square Alpine-Himalayan seismic belt is model (Brune, 1970), which has ω2 decay recognized as one of the seismically for frequencies higher than the corner active areas of the world. The Iranian frequency. Acceleration spectrum is one plateau, situated on this belt, has of the most direct and common data used experienced several major and to describe the frequency content of destructive earthquakes in the recent past. strong ground earthquake shaking Deformation and seismicity in this region (Hudson, 1962). is mainly due to the continental shortening between Eurasian and Arabian Estimation of the kinematic source parameters and frequency independent shear wave … 23 plates. Iranian plateau is principally its formation is the result of Arabian- divided into five major geological units- Eurasia convergence moments. It is based on remarkable tectonic history, homologous to the Zagros, which forms magmatic events or sedimentary features the corresponding southeastern limit of (Nabavi, 1976). These units are Zagros, the belt (Tchalenko and Berberian, 1975). Sanandaj-Sirjan, Central Iran, East and Ocean continental subduction zone in South-East zones and Alborz, each of southeast Iran, Makran, region of which is subdivided into a number of southeastern Iran and southern Pakistan is sub-units with specific characteristics. a 1000 km section of the Eurasian- The study area is located in Zagros region Arabian plate boundary extending from shown in Figure 1. The deformation in Strait of Hormoz in Iran to the mouth of the Iranian plateau is related to the the Indus River in Pakistan, where continuous convergent movement consumption of oceanic crust has between Arabian plate to the southwest occurred continuously since the Early and Turan platform to the northeast with Cretaceous along a north-dipping the north-northeast drift of Afro-Arabian subduction zone (e.g., Page et al., 1979; plate against Eurasia. Iran is one of the Byrne et al., 1992). The covered area in seismically active regions of the world in this study is situated in the Zagros zone which destructive earthquakes occur in western and southwestern Iran at the almost every year, causing costly losses eastern edge of the Persian Gulf. Figure 1 in human life and widespread damages. shows the location of Zagros Mountains. The Zagros Fold-Thrust Belt as a part of The work presented here is complicated the Alpine-Himalayan orogenic belt is information on earthquakes that occurred one of the youngest and most active 300 km far from Bushehr province, as a collision zones on the earth (Snyder and result, the study area is situated in the Barazangi, 1986) which extends about Simply Folded Belt of the Zagros 1500 km from Tarus Mountains in Mountains in South-Western of Iran. It southeastern Turkey to the fault in the seems that earthquakes in the Zagros belt east of Strait of Hormoz in the Persian are limited to the upper continental crust Gulf. Highly seismic regions of Alborz- with depths less than 20 km with average Azarbayjan covered the north and the magnitude less than 6 (Jackson and Fitch, northwest of Iran, which constitute a part 1981). The largest earthquake in recent of the northern limit of Alpine- decade has occurred in this region was Himalayan orogenic belt. Alborz- Kaki earthquake with Mw=6.2. The Azarbayjan major province is a distribution of historical earthquakes significant belt of seismicity that covers around Bushehr is depicted in Figure 2, the northwestern Iran and southern of the showing that the area has experienced Caspian Sea. Continental collision zone more than five large earthquakes with in the northeast, Koppeh-Dagh, magnitudes greater than 6 from 4th B.C to constituting a part of the northern limit of 1900 (Ambraseys and Melville, 1982; the Alpine-Himalayan mountain belt and Ambraseys, 1988). 24 Mahmoodi et al. Iranian Journal of Geophysics, 2019 Figure 1. Tectonic and Geomorphological map of the SE Zagros (Mohammadi and Bayrak, 2015) Figure 2. Historical and instrumental earthquake around Bushehr city 1900-2013. Estimation of the kinematic source parameters and frequency independent shear wave … 25 3 Data and analysis the shear wave window is shown in The strong motion data recorded by Figure 4. For this aim, each acceleration BHRC network are used in this study. All record was corrected for drift and accelerographs are the Digital SSA-2 baseline. Next, the data of two horizontal type with the threshold of 10 gals at the components were rotated to determine the sampling rate of 200 samples, recording transverse and radial component of the signals from three components. This ground motion along the source-receiver configuration yields a flat acceleration azimuth direction. The time series, 푥(푛) response between the frequencies of 0.01 of both components (n is the sample to 50. In the study area more than 200 number) were processed in the following strong motion time series were recorded manner. First, 푥(푛) was band pass by BHRC network during 1999 to 2014; filtered to obtain, say, 푏(푛). Following however, 121 accelerograms which have Kinoshita (1994), for the selection of S- a good signal to noise ratio were selected wave section in 푥(푛), an acceleration to estimate the source parameters and envelope (푛) = [푏(푛) + 퐻푏(푛)] , was shear wave quality factor (Figure 3).